ISSN:1359-6640    

DOI:10.1039/b506850a

出版商:RSC    

年代:2005

数据来源: RSC

ISSN:1359-6640    

DOI:10.1039/b506851g

出版商:RSC    

年代:2005

数据来源: RSC

ISSN:1359-6640    

DOI:10.1039/b507787g

出版商:RSC    

年代:2005

数据来源: RSC

摘要:

Dr Monksopened the discussion of Professor Platt’s paper: The theme of this session is detection and understanding of small scale structure in controlling atmospheric composition. Prof. Platt has clearly demonstrated the power and application of MAX-DOAS. I would like to show some recent data that further illustrates the points made in Prof. Platt’s paper. Using a CMAX-DOAS,1where all the viewing angles are imaged concomitantly it is possible to follow plume evolution owing to the ability to collect high time resolution data. Fig. 1 shows NO2slant column data taken from the roof of the space research centre (52.62 °N, 1.12 °W) at the University of Leicester, UK on the 17th January 2004. The data shows a clear progression of a tropospheric NO2signal through the different viewing geometries.Slant column NO2as retrieved from a CMAX-DOAS instrument on January 17th 2004.The wind speed on the 17th January 2004 was approximately 2 m s−1coming from a northerly direction. As the instrument is placed to the south of the city centre, plumes with an origin in the city centre are transported slowly through each viewing direction of the CMAX-DOAS instrument under these conditions. Analysis of structure of the peaks and their timings in a given viewing geometry provides information on the transport of the two plumes (09.30–35 and 09.40–45), their spatial extent and the concentration of NO2.A relatively straightforward geometric analysis can be performed to essentially “triangulate” the origin of the plume.These data demonstrate the power of MAX-DOAS for being able to measure, essentially, plume dynamics and evolution, the small-scale structure of the atmosphere.1 R. J. Leigh, G. K. Corlett, U. Friess and P. S. Monks,J. Appl. Opt., paper in preparation.Professor Planeasked: With a ground-based MAX-DOAS, the azimuthal angle can be fixed. But in a mobile system on a ship or plane, the azimuthal viewing angle will vary. Is the resulting complication of taking multi-axis spectra as a function of solar zenith angle at varying azimuthal angle a major challenge?Professor Plattreplied: There is an effect of the relative azimuth angle (i.e.the angle between observation azimuth and solar azimuth) on the result of the measurement because Mie and Rayleigh scattering are angle dependent. However, the effect of different azimuth angles can be modelled and—we think—its effect on the measurements can be corrected for. In fact such a correction would also be necessary to use MAX-DOAS with different observation azimuth angles to derive the spatial distribution of aerosol and trace gases around the instrument or to perform tomographic measurements using several instruments.Professor Stutzasked: Is it going to be possible to deriveuniquevertical profiles of trace gases if noa prioriinformation on the aerosol and the gas is available?Professor Plattreplied: We showed in our manuscript that a MAX-DOAS instrument can measure some elements of the vertical trace gas and aerosol distribution, for instance the concentration in the boundary layer, the vertical extend of the boundary layer, as well as trace gas mixing ratios and aerosol optical density above the boundary layer. The retrieval of these elements should be independent ofa prioriinformation. In the future retrieval of more elements could be possible, for instance the concentration in several layers.Dr Kolbsaid: Your paper demonstrates clearly that MAX-DOAS measurements can provide important data on the boundary layer concentrations of absorbing pollutants, provided information on the aerosol particle properties in the interrogated air mass are well enough known to describe the radiation transport. It has long been recognized that the polarization properties of detected solar radiation contain valuable information about the aerosol particles it has encountered.1Would you comment on the potential for polarization data to improve the aerosol extinction data obtained by MAX-DOAS observations? I note that colleagues in my laboratory have recently demonstrated a novel and agile method for measuring full Stokes vector polarization information for passively sensed near infrared and visible radiation.21 M. Mishchenko and L.Travis,J. Geophys. Res.1997,103, 16989–17013.2 S. H. Jones, F. J. Iannarilli and P. L. Kebabian,Opt. Express2004,12(26), 6559.Professor Plattresponded: As I described in the manuscript, the essence of our new technique is to obtain the required information on the aerosol properties from DOAS measurements of the absorption due to species with known distribution in the atmosphere (e.g.O4or O2). However, as you correctly point out, additional information may be gained from the polarisation properties of the analysed radiation. This information mostly comes from the fact that Mie-scattered radiation is unpolarised (on average), while Rayleigh scattered light is (in most viewing geometries). I think that your new method to determine full Stokes vector information of scattered radiation is likely to be of great value in this context and should be evaluated for that purpose.Professor Burrowscommented: The vertical resolution of the MAX DOAS retrieved depends on the radiation transfer conditions and the number of azimuthal and off-axis angles. This needs to be taken into account in the retrieval algorithm and I believe is taken into account. In addition the resolution depends on the multiple scattering in the atmosphere for cloud-free conditions. This depends on the aerosol, which can be determined from the O4absorption.Professor Plattreplied: I agree with your comment. In fact our radiative transfer modelling does take multiple scattering into account. The number of different azimuthal and off-axis angles used for the measurement does influence the retrieval, but we think that taking more than 5–10 off-axis angles does not much improve the retrieval of trace gas concentrations and aerosol data. You are correct in pointing out that up to now we largely limited ourselves to cloud-free conditions, however I see no fundamental limitations for retrieving trace gas and aerosol distributions in the presence of clouds.Dr RemediosAlso Dr U. Frieß, EOS, Departments of Chemistry and Physics/Astronomy, University of Leicester, UK.commented: We have been working in Leicester on the derivation of vertical information on aerosols from MAX-DOAS instruments. The results clearly depend on the number of wavelengths observed and the choice of angles with the combination of both being an important factor. However, for four wavelengths (360 nm, 477 nm, 577 nm and 640 nm), and utilising both O4optical depth and relative intensity, we find that up to four independent pieces of information can be obtained for retrieved aerosol; typical averaging kernels are illustrated in Fig. 2. We have tried to optimise, in simulations, the angles which should be used and find this depends on the type of aerosol situation that one wants to observe but the dependence is also relatively weak on instrument set-up. There might be some variation in instrument set-up, therefore, from location to location such as marine to polluted urban environments but clearly, there is potential for much well-characterised information on aerosols.Averaging kernels for the aerosol extinction vertical profile from MAX-DOAS measurements of the O4optical depth and relative intensity measured at four wavelengths (360, 477, 577 and 640 nm) and elevations of 20°, 10°, 5° and 2° for a visibility of 25 km. Averaging kernels represent the sensitivity of the retrieved profile to changes in the true profile with the altitudes of the retrieval levels indicated on the diagram.Dr Jonesasked: Would the use of spectrally resolved pressure broadened vibrational–rotational lines of O2—a species of known distribution; be useful in order to determine optical depth (often enhanced due to multiple scattering) as a function ofpressure—i.e.direct information on AMFs in aerosol and cloud?Professor Plattreplied: Many thanks for your suggestion. In principle, of course O2could be used as a species of known distribution just like O4. The disadvantage of the former gas would be that the O2spectrum (e.g.the A-band) consists of many very narrow, mostly highly saturated rotational lines. To make full use of the information a high resolution spectrometer would be required, but then the shape of the individual rotational lines could yield further information on the aerosol profile. In fact we made use of high resolution measurements of O2spectra to study radiation transport in clouds, these results are published.1 K. Pfeilsticker, F. Erle, O. Funk, H. Veitel and U. Platt,J. Geophys. Res.1998,103, 11483–11504.2 K. Pfeilsticker,J. Geophys. Res.1999,104, 4101–4116.Dr Sarkarasked: How does your MAX-DOAS compare with a combination of (MICROTOP sun photometer + micropulse lidar) with respect to profile retrievals for aerosols for both day and night? Please mention economic viability also.Professor Plattsaid: A MAX-DOAS instrument can measure some elements of the vertical aerosol distributione.g.aerosol optical density in the boundary layer, the vertical extend of the BL and aerosol optical density above the BL. In the future retrieval of more elements could be possible, for instance trace gas concentrations in several layers. MAX-DOAS measurements are only possible during daylight hours (including the twilight period). A micropulse LIDAR would probably give somewhat better vertical resolution in the lower atmosphere, also it would work during periods of darkness. However I would estimate the combination of MICROTOP plus micropulse LIDAR to be more expensive and more bulky, moreover it would not measure trace gas distributions.Dr Monksopened the discussion of Dr Jones’s paper: One aspect missing from your discussion is the effect of ozone–alkene chemistry of night-time chemistry. It has been shown that ozone–alkene chemistry can be as important as NO3chemistry in controlling night-time oxidation chemistry.1Under the conditions encountered at Mace Head in Ireland, night-time NMHC chemistry oxidation was dominated by ozone reactions up to a factor of four over NO3. Interestingly, the nitrate radical remains critical to the RO2to HO2propagation. In the context of overall oxidation chemistry the night-time reaction fluxes, particularly for alkenes, contributed up to 20% of their overall loss. How similar would your findings be for the ozone–alkene chemistry?1 G. Salisbury, A. R. Rickard, P. S. Monks, B. J. Allan, S. Bauguitte, S. A. Penkett, N. Carslaw, A. C. Lewis, D. J. Creasey, D. E. Heard, P. J. Jacobs and J. D. Lee.,J. Geophys. Res. [Atmos.]2001,106, 12669–12688.Dr Jonesreplied: We are well aware of the importance of ozone–alkene (OA) chemistry at night-time and integrations using a more detailed chemistry model show that one moves from a regime where OA chemistry dominates to one where NO3chemistry dominates as the level of NOxin the ‘plume’ is varied. The issue of the importance of mixing for OA chemistry would be bound up with the level of NOxin the plume, and might be expected to be very different in clean marine boundary layer conditions as compared with more polluted (higher NOx) urban conditions. However, even in the absence of NOx, there would still be a mixing issue, where O3levels would be expected to increase away from the surface, while biogenic VOC concentrations are maximum at the surface, and would decrease with altitude. This is another example of the mutual exclusivity of two species that can react with each other (cf. NO3and biogenic HC). In this respect, while the idealised model, as described, represents horizontal structure, it could equally well be applied to represent vertical structure and other chemical systems.Dr Shallcrosscommunicated: Indeed both ozonolysis of alkenesetc. and NO3chemistry are important in night time chemistry. We have observed, in a model containing more detailed chemistry, conditions where ozone chemistry dominates right through to where NO3dominates. The transition from one to another depending on the level of NOxemission introduced into the model. However, the major point of this paper is that inspection of any one emission scenario for NOxwill generate very different levels of NOxand its partitioning, ozone depositionetc. depending on mixing timescales.Professor Stutzcommented: The contribution by Dr Joneset al. addresses nocturnal chemistry, and in particular the reactions of NO3and the impact of transport and mixing. My comment addresses a fundamental problem when modeling the turbulent transport of reactive trace gases, which are currently treated as scalars in most chemical transport models of the atmosphere. While this topic has been addressed in a number of publications,1–10it is often described in a rigorous mathematical form that does not provide a good understanding of the nature of this phenomenon. Here I want to present a simplified derivation of the turbulent transport of reactive gases, which serves as a more intuitive explanation of this effect. It will become clear below that the modeling of some nocturnal trace gases does indeed require the consideration of chemical reactions during turbulent mixing and transport (see also ref. 11).The derivation starts from the mixing length approach first introduced by Prandtl,12and expands it by including a loss of a reactive species (Fig. 3). We will concentrate on vertical mixing, although the derivation will apply in the same way for all other directions. Turbulent mixing will be simplified by considering two adjacent volumes with trace gas concentrationsc(z1) andc(z2) at heightz1andz2(Fig. 3). We will distinguish upward and downward trace gas flux,JupandJdown, which, in the case of a scalar quantity or unreactive gas, are determined by1Jup=ωc(z1) andJdown= −ωc(z2)whereωis a vertical velocity describing the turbulent mixing (Fig. 3) and is the same in upward and downward direction. By calculating the net trace gas flux,i.e.the sum ofJupandJdown, we can then derive the gradient law, also often referred to asK-theory,12for vertical turbulent transport betweenz1andz2.2Heredis the distance between the center of our two boxes,d=z2–z1. The product ofωanddis the turbulent diffusion coefficient,K.12Eqn. (2)is the basis of the description of vertical mixing and transport in most chemical transport models.Sketch of the expansion of Prandtl’s mixing length theory to include the chemical lossL1andL2.We can now expand this derivation by introducing chemistry. As an example we will consider that only a chemical loss of the trace gas, with loss frequenciesL(z1) andL(z2), occurs during upward and downward transport, respectively. Please note thatL(z1) andL(z2) can depend on the concentrations of other trace gases in the upper and lower box, and do not have to be equal. The concentrationsc(z1,2) are reduced by a factorL(z1,2)τ/c(z1,2), whereτis the timescale for turbulent mixing. Including this loss for both the upward and downward fluxes ineqns. (1) and (2)leads to the following mathematical equation:3The final result of this calculation (last line ineqn. (3)) shows that the flux not only depends on the gradient of the concentration ∂c/∂z, but also on the gradient of the loss rate ∂L/∂zscaled with the turbulent time scaleτ. In the general case, where both formation,P*, and loss,L*, of a trace gas occur,Lcan be replaced by the net loss frequencyL=L* −P*. In cases where significant net loss can be observed the classical gradient approach (eqn. (2)) does not describe turbulent transport correctly and an approach based oneqn. (3)must be used instead.11The effect of chemistry is typically largest when the turbulent timescale is similar to the chemical timescaleL(z1,2)/c(z1,2). If the chemical timescale is much smaller thanτ, the trace gas will not be efficiently transported betweenz1andz2. In contrast, when the chemical timescale is much larger thanτthe gradient approach (eqn. (2)) can be used.Based on the above concept Hambaet al.13,14have described an approach that provides a relatively simple method to treat the transport of reactive gases in the atmosphere. We have applied and expanded this method in our modeling studies of the vertical dependence of nocturnal chemistry.11We found that at night the O3+ NO reaction, which leads to a loss of both species, can lead to a reduction of the vertical transport of NO of up to a factor of five compared to the classical gradient approach (eqn. (2)).The derivation ofeqn. (3)and the results from our modeling study show that chemistry has to be considered when describing turbulent transport of reactive trace gases in chemical transport models, in particular at night.1 D. R. Fitzjarrald and D. Lenschow,Atmos. Environ., 1983,17(12), 2505–2512.2 W. Gao and M. L. Wesely,J. Appl. Meteorol., 1994,33(7), 835–847.3 W. Gao, M. L. Wesely and I. Y. Lee,J. Geophys. Res., 1991,96, 18,761–18,770.4 S. Galmarini, P. G. Duynkerke and J. Vilà-Guerau de Arellano,J. Appl. Meteorol., 1997,36(7), 943–957.5 O. Hov, Atmos. Environ., 1983,17, 535–550.6 G. Kramm, H. Mueller, D. Fowler, K. D. Hoefken, F. X. Meixner and E. Schaller,J. Atmos. Chem., 1991,13, 265–288.7 D. H. Lenschow,J. Meteorol. Soc. Japan, 1982,1, 60.8 D. H. Lenschow and A. C. Delany,J. Atmos. Chem., 1987,5, 301–309.9 A. M. Thompson and D. Lenschow,J. Geophys. Res., 1984,89(D3), 4788–4796.10 J. Vila-Guerau de Arellano,Bull. Am. Meteorol. Soc., 2003,84(1), 51–56.11 A. Geyer and J. Stutz,J. Geophys. Res., 2004,109, DOI: 10.1029/2003JD004211.12 R. B. Stull,An Introduction to Boundary Layer Meteorology, Kluwer, Dordrecht, 1988, p. 666.13 F. Hamba,J. Phys. Soc. Jpn., 1987,56, 79–96.14 F. Hamba,J. Geophys. Res., 1993,98(D3), 5173–5182.Professor Planeasked: Vertical deposition velocities are a crude parameterisation of vertical transport to the surface, which assume the species are well mixed (usually within the planetary boundary layer). This will not be the case for the plume model. Will this affect the deposition of O3as treated in the model?Dr Jonesresponded: The model makes no attempt to reproduce the range of scales likely to be present in the real planetary boundary layer (PBL). Nor does it include realistic surface chemical processes. In this sense it cannot be expected to reproduce certain aspects of the interaction between composition and transport. This was made clear in the paper where it is stated that the model results cannot as they stand be used to produce parameterisations for use in lower resolution models.In the case of O3deposition, the major night time influence is likely to be titration by NO, which, as observations show, in the urban situation is to be emitted in highly variable localised structures. While of course the detail would be different in, for example, an LES representation of the PBL, small scale structure in O3would still be expected to be present in the more sophisticated model, leading to the non-linear effects described in the paper.Professor Heardcommented: The paper has demonstrated the potential importance of structure, chemical variability and mixing processes in the evolution of the composition of the nighttime boundary layer. We have observed behaviour which suggests similar conclusions also hold during the daytime when oxidation is driven primarily by the hydroxyl radical, OH, and ozone is generatedin situthrough the reaction of peroxy radicals with NO. In recent field measurements undertaken in Leeds by Dr J. D. Lee and Ms S. C. Smith, we have recorded signals from atmospheric OH and HO2with a 1 s integration period. The atmospheric lifetimes of these species under urban conditions of elevated NOxare very short (∼0.1 s for OH and a few seconds for HO2), and as mentioned by both Professor Ravishankara in his introductory lecture, and by Dr Jones in this paper, the atmospheric variability of such short lived species is expected to be high when observed on short timescales. Indeed, the atmospheric variability of both OH and HO2during measurements in Leeds is larger than the variability due to the precision of the instrument alone (obtained from inspection of the variation of background signal) for a 1 s integration period.The data indicate that on a 1 s timescale, both OH and HO2concentrations are highly variable, suggesting that their rates of production and/or loss (controlled byJ(O1D), otherJvalues, [H2O], [O3], [NOx], [CO] and [VOCs]) are also varying on very short timescales. Such behaviour is expected due to mixing of spatial concentration gradients in sources and sinks by turbulent eddies. The key questions are: Can we understand the origin of these rapid fluctuations, and if so, are the results important? For example, are the variations significant compared with the integrated average, and if so, are there implications for the predictions of numerical models? Does the model output depend upon the integration period of the inputs with which it is constrained? It is clear from this paper that 20 s fluctuations in NOxare significant compared with the average value taken over a longer period. The instantaneous rates of many processes, for example, the rate of ozone formation, or the rate of secondary organic aerosol formation, will depend upon concentrations of free-radicals and NOx, which are highly variable. Hence the quantity of ozone generated by the model in a given period, for example 15 min, may be different depending on whether the model is constrained by 1 s input data, and the 1 s ozone production is computed, and then averaged up to 15 min, or whether the model is constrained by a single set of 15 min averaged input data, and one value of the ozone production is calculated. The relevant chemistry is highly non-linear and a different result may be expected for the two quantitiesDr Sarkarcommunicated: I have some doubts on your ‘Representation of mixing in the model’. In one hand you neglected multi-scale turbulent mixing processes, in the other hand you used a range of eddy diffusion coefficients (104cm2s−1–3 × 107cm2s−1) which is true for a realistic range of PBL conditions.1Do you mean that the realistic range of PBL conditions does not include turbulent mixing processes?1 R. B. Stull,An Introduction to Boundary Layer Meteorology, Kluwer, Dordrecht, 1988.Dr Jonesreplied: The representation of mixing in the model is clearly not adequate to represent the detail of the processes known to be important in influencing PBL structure and mixing. The use of eddy diffusion coefficients, as per Stull’s book, provides a mechanism to reproduce, in a numerically tractable manner, realisticaveragePBL conditions for a given set of conditions (e.g.stability). In this sense the use of eddy diffusion coefficients includes the effects of turbulent mixing processes. The use of a range of eddy diffusion coefficients used in the simplified model was chosen to encompass different rates of mixing, from incomplete (low mixing) to rapid.Dr Stevensonasked: Global models have resolutions of ∼1° to 5°, and only coarsely resolve the boundary layer (∼100 m). They contain nighttime chemistry (NO3, N2O5). These models can reproduce column NO2(satellite observations) to maybe ∼30%. Is this result fortuitous, or does it indicate that small-scale structure (i.e.structure not resolved by models) is relatively unimportant?Dr Jonesresponded: Given that the range of plume and mixing conditions in the real atmosphere is expected to be large, the relevant comparison is between some average of the model cases and the unstructured model case. Simple inspection of the model results shows that there would be biases in virtually all aspects of the model results, O3deposition, NOxlifetime, VOC oxidationetc. Thus, agreement to 30% for NO2does not necessarily provide a useful constraint for other aspects of the chemistry. There are also well documented situations (e.g.ship plumes referenced in the paper) where instantaneous mixing leads, for example, to unrealistic ozone production. The conclusion is that the agreement for NO2may be to a degree fortuitous, and does not imply that large scale models are reproducing many of the subtleties of the chemistry.It would be interesting to know whether much higher resolution integrations have a significant impact on the NO2columns produced by global models. Global and indeed many regional models effectively assume instantaneous mixing and are therefore represented by one extreme of the model integrations performed in this study. The impact of the rate of mixing on nitric acid formation can be very important under certain NOxemissions and therefore NO2levels are intrinsically linked.Dr Kolbcommented: Your paper demonstrates that steep pollutant concentration gradients can be expected to occur in evolving emission plumes and, due to strong photochemical kinetics nonlinearities, that significant effects on the resulting secondary pollutant concentrations and deposition rates can be expected. You show that steep variations in urban ozone concentrations down to 20 s measurement integration times are consistent with this supposition. We routinely deploy tunable infrared laser instruments during urban measurements with measurement integration times of 1 s.1,2We often observe significant fluctuations of both primary (NO, NH3, CO2) and largely secondary (NO2, O3, HCHO) pollutant concentrations down to time scales of a few seconds, confirming your hypothesis. It is probable that not only atmospheric pollutant concentrations and deposition rates are strongly impacted by the concentrations gradients you discuss. The damage to human health and ecosystem viability may also be strongly non-linear in pollutant exposure and uptake.1 C. E. Kolb, S. C. Herndon, J. B. McManus, J. H. Shorter, M. S. Zahniser, D. D. Nelson, J. T. Jayne, M. R. Canagaratne and D. R. Worsnop,Environ. Sci. Technol., 2004,38, 5694–573.2 S. C. Herndon, J. T. Jayne, M. S. Zahniser, D. R. Worsnop, B. Knighton, E. Allwine, B. K. Lamb, M. Zavala, D. D. Nelson, J. B. McManus, J. H. Shorter, M. R. Canagaratna, T. B. Onasch and C. E. Kolb,Faraday Discuss., 2005,130, DOI: 10.1039/b500411j.Professor Duxburyopened the discussion of Dr Tuck’s paper:(1) The absorption lines in the IR spectrum of water have a small subset where collisions don’t change the vibration-rotation state. By comparing the behaviour of the special (Dicke-narrowed) lines with the normally pressure broadened lines it may be possible to unpick the collision effects of high speed atoms produced by for O3photodissociation.(2) Recent measurements in the centre of Glasgow by Wright and Duxbury using a rapid sweep quantum cascade (QC) laser spectrometer have demonstrated rapid fluctuations in methane concentration on a time of seconds, where the other traces measured at the same time, water and nitrous oxide, show little or no fluctuation. This shows the experimental measurement of rapid fluctuations of one component of a multicomponent mixture are possible using the wide tuning range an intra-pulse QC laser spectrometer.Dr Tuckresponded:(1) The ability to examine molecular behaviour quickly is very important in the present context, and I therefore await publication of Prof. Duxbury’s spectroscopic results with interest. A direct experiment using molecular beam methods to measure the velocity PDFs of molecules in the real atmosphere would be a powerful complement to fast high resolution optical spectroscopy, but it looked to me to be a very difficult experiment, even with modern rotating, slotted disc velocity selectors and high vacuum techniques.(2) Variations in line shape taken very fast and at very high resolution might be registering other processes as well as concentration fluctuations, for example a different ‘local’ velocity PDF arising from variation in one or more ‘local’ anisotropies.Professor Plattasked:(1) How do your findings relate to the phenomenon of non-local thermal equilibrium (non-LTE) in the upper atmosphere?(2) What is the role of excited oxygen atoms (O21Δ)?and commented: (3) Generally it is assumed that the dominant source of O2atoms in the singlet delta state is the UV-photolysis of ozone. However it was found by Schurath1that the photolysis of O4(oxygen dimers) by visible light typically produces about one order of magnitude more O2singlet delta molecules.1 U. Schurath,Free Radical Res. Commun., 1987,3(1–5), 173–184.Dr Tuckreplied: The phrase ‘local thermodynamic equilibrium’ conceals many difficulties hidden in the word ‘local’. How local is local? Alder’s calculations suggest that it could be limited to very brief time scales and to very short space scales. It should also be noted that, to the best of my knowledge, even the fastest atmospheric measurements show non-instrumental variability characterized by fat-tailed PDFs in temperature, wind speed and chemical mixing ratios, on scales from an earth radius down to lengths of order 10 m. I recommend ref. 1, which argues statistically mechanically for the operation of a fluctuation–dissipation theorem in association with the need to maximize entropy production; temperature remains well-defined in a system far from equilibrium.The role of O2(1Δg) could be very interesting, as you suggest, depending on how the large production rate you cite from the O4photodissociation studies by Schurath has its energy flux redistributed in air, presumably mainly at rather low altitudes. It is a good example of the need for laboratory experiments and possibly molecular dynamical simulation studies to understand the quantitative relevance for the atmosphere.1 R. Dewar,J. Phys. A: Math. Gen., 2003,36, 631.Professor HeardandProfessor Ashfoldcommented: A number of laboratory studies have highlighted ways in which the exoergicity of the various O3photodissociation pathways will map through into local excitation within the stratosphere. Dr Tuck’s paper references the work of Takahashiet al.,1who have investigated the Doppler lineshapes (and thus the translational energies) of O(1D) atoms resulting from O3photolysis at 248 nm, in the presence of various pressures of N2. Analysis allowed determination of the relative efficiencies of translational and electronic (to O(3P)) relaxation pathways, and led to the conclusion that the average translational energy of O(1D) atoms in the stratosphere at an altitude of 50 km, for example, will be about twice as large as the thermal energy of the ambient air. Standard models treat photolytically produced stratospheric O(1D) atoms as being fully thermalised before reacting with trace gas molecules like H2O, CH4or N2O. Clearly, inclusion of a more realistic, non-thermal, distribution of collision energies (and, particularly, the high energy tail to the distribution) could have implications both for relative and absolute reaction rates, and product channel branching ratios. Kharchenko and Dalgarno have investigated such effects, for the specific case of the O(1D) + N2O reaction.2Photodissociation can also lead to non-thermal product internal state population distributions. O3constitutes a text-book case in this regard also. Ground state (O(3P) + O2(X3Σ−g)) products account for ∼10% of the total dissociation yield following O3photolysis at wavelengthsλ< 310 nm; the resulting O2(X) products are formed in a broad spread of vibrational energy levels.3The O2(X) vibrational state population distribution measured atλ∼ 233 nm is bimodal, with a component attributable to formation of very highly vibrationally excited fragments carrying almost all of the available excess energy. The relative yield of these very highly vibrationally excited fragments grows substantially as the photolysis wavelength is decreased further, and accounts for some 15% of the total O2(X) yield atλ∼ 226 nm. Such findings have attracted previous interest within the stratospheric modelling community, given the realisation that reaction of ambient O2molecules with the fraction of photolytically produced O2(X) fragments formed withv≥ 26 could constitute an exoergic route for reforming O3molecules (the reverse of the fourth reaction in the Chapman mechanism O + O3→ O2+ O2). The stratospheric importance, or otherwise, of this pathway will depend on the relative probabilities of reaction and vibrational relaxation; in practice, neither experiment4nor theory5has yet found compelling evidence for the reactive pathway.1 K. Takahashi, N. Taniguchi, Y. Sato and Y. Matsumi,J. Geophys. Res., 2002,107(D20), ACH-11, DOI: 10.1029/2001JD002048.2 V. Kharchenko and A. Dalgarno,J. Geophys. Res., 2004,109(D18311), DOI: 10.1029/2004JD004597.3 J. D. Geiser, S. M. Dylewski, J. A. Mueller, R. J. Wilson, R. Toumi and P. L. Houston,J. Chem. Phys., 2000,112, 1279 and references therein.4 C. A. Rogaski, J. A. Mack and A. M. Wodtke,Faraday Discussions, 1995,100, 229.5 A. J. C. Varandas,J. Phys. Chem. A, 2004,108, 758, and references therein.Dr Tuckresponded: The comments of Profs. Heard and Ashfold are germane. One of the predictions of the mechanism we suggest is that the ‘steady state’ ratio of ground state oxygen atoms O(3P) to ozone will be higher than that observed in well-thermalised laboratory experiments, or in numerical model calculations using thermal rate coefficients. An over-population of O(3P) could conceivably have chemical consequences, particularly if there is a fat tail in its atomic speed PDF. It may further be necessary to re-examine the ‘odd oxygen’ concept in a quantitative context.Professor Planeasked: Self-sustaining vortices through the creation of ring currents are understandable in the case of a molecular-beam gas experiment. However, in the atmosphere O3photolysis creates translationally hot fragments at very low concentrations and moving isotropically. How will this generate self-sustaining vortices?Dr Tuckreplied: There is a big difference between a molecular beam of gas propagating into a vacuum and an atmospheric photodissociation event, as Professor Plane says. The former has its own anisotropy, a direction arising from the large, generating pressure gradient. It is, however, depending on experimental design, a largely collision-free environment off-axis. Rarefied gas dynamics is involved rather than the fluid mechanics appropriate to the atmosphere below the turbopause. It is important to remember that the translationally hot photofragments from ozone photodissociation recoil into a pre-existing set of anisotropies, in particular into a set of generating, distorting and dissipating vortex tubes and filaments appropriate to a fluid with a Reynolds number of order 1012and which of course involves all air molecules. The translationally hot photofragment energy can feed into, be sustained by and in turn help to sustain these pre-existing vorticity structures. In traditional radiative transfer calculations, the absorption of solar and terrestrial radiation is significant in the atmospheric heating rate—for example as witness the extension of the life of the Antarctic vortex induced by the loss of such heating consequent upon the almost complete destruction of ozone by the halogen-based chemistry in the ‘ozone hole’. The behaviour described in Figs. 4–6 in our paper is direct evidence of the tangible role of ozone heating. I emphasise again that the heavy tails of the molecular speed distribution are sustained by and sustain the ‘ring currents’ in the molecular dynamics simulations.I would also like to make a final point about non-linear correlations, the occurrence of fluctuations and the generation and maintenance of the heavy-tailed PDFs we see in atmospheric observations for which the atmospheric variability is greater than the instrumental noise. Suppose we have two chemical species A and B, and that we have a reaction rate coefficientkbetween them1,2, and that we also have wind speedvand temperatureT. Then if we calculate the rate of reactionk[A][B] we are convoluting two heavy-tailed PDFs; the result will be an even more skewed distribution in the rate than in either reactant, with a large contribution to the mean from rare but intense events. The same will apply to the advection of temperature,v∇T. The atmosphere, far from acting like a giant, smoothing integrator generates fluctuations and sharp gradients by virtue of its non-linearity. The variability is in some ways more important than the mean; it is what we and our instruments actually experience.1 A. F. Tuck,Philos. Trans. R. Soc. London, Ser. A, 1979,290, 477–494.2 A. F. Tuck, S. J. Hovde, R.-S. Gao and E. C. Richard,J. Geophys. Res., 2003,108(D15), DOI: 10.1029/2002JD002832, Art. no. 4451Professor Herrmannasked: Could heterogeneous reactions of O(3P) be important and represent an indirect ozone loss process?Dr Tuckresponded: Yes; it is a prediction of the mechanism that there will be an increase in the ratio [O(3P)]/[O3]. Moreover, it has been demonstrated in the laboratory that ground state oxygen atoms react with films of surfactants with almost no activation energy,1a finding which could be relevant to the atmospheric oxidation of hydrocarbon chains, given the presence of long-chain carboxylic acid films on the surfaces of both marine and continental aerosols.21 Y. Paz, S. Trakhtenberg and R. Naaman,J. Phys. Chem., 1994,98, 13517–13523.2 H. Tervahattu, J. Juhanoja, V. Vaida, A. F. Tuck, J. V. Niemi, K. Kupiainen, M. Kulmala and H. Vehkamaki,J. Geophys. Res., 2005,110(D6), DOI: 10.1029/2004JD005400, Art. no. D06207.Dr Jonessaid: You say in the paper that the relation between intermittency inTdoes not correlate with [O3]. Given the importance of O3photolysis in establishing theTintermittency implicit in your argument, can you explain why this correlation does not appear?Dr Tuckanswered: We do not have an explanation based upon direct experiment or molecular dynamical simulation. However, an argument may be made as follows. In an environment with O2and O(3P) having heavy-tailed PDFs in their speeds, the reaction O + O2+ M → O3+ M, virtually the sole atmospheric means of producing ozone, will select for the slower-moving atoms in the O(3P) population and the slower-moving molecules in the O2and M populations. It will do so because the recombination reaction has an inverse temperature dependence, and one might not therefore expect a correlation between the product [O3] itself and the intermittency of temperature. Recall that the great majority of ozone photodissociations are followed by such a recombination, at a rate sufficient to put every oxygen atom in the lower stratosphere in an oxygen molecule, in an ozone molecule and in its free state within a few months. The correlation is not withJand not with [O3] but with their product. My earlier remarks in response to Prof. Plane about the photofragments recoiling into pre-existing vorticity structures also seem apposite.Professor Planeasked: There is a clear correlationbetweenthe winter and summer flight data, rather thanamongeach group of data points in Fig. 2 of the paper. Is the overall correlation therefore really clear-cut?Dr Tuckreplied: Both the summer POLARIS flights and the winter SOLVE flights were designed primarily to test ozone photochemistry in the Arctic lower stratosphere, and often the flight paths were not ideal for our statistical multifractal analysis, which requires great circle flight legs for as long as possible under entirely autopilot control, with no vertical profiling. We have selected all such flight legs possessing an adequate number of decades of scale, see references 10 and 14 in our paper. In viewing Figs. 2 and 3, it should be remembered that each point on those graphs cost of order ∼105dollars at least and are unlikely to be repeated any time soon. The error bars on the points should be noted. Finally, the chemistry in the vortex during the 48 day period 20000123–20000312 was changing significantly, as evidenced by the large change in the ClO scaling (ref. 11 of our paper) and this could affect the ‘within SOLVE’ correlation. The ‘within POLARIS’ correlation involves a much longer period, 136 days from April to September and is certainly spread out over substantial air mass changes. Note that SOLVE does yield in Fig. 3 of the paper two or three points intermediate between the main POLARIS and SOLVE clusters. Perhaps future flights with high altitude, long endurance drone aircraft will provide better data—we are unlikely to examine this problem better through either satellites or numerical models.Dr I. W. M. Smithsaid: Your paper states (on the basis of calculations reported in ref. 1) that ‘memory of initial velocity can persist for ∼100 collisions’. I wonder about the nature of the interaction between the collision partners that is assumed in those calculations. I ask because, at least for collisions between O(3P) atoms and O2, the multiple intermolecular potentials will be anything but ‘hard-sphere’, reflecting the ability of these species to interact chemically.1 S. Chapman and T. G. Cowling,The Mathematical Theory of Non-Uniform Gases, Cambridge University Press, Cambridge, 3rd edn., 1970, pp. 93–96, 327.Dr Tuckreplied: Chapman and Cowling briefly consider the persistence of molecular velocity for ‘real’,i.e.quantum mechanical, intermolecular potentials on p. 327. On the one hand, as you say, an attractive potential might enhance energy exchange possibilities between collidants. On the other hand, compared to ‘billiard ball’ collisions, ‘real’ collisions will be fuzzier and involve structure in both the distance and angular parts of the potential, so weakening the validity of the strong collision assumption, the lack of correlation in position and momentum between collision partners before and after collision. This assumption of course underlies such common formulations as Maxwellian distributions and pressure-broadened Lorentzian line shapes, for example. I believe some molecular dynamical simulations have been done with square well potentials rather than an infinitely steep repulsion. It is possible that MD approaches could help here, given that the current state of the art is 107atoms, a number which would permit simulation of the absorption of a solar photon by a single ozone molecule in a realistically-sized bath of air molecules.Dr Taatjescommunicated: In reference to the question, raised both in the paper and in the discussion, of whether experimental gas kineticists know or appreciate the importance of velocity relaxation I would like to point out that many measurements and calculations have in fact been made on the relaxation of nonequilibrium velocity distributions of atoms. “Hot atom” chemistry in the presence of velocity moderation was described for products of nuclear reactions by Libby in the 1940s.1Non-Maxwellian distributions of velocity in Earth’s atmosphere have been calculated by application of the Boltzmann equation.2–4Laboratory measurements of, for example, translational relaxation of I atoms in C3F7I and in He,5or of hot O(1D) atoms in rare gases, N2, and O2,6have addressed the persistence of the direction of velocity as well as the relaxation of the speed distribution towards equilibrium. These experiments tend to confirm simple gas kinetic calculations and the experience of laboratory kineticists; hundreds of collisions may be necessary to relax hot atoms in the case of a large mass mismatch, but thermalization of velocity in both magnitude and direction is nearly complete within a few tens of collisions for similar-mass particles. Prudent kineticists allow many more average collisions for effective thermalization. The fact that the decay of velocity correlations is described by a power-law rather than an exponential7probably has no effect on the results of careful laboratory kinetics measurements.1 W. F. Libby,J. Am. Chem. Soc., 1947,69, 2523–2534.2 B. D. Shizgal,Planet. Space. Sci., 2004,52, 915–922.3 B. Shizgal and M. J. Lindenfeld,Planet. Space. Sci., 1979,27, 1321–1332.4 E. C. Whipple Jr., T. E. VanZandt and C. H. Love,J. Chem. Phys., 1975,62, 3024–3030.5 J. I. Cline, C. A. Taatjes and S. R. Leone,J. Chem. Phys., 1990,93, 6543.6 Y. Matsumi, S. M. Shamsuddin, Y. Sato and M. Kawasaki,J. Chem. Phys., 1994,101, 9610–9618.7 B. J. Alder and T. E. Wainwright,Phys. Rev. A, 1970,1, 18–21.Dr Tuckcommunicated in reply: Matsumiet al.contains work which was cited in our paper as refs. 29 and 38. Their conclusion was that there are likely to be very substantial overpopulations of hot O(1D) atoms, ranging from 20% to a factor of two, throughout the stratosphere. In my opinion, these are likely to be lower limits, because of the mutually self-sustaining interaction between ‘ring currents’ and translationally hot atoms and molecules; the anisotropies which have existed in the atmosphere for geological time scales will feed and be fed by the production of translationally hot atoms, as I have remarked in response to earlier questions. In the context of laboratory experiment, I am happy to acknowledge Dr Taatjes’s remarks about prudent kineticists, but I observe that measuring velocity distributions in a flow tube with pressures high enough to exhibit fluid mechanical behaviour is a far from trivial exercise, as is also true for the atmosphere.Professor Herrmannopened the discussion of Professor George’s paper: P. Warneck has suggested a heterogeneous HONO source involving PNA (peroxynitric acid) which has been included in CAPRAM 2.4.11 B. Ervens, C. George, J. E. Williams, G. V. Buxton, G. A. Salmon, M. Bydder, F. Wilkinson, F. Dentener, P. Mirabel and H. Herrmann,J. Geophys. Res., 2003,108(D14), 4426.Professor Georgereplied: The formation of nitrous acid through the chemistry of peroxynitric acid is indeed important. It has be to be underlined that peroxynitric acid can be formed in the gas phase as well in the liquid phase through the reaction of NO2and O2−as shown by the paper by Ervens, Georgeet al.1However, this chemistry is secondary in naturei.e., as it requires formation of different intermediates (as peroxynitric acid) from NO2. In this context, Lahoutifardet al.,2showed for aerosol dark chemistry that the peroxynitric acid HONO source is at a comparable level when compared to the dark chemistry between phenolates and NO2. Peroxynitric acid decomposition into nitrite is mainly important at higher pH, while at low pH (where HONO could be evaporating from the liquid) the decomposition into NO2and HO2might be at least as important.Also, this chemistry is occurring in aqueous droplets while we considered here chemistry of organic surfaces as proxies for ubiquitous organic surfaces.We demonstrated that under such conditions the NO2conversion to HONO is drastically enhanced by photochemical processes, which exceeds the rates observed on pure phenolic substrates by at least an order of magnitude. Therefore the source suggested here may be of major importance.1 B. Ervens, C. George, J. Williams, G. V. Buxton, G. A. Salmon, M. Bydder, F. Wilkinson, F. Dentener, P. Mirabel, R. Wolke and H. Herrmann,J. Geophys. Res., 2002,108, 4426.2 N. Lahoutifard, M. Ammann, L. Gutzwiller, B. Ervens and C. George,Atmos. Chem. Phys.2002,2, 215.Professor Plattcommented: Regarding the NO2+ HO2mechanism for HONO formation: peroxy radicals (HO2) can also form at night by NO3+ olefin reactions.Professor Georgereplied: This comment refers to Professor Herrmann’s question. As we have already argued, the photochemical source identified during this study is much more important than a hypothetical source by pernitric acid formed from NO2and HO2during the day; it is unlikely that HO2could contribute to HONO formingviathis pathway during the night.Professor Ravishankarasaid: I have a very simple minded comment. It appears to me that one of the needs is to get NO2−, which then would make HONO. So, is there some way of making NO2−in your system?Professor Georgeresponded: Indeed, nitrite ions might play a very important role as precursor of nitrous acid (as it is the conjugated base associated to this acid). However, in our experiments we cannot distinguish if nitrites anions are first produced or if HONO is directly produced.The mechanism we are suggesting is based on different redox reactions where electrons will be transferred, therefore nitrite anions can be formed from the reduction of NO2subsequent to a photoinduced electron transfer (see proposed mechanism).Dr Baltenspergerasked:(1) You mentioned in your paper that HONO was only formed in the presence of water. Does this mean that also the NO2uptake is enhanced in the presence of water? One could imagine that removing a product might result in an enhancement of the uptake.(2) I like your paper because it nicely explains the additional HONO source observed in smog chambers, which makes calculations of reaction kinetics complicated.Professor Georgereplied:(1) We observed that the photochemical enhancement of the NO2uptake was independent of the relative humidity but that the product distribution was affected. Traces of water vapour were indeed necessary to see some traces of gas phase HONO, even if no dependence on humidity was established. This is in line with the mechanism we are suggesting where a photoinduced electron transfer produces nitrite anions, followed by HONO production. The latter step would require some amounts of water (see our response to Professor Ravishankara’s comment). In such a two-step process, and as long as the surface is not saturated, the removal of a product would not lead to an additional uptake enhancement.(2) We appreciated your second comment. Indeed we also believe that a similar mechanism (while not identical) may occur in smog chambers. However, in a recent study in a simulation chamber a contribution by NO2to the photoenhanced HONO formation could be excluded.1Thus there might be different mechanisms explaining the “background reactivity” of smog chambers.1 F. Rohrer, B. Bohn, T. Brauers, D. Brüning, J.-F. Johnen, A. Wahner and J. Kleffmann,Atmos. Chem. Phys. Discuss., 2004,4, 7881.Dr Brauerscommented: Our recent chamber experiments (see ref. 1 or the poster by Brauerset al.) show a photo-enhanced HONO source dependent on humidity, temperature, and light, but not on the gas phase NO2concentration. This finding is not covered by the NO2uptake and HONO formation introduced in the presented paper.1 F. Rohrer, B. Bohn, T. Brauers, D. Brüning, J.-F. Johnen, A. Wahner and J. Kleffmann,Atmos. Chem. Phys. Discuss., 2004,4, 7881.Professor Georgereplied: We agree, that our mechanism cannot explain the observations in the SAPHIR chamber. The organic substrates used in the present study are most probably not present on the surface of the SAPHIR chamber, caused by the kind of chemistry and the low concentration levels used in this chamber. Thus, there might be different mechanism explaining the photoenhanced HONO formation in this simulation chamber and in the atmosphere, respectively.Dr Ammannsaid: This comment addresses the comparison made by Dr Brauers between the photochemical HONO source observed in atmospheric simulation chambers also in absence of NO2and the results presented in the paper by Georgeet al. The chemistry we are proposing is generating nitrite ion on the surface. Depending on the water and proton availability (related to the acidity of the surface), HONO may be formed and released to the gas phase. Porous media will additionally complicate the situation allowing diffusion of NO2or HONO in the bulk underneath the surface. In the end, under given specific conditions, emission of HONO must not necessarily be coinciding with the presence of NO2, but could be related to previous processing of a surface. With respect to the HONO emission observed in absence of NO2in smog chambers, nitrite or adsorbed HONO could have been formed in absence of NO2under dry conditions. After the admission of humidity later on under otherwise clean conditions, HONO could be emitted from the surfaces.Professor Georgeadded: This might play a role in other chambers, however, it cannot explain the HONO formation in the SAPHIR chamber. In the SAPHIR chamber, the addition of water in the dark did not result in release of HONO from the walls in contrast to other chambers.Professor Burrowsasked: Can you include the role of N2O4in your system as an intermediate, which may also produce a lead to the production of HONO? The presence of N2O4may complicate the interpretation of your interesting observations.Professor Georgereplied: The first experiments we made were with high concentrations of NO2(i.e., 50–100 ppm) where the influences of N2O4could have been important. Therefore, the gas phase concentration was lowered by three orders of magnitude (i.e., up to 20 ppb) without any change in our results.As the N2O4fraction depends on the square of the NO2gas phase concentration and since the surface is not saturated, we are quite confident that N2O4is an unimportant intermediate at atmospherically relevant concentration. In addition, even if a theoretical uptake coefficient for N2O4of unity is used as an upper limit, the observed uptake coefficients of NO2ofca.10−5from the present study cannot be explained by uptake of N2O4at the low NO2concentrations applied (i.e.20 ppb corresponding to <10−3ppt N2O4).Professor Donahueasked: Is this redox mechanism a significant sink of aromatics in the organic aerosol fraction?Professor Georgereplied: The redox chemistry involving NO2might not be such an important sink. However photochemistry of organics in the aerosol might an important sink with a mechanism similar to the one proposed here, see,e.g.ref. 1.1 A. Feilberg and T. Nielsen,Environ. Sci. Technol.2000,34, 789–797.Dr Coxsaid: The discovery of a photochemically induced heterogeneous conversion of NO2into HONO on organic material is an important contribution to the longstanding puzzle of the formation process for atmospheric HONO. Have the authors seen any evidence for a photochemical reaction of this type on inorganic surfaces such as mineral dust or on Pyrex glass surfaces?Professor Georgeanswered: The photochemical conversion of NO2to HONO was not observed on Pyrex or glass once these materials were extremely well cleaned. Note that only really thorough cleaning of the glass (either mechanically or using HF) completely removed any photoenhancement of the dark HONO formation on glass. We assume that this is due to the formation of products, which were not soluble in any of the solvents we used.However, such a conversion might occur, and have been observed, when redox cycles might be photoinduced, as on some specific minerals.Dr Kingcommented: The work described in this paper is exciting and it will be very interesting to apply to snowpack photochemistry.The question relates to the following chemistry1NO3−+hν→ [NO3−*]2[NO3−*] → NO2−+ O(3P)3a[NO3−*] → NO2+ O−3bO−+ H2O → OH + OH−In their paper Georgeet al. state that “In the laboratory, the effective yield of nitrite formation was found to be two orders of magnitude smaller compared to the NO2formation.” When discussing the emission of HONO and NO2from Arctic snowpacks they reference Market al.1However, a later review by Mack and Bolton2suggests that values for the quantum yields forchannels (2) and (3)are of the order 0.6–2% and ∼6%, respectively. The work of Dubowskiet al.3on the photochannels in ice demonstrates a quantum yield for channel 2 of ∼0.5% and an apparent quantum yield ofchannel (3)of 0.1%. Also the work of Chu and Anastasio4reports a quantum yield forchannel (3b)of about 0.3% in ice. These works suggest that there are not two orders of magnitude difference in the yield between these channels. Thus, these values are not inconsistent with the 1 : 2 yield of HONO : NO2from snowpack at Alert5as detailed in their paper. It is worth noting that a yield of 1 : 3 was noted at Summit Greenland6and that unpublished data from other polar snowpacks suggest that emissions of HONO from snowpacks can be zero.1 G. Mark, H.-G. Korth, H.-P Schuchmann and C. Von SonntagJ. Photochem. Photobiol. A, 1996,101, 89.2 J. Mack and J. R. Bolton,J. Photochem. Photobiol. A, 1999,128, 1.3 Y. Dubowski, A. J. Colussi, C. Boxe and M. R. Hoffman,J. Phys. Chem.2001,105, 4928–4932.4 L. Chu and C. Anastasio,J. Phys. Chem. A, 2003,107, 9594.5 H. J. Beine, I. Allegrini, R. Sparapani, A. Ianniello and F. Valentini,Atmos. Environ., 2001,35, 36456 J. E. Dibb, M. Arsenault, M. C. Peterson and R. E. Honrath,Atmos. Environ., 2002,26, 2501Professor Georgereplied: We agree, as very different numbers of the quantum yields are reported for both processes (i.e.reactions (2) and (3)). For example, referring to Table 3 of the paper of Mack and Bolton, values for&phis;(OH)(reaction (3)) between 0.8–9% are reported, whereas a number of ∼0.1% is given for&phis;(O)(reaction (2)). Thus, there is a large range published for the relative contribution of both channels. In addition, the situation might become even more complex in snow depending on the pH, amount of water in a quasiliquid layer of the ice,etc.In our paper we only would like to propose, that one reason for the variable HONO/NO2ratio observed on ice surfaces in the field may be the conversion of NO2(formed by the nitrate photolysis) into HONO by the proposed photolytic mechanism and a variable content of organics, similar to those used in our study. Since there is a tendency from the literature data thatreaction (3)is of higher importance and since additionally, HONO is much more soluble in water and shows a much higher adsorption enthalpy on ice compared to NO2, a conversion of NO2into HONO would help for the explanation of field data, at least for which a high HONO/NO2ratio was observed. This was already discussed in the paper of Zhouet al. However, the dark reaction of NO2+ H2O, as discussed in their study, is too slow to explain a fast conversion, in contrast to the mechanism proposed in the present study.From many snowpack photochemical studies it is known that not only nitrogen oxides are emitted but also many low weight organic compounds which indicate that organic photochemical processes are occurring in snow. When we take the data from Grannas, Shepson and Filley2who found organic contents of around 0.5 mg l−1of molten snow and showed a high content of lignin phenols and of high molecular weight organic compounds, we conclude that organic chromophores could be the most important light absorbers in snow over a wide range of the solar spectrum. If such organic compounds and nitrate are collocated in quasiliquid layers of snow a reaction of the photo-produced NO2with photo-activated organic mixtures is likely.1 X. L. Zhou, Y. He, G. Huang, T. D. Thornberry, M. A. Carroll, S. B. Bertman,Geophys. Res. Lett., 2002,29, 4087–4090, DOI: 10.1029/2002GL015080.2 A. M. Grannas, P. B. Shepson and T. R. Filley,Global Biochem. Cycles2004,18.Mr Glowackisaid: In your “Proposed reaction mechanism” section you mention that you take 2π spectral irradiance measurements and use them to calculateJ(NO2) by multiplying by a factor 4, giving a calculatedJ(NO2) of 0.046 s−1. The 1987 Madronich paper which you cite1as informing this factor of 4 pertains to treating the sun as both an isotropic and collimated source which is isotropically scattered. However, in this case of a photoreactor with cylindrical symmetry, the light source is not collimated. Simply multiplying the 2π spec irradiance measurements by a factor of 2 would give aJ(NO2) of 0.046/2 = 0.023 s−1, which would match much better your experimentally measuredJ(NO2) of 0.024 s−1and still be theoretically justified because of the chamber symmetry (i.e.2 × 2π = 4π, which represents spherical surface area). Can you explain why your radiation field/J(NO2) calculations have been performed this way?1 S. Madronich,J. Geophys. Res. [Atmos.], 1987,92, 9740.Professor Georgereplied: In our article we intended to calculate the actinic flux in our reactor from the 2π-irradiance measurements taken at the reactor surface. We assumed an isotropic radiation and neglected any influence of the glass reactor. By integration of the radianceL(λ) over a hemisphere with unit radius, we obtained, as shown in the cited paper by Madronich (1987), an actinic flux ofF(λ) = 2πL(λ). For the 2π-irradiance at the reactor surface we obtainedE(l) = πL(λ) by integration of the radianceL(λ) weighted by the cosine of the zenith angle over the hemisphere. We concluded that the hemispheric actinic flux is therefore twice the measured irradiance. As described by Madronich (1987), we introduced a second factor of two to account for the actinic flux originating from both hemispheres with identical magnitude, and hence used in total a factor of four to convert the measured 2π-irradiance into the total spherically integrated actinic flux in the reactor. Due to your question we reconsidered our approach and conclude that our model is likely inadequate for the given conditions, because we assumed an isotropic radiance over all angles. But actually we confirmed experimentally only that the irradiance in the reactor was axial symmetric and the assumption of a spherical isotropic radiance is not supported by the reactor geometry. Hence we should calculate the flux of an axial symmetric radiance trough a sphere, which we are presently not capable to do. Therefore, we can not judge your proposal of a factor two for the conversion. In conclusion we recommend to rely rather on the measuredJ(NO2) and accordingly scale the photolysis frequenciesJ(HONO),J(HNO3),J(NO3−) predicted by the model down by a factor of two.Professor Donaldsonopened the discussion of Professor Abbatt’s paper; Professor Abbatt indicated that nitric acid adsorbed on cirrus particles could perhaps undergo photochemical reaction. We have measured photochemistry involving nitric acid adsorbed on cold ice surfaces as well as from frozen nitric acid solutions at several acid concentrations.1Our experiments utilize a quartz-windowed Pyrex ice chamber coupled to the CIMS apparatus described in your paper. This chamber could be cooled to approx. 220 K using a circulating chiller bath. The output of a 1000 W Hg–Xe lamp was passed through a 10 cm long water filter to remove infrared light, then mildly focussed through the top quartz window to the ice surface. Various long-pass optical filters could be inserted following the IR filter to measure an approximate wavelength dependence to the photochemistry. Gas phase products which evolved from the ice chamber were entrained in a flow of N2and introduced to the CIMS flowtube. NO2and HONO were detected as NO2−and F−HONO, respectively.Fig. 4 shows the results of a representative experiment using a frozen 10 wt.% nitric acid solution, performed at 228 K at a relative humidity of 0% (relative to the ice sample). Upon exposure to the full (except for IR) output of the lamp, a rapid rise is observed in both NO2and HONO signals. These both become much less intense when light ofλ< 295 nm is removed; the relative changes are quite different for the two species, however. There is a monotonic decrease in product flux as the wavelength cutoff is moved further to the red; again, the NO2signal is relatively more sensitive to these changes than that from HONO.Plots of NO2−(upper trace, dark line) and F−HONO (lower trace, lighter line) mass spectrometer signalsvs. time from illumination of a frozen sample of 10 wt.% nitric acid at 228 K and 0% RH. The various bars indicate the different long-pass wavelength cutoff filters used, with the cutoff wavelength (in nm) indicated.Qualitatively similar results were observed for different concentrations of frozen nitric acid solution and different relative humidities (up to 100%) and temperatures (between 220 K and 250 K). No product evolution was seen (at these low temperatures) using frozen solutions of KNO3. Both NO2and HONO products were observed upon illumination atλ> 295 nm when gas phase nitric acid was deposited onto pure water ice in the monolayer coverage regime.A few recent studies have reported NO2production from illuminated ice and snow samples doped with nitrate.2–4To our knowledge the present report represents the first direct laboratory observation of HONO production from illuminated frozen nitric acid solutions. The low temperature regime probed here suggests that NOxcould be photolytically released from nitric acid adsorbed to cirrus cloud particles or from NAT- and NAD-containing PSCs.1 T. Bartels-Rausch and D. J. Donaldson, to be published.2 Y. Dubowski, A. J. Colussi and M. R. Hoffmann,J. Phys. Chem. A, 2001,1054928.3 L. Chu and C. Anastasio,J. Phys. Chem. A, 2003,107, 9594.4 E. S. N. Cotter, A. E. Jones, E. W. Wolff and S. J.-B. Bauguitte,J. Geophys. Res., 2003,108(D4) 4147; DOI: 10.1029/2002JD002602.Professor Abbattreplied: These are very interesting results. They highlight the need to understand the chemical state in which nitric acid exists when present on an ice (or other) surface. If nitrate is formed, then its absorption coefficient is shifted into the red compared to that of nitric acid. Together with the ability of ice particles to scavenge nitric acid from the upper troposphere, andviagravitational settling to then give rise to vertical redistribution of NOyspecies, this photochemical degradation mechanism may also have a role to play in establishing the NOxto HNO3ratio.Professor Cohencommented: Working at the atmospherically appropriate concentration matters for NO2release from NO3−photolysis mentioned in Professor Donaldson’s comment as well as the uptake described by Professor Abbatt. We observe NO2release proportional to [NO3−]01/2Dr Kärcherasked:(1) Most laboratory experiments have been performed under ice-saturated conditions, while atmospheric ice crystals frequently experience substantial super-saturations. Would you agree that uptake mechanisms in cirrus clouds involve growing ice surfaces?(2) Trace gas uptake in growing ice crystals imply a volume uptake, butin situdata are often plotted as surface coverages. Shouldn’t this way of interpretation be abandoned, as it is not reflecting the true physical mechanism of uptake? At most theΘ’s are inferred upper limit values, but adsorption enthalpiesetc. must not be inferred.(3) How should lab. measurements be designed (ideally) to capture the salient features of uptake on growing surfaces?Professor Abbattreplied: In work that is soon to be submitted for publication, we have extended the work presented here at the Faraday Discussion by studying the HNO3uptake profiles under conditions where ice is growing. We do this by increasing the relative humidity of the carrier gas entering the flow tube. Average growth rates are a few microns per minute, not unlike those experienced in some portions in the atmosphere. We see significant steady state uptake of nitric acid in these experiments indicative of burial. And so, in response to the first question, I would say that uptake by a burial/trapping mechanism under conditions where the ice is growing is a highly viable mechanism in cirrus clouds. I don’t believe we can yet say whether it dominates over the uptake due to surface adsorption for ice growth rates, residence timesetc. in the upper troposphere. However, it is an important uptake mode that has been overlooked until recently, as nicely highlighted by Dr Kärcher’s recent model on this subject.1In this context I fully agree that the community should be cautious in interpreting nitric acid contents of cirrus cloud particles in terms of only a surficial uptake model. And I also agree that it would be extremely difficult, if not impossible at this stage, to try to extract adsorption enthalpies from field measurements, given how strongly affected they are by varying temperature, partial pressure, total ice surface areas, and particle history.The new work on uptake under growing ice conditions that I just described is a first cut but a weakness is that the ice growth rate is not uniform down the flow tube. An experiment where the ice growth rate is uniform across the ice sample area, as could be attained in a Knudsen cell, would be preferable. However, the Knudsen cell would be constrained to relatively low temperatures. An experiment where ice particles exposed to nitric acid, where the nitric acid content is measured as the particles grow, is feasible but would not be an easy experiment to make work quantitatively.1 B. Kärcher and M. M. Basko,J. Geophys. Res., 2004,109, Article No. D22204.Dr Coesaid: In the atmosphere, homogeneous freezing at temperatures greater than −70 °C may occur from the surface inwards as the latent heat generated in the centre of the drop may heat it and prevent freezing whereas convective cooling at the surface will lead to freezing. This may trap significant HNO3, already dissolved in the drop, before freezing takes place.Professor Abbattresponded: The topic of whether homogeneous freezing occurs at the surface or in the bulk is a current, controversial topic. Assuming for the time being that freezing does indeed occur at the surface, then the mechanism you describe for trapping nitric acid seems viable. The retention efficiency will be driven by the relative rates of ice growth after nucleation and diffusion of nitric acid from the solution droplet. From recent field studies, it has been reported that gases such as HNO3are efficiently retained in the ice crystal upon rapid freezing of supercooled cloud droplets, whereas gases such as H2O2and SO2are not.1,2In an earlier publication, where we show that both SO2and H2O2have a much lower affinity for adsorption to ice than does HNO3, we suggested that all the gases are excluded from the droplet as it freezes but that the HNO3retention efficiency is much higher than the others because it is retained on the surface of the ice crystal.3In that publication we noted the consistency of this model with observations of convective enhancement factors,i.e.ratio of trace gases in convective outflow compared to levels in the absence of convection, observed during the TRACE-A field campaign.4In particular, it was observed in those field measurements that the convective enhancement factor for HNO3was less than one whereas it was larger than one for H2O2. Although both gases are very soluble in supercooled water, nitric acid remains in the ice upon freezing and so is removed from convective outflow by ice gravitational settling whereas H2O2is released from the ice particle upon freezing, and so the gas can be transported from the boundary layer to the upper troposphere.4Given the importance of trace gas retention upon droplet freezing, this phenomenon is clearly worthy of more study.1 D. Voisin, M. Legrand and N. Chaumerliac,J. Geophys. Res., 2000,105, 6817.2 J. R. Snider and J. Huang,J. Geophys. Res., 1998,103, 1405.3 S. M. Clegg and J. P. D. Abbatt,J. Phys. Chem. A, 2001,105, 6630.4 C. Mari, D. J. Jacob and P. Bechtold,J. Geophys. Res., 2000,105, 22255.Dr Coxcommented: I would like to report results obtained with a numerical simulation model which helps to interpret observations of uptake on surfaces in a flow system such as reported by Prof. Abbatt. This is illustrated in the accompanying figure which demonstrates the need to include diffusion into the ice film as well as Langmuir type surface adsorption to reproduce the adsorption curves for HNO3exposed to ice at 229 K. The surface coverage is overestimated if transfer into the bulk is not taken into account. The model is also able to reproduce the patterns observed for repeated exposure to the same film and also competitive adsorption of HNO3and HCl on an ice film.Comparison of experimental data (points) and simulated HNO3concentration changes following exposure to an ice film. Conditions as Fig. 1A of Ullerstamet al. Thin continuous line using Langmuir constant,Keq, reported in the paper; other lines usingKeqreduced by 30% and with different rates of diffusion into bulk ice film.Professor Abbattresponded: The results of Prof. Cox’s model, which accounts for both surface adsorption and diffusion into the ice film, nicely highlight a number of issues associated with nitric acid interacting with ice surfaces. First, as Prof. Cox’s model results illustrate, a significant component (30%, in this case) of what may be interpreted as surface uptake may in fact be arising from diffusive processes that occur subsequent to surface adsorption. Second, it is not clear whether the diffusive component arises from diffusion into the bulk of the ice or along grain boundaries. Third, is the chemical state of the nitric acid different for that component diffusing into the iceversusthat residing on the surface. For example, is dissociated nitric acid diffusing whereas undissociated nitric acid resides on the surface?Professor Zellnersaid: We have recently published1a simple kinetic model to describe reversible gas adsorption in CWFT experiments which helps to quantify observed adsorption and desorption profiles as well as to extract the associated kinetic quantities.In parallel to this modelling work we have extended our experimental investigations of the adsorption of acetone on ice surfaces. In particular we have concentrated on the ageing effect of the adsorption capacity. Our most recent explanation is that there exist two different types of adsorption sites which both have their own adsorption thermodynamics and kinetics. Ageing of the ice surface is interpreted as a change from one type of surface adsorption into the other which probably is associated with structural change of the ice surface.Unlike most other surfaces the ice surface is extremely dynamic implying that even at temperatures around 200 K there will be continuous flux of water from and to the surface. What do you think does a monolayer adsorption mean in the light of this dynamics? Will it hamper dynamic exchange or will adsorption always reflect heteromolecular condensation including water?1 P. Behr, A. Terziyski and R. Zellner,Z. Phys. Chem.2004,218, 1307–1327Professor Abbattresponded: This is an interesting observation commensurate with a number of studies that show that snow samples collected in the field also exhibit aging phenomena where the specific surface area decreases as a function of time. One might suspect that the nature of the binding interaction may also change as the surface structure changes. In addition to time, I can expand upon this comment by noting that interpretation of the mechanism of uptake by ice surfaces is complicated by the changing structure of the ice surface as a function of temperature. At low temperatures, the surficial water molecules have low mobility whereas at higher temperatures the “quasi-liquid” layer grows in. How does this affect the nature of adsorbate-surface interactions? In studies of the adsorption of small alcohols to ice we found typical van’t Hoff behaviour,i.e.the logarithm of the gas-surface equilibrium constant varied linearly with inverse temperature, with the larger uptake at lower temperatures, as is normally the case.1For the uptake of SO2to ice we found the opposite behavior with uptake increasing with increasing temperature, an observation which we interpreted to imply that water mobility was essential for SO2uptake to proceedviaHSO3−formation.2Lastly, in the nitric acid study presented today, the van’t Hoff plot was not linear. It does not seem unreasonable to suggest that the changing nature of the surface might be to blame.With regards to your question concerning the dynamic nature of the ice surface, I point out that the average residence time on the surface for a nitric acid molecule will be orders of magnitude longer than for water molecules. In this sense, any one adsorbed nitric acid molecule will experience a mean “field” of water molecules over its lifetime on the surface as the waters evaporate and condense. Does the interaction between the nitric acid and the water molecules affect the volatility of the water molecules? Studies from the research group of Tolbert have shown that the evaporation rate of ice surfaces are not affected when exposed to roughly 10−6torr of nitric acid,i.e.in the monolayer regime.31 O. Sokolov and J. P. D. Abbatt,J. Phys. Chem. A, 2002,106, 775.2 S. M. Clegg and J. P. D. Abbatt,J. Phys. Chem. A, 2001,105, 6630.3 M. S. Warshawsky, M. A. Zondlo and M. A. Tolbert,Geophys. Res. Lett., 1999,26, 823.Dr Kolbsaid: Your paper nicely demonstrates the complexity of heterogeneous chemical processes on real atmospheric aerosol particles and environmental surfaces, which are often mixtures of deliquesced inorganic and organic species. The role of both organic films on water and water condensed on organic surfaces needs to be understood. You measurements of anthracene uptake to and reaction on both water and organic surfaces demonstrate how complicated understanding real-world surface chemistry will be.We have recently published droplet train flow reactor uptake studies of hydrogen halides, HCl, HBr, and HI,1and the organic species, acetic acid,1as well as α-pinene, γ-terpinene,p-cymene, and 2-methyl-2-hexanol2on 1-octanol surfaces. Mass accommodation coefficients for the organic compounds, including acetic acid, were fairly large, with ∼265 K values of 0.4 for acetic acid, 0.25 for 2-methyl-2-hexanol, 0.20 forp-cymene, 0.12 for γ-terpinene, falling off gradually with increasing temperature.1,2The uptake of α-pinene was solubility limited, but exhibited an uptake coefficient >0.1 at the shortest gas/droplet contact times, setting a lower limit for itsα. The uptake of benzene was too small for us to detect, setting an upper limit for an uptake coefficient on 1-octanol of 0.001 for a 1.5 ms interaction time, consistent with your previous measurement of small anthacene and pyrene uptake coefficients on 1-octanol coated water surfaces,3but much smaller than the uptake coefficient on neat 1-octanol surfaces you report here. Interestingly, none of our measured mass accommodation coefficients for organic species showed any significant change as the relative humidity was varied between 0 and 100+%.The influence of water on the uptake behavior of the hydrogen halides on water vapor was much more dramatic (1). The measured mass accommodation coefficients for showed no significant temperature dependence. However, that for HCl, was 0.008 ± 0.001 and exhibited anegativetemperature dependence. The effect of water vapor was dramatic, at 100% relative humidity (which corresponds to less than a 10% liquid water surface coverage), the 273 K mass accommodation coefficients for HBr and HCl, had fallen to 0.21 ± 0.03 and 0.18 ± 0.05, respectively, while that for HCl had risen to 0.17 ± 0.03(i). All of the 100% relative humidity values are equal, within measurement error, to their values on pure water.1These measurements along with yours help demonstrate just how rich and complex the heterogeneous chemistry of mixed organic/water surfaces can be.1 H. Z. Zhang, Y. Q. Li, P. Davidovits, L. R. Williams, J. T. Jayne, C. E. Kolb and D. R. Worsnop,J. Phys. Chem. A, 2003,107, 6398–6407.2 H. Z. Zhang, Y. Q. Li, J. R. Xia, P. Davidovits, L. R. Williams, J. T. Jayne, C. E. Kolb and D. R. Worsnop,J. Phys. Chem. A, 2003,107, 6398–6407.3 B. T. Mmereki, S. R. Chaudhuri and D. J. Donaldson,J. Phys. Chem. A, 2003,107, 2264–2269.Professor Donaldsonreplied: When comparing the uptake coefficient of benzene on octanol (<10−3as reported here by Dr Kolb) with our higher uptake of anthracene by octanol, it might be helpful to consider the vapour pressures and octanol–air partition coefficients (KOA) of these compounds as well. We note in our report that the uptake of anthracene by oleic acid seems larger than the uptake of naphthalene by the same substrate (although one should be aware that these are measured using different methods). The room temperature vapour pressures of these compounds differ by four orders of magnitude in Pa; theirKOAvalues by two orders of magnitude. The differences between the values of these parameters for benzene and anthracene are larger still. One might therefore expect a higher propensity for anthracene to partition to the condensed phase than for benzene to do so, as we observe.Professor Herrmanncommented: I would like to stress the importance of Professor Donaldson’s studies. Current field work suggests that cloud droplets might be much enriched with regards to less water-soluble organics. Hence, it might be necessary to introduce a third compartment,i.e.the droplet surface, into advanced tropospheric multiphase models for aerosol and cloud chemistry.Professor Donaldsonresponded: Professor Herrmann raises an important point. Field studies have identified higher concentrations of organics, including polycyclic aromatic hydrocarbons (PAHs), in fog-water and aqueous aerosol particles than could be explained by Henry’s law and the surrounding gas phase concentrations.1–4Gill and Graedel5suggested that such an organic film exists on atmospheric aerosol particles, fog droplets, cloud droplets, rain droplets, and snowflakes. This suggestion has been confirmed by recent field measurements.6,7Lo and Lee8suggested that such a coating could enhance adsorption of more hydrophobic compounds such as PAHs, perhaps explaining the observed non-Henry’s law behaviour. We have recently reported both an increased solublization9and an increased mass accomodation coefficient10for PAH compounds at organic-coated aqueous interfaces. Interestingly, the effect seems to depend on the chemical nature of the organic coating, with octanol proving a better surface “solvent” for PAHs than hexanoic acid.1 (a) D. E. Glotfelty, J. N. Seiber and L. A. Lilje,Nature, 1986,325, 602; (b) D. E. Glotfelty, M. S. Majewski and J. N. Seiber,Env. Sci. Tech., 1990,24, 353.2 P. D. Capel, C. Leuenberger and W. Giger,Atmos. Environ., Part A, 1991,25, 1335.3 C. J. Schomburg, D. E. Glotfelty and J. N. Seiber,Environ. Sci. Technol., 1990,25, 155.4 J. C. Sagebiel and J. N. Seiber,Environ. Toxicol. Chem., 1993,12, 813.5 P. S. Gill and T. E. Graedel,Rev.Geophys., 1983,21(4), 903.6 H. Tervahattu, J. Juhanoja and K. Kupiainen,J. Geophys. Res., 2002107, Art. no. 4319.7 H. Tervahattu, J. Juhanoja, V. Vaida, A. F. Tuck, J. V. Niemi, K. Kupiainen, M. Kulmala and H. Vehkamaki,J. Geophys. Res., 2005110, Art. no. D06207.8 J. H. A. Lo and W. M. G. Lee,Chemosphere1996,33(7), 1391.9 B. T. Mmereki and D. J. Donaldson,Phys. Chem. Chem. Phys., 2002,4, 4186.10 B. T. Mmereki, S. R. Chaudhuri and D. J. Donaldson,J. Phys. Chem. A, 2003,107, 2264.Dr Ammanncommented:(1) This paper nicely highlights the importance of surface processes. Several aspects associated with these include the competition for two-dimensional space, enhancement of surface reactions compared to bulk reactions, effects of surfactant layers,etc. We have developed a framework model for aerosol and cloud surface chemistry and gas particle interactions with consistent terminology for transport and physico-chemical processes, which are established in a simple and straightforward flux formalism.1Exemplary simulations of time-dependent uptake processes and steady-state situations show the general applicability and convenience of the proposed framework, which are also relevant for the issues addressed in the present contribution.2(2) A specific comment concerns the slow recovery of the ms signal in Fig. 1a of the paper. From the fast drop at the moment, when the surface is exposed to the gas-phase naphthalene, I presume that memory effects are not significant in the experimental setup, and I would then expect an as fast recovery, when the lid is closed. If this slow recovery should be related to partitioning of the oleic acid substrate to the chamber walls and associated adsorption of naphthalene to this after the lid is closed, I wonder whether this does not affect the apparent uptake kinetics.1 U. Pöschl, Y. Rudich and M. Ammann,Atmos. Chem. Phys. Discuss., 2005,5, 2111–2191.2 M. Ammann and U. Pöschl,Atmos. Chem. Phys. Discuss., 2005,5, 2193–2246.Professor Donaldsonresponded: Dr Ammann points out that the recovery times observed for the mass spectrometer signal in the Knudsen cell when the substrate is isolated following exposure to the gas phase are longer using liquid substrates than solid (cf. Fig. 1 of our paper). This might be due to “creep” of the liquid substrate out of the substrate holder into the Knudsen cell proper. In this case, signal recovery would depend on the gas phase probe species (i.e., naphthalene) reaching equilibrium with this small amount of exposed liquid. As Dr Ammann correctly points out, this would introduce an error in the inferred uptake coefficient, since the substrate surface area exposed to the gas phase is no longer just that in the sample container. We believe that this effect may well be responsible for the low precision of the uptake coefficients we measure. The error estimates on the results given in Table 1 of the paper are on the order of 20–50%, which should account for this possible artefact.Professor Planeasked: In Fig. 6 in the paper, a linear regression through the experimental points at [O3] > O has a clear intercept on the ordinate. What does this indicate? One clue might emerge from comparing to the plot in Fig. 5 for oxidation of anthracene on a 1-octanol coated water surface. This shows a rapid increase tokobs∼ 0.0015 s−1when [O3] reaches 1 × 1015cm−3. If there was a similar increase in Fig. 6, this might imply that at low [O3] there is a surface-only reaction.Professor Donaldsonreplied: Professor Plane suggests that there could be a surface component to the reaction of gas phase ozone with anthracene dissolved in octanol, which is only important at low ozone concentrations. This is a very exciting possibility which we shall certainly be testing. With the experimental setup we used to obtain the results illustrated in Fig. 6, the S/N ratio becomes poor for ozone concentrations less than about 1015molecule cm−3. We are improving the experimental system and hope to have kinetic results for lower ozone concentrations shortly.Dr McFigganscommented: Results are presented for uptake and ozone reaction on expanded and compressed films. Would the authors like to comment on the atmospheric likelihood of each and the relevance? What would be the effect inmixedorganic systems where a uniform compressed film will not form at the molecular area corresponding to the stearic acid concentration shown?Professor Donaldsonresponded: Dr McFiggans asks an excellent question about whether organic-coated surfaces in the “real world” (as opposed to the laboratory) may be considered to be compressed, and also what the effect of mixed organic films might be on the results presented here. Fatty acid-coated particles have been observed in the field;1,2it is not at all clear whether the coatings comprised compressed or expanded films, however. We have recently proposed various mechanisms by which film compression might take place in atmospheric aqueous particles.3A report by Pogorzelski4on the surface thermodynamics of real sea-surface microlayers suggests that, at least on the ocean surface, the film is clearly a multicomponent mixture, and that it is in an expanded state.1 H. Tervahattu, J. Juhanoja and K. Kupiainen,J. Geophys. Res., 2002,107, Art. no. 4319.2 H. Tervahattu, J. Juhanoja, V. Vaida, A. F. Tuck, J. V. Niemi, K. Kupiainen, M. Kulmala and H. Vehkamaki,J. Geophys. Res., 2005,110, Art. no. D06207.3 D. J. Donaldson, A. F. Tuck and V. Vaida,Phys. Chem. Chem. Phys., 2001,3, 5270.4 S. J. Pogorzelski,Colloids Surf., A, 2001,189, 163.Dr Eisfeldaddressed Professor Abbatt:(1) It is discussed that real atmospheric ice particles are quite different from the ones produced in the laboratory. They seem to consist of an ice core surrounded by a shell of liquid water containing up to 40% of electrolytes. How can the presented laboratory results be compared to the situation in the real atmosphere?(2) The repeated exposure results (Fig. 3) indicate some aging of the surface. Could this be related to the effect of a liquid water/nitric acid layer forming, making the particles more similar to the ones proposed in the real atmosphere?Professor Abbattreplied: It is possible that the surfaces of the cirrus cloud particles contain electrolytes, perhaps from sulfate aerosol scavenging or if the ice particle formedviahomogeneous freezing of an aqueous aerosol. If electrolytes exist on the surface of the ice then the uptake of nitric acid may be affected. The question is how much of the surface is “contaminated” in this manner. Given the relatively high surface areas that can exist in cirrus compared to the total sulfate aerosol loading, and the rapid growth conditions that cirrus frequently experience, the total surface coverages may not be high. I am not aware of any measurements that support a liquid water shell with up to 40% electrolytes but if such a shell is present it will have a huge effect on the uptake of trace gases such as nitric acid.With respect to your second point, yes, we believe the uptake of nitric acid is suppressed in the repeated exposure experiments because some nitric acid remains on/in the ice after the initial exposure. In this regard, the subsequent exposure may well be more representative of uptake of nitric acid to surfaces that have been previously exposed.

ISSN:1359-6640    

DOI:10.1039/b507789n

出版商:RSC    

年代:2005

数据来源: RSC

摘要:

Professor Rudichopened the discussion of Dr Baltensperger’s paper: Using molecular weight resolved humic and fulvic acids (by microfiltration) we measured the activation diameter of these protonated species. We use these species as simple proxies for the atmospheric HULIS.We showed that the activated diameter increases with the molecular weight for Suwannee River fulvic acid the activation diameter at 0.2%ss increases from 85 nm for the 0.5–1 kDa fraction to 112 nm for the 10–30 kDA fraction. The bulk activates at 93 nm. The bulk of Suwannee River fulvic acid behaves like the 1–3 kDa fraction. In humic acid the bulk activates at a higher diameter than the very large (>30 kDa) fraction.Dr Baltenspergerreplied: This makes complete sense and supports my statement that the increase in molecular weight by the oligomerization process should be reflected in a decrease of the hygroscopic growth factor. This is not the case, suggesting that this effect might be compensated by a further increase in the oxidation state of the molecules within the SOA.Dr McFiggansasked a technical question: Fig. 5 of the paper shows the volatility of the chamber products and Nordic reference fulvic acid. Could the authors comment on how appropriate Nordic fulvic acid is (given the proposed model structure, Suwannee River fulvic acid has been widely proposed)?Professor Rudich showed Suwannee River fulvic acid and humic acid activation spectra as a function ofMw. My first query: Given the curves, it appears that they may be readily accessible to explanation by conventional Köhler theory. My comment is that these compounds (and K‐puszta ambient aerosol) are thought to weakly dissociate based on osmolality measurements and should be accessible to such modelling techniques.Professor Rudichreplied: We agree with the query and will soon be working on modelling the growth curves of size‐segregated humic and fulvic acids at various supersaturation.Dr Baltenspergersaid: I agree that the Suwannee River FA is probably the better proxy. We simply haven’t done that experiment yet, but will do so soon. We present this example here in order to show that agreement in functional groups does not necessarily mean also agreement in physical properties, but that parameters such as molecular weight also play a vital role.Concerning the explanation by conventional Köhler theory and potential dissociation of oligomers at high relative humidity, it will be very interesting to compare H‐TDMA measurements at subsaturated conditions with CCN measurements at supersaturated conditions. It may be speculated that dissociation might be promoted by a higher water activity.Professor Herrmanncommented: Care should be taken when using commercially available humic substances for the study of systems of atmospheric relevance. The oligomeric products identified recently can have fully different properties than such proxies.Dr Baltenspergeragreed: Yes, this is exactly what Fig. 5 shows.Professor Rudichadded: Prof. Hermann is correct, and we are very cautious about extending the laboratory results to the atmosphere. We also do not argue that these compounds accurately represent all high molecular weight compounds in aerosols, but can be used to study some of the relevant processes. Some of the aerosol‐bound high molecular weight compounds are polymeric and may be different from humic-like substances (HULIS). However, a large body of literature suggests that a substantial part of organic fraction of atmospheric particles resemble humic substances in general and of Suwannee river fulvic acid in particular. We presented results of a study that explores the properties of such reference compounds as a function of molecular size, and various other parameters such as O/C ratio, surface tensionetc. The logic behind it is to see which fraction behaves closely to the atmospheric‐derived HULIS and to understand which model compounds can be used to study the properties of real particles. Indeed we can see that using different size fractions of these compounds have different behaviour and properties and care should be taken when using them.Professor Donahuesaid: One needs to be very careful about attributing secondary organic aerosol (SOA) yields to one particular source—for example seeingα‐pinene SOA and asserting that this is biogenic—because the fraction of material partitioning into the condensed phase depends on thetotalorganic mass:ξi= 1/[1 + (c*i/Mom)];Mom= ∑iξiMithus polluted urban air will attract relatively volatileα‐pinene oxidation products.Dr Baltenspergerresponded: I agree. This is however not the only uncertainty in extrapolating SOA yields. As we have shown, oligomerization is also important, providing a means for highly volatile molecules to partition to the aerosol mass which otherwise would be expected to be found exclusively in the gas phase.Professor Georgeasked:(1) During SOA formation, the particle change in composition but possibly also its oxidative properties may increase. Are therefore oligomers be oxidised back to short chain compounds during the aging of the aerosols?(2) As you are starting with a pure gas phase, the first particles must be formed through a gaseous process (e.g.gas phase oligomerisation) followed by multiphase oligomerisation. Both process being in competition. Have you observed such a transition?Dr Baltenspergerreplied: From the limited number of experiments we have done so far we cannnot tell for sure yet. However we would expect that oxidation to short chain compounds does not take place, since then the hygroscopic growth factors as measured with the HTDMA should continue to increase, which is not the case. As already mentioned, CCN measurements will be highly interesting to see if dissociation takes place at higher water activity.We do see some peaks in the PTR‐MS spectra which might be an indication of dimers, however, we have not confirmed the structures of these compounds yet. Indeed we observe homogeneous nucleation in experiments with trimethylbenzene, α‐pinene, and isoprene. While for the former two precursors one could imagine that molecules with sufficiently low volatility might be formed to allow for homogeneous nucleation, such new particle formation from isoprene might indeed need dimer formation in the gas phase.Professor Rudichadded: In urban and rural environments HULIS can be directly emitted by wood burning, especially in winter in northern latitudes.Professor Ravishankaramade a general comment: In addition to the beautiful and careful work you all have performed and shared with us today, may I suggest that it is very worthwhile to measure aerosol properties in the atmosphere with atmospheric aerosols? Such an approach will be very useful and add to the results presented here because of the complexity of the aerosols in the atmosphere regarding their chemical composition, physical state, and structure. Further, these aerosols are processed in the atmosphere such that the aerosol change with time. They may also redistribute components as time goes on.A specific question to Dr Baltensperger: would you consider putting other atmospheric constituents,e.g., ammonium sulfate, along with the organics in your chamber to study what their properties are?Dr Baltenspergerreplied: I agree. We indeed need a balance between a fundamental understanding of individual mechanisms and an overview of complex systems in a less detailed way. This was actually also the intention of this paper, by combining results from our smog chamber with characteristics of aerosol samples from the real world.Concerning the addition of ammonium sulphate into our chamber: we hate to do so in the moment, because, once you have put sulphate in, it is difficult to get rid of it again, unlike for organic compounds where oxidation by high concentration of ozone is a good means of cleaning the bag. What we plan to do, however, is to sample the SOA from the chamber, mix it with variable amounts of ammonium sulphate and then re‐nebulize it in order to test for the properties of these mixtures.Professor Waynecommented: Professor Ravishankara has asked about the influence that sulphate—not present in Baltensperger’s system, but ubiquitous in atmospheric aerosol—might have on he results obtained, and he suggested that the experiments might be more appropriately conducted in the presence of sulphate. I was startled to hear Professor Ravishankara make such a suggestion, as he has spent years preaching the virtues of isolating individual elementary steps for study. Now he seems to be advocating mixing all the bits together!Professor Ziemannasked: In your paper you propose that oligomers formed in your chamber reactions could be a major source of humic‐like substances (HULIS) in the atmosphere. How do your AMS mass spectra compare with those of oxidized organic matter that have been extracted from AMS analysis of ambient organic matter?Dr Baltenspergerreplied: We do not say this is the only process to form HULIS. In fact other processes have been shown to work as well. AMS spectra of fulvic acid have been shown to compare well with the organic fraction of aerosols from various remote locations (see Fig. 9 in the paper by McFigganset al.). AMS spectra fromα‐pinene SOA also compare well with those of processed aerosol samples from remote locations such as the high‐alpine station Jungfraujoch.11 Alfarraet al.,Environ. Sci. Technol., submitted.Professor Rudichcommented: Using VUV‐AMS one could obtain a better idea of the molecular weight of the HULIS, and avoid fragmentation.Dr Baltenspergerreplied: Yes, softer ionization does indeed give potential for less fragmentation. We hope to show this in the near future.Professor Planeasked: The “dinosaur” plots such as Fig. 4 in the paper do not show data belowm/z= 280. Presumably smaller polymers are present, obviously at short irradiation times but what about after a few hours?Plots such as Fig. 3 in the paper contain a wealth of kinetic information. Have you attempted to reproduce this data with a model?Dr Baltenspergerresponded: Indeed, peaks from molecules with a lower molecular weight are present as well, throughout the experiment. However, their relative intensity becomes smaller with time.Concerning the analysis of particle growth curves as a function of time: yes we are doing this, partly in our own laboratory and partly through collaboration with other groups. We presented preliminary results at the EGS 2003 in Nice, showing that growth rates were in the range of 10 to 60 nm h−1, with the lower end being in the range of those found under atmospheric conditions. The shift of the size distribution due to condensational growth can be described by free molecular condensation theory. From the comparison between experiments and theory the concentration of condensable vapor can be estimated. Furthermore, from the time evolution of the size distribution and calculated condensational growth rate, it is possible to back‐calculate the rate of particle formation of the nucleation event as a function of time.Dr Allanopened the discussion of Professor Herrmann’s paper, which was presented by Dr Iinuma:(1) In the work presented, only the first order ozonolysis products were studied. If higher order products were to be generated, would the effect of OH on the SOA yield be as great?(2) Inorganic salts were used as seed particles; however particles in the real atmosphere almost always contain some organics. Also, the RH used was quite low: ambient boundary layer accumulation‐mode particles are usually deliquesced. Would these factors affect the results?Dr Iinumareplied:(1) Fig. 2 shows that the SOA volume change almost reaches a plateau approximately at 2 h. This suggests that the effect of OH would be similar to the finding from this study even with the generation of higher order products.(2) These parameters (seed particle properties and RH) may affect the results though there degree of the influence is difficult to predict. For example, Cockeret al.1have investigated the effect of water on the partitioning of SOA from α‐pinene ozonolysis. It has been shown that deliquesced inorganic seed particles reduce the SOA yields in comparison to those from dry inorganic seed particles or inorganic seed free cases. In the same study, it has been shown that the type of the inorganic seed particles (the highest reduction by ammonium sulphate followed by ammonium bisulphate and calcium chloride) had a strong influence on the yields of SOA when the seed particles are deliquesced. On the other hand, little effect of RH on SOA yield was found from the same study. Our experimental conditions are similar to that of Cockeret al.1where aqueous seed particles and 50% RH are used. Bonn and Moortgat2have reported that the influence of water vapour on the SOA volume was significantly reduced at a high concentration of α‐pinene (1 ppmv) and O3(0.5 ppmv) compared to lower concentrations (50 ppbvα‐pinene and 110 ppbv O3).Little is known about the influence of organic seed particles to SOA yields and this needs to be addressed in the future studies concerning the SOA.Although, the conditions chosen for the various chamber studies appear to be unrealistic or too simplistic compared to the real atmosphere, these studies are an essential step towards understanding a complex phenomenon such as SOA formation.1 D. R. Cocker, S. L. Clegg, R. C. Flagan and J. H. Seinfeld,Atmos. Environ., 2001,35, 6049.2 B. Bonn and G. K. Moortgat,Atmos. Chem. Phys., 2002,2, 183.Professor Ravishankaraasked: Could you please comment on how the presence of water vapor would affect the formation (and supression) of SOA in your work?Dr Iinumareplied: It is difficult to comment on the effect of water vapour on the formation of SOA in the current work because the experiments were performed at nearly constant relative humidity. However, as mentioned in my previous answer, one would expect that it would follow similar trends to the previous studies with comparable experimental conditions. It is known from the literature that water vapour influences product distribution inα‐pinene ozonolysis and enhances the formation of new particles.1It is also known that the yield of organic aerosol varies little with RH and that aqueous salt particles reduce the yield of SOA compared to dry seed conditions.21 B. Bonn and G. K. Moortgat,Atmos. Chem. Phys., 2002,2, 183.2 D. R. Cocker, S. L. Clegg, R. C. Flagan, and J. H. Seinfeld,Atmos. Environ., 2001,35, 6049.Dr Baltenspergercommented: While the first papers on this topic strongly implicated the need for acidic seed particles, the importance of these acidic seed particles appears to be less important today, with your paper showing even a lower yield with acidic particles in one case. Could you tell if this yield increase is a question of thermodynamics or just kinetics? If the latter is true, the effect would be less important because in the atmosphere typically longer time scales are involved than in a chamber experiment.Dr Iinumaresponded: Discrimination between thermodynamic and kinetic effects is very difficult to achieve at this moment. Both phase transfer parameters as well as condensed phase kinetics do need more detailed investigation.Dr Alfarrasaid: All experiments reported in this paper were limited to 2.5 h time duration. It has been reported in many other studies (e.g.by Kalbereret al.and Tolockaet al.in 2004) that oligomers can be formed after more than 4 h of light on in photooxidation experiments or ozone introduction in ozonolysis studies. I am concerned that some of the results in this paper and in particular the absence of oligomer formation in some experiments might have been influenced by the relatively short time duration and is not a true reflection of the experimental variables discussed.Dr Iinumareplied: The experiments without scavengers as well as our previous study1show the presence of dimers and oligomers even with an experimental duration of 2.5 h. This suggests that these dimers and oligomers reported from α‐pinene ozonolysis can be formed in a relatively short time scale (<2 h). Therefore, experimental duration cannot be the sole reason for the reduction of dimers and oligomers in the presence of OH scavenger.Our current chamber setup does not allow much longer experimental duration due to its size but our new and larger chamber facility will address the influence of experimental duration in the future studies.1 Y. Iinuma, O. Böge, T. Gnauk and H. Herrmann,Atmos. Environ., 2004,38, 761.Professor Georgeasked: In your experiments using radical scavengers, you observed an lowering of the SOA yields. What are the processes involved in this lowering? Could the scavengers inhibit not only OH formation but also inhibit the formation of other gaseous intermediates?Dr Iinumareplied: One major reason for the observed lower SOA yields in the one set of experiments using cyclohexane is indeed the inhibition of OH formation. But there could also be a lot of other reactions of the ozonolysis intermediates with the scavenger, or products formed from these as it is known from organic chemistry as well as atmospheric chemistry. Because of the complexity of this reactions which may inhibit the formation of some products and may also enhance the formation of other products the effect on SOA formation is hard to predict.Dr Wengersaid: The use of 2‐butanol and cyclohexane as OH radical scavengers will produce carbonyl compounds that may partition into the aerosol phase. Have you considered using alternative scavengers (e.g.CO)?Dr Iinumareplied: Indeed, the choice of OH scavenger is extremely important. Various OH scavengers were considered during the experimental planning stage. CO was also one of the candidates for OH scavenger but it has not been used in our current chamber setup due to the safety reasons but our new chamber facility should be able us to avoid the safety problem. CO is attractive as it is inorganic and does not produce alkylperoxy radicals which might perturb SOA yields. On the other hand, it has been reported that produced hydroperoxy radical from an OH and CO reaction does also influence on SOA yield from alkene ozone reaction systems1,2as well.1 M. D. Keywood, J. H. Kroll, V. Varutbangkul, R. Bahreini, R. C. Flagan and J. H. Seinfeld,Environ. Sci. Technol., 2004,38, 3343.2 K. S. Docherty and P. J. Ziemann,Aerosol Sci. Technol., 2003,37, 877.Professor Ziemanncommented: In our laboratory we have recently measured the organic peroxide content of secondary organic aerosol particles and conclude that organic peroxides contribute approximately 50% of the aerosol formed from the reaction ofα‐pinene with ozone. The contributions are similarly large for other monoterpenes. These products have not been identified previously because of the limitations of analytical methods.Dr Iinumareplied: There is definitely the need for better analytical method not only concerning the peroxides but also for oligomers. Our preliminary CE‐TOFMS analysis shows the presence of peroxy or peroxyacid compounds from α‐pinene ozonolysis. Although the peak areas of these peroxides are as not as big as major oxidation products such as pinic acid in our analysis, the peroxides appear to be an important fraction of the SOA formed from the α‐pinene ozonolysis.Professor Donahuecommented: For some time many people tended to ignore the gas‐phase oxidation mechanism following ozonolysis, effectively assuming that a set of reaction products follows inevitably from an ozone–alkene reaction. However, this reaction produces organic fragments that will eventually participate in more or less standard radical cycling. The products of this chemistry will depend on atmospheric composition, and so will SOA yields. We must not neglect the gas phase.Dr Iinumasaid: I cannot agree more.Professor Heardaddressed Dr Iinuma and Professor Donahue: It is important that the gas‐phase field measurement community, for example those involved in the measurement of free radicals, are included in measurements within aerosol chambers to understand the mechanisms of formation of secondary organic aerosol. Measurements of the composition of both the gas and aerosol phases are necessary in order to extract a full understanding of the key controlling processes.Dr Iinumareplied: Although the recent development on the particle phase chemistry leading to higher molecular mass compounds is an extremely exciting field, it is essential for the aerosol chamber community not to forget that the gas phase still plays an important role for the systems we’ve been studying.Professor Donahueadded: I fully agree. I believe that a major conclusion from this and several other recent studies is that gas‐phase chemistry clearly matters for SOA formation, with first‐order implications for product distributions and aerosol yields. It is especially worrisome that the vast majority of chamber studies addressing ozonolysis are conducted with relatively high ozone and terpenes in the absence of NOx. There is certainly a great risk that peroxy radical chemistry under these conditions does not reflect atmospheric behaviour. Consequently, I strongly advocate a minimum of peroxy radical measurements in all chamber studies dealing with hydrocarbon oxidation, no matter what the other objectives may be.Dr Baltenspergeropened the discussion of Professor Donahue’s paper: The increase in the activated fraction of β‐caryophylene SOA with time differs from our experience at subsaturated conditions for α‐pinene SOA (hygroscopicity TDNA measurements).1There, the hygroscopic growth factor increased in the first hour, but then stayed constant at the typical value of 1.1 until the end of the experiment. I see two possible explanations for this: first, this might be due to an increase in multiple charged particles (with increasing aging time the SOA particles will grow, and the 100 nm cut will thus be at the lower end of the site distribution, enhancing the chance of multiply charged particles). Second, it might be the result of reversible oligomerisation at supersaturated conditions where the water content of the aerosol is much higher. If this is true, this would be extremely important for cloud formation and would urgently need more research.1 H. Saathoff, K.-H. Naumann, M. Schnaiter, W. Schöck, O. Möhler, U. Schurath, E. Weingartner, M. Gysel and U. Baltensperger,J. Aerosol Sci.2003,34, 1297–1321.Professor Donahuesaid: A far more detailed study of the activation is underway. I believe that the bulk of the evidence in the β‐caryophyllene system supports secondary oxidation of theexodouble bond as the main cause of the evolving activation. We also do not see significant changes from a‐pinene (we may or may not be able to resolve the initial change in hygroscopic growth, depending on the integrated exposure, as the CCN measurements are relatively slow). Also, the D50 for β‐caryophyllene SOA is initially quite large, consistent with these substantially reduced products starting out quite hydrophobic, whereas a‐pinene SOA typically activates readily. For that reason, we also expect any signal from further processing to be much more pronounced in the sesquiterpene.Dr Monksasked: With your NOx‐aerosol work, you can have long‐exposure times. Is there a possibility of NO2+ diene chemistry? Though this is quite slow (ca. 10−17to 10−18cm3molecule−1s−1) it has been implicated in stagnant winter‐time particulate production.Professor Donahuereplied: Sure. A major point of this paper is that gas‐phase chemistry clearly plays an important role in all aspects of SOA formation—yields of condensable products will depend on gas‐phase conditions. That may be preaching to the choir for this audience. The hypothesis being tested with our VOC/NOxwork was that ‘standard’ peroxy radical branching would turn out to be important to SOA yields following ozone‐terpene reactions. At this point, I would say that our data are consistent with this hypothesis, but I would not begin to claim that the hypothesis is confirmed to the exclusion of alternate hypotheses.Another goal of this paper is to suggest that certain key areas deserve the intense attention (again, I may be preaching to the choir). To your joy, I think peroxy radicals involved in SOA, including from ozonolysis fragments, are clearly one of those areas.Professor Ziemannasked: Do you have an explanation for how the addition of NO changes the chemistry of aerosol formation and results in lower aerosol yields?Professor Donahuereplied: Sure—peroxy radical branching. The response to Dr Monks’ question delves into alternate possibilities as well, which I can not exclude, but thea priorihypothesis in the VOC/NOxstudy was that peroxy radical branching would cause changes in the SOA yields, and we see a transition at roughly the same VOC/NOxwhere ozone production changes from VOC‐limited to NOx‐limited conditions for the same reason. All told I believe the results are very consistent with thata priorihypothesis.Dr Jenkincommented: Based on current understanding of the detailed mechanisms leading to known products of α‐pinene ozonolysis (e.g.as represented in the MCM) the presence of NO and NO2would be expected to inhibit the formation of species implicated as important contributors to SOA (e.g.pinic acid; 10‐hydroxypinonic acid; hydroperoxides). This is because they are formed as a result of permutation reactions of sequentially‐formed RO2radicals, and their reactions with HO2. NO2will interrupt these sequences through generation of PAN compounds, whereas NO promotes competing propagation reactions leading to carbonyl products, and possible fragmentation to smaller (less volatile) species. Your results are therefore fully consistent with the qualitative response which would be predicted.Professor Donahueresponded: As this thread of questions shows, yes, we agree that the result is consistent with expectations. Operational air quality models contain highly reduced mechanisms—at this point usuallyterpene + O3→βSOAwhereβis some fixed yield. That is clearly inadequate. On the other hand, it is not at all clear to me that we have good constraints on the behaviour of the sorts of peroxy radicals we expect following terpene‐CI decomposition, and we have to be careful even when assuming that the reaction pathways are the same as with simpler peroxy radicals. One example of this is the strong effect we observe when UV light is added (resulting in an approximately 0.03 reduction in yields, meaning 50% under atmospheric conditions). It appears that some product has a high cross section in the near-UV, but we don’t yet know what it is.Professor Ziemannsaid: Your conclusion that excited Criegee intermediates (from cyclic alkene ozonolysis) do not become stabilized is inconsistent with our experiments with cyclic alkenes in which we have identified alkoxyhydroperoxides formed from the reactions of stabilized Criegee intermediates with propanol.Professor Donahuereplied: I would really love to observe the Criegee some day (we are trying). I agree there is an apparent inconsistency. We hope to carry out much higher‐level theory on the secondary cycloreversion (producing the SOZ) in the future. Certainly one possibility is that this pathway has a higher barrier than our density functional calculations suggest. However, the thermal, unimolecular lifetime of the SCI against formation of SOZ, based on our current potential energy surfaces, is of order 10 ns (the microcanonicalA‐factor is 1(8) s−1, and the process has almost no activation energy). It would seem that this would need to slow by at least a factor of 1000, if not much more, to permit bimolecular scavenging. We are using hexafluoroacetone as a scavenger for the SCI in ongoing experiments, but direct measurements would be a huge help. Just as peroxy radicals are a critical area deserving intensive study, I believe that the Criegee intermediates are another.Professor Waynesaid: If I have understood correctly, you explain your observations on the basis of a very short lifetime of the Criegee intermediate for the monoterpenes, but a long lifetime for the sesquiterpenes. Are the lifetimes you are talking about for the unstabilized or stabilized Criegee radical: do the C10compounds form the secondary ozonide direct from the excited radical? What values of the lifetimes could you estimate in the C10and C15cases, and how do they compare with lifetimes against collisional deactivation?Professor Donahuereplied: These issues are discussed in detail in ref. 3. One of the many factors at play in this paper is Criegee stabilization. We do indeed suggest that the Criegee intermediates arising from endocyclic monoterpene ozonolysis will undergo unimolecular reaction before being stabilized (and thus have a very short lifetime, of order 10 ps) while the CI arising from endocyclic sesquiterpene ozonolysis will be collisionally stabilized and thus have a longer lifetime (of order nanoseconds if SOZ formation occurs, as our model suggest, or up to 1 s if it does not). The collision frequency is around 1 × 1010Hz, and the unimolecular microcanonical rate constants at the reaction energy are of order 10−10for C10 and 10−9for C15 compounds. The energies are of order 25 000 cm−1and the collisional energy transfer is taken to be of order 500 cm−1per collision, so multiple collisions are required to achieve stabilization—this is why we use the master equation. The essential point for endocyclic alkenes is that for low carbon number essentially no stabilization will occur, because the activiation is completely localised in the single reaction product, while at some carbon number a fairly abrupt transition to complete stabililzation will occur.The C10 compounds should not form the SOZ directly from the excided CI because this pathway has very, very low entropy (the transition state is bicyclic and the reactant is linear, leading to a microcanonical A factor of about 10−8for SOZ formation)—thus at high energy the higher entropy pathways involving rearrangements close to the C–O–O center are strongly favored (these are the ‘standard’ pathways like vinyl hydroperoxide and dioxirane formation). In this regard the system is very much like thermal systems showing a strong negative temperature dependence, like nitric acid + OH, where one pathway (formation of products for OH + nitric acid) dominates at low energy and another dominates at high energy.Dr Baeza Romerosaid: I would like to know in the experiments of NOxand aerosols how you are sure that the chemistry of NO3radical is not going to be important.Professor Donahueresponded: We know enough of the mechanisms and rate constants to do a reasonable job modelling the system. At high NO2, any NO3is scavenged by the NO2to make N2O5. In earlier flow‐tube studies (where we used NO2to scavenge the Criegee intermediate as well as essentially all radicals) we clearly observed this N2O5formation (ref. 20). There is an intermediate regime with VOC/NOxaround 5 where NO3could be responsible for 20% or so of the terpene removal; however, the smooth transition from very low NOx(where we know NO3is not important) to very high NO2(where we are confident that NO3is scavenged before it can react with the terpene, and where SOA production is dramatically reduced) indicates that NO3chemistry is not driving what we observe.Dr Baltenspergerasked: Did you measure the temperature dependence of the SOA volume (Fig. 5) also after prolonged aging times of the SOA in the chamber? In the light of the discussed oligomerization processes one would expect a less volatile aerosol with increasing aging time.Professor Donahuereplied: Yes. The data in the figure include experiments with several hours of exposure at high ozone levels. In absolute time, experiments span 4 h following initial particle formation. In terms of ozone, the total exposure corresponds to several days under typical conditions, and in terms of OH exposure is minimal as we always include an OH scavenger. The data included in Fig. 5 span a range of ozone exposure from relatively fresh aerosol to more than 2 ppm h−1. All of our experiments are carried out with a large excess of ozone and 2‐butanol for OH scavenging. Consequently, there is continuous exposure to ozone but little exposure to OH. Presumably, the SOA includes significant acidity from the organic acids produced during α‐pinene ozonolysis. There are no obvious signs of decreasing volatility or increasing ΔHover that timespan. Furthermore, the first‐order loss shown in Fig. 4 of the paper is consistent with typical dark‐bag particle loss rates, indicating that the effective SOA yield is not increasing with time in this experiment. By contrast, the first‐order loss shown in Fig. 8 of the paper (for β‐caryophyllene) is much less than typical wall loss; this does suggest increasing yields with increasing exposure where a second double bond is available. Thus, any oligomerization in these experiments does not appear to dramatically alter the SOA yields from α‐pinene; for β‐caryophyllene we cannot yet say.Dr Shallcrossopened the discussion of Dr Jenkin’s paper:(1) In your model you have a set diurnal emission pattern for anthropogenic species. If this emission profile was changed, whilst preserving the overall mass emitted, to either a fixed amount,i.e.no diurnal change, or highly spiked in early morning and in the evening, would it make a difference to total ozone generated along the trajectory, NOyand its partitioning and primary to secondary VOCs? If so does this have implications for daily emission patterns and their impact on air quality?(2) Is it possible that your underprediction of aldehydes and overprediction of ketones is due to the way the MCM partitions primary, secondary and tertiary peroxy radicals,i.e.it is biased towards secondary and tertiary peroxy radicals?Dr Jenkinreplied:(1) We feel it is important to apply some representation of the temporal variation of pollutant emissions, based where possible on measured statistics (e.g.traffic volumes). The seasonal and hebdomadal variations actually influence the emitted VOC/NOxratio and therefore clearly need to be represented as realistically as possible. For the scenarios relevant to this study, however, the effect of the diurnal emissions pattern on the formation of secondary pollutants such as ozone is more subtle: this is because the quantity of ozone (and indeed that of oxidised organic products of VOC oxidation) is mainly governed by quantity of VOC and NOxcollected and processed prior to arrival. Unless imposition of the diurnal profile changes the quantity of emissions collected along the trajectory (which can occur), the form of the profile would not be expected to have a large impact on simulated secondary pollutant concentrations. For primary pollutants such as the emitted VOC and NOx, the imposition of the diurnal profiles has a much more important influence on the simulated concentrations, allowing an improved comparison with the observations.(2) The partitioning of the chemistry into primary, secondary and tertiary radicals depends mainly on the speciation of the emitted VOC and the relative attack rates of OH at different sites within the VOC, which is represented rigorously in the MCM. Although this has an influence on the relative formation of aldehydes and ketones, there are many other factors which are also important. For example, secondary oxy radicals often have significant decomposition channels (particularly if they contain β‐hydroxy or α‐carbonyl groups) which generate aldehydes. In practice, the simple aldehydes in our model‐measurement comparison are largely formed by such processes during the breakdown of larger VOC, rather than from reactions of the corresponding primary oxy radicals with O2, these reactions becoming uncompetitive for primary oxy radicals with chains ≥C4.Dr Monkscommented: With respect to the GC measurements of aldehydes and ketones, it is worth noting that the sampling period is 5–10 min in a given hour. The Waldhof measurements you show as another comparator are probably cartridge measurements, so are integrated over a longer time.Dr Jenkinresponded: The GC measurements of the smaller carbonyls (e.g., acetaldehyde and acetone) are based on 10 min sampling in a given hour. The GC × GC measurements of the higher carbonyls are based on 2 h integration in a 3 h period. The historical measurements from the EMEP network are based on 8 h daytime integrations (centred on noon), with DNPH derivitisation and analysis with HPLC.Dr Monksthen asked: From the data in Fig. 8 on radical production rates, HCHO is clearly significant. Do you have any idea on what the primary to secondary ratio of HCHO is?Dr Jenkinreplied: The simulated concentrations of all the carbonyls at the arrival point are totally dominated by their secondary production from emitted VOC oxidation. In the case of HCHO, it is generated as a degradation product of most (if not all) of the VOC listed in Fig. 1 (e.g.the oxidation of 1 kg of ethene generatesca. 1.7 kg HCHO), such that its direct emission is only a significant contributor comparatively close to the point of release. We have previously simulated the impact of omitting all primary aldehyde emissions on simulated aldehyde concentrations at a rural southern UK location under similar summertime anticyclonic conditions. The simulated concentration of HCHO was reduced by 1.3% (3.83 to 3.78 ppb), which is comparable to the 1.7% reduction in total VOC input resulting from omitting the aldehyde emissions.Dr Arnoldasked:(1) Due to lack of mixing between parcels in the model, in effect the emissions grid is relied upon to introduce horizontal mixing between trajectories. To what extent does the emissions resolution affect not only the variability of the time‐series model output, but also the absolute concentrations?(2) Due to the above, species which are not emitted have no opportunity to mix with surroundings or neighbouring trajectories. Consequently, any longer‐lived intermediate species will build up in trajectories over the course of advection, perhaps to unrealistic concentrations. To what extent may this impact chemistry in the model?Dr Jenkinresponded: The trajectory modelling approach that we necessarily employ for applications using a 14 000 reaction chemical mechanism inevitably has some significant simplifications in the way it represents transport and mixing. However, previous application and testing1has demonstrated that the approximations of the method are most appropriate for simulating summertime anticyclonic conditions associated with regional scale photochemical episodes in NW Europe. Under such conditions, vertical mixing is inhibited, and the broad parallel airflow leads to similar pollutant concentrations over widespread areas (e.g.almost identical temporal patterns of ozone and particulate mass concentrations were observed at southern UK sites separated by up to 100 km in the August 2003 episode). Under such circumstances, absence of horizontal mixing between trajectories is a better approximation because neighbouring air masses are of a similar composition. As a result, long‐lived intermediates do not appear to build‐up to unrealistic concentrations. Acetone is the probably the least reactive oxidised organic generated in the mechanism (lifetime of about two to four weeks), and it provides an excellent illustration. Its regional background concentration is about 0.6 ppb, and this was used to initialise the model. The quantities generated on the 96 h case study trajectories elevated its concentration by factors of betweenca. 2 and 7 over this background. As shown in Fig. 7, the simulated concentrations were consistent with those observed.You are correct that the emissions grid introduces a degree of horizontal mixing. Because the resolution of the grid decreases from 10 × 10 km in the UK, to 50 × 50 km in the EU, to 150 × 150 km elsewhere, this serves to average out the emissions which are more remote from the arrival point, notionally accounting for off‐trajectory contributions. The model is normally used to simulate rural background arrival locations, and is inappropriate to simulate locations close to emissions sources.1 M. E. Jenkin, T. J. Davies and J. R. Stedman,Atmos. Environ., 2002,36, 999.Dr Coxcommented: It is interesting to note the substantial yields of 1,4 hydroxycarbonyl products from VOC degradation, which was reported by Atkinsonet al.1We have recently reported measurements of the temperature dependent rate coefficients of the isomerisation of >C4alkoxy radicals which give rise to these products.2We find a correlation of the activation energy with C–H bond strength which allows estimates of the rate constants for a range of VOC structures. Our results confirm the importance of this process in VOC degradation and give improved capability for accurate description in models.1 R. Atkinson, E. S. C. Kwok, J. Arey and S. M. Aschmann,Faraday Discuss., 1995,100, 23.2 D. Johnson, P. Cassanelli and R. A. Cox,J. Phys. Chem. A, 2004,108, 519.Dr Jenkinreplied: Your work is very valuable for the construction of detailed chemical mechanisms, such as the MCM, and will certainly allow further improvements and refinements to be made. The rate coefficients we currently use for the alkoxy radical isomerisation reactions (which lead to the formation of the 1,4‐hydroxycarbonyl products), and for the potentially competitive decomposition reactions and reactions with O2, are based on the experimental results and estimation methods of Professor Atkinson. For atmospheric boundary layer conditions, the parameters are comparable with those you have reported. However your measurements of the temperature dependences, and the associated development of the basis of an estimation method, will help improve the representation of these reactions for a range of atmospheric conditions.It should be noted that our reported concentrations of the 1,4‐hydroxycarbonyls also (notionally) include contributions from possible isomerisation products observed in laboratory systems,1generated from cyclisation reactions which are currently not represented in the MCM. The importance of these reactions under atmospheric conditions, and the details of the subsequent chemistry, are a current area of uncertainty where further information is required.1 P. Martin, E. C. Tuazon, S. M. Aschmann, J. Arey and R. Atkinson,J. Phys. Chem. A, 2002,106, 11492.Dr Wengercommented: Measurement of 1,4‐hydroxycarbonyl, was raised during the discussion. I would like to point out that Atkinson’s group have recently published a method for measuring these compounds under laboratory conditions using SPME fibres coated with the derivatizing agent PFBHA and subsequent GC and GC‐MS analysis. This technique can, in principle, be used for field measurements.Dr Tuckcommented: Very recently, two astrobiologists have noted that the complexity of the Earth’s stratospheric chemistry constitutes a scale‐free network when molecules are represented as nodes and reactions are represented as links (connections).1It may be possible that the power law expressions associated with such a network could help in organising a complicated chemical scheme like the MCM—you had four decades of concentrations, and plotting them against time per the techniques of generalized scale invariance might yield some organizing principles. I note that you had a linear log–log plot over four to five decades in your model–observation comparison.1 R. V. Solé and A. Munteanu,Europhys. Lett., 2004,68, 170.Dr Jenkinresponded: I am not familiar with the work to which you refer, but the much greater complexity of tropospheric chemistry might make the procedure a challenging task, involving a network of 5000 nodes and 14 000 links (based on the MCM). Use of a reduced chemical mechanism might make the task more tractable.It is not feasible at present to plot the results as a function of time. The MCM output is essentially representative of hourly average concentrations at the arrival point for the five case studies. The GC measurements of the smaller carbonyls (e.g., acetaldehyde and acetone) are based on 10 min sampling in a given hour. The GC × GC measurements of the higher carbonyls are based on 2 h integration in a 3 h period.The distribution of the simulated (and observed) species concentrations over several orders of magnitude is influenced by a number of factors, but is mainly due to the availability of appropriate emitted precursor VOC. For example, the simple aldehydes all have comparable atmospheric lifetimes (ca. 2–3 h): the range of concentrations reflects that there are systematically fewer reactions which generate the larger species,i.e.almost all emitted VOC are degraded to form formaldehyde, but the number (and emitted quantity) of VOC precursors progressively diminishes as the carbon number of the aldehyde increases.Professor Donahuecommented: Chemical variability can be described by Reynolds averaging,i.e.RAB=kCACBR&cmb.macr;AB=kC&cmb.macr;AC&cmb.macr;B+kC&cmb.macr;′AC&cmb.macr;′BMechanisms with full chemical complexityi.e.The MCM may be ideal for exploring this variability along with high‐time resolution ambient radical data (i.e.Professor Heard’s comment).Dr Jenkinreplied: I agree that detailed explicit chemical mechanisms might be helpful in interpreting such high temporal resolution data. However, even the MCM possesses some strategic simplifications to limit its size:e.g., the formation and back‐decomposition of short‐lived (<1 s) peroxynitrates from the reactions of many RO2radicals with NO2are not included. I suspect that a carefully expanded subset of the MCM might be an appropriate place to start.Professor Duxburyopened the discussion of Dr Kolb’s paper: Quantum cascade lasers are a new type of semiconductor laser in which the mini‐band structure is determined by the way in which layers of semiconductor are deposited epitaxially.1The emission wavelength is then determined by the separation of the minibands. Since the light emitted results from the electrons cascading down a staircase like structure of minibands, emitting a photon at each step, the quantum cascade (QC) laser is a unipolar laser. In a normal semiconductor laser, such as a lead salt laser, the emitted wavelength is determined by the band gap of the semiconductor material, and results from the recombination of holes and electrons.There have been two main ways developed for using pulsed QC lasers, the inter‐pulse and the intra‐pulse methods. The first method has been used in QC spectrometers described in Paper 18. In this type of spectrometer a short duration current pulse is applied to the laser to limit its frequency sweep during the pulse, a frequency chirp. The tuning is then provided by a sub‐threshold current ramp, leading to a short current tuning regime. In an alternative method which we have developed, the intrapulse method, we use a long duration current pulse and allow the laser to chirp over a wide frequency interval, recording the entire spectral window during each current pulse. Using this method we can record up to 3 cm−1tuning during a single pulse.1The laser sweeps to lower frequency during the current pulse, a downchirp.In order to compare this method with that described in Dr Kolb’s paper we have recorded the spectrum of the ambient atmosphere above the Physics Department of Strathclyde University on the fourth, fifth and seventh of April 2005. Examples of the variations of the concentrations of methane, nitrous oxide and water are shown in two micro‐windows inFigs. 1 and 2. These two micro‐windows were recorded during a single frequency chirped pulse. The laser repetition frequency used was 20 kHz, and 64 000 spectra were averaged, so that the time between consecutive samples is 3.2 seconds. InFigs. 1a and 2athe methane concentration was fluctuating rapidly, with 12.8 seconds between the maximum and minimum concentrations recordedviathe change in absorbance. Within this time scale neither the water nor the nitrous oxide showed any significant variation in concentration. The atmospheric conditions under which this behaviour was observed on April 4th were very gusty with heavy showers of rain. The wide variation of the amplitude of the absorbance of methane is suggestive that various localised sources contribute to the methane concentration in the vicinity of the Physics building. InFigs. 1b and 2brecorded on April 5th, heavy rain in the morning was followed by a dry afternoon with light winds. The methane concentration was stable and the amplitude of its absorbance inFig. 1bwas similar to that of nitrous oxide. This is to be expected if both gases have concentrations close to their global average, since the absorption coefficients of these lines of nitrous oxide and methane are almost inversely proportional to their ambient concentrations, with the absorbance of methane calculated to be about 97% of that of nitrous oxide. In the final spectra ofFigs. 1c and 2cthe methane concentration has returned to the level which we have generally measured in the centre of Glasgow, slightly above the global average value. However, as it was a very dry and cold day, the water absorbance seen inFig. 2cis much lower than that measured earlier in the week.A section of the spectrum of ambient air recorded towards the end of the current pulse when the laser tuning rate has slowed. The path length was 39 m, the pulse duration was 1.25 μs, the pulse repetition frequency 20 KH, and the number of averages to create a single spectrum 64 000. For spectrum (a) the drive voltage was 157 V, and for (b) an (c) 150 V. The cell pressure was held at 29.6 Torr. The cell was purged with zero air every four minutes to provide a background spectrum against which the atmospheric spectra was ratioed to provide transmission and absorbance spectra. (a) 4th April, squally rain showers, (b) 5th April, recorded following a period of heavy rain. (c) 7th April, a cold, dry and still day. The Physics building of Strathclyde University is situated on top of a hill in between two busy main roads which run East–West. The sampling inlet of the spectrometer feed pipe was located on the southern edge of the roof.A section of the spectrum of ambient air recorded near the begining of the current pulse when the laser tuning rate is rapid. The remaining conditions are as those inFig. 1.In summary the use of a long duration current pulse allows us to measure the concentrations of several molecules simultaneously, and also to measure rapid fluctuations of the type described by Kolb and his colleagues in their mobile laboratory experiments.1 G. Duxbury, N. Langford, M. T. McCulloch and S. Wright,Chem. Soc. Rev., 2005,34, DOI: 10.1039/b400914m.Dr Kolbreplied: The intra‐pulse application of pulsed quantum cascade (QC) lasers being developed at the University of Strathclyde and described by Professor Duxbury in his comment is a highly innovative technique that we have been following with great interest. We agree that its potential to provide wider spectral coverage on very short time scales is interesting and we look forward to future developments.The challenge of adapting innovative laboratory laser measurement techniques for successful deployment on real world field measurement platforms, like the mobile laboratory described in our paper, is one that we have been working on for over two decades. One key issue is ensuring that the measured spectra can be reliably related to the atmospheric concentration of the pollutant of interest. We have expended a great deal of effort to develop data acquisition hardware and software and data analysis algorithms that allow us to be confident that the spectra we measure with our inter‐pulse quantum cascade systems meet this challenge with both high precision and accuracy. The fast background definition, spectral line isolation, spectral parameter fitting, and spectral line integration steps we perform allow us to derive trace gas atmospheric mixing ratios from our measured spectra with great confidence. A spectral analysis method that makes direct connection to the known spectral line strengths and collisional broadening coefficients greatly reduces the logistical and measurement burden of frequent trace gas calibration measurements. A detailed discussion of our signal processing approach applied to the two trace gases, nitrous oxide and methane, addressed in Professor Duxbury’s comment has recently been published.1We look forward to comparing the accuracy and precision of our inter‐pulse QC laser systems with the intra‐pulse system being developed at Strathclyde when it is ready for field deployment.1 D. D. Nelson, B. McManus, S. Urbanski, S. Herndon and M. S. Zhaniser,Spectrochim. Acta A, 2004,60, 3325–3335.Professor Wayneremarked: The quantum‐cascade lasers described by Duxbury heat up during a pulse to produce chirping, and in Duxbury’s implementation, the sweep is used to obtain the spectrum. Do second and subsequent pulses start at the original temperature (and thus wavelength), or somewhere towards the terminal one? Is there an offset (T≡λ) in each successive sweep, or is the heat dissipated and the internal temperature rise fully relaxed before the next pulse? If the latter, how does it occur, and on what time scale?Professor Duxburyreplied: The quantum cascade lasers do retain some residual heat immediately after the cessation of the current pulse.The decay rate of the residual heating, by heat conduction to the heatsink attached to the cooled outer surface of the copper laser enclosure, may be obtained from the results of a double pulse experiment which we have carried out. InFig. 3we show the form of two successive output pulses from an 8 μm QC laser. The pulse durations are both 500 ns and the pulse separation was also 500 ns. The pulse repetition frequency of the pulse pair was 20 kHz. InFig. 4the transmission spectra of a low pressure sample of nitrous oxide are compared. It may be seen that the residual heat which remains in the device after 500 ns is sufficient to shift the origin of the spectrum recorded using the second pulse by about −0.76 cm−1, corresponding to an excess temperature of about 11 °C.The average of 20 000 successive laser pulses recorded using a fast digitiser with 0.5 ns time resolution. The laser substrate temperature was held at 2.75 °C and the laser drive voltage was 150 V. The negative going signal results from the characteristics of the detector amplifier combination. The laser power reduces by about 20% during each laser pulse. The electrical interference occurs away from the optical signal owing to the time delay between the electrical current pulse applied to the laser and the optical pulse arriving at the detector, since the optical path length is 110 m (S. Wright, M. T. McCulloch, G. Duxbury and N. Langford, unpublished spectra.)The transmission spectra of a 2 mTorr of nitrous oxide contained in the multiple pass absorption cell. The spectra are offset for clarity. The effect of the residual heating from the first pulse in displacing the origin of the second pulse to longer wavenumber is evident, since it is almost equal to the separation of successive vibration‐rotation lines of nitrous oxide. The unusual line shape is due to rapid passage, since at the low gas pressure in the absorption cell there are no collisions within the time taken to scan through a Doppler broadened line, about 1 ns. This effect is described in detail in ref. 1. (S. Wright, M. T. McCulloch, G. Duxbury and N. Langford, unpublished spectra.)Is the heat dissipated and the internal temperature rise fully relaxed before the next pulse? Since the separation of the pulse pairs is 50 μs at the repetition frequency of 20 kHz, this is quite sufficient to allow the excess heat to have dissipated by conduction before the start of the next pulse pair. As a result we have seen no evidence for the reduced resolution which would result from the superposition of the cumulatively shifted patterns of up to 64 000 successively averaged spectra.1 G. Duxbury, N. Langford, M. T. McCulloch and S. Wright,Chem. Soc. Rev., 2005,34, DOI: 10.1039/b400914m.Dr Seakinscommented: In their paper, the authors present some high quality data on a variety of different types of measurements obtained from a mobile laboratory. One of the issues that we have come across in our work on mobile measurements of NOxand O3is the reproducibility of the data. Modellers can be sceptical of the results believing that all that is being measured is the plume of a recently passed vehicle.We present some preliminary data showing that mobile measurements can indeed produce reproducible data, although in the complex and highly variable urban environment, averaging is required to obtain representative data.The University of Leeds has operated a mobile laboratory for several years.1The laboratory can analyse for NO, NO2, O3, CO and SO2using commercial instrumentation, sampling either ‘on‐the‐move’ or at fixed locations using battery packs. The chemical measurements are supported by basic meteorological data and location and speed are recordedviaGPS.The first example is from the second NERC funded TORCH campaign and seeks to provide experimental confirmation of predictions of significant decreases in boundary layer ozone as air moves from a marine (low ozone deposition) to terrestrial environment.2Predicted air trajectories were followed from the Weybourne Observatory, Norfolk, up to 50 km inland during both day and night. Measurements were made under both modes of operation.Fig. 5shows that any slight deviations of the route from the actual trajectory are unlikely to cause significant errors as ozone concentrations were constant along the whole of the Norfolk coast.Fig. 6shows an example of day and night time ozone concentrations, obtainedviameasurements at fixed locations with theFig. 7showing the time profile of the reverse nighttime trip back to Weybourne. The mobile data have yet to be fully processed (to account for any local sources at road junctionsetc.), but clearly show the inverse dependence from the fixed site measurements, with ozone concentrations increasing towards the coast. Due to lower boundary layer levels, ozone deposition at night is predicted to be more significant than during the daytime and this prediction is confirmed by the experimental measurements.Mobile measurements along the Norfolk coast. Concentrations of ozone are virtually constant (note significant false origin).(a) O3profiles day and night with distance from coast. (b) Mobile measurements of O3, NO and NO2from nighttime return to coast.NOxconcentrations around the Crossgates circuit.The second example is some very recent measurements from the Crossgates area of Leeds. The aim of this work is to assess the viability of mobile measurements to produce ‘pollution maps’ that can be used to validate the air quality models used by local councils to assess air quality and exceedences of limit values.The data presented inFig. 7are NOxconcentrations averaged over 30 circuits. Careful processing is required to average the data, but the resulting profile clearly shows significant trends in air quality with the highest concentrations being observed in street canyons and along busy dual carriageways with minor peaks associated with traffic lights or bus stops in residential areas. These are the expected qualitative variations, however, with the mobile laboratory we are now able to quantify such effects and compare with model predictions.1 P. W. Seakins, D. L. Lansley, N. Huntley and A. Hodgson,Atmos. Environ., 2002,36, 1247–1248.2 J. Entwistle, K. Weston, R. Singles and R. Burgess,Atmos. Environ., 1997,31, 1925–1932.Dr Kolbreplied: The data reported in this comment are very similar to that obtained by our mobile laboratory (ref. 1 and our paper in this Discussion). In particular, the urban pollution map from Leeds is very similar to our published observations from Boston. Because we deploy many instruments with ∼1 s integration times our on‐road mapping data show sharp (1–3 s duration) trace gas and fine particle signals due to individual vehicle emission plumes superimposed over the mean on‐road background pollution levels. In ref. 1 we discuss automated methods for removing these fresh emission plumes to produce maps more representative of the urban background. As discussed in our paper in this volume, we also process these above background exhaust plume signatures to develop on‐road fleet averaged emission indices for both gas phase and particle exhaust pollutants by measuring above background pollutant to above background carbon dioxide (or carbon dioxide + carbon monoxide) levels in each encountered vehicle exhaust plume and processing the results from hundreds to thousands on‐road plumes.1 C. E. Kolb, S. C. Herndon, J. B. McManus, J. H. Shorter, M. S. Zahniser, D. D. Nelson, J. T. Jayne, M. R. Canagaratne and D. R. Worsnop,Environ. Sci. Technol., 2004,38, 5694–573.Dr Monkssaid: Studies of air quality and health require exposure assessment at head height (1–1.5 m). Many of the sampling inlets on your platform are significantly higher. Is this an issue?Dr Kolbreplied: The main sampling inlet on our current mobile laboratory is ∼2.5 m above ground level. We have tried sampling inlets at various height levels and have seen no significant difference in measured vehicle exhaust plume pollution ratios or background pollution levels. The turbulent wakes behind on‐road vehicles very effectively and rapidly mix and disperse exhaust plumes ambient both vertically and horizontally (1,2). The continuous passage of vehicles thoroughly mixes the air from the roadway surface to a height well above our sampling inlets. There is obviously no problem deploying a mobile laboratory sampling probe at the 1–1.5 m height if regulations require, but our experience indicates that this is unnecessary.1. P. Jiang, D. O. Lignell, K. E. Kelley, J. S. Lightly, A. E. Sarofim and C. J. Montgomery,J. Air. Waste Manage. Assoc., 2005, 437–445.2. S. R. Ahmed and G. Ramm,SAE Tech. Pap. Ser., 1984, 840300.Dr Seakinsreplied: I am sure that it is possible to envisage a scenario in which vertical resolution in the lowest 0–3 m of the urban atmosphere was required. However, in most instances the actual movement of the vehicles generates significant turbulence which rapidly mixes pollutants in this spatial domain. I would refer interested readers to pictures within the paper by Pirjolaet al.1describing their mobile monitoring vehicle. It is of course possible to sample at various heights.Our primary objective is to compare experimental measurements with model predictions and we are careful to ensure that the correct receptor height is used in the model calculation.1 L. Pirjola, H. Parviainen, T. Hussein, A. Valli, K. Hameri, P. Aaalto, A. Virtanen, J. Keskinen, T. A. Pakkanen, T. Makela and R. E. Hillamo,Atmos. Environ., 2004,38, 3625–3635.Dr Shallcrossasked: Building geometry and local meteorology can play an extremely important role in determining concentration—time profiles of pollutants, emitted by vehicles, in urban streets. Particularly in your stationary observations, how important were these factors for the pollutant levels you have observed during your investigations?Dr Kolbreplied: It is certainly true that tall and densely packed buildings can create a “street canyon” effect, trapping pollutants, especially in low wind situations. Urban tracer measurements have confirmed that both horizontal and vertical plume spread is highly dependent on boundary layer meteorological parameters.1We often deliberately choose windy days for stationary mobile lab deployments to avoid measurements unduly dominated by nearby sources and low wind, inversion conditions that integrate pollutants from surface sources for mobile experiments designed to locate fixed point and area source pollutants.1 A. Venkatram, V. Isakov, D. Pankratz and J. Yuan,Atmos. Environ., 2005,39, 371–380.Professor Heardasked: How will the data presented in Table 2 of the paper (in particular the ratio of in plume to background concentrations) by exploited by pollution regulators, or indeed atmospheric modellers?Dr Kolbresponded: The data presented in Table 2 of the paper show our first attempt to develop and use a fast conditional sampling method to determine emission ratios (excess exhaust pollutant/excess exhaust CO2) for volatile organic compounds (VOCs) for which we do not have ∼1 s response instruments that allow us to measure these ratios directly. We collect “exhaust plume” samples into an evacuated cylinder when our fast response CO2sensor shows high, above ambient, levels characteristic of exhaust plumes and “background” samples when the CO2readings are at background levels. A high ratio (≫1) of plume to background for a specific VOC indicates that it has a strong vehicle exhaust source, a ratio below 1 shows that the VOC’s vehicle emission source is not significant for the class oWe are currently converting these data to emission ratios two ways: (1) by integrating the fast response instrumental CO2data to compute the CO2levels in both the plume and background samples, which will allow use to determine the excessVOC levels and the excess CO2levels by subtracting the background levels from the plume levels; and, (2) by using direct PTR‐MS fast response measurements of exhaust VOCs, like benzene and toluene, where we have both real‐time and conditional sampling canister data and using their real‐time excess VOC/excess CO2values to normalize the data for the other VOCs with high plume/background ratios shown in Table 2. For the vehicles sampled, we will then be able to compute emission indices (g pollutant emitted per kg of fuel burned) that are useful for mobile source regulatory evaluations and emissions inventory development.Dr JenkinandDr JohnsonAlso Dr Steven Utembe, Department of Environmental Science and Technology, Imperial College London, Silwood Park, Ascot, Berkshire, UK SL5 7PY.opened the discussion of Dr McFiggans’s paper: As an extension to the simulations of gas phase organics in our paper, we have also carried out simulations of organic aerosol formation for the TORCH campaign in 2003. A representation of the transfer ofca. 2000 lower volatility products in MCM v3.1 to the aerosol phase (i.e.SOA formation) has been included, based on the Pankow equilibrium absorptive partitioning theory, using estimated vapour pressures. The air parcels are also simulated to receive direct emissions of organic aerosol (i.e.‘primary organic aerosol’, POA) from sources such as road transport. The magnitude of the POA emissions was defined relative to NOxemissions, on the basis of observed correlations of fine organic particles and NOxat urban locations, made by the University of Manchester.The simulated campaign mean concentration of POA accounts for 18% of the observed organic aerosol (OA) concentration reported in Dr McFiggans’s paper, being fully consistent with the data presented in Fig. 12 of that paper. The simulations therefore also suggest that most of the observed OA was not emitted directly, but formed from chemical processing of emitted VOC.Concentrations of total OA were simulated for selected events using MCM v3.1 with an optimised representation of SOA formation, based on the method described above. The aim was to force the simulated OA concentrations into agreement with those observed, as presented in Dr McFiggans’s paper. The simulations imply that the OA can be ascribed to three general sources, as follows: (i) POA resulting from direct emissions; (ii) Gas to aerosol transfer of lower volatility products of VOC oxidation generated by the regional scale processing of emitted VOC. However, the observed aerosol can only be accounted for if the gas–aerosol equilibrium partitioning coefficients are increased by a large factor of 500; (iii) A ubiquitous background concentration of OA of 0.7 μg m−3, represented as an initialisation of the model.The contributions of sources (ii) and (iii) (which are collectively SOA) are both indicative of the occurrence of chemical processes within the aerosol which allow the oxidised organic species to react by association and/or accretion reactions which generate even lower volatility products. Such processes are likely to be more important in acidic aerosols, and with prolonged timescales available for chemical processing. As a result, the thermodynamic equilibrium approach severely underestimates the transfer of organic material to the aerosol (hence the large scaling factor required), although it does appear to provide a practical framework for describing the process. The resultant involatile organic aerosol is therefore very persistent, leading to the ubiquitous background concentration. Despite the large elevation of partitioning parameters required, detailed analysis of one event suggests that only a small fraction (ca. 5%) of the simulated oxidised organic material needs to be in the condensed phase to account for the observed OA.Dr McFiggansreplied: The agreement between the simulations shown by Dr Jenkin and the ratio of hydrocarbon‐like to oxygenated aerosols measured during the TORCH experiment are striking. These results provide confidence in the description of the observed aerosol as an aged contribution from the regional background into which the “primary” urban emissions dilute. Superimposed on this is a largely internally‐mixed inorganic and organic aerosol population, most likely formed by a combination of gaseous and condensed phase oxidation. The simulations presented allow a good estimate of the amount of gaseous VOC which is required either to form new aerosol particles or to condense onto the pre‐existing aerosol distribution.However, great care must be taken in extrapolating these results to a more general statement of the contributions of secondary organic material to aerosol loading in other environments. The optimisation requires a suppression of vapour pressure of the VOC oxidation products by a factor of 500. That only a single factor is required across all condensing products may be a profound finding in itself. However, the use of such a factor indicates that some aspect of the mechanism is poorly understood. It is very possible that condensed phase reactions are responsible for such a suppression of vapour pressure, but these must be identified and their effects quantified before the approach can be used as a predictive tool under other conditions. It is also of some concern that the required optimisation factor is around an order of magnitude greater than that required in chamber experiments upon which much of the gas phase mechanism is validated. This implies that the mechanism leading to production of organic aerosol mass in chamber experiments is not the same as that occurring in the atmosphere. This is not unexpected, since the chamber systems are very much simpler and the precursor molecules are available at much greater than ambient concentrations.Professor Abbattcommented: In support of the low hygroscopicity of fresh organic emissions, we have found that the organic component is best modeled as fully insoluble in a recent, as‐yet‐unpublished CCN closure experiment. With simultaneous CCN, SMPS and AMS measurements of particles on a busy urban street, there is good agreement between the number density of modeled and measured CCN if the organics are assumed to be fully insoluble, whereas the inorganic components are soluble. Even though the organics represent up to 90% of the particle mass in these observations, their only effect on cloud activation is a particle size effect and they do not appear to give rise to vapour pressure lowering. Nor do they appear to lower the surface tension, as the surface tension of the particles in the Köhler model is assumed to be that of water.Dr McFiggansreplied: The ambient CCN closure does indeed support the low hygroscopicity of fresh organic emissions, extending the consideration from the sub‐saturated to supersaturated environment. Our paper makes a statement beyond the fact that fresh organic emissions are of low hygroscopicity; we contend that realistic atmospheric mixtures of organic compoundsof any age and processing historydo not contribute significantly to the aerosol water content in the sub‐saturated environmentin Northern Hemisphere mid‐latitude locations.Concerning the CCN closure, there are very many potential effects of organic materials. Sorjamaaet al.1have recently reported that surface partitioning must be considered in cloud activation prediction when strongly surface active compounds are present. This will suppress the Raoult effect, so counteracting the effect on the Kelvin term through surface tension suppression. Shulmanet al.2discussed the interaction between solubility and activation. In the urban environment such effects may be significant. In addition, McFigganset al.,3shows that the reduction of the number of molecules in the bulk droplet interior due to the presence of insoluble films may suppress activation although the surface tension will be greatly reduced, and Clegget al.,4showed that immiscible phases of organics may be formed in even fairly simple multicomponent mixtures. To my knowledge, there have only been experiments in the laboratory into the first two of the above effects on extremely simplified systems,1,5–8and no experiments into the other effects. Furthermore, there have been few experiments exploring realistic atmospheric combinations of compounds in a systematic manner.8Whilst there may be significant evidence from simple systems that the presence of only a little inorganic salt will lead to activation, it is unlikely that an atmospheric closure study will be sensitive enough to explore whether these competing effects are playing a role in droplet activation. The fact that good closure is realised in such studies with a simple Köhler approach may not show that a surface tension effect is not taking place, but that if there is a surface tension effect, it is counteracted in some way. It is not clear that such a cancellation will occur under all conditions. Whether this is indeed the case should be the subject of further study. If so, a simplification for the supersaturated regime analogous to that presented in our paper for sub‐saturation, may be possible.1 R. Sorjamaa, B. Svenningsson, T. Raatikainen, S. Henning, M. Bilde and A. Laaksonen,Atmos. Chem. Phys., 2004,4, 21072 M. L. Shulman, M. C. Jacobson, R. J. Carlson, R. E. Synovec and T. E. Young,Geophys. Res. Lett., 1996,23, 277–2803 G. McFiggans, P. Artaxo, U. Baltensperger, H. Coe, M. C. Facchini, G. Feingold, S. Fuzzi, M. Gysel, A. Laaksonen, U. Lohmann, Th. F. Mentel, D. M. Murphy, C. D. O’Dowd, J. Snider and E. Weingartner, The Effect of Physical and Chemical Aerosol Properties on Warm Cloud Droplet Activation, submitted toAtmos. Chem. Phys. Discuss., 20054 S. L. Clegg, J. H. Seinfeld and P. Brimblecombe,J. Aerosol Sci., 2001,32, 713–7385 M. Bilde and B. Svenningsson,Tellus B, 2004,56B, 128–1346 K. Broekhuizen, P. Pradeep Kumar, J. P. D. Abbatt,Geophys. Res. Lett., 2004, 31, DOI: 10.1029/2003GL0182037 S. Henning, T. Rosenørn, B. D’Anna, A. A. Gola, B. Svenningsson and M. Bilde,Atmos. Chem. Phys., 2005,5, 575–5828 B. Svenningsson, J. Rissler, E. Swietlicki, M. Mircea, M. Bilde, M. C. Facchini, S. Decesari, S. Fuzzi, J. Zhou, J. Mønster and T. Rosenørn,Atmos. Chem. Phys. Discuss., 2005,5, 2833–2877Dr Coeasked Professor Abbatt: How have you dealt with the organic contribution to surface tension and osmotic coefficient?Professor Abbattreplied: In this Köhler calculation for closure between modeled and measured CCN numbers, we have assumed the inorganics are fully insoluble and that the surface tension of the growing droplet is that of water. In work now accepted for publication inAtmospheric Environmentwe have been searching for the surface‐tension lowering effect that might arise when surface active organics are present together with ammonium sulfate.1So far we have been able to explain all of our activation results in these lab experiments by using the bulk solubility of the organic and by assuming the droplets have the surface tension of water.1 J. P. D. Abbatt, K. Broekhuizen and P. P. Kumar, Cloud Condensation Nucleus Activity of Internally Mixed Ammonium Sulfate/Organic Acid Aerosol Particles,Atmos. Environ., 2005, accepted for publication.Professor Donahuecommented: The asymptotic value for primary/oxidized aerosol of ∼0.1 is quite interesting. Why does the ‘aged’ aerosol retain this reduced fraction—is it local fresh emission or a sequestered fraction of the OA?Dr McFiggansreplied: This has been correctly identified as a very interesting point which can originate from one of two sources. Either the analytical technique used to separate the fractions according to their representative fragmentation pattern is not capable of fully resolving the fractions or this is a real atmospheric phenomenon.In the cleaner background air, the aerosol mass loading is lower and the AMS signal to noise ratio will be lower. It is in the cleaner airmasses that the HOA/OOA ratio approaches its asymptotic value. The error propagation through the analysis will lead to an unresolved residual which may be of the order of the HOA loading (there are more contributing peaks across the spectrum for the HOA fraction). In addition, the aliphatic chain of the compounds within the OOA fraction will also contribute fragments found in the characteristic HOA fraction. The low signal to noise in background air will make it difficult to disentangle the ratios around detection limit. A full error propagation through the analytical procedure is required.Whilst this is a possible explanation, there is some independent evidence that an externally‐mixed population is a real phenomenon. Instruments which indirectly probe the composition mixing state of an aerosol population sometimes find more than a single mode at a given size even in the cleanest environments. For example, HTDMA measurements of any hydrophobic particles at smaller sizes in these circumstances may contain hydrocarbon‐like compounds. However, normally such a mode has been thought to comprise externally mixed organics, not necessarily from the HOA fraction, (e.g.those found in marine airmasses during ACE 2). Important information may be gained from more frequent deployment of composition and mixing‐state measurements in a variety of locations. Whether these particles are formed in some “secondary” process is highly uncertain.Dr Jenkinadded: Our trajectory model simulations (described in an earlier comment) allow emitted organic aerosol to be estimated for the whole TORCH campaign, using a ‘no chemistry’ version of the model. The simulated concentrations match the ‘HOA’ profile in Fig. 12 very closely, with the highest contributions to OA during the south‐westerly period and contributions of the order of 10–15% in the more aged air during the anticyclonic period. This suggests that the observed HOA contribution can reasonably be attributed to fresh emission.Professor Rudichcommented: The aerosols measured on the Toronto Street may be still ‘fresh’ and hence the organics are less oxidized and hence not so much CCN‐active. In a recent study of biomass burning aerosol in Brazil we show that the organic soluble fraction’s properties (solubility, surface tension and molecular weight) are needed to accurately predict water uptake and CCN activity.1 M. Mircea, M. C. Facchini, S. Decesari, F. Cavalli, L. Emblico, S. Fuzzi, E. Swietlicki, G. Frank, Y. Rudich and P. Artaxo,Geophys. Res. Abstr., 2005,7, 04667.Professor Abbattagreed: Yes, we fully believe that the organic fraction in the size range of particles that act as CCN (50 to 100 nm or so) in our downtown Toronto setting are fresh emissions, and that they will have lower solubility than might arise in other environments. The behavior that we have observed with respect to the organic contribution to the CCN properties will not necessarily be the same in rural, remote or biomass burning regions.Mr Toppingasked Professor Abbatt what size range the cloud activation predictions/measurements were made for since it is likely that one might capture an ‘insoluble’ mode where an assumption that the surface tension of water is likely to hold. There have been various theoretical studies however that show activation is sensitive to varying organic composition.Professor Abbattreplied: All the CCN predictions and measurements were for a supersaturation of 0.58%. For the particles size distribution present at our sampling site in Toronto at that time, essentially all the CCN were particles in the size range between about 60 and 200 nm. Although there was substantial organic (and inorganic) mass present in the particles with sizes larger than 200 nm, the numbers of particles present in this size regime is so small that they do not contribute substantially to the total number of CCN. But your point is well taken—the organic fraction in different size regimes may have different composition and hygroscopic properties.Professor Herrmanncommented: According to your manuscript the hygroscopic properties of particles are largely governed by their inorganic composition. However, this might largely differ for particles of different character and in different environments. Hence, maybe it is somewhat too early for a very broad statement as mentioned before?Dr McFiggansresponded: Professor Herrmann has indeed raised an important point. In our paper we claim that the ratio of inorganic to organic material determines the hygroscopic properties. This will be true whenever there is a significant amount of internally mixed inorganic material mixed with organic materialof a similar composition profileto those sampled during the TORCH experiment. We compared the range of representative model compounds from the TORCH experiment with those representing the Po Valley organic composition. The difference between the predicted hygroscopic growth factor of particles of totally organic particles from each location was found to be very much less than the resolution of instruments which can measure ambient growth factor.There are now detailed compositional measurements from which model compounds can be derived to represent the functionality for a variety of locations (Rondonia in Brazil, Mace Head in Ireland, Hyytiälä in Finland, Po Valley in Italy). Since broadly similar contributions are made in each location from the NC, MDA and PA fractions and the oxygen to carbon ratio in each location is comparable, the amount of water associated with a given number of moles will be comparable. Since the average molecular weight of the organics is of the order of several hundred in almost all locations, there will be a comparable number of moles per unit mass. The inorganic components, in contrast, are associated with much more water per unit mass due to their lower molecular weights and increased polarity. Of course, if there are regions found where a large majority of the mass comprises solely organic compounds, they will generally be lower in growth factor, but this growth factor may be affected by the nature of the organic components. Such situations may arise in biomass burning plumes above Amazonia, for example.If there are significant amounts of internally‐mixed organic components (as are ubiquitous in Northern Hemisphere mid‐latitudes), for the inorganics and organics to contribute to the associated water equally, it would be necessary for comparable masses of low molecular weight polar organic compounds and inorganic electrolytes to be present in the same particles. To our knowledge, this type of particle has not been reported in atmospheric samples. The hygroscopic properties of ambient aerosol particles in NH mid‐latitudes may therefore be assumed to be determined by the ratio of inorganic to organic mass, unless contrary evidence arises.

ISSN:1359-6640    

DOI:10.1039/b507793c

出版商:RSC    

年代:2005

数据来源: RSC

摘要:

Dr Remediosopened the discussion of Dr Burrows’s paper: Dr. Burrows has shown results from measurements of three species: ozone, nitrogen dioxide and formaldehyde. They imply that further information separating sources such as biomass burning from biogenic emission and stratosphere–troposphere exchange would be very useful. Measurements of carbon monoxide (CO) from the MOPITT instrument can provide a diagnostic of biomass burning signals, assuming that large enhancements of its concentrations are due to this source alone. In principle, the CO data also provide tracer information, allowing for evolution of CO in biomass plumes, with MOPITT daytime “surface” data over land demonstrating lower-to-mid-troposphere transport and nighttime “surface” measurements over ocean indicating mid-to-upper troposphere transport; the “surface” retrieved level for MOPITT data can shift significantly in altitude sensitivity and so characterisation data or averaging kernels have been used to interpret the data (not shown). In the figure shown for September 2000, we see that nighttime CO data indicate upper troposphere outflow over Madagascar, similar to the transport indicated in the paper albeit for a different year. We also observe a strong pattern of CO sources in western to central Africa (indicated by “daytime − nighttime surface” CO over land which our analyses have shown can be sensitive to near-surface enhancements of CO), raising questions of relations between fire types and emissions of trace gases both as primary and secondary products. A further study to the paper presented would therefore be to combine results from GOME or SCIAMACHY on ENVISAT with analyses of MOPITT data.Retrieved “surface” level carbon monoxide (CO) data from the measurements of pollution in the troposphere (MOPITT) instrument on EOS-Terra. Data are global monthly means for September 2000: (a) daytime measurements; (b) nighttime measurements; (c) difference between daytime and nighttime measurements (J. J. Remedios, N. A. D. Richards, U. Friess)Professor Burrowsreplied: The comment and question are very relevant. For our study, which focuses on the September 1997, unfortunately there are no MOPITT data available as it was launched aboard the NASA Terra satellite in December 1999 and began measurements in 2000. Clearly the assimilation of satellite data from MOPITT and GOME and SCIAMACHY for the period from August 2002 to June 2003 when all are working represents a potentially very interesting study. This was however outside of the scope of this study. This should stimulate more studies using the complete set of data from MOPITT, GOME and SCIAMACHY. MOPITT CO data of a different El Niño year (e.g.2002) can be compared with the results of the transport analysis for September 1997. The MOPITT data from September 2002 indicates an eastward outflow of biomass burning emissions over Madagascar, not very dissimilar to that observed by GOME for NO2.Dr Chipperfieldsaid: The authors say that 1997 is studied due to a ‘strong signal’. What do they mean exactly by this—what quantities are large in this period? This may have implications for the discussions which compared this paper with other GOME periods.Professor Burrowsreplied: The 1997 El Niño Southern Oscillation (ENSO) event was the largest yet recorded. Over Indonesia but also over the Horn of Africa and related regions of East Africa, the El Niño phase of the ENSO results in low rainfall and consequently the amount of fire both natural and anthropogenic in origin was high. This results in large biomass burning/biofuel emissions and the month of September 1997 is a particularly good example of this phenomena. Overall a significant enhancement of tropospheric columns of O3, NO2, and HCHO derived from GOME is observed during September 1997, when compared to the wetter September of 1998. The total column O3 retrieved from GOME agree well with that determined the sonde network, SHADOZ (Thompsonet al.), which started in 1998. Over a significant area of Africa and the western parts of the Indian Ocean the tropospheric O3column during September 1997 exceeded that observed in September 1998 by 25 to 100%. So as a result of the strong ENSO 1997 was an extraordinary year in the tropics and the tropospheric amounts of O3, HCHO and NO2were large.1 A. M. Thompson, J. C. Witte, R. D. McPeters, S. J. Oltmans, F. J. Schmidlin, J. A. Logan, M. Fujiwara, V. W. J. H. Kirchhoff, F. Posny, G. J. R. Coetzee, B. Hoegger, S. Kawakami, T. Ogawa, B. J. Johnson, H. Vömel and G. Lebauw,J. Geophys. Res., 2003,108(D2), 8238, DOI: 1029/2001JD000967.Mr Pfrangcommunicated: I would like to comment on the differences between retrieval and modelling results. Meyer-Arneket al. conclude that uncertainties in the meteorological input data (ERA-40 re-analysis data from ECMWF) are the main reason for inaccuracies in the model representation of transport processes in the upper troposphere without sufficiently taking into account simplifications connected with the transport modelling approach employed in the study.In discussions with Johannes Flemming and Sakari Uppala at ECMWF, a number of questions emerged that would benefit from clarification. First, it remains unclear how vertical exchange processes are treated by the model as we assume that the kinematic trajectories are only isobaric. Convection is highly important in the tropics and may force the trajectories to be located on changing height levels. Secondly, we would be interested to learn how the starting levels for trajectories were chosen and how sensitive is the model with respect to changes in the apparently arbitrarily selected pressure levels. The latter point might be of particular interest when considering uncertainties in injection heights of fires. Thirdly, it is mentioned in the paper that 230 000 trajectories were calculated, but it remains somewhat unclear how these trajectories were chosen and partitioned between the different pressure levels and what effect a different choice and vertical partitioning of trajectories would have had on the results of the model. Fourthly, assuming chemical processes are of importance, why would the maximum trajectory density necessarily coincide with the maximum of the tropospheric O3column as indicated in the discussion on page ten? If the ERA-40 data would remain the main reason for inaccuracies in the transport model it would be very useful to quantify the error of the ERA-40 data itself and the error due to temporal interpolation to enable the ECMWF to analyse and possibly further improve the next generation re-analysis data.To summarise, we are certainly aware of uncertainties in the ERA-40 data, but it appears to be premature to conclude from the results presented in this paper that inaccuracies detected in the representation of transport processes in the model are solely due to the meteorological data provided by the ERA-40 re-analysis (as stated on the bottom of page thirteen).ECMWF highly appreciates your use of ERA-40 re-analysis data, since constructive criticism from independent researchers is one of the important tools for the improvement of our products. We would like to draw your attention to a survey of current and future applications of the ERA-40 re-analysis data which is now available athttp://www.ecmwf.int/research/era/era40survey/. We hope the ERA-40 survey will prove valuable to (i) assess the impact of the ERA-40 re-analysis project, (ii) facilitate exchange of information between ERA-40 users, (iii) compile a review of research work using ERA-40 and (iv) make a case for a more comprehensive and longer re-analysis later this decade, which will benefit from the lessons learned from ERA-40. If you are aware of any project using ERA-40 data not being included in the current version of the ERA-40 survey, we would be most grateful if you contact us at era40survey@ecmwf.int.Professor Burrowscommunicated in reply: We would like to thank the ECWMF for their comments. We used ERA-40 data because we consider it to be suitable and an excellent set of data. However as pointed out on the ERA-40 web site, there appears to excessive tropical oceanic precipitation in the ERA-40. This indicates that in the tropics there are some as yet unresolved issues. Although total ozone has been assimilated into ERA-40 from satellites, this is dominated by the stratospheric ozone burden. In our study the tropospheric columns of O3, NO2and HCHO, retrieved from the measurements of the satellite GOME during the strong ENSO of September 1997 are presented and compared with trajectory calculations, determined using the ERA-40 data with and without chemistry calculated along trajectories. This approach has its limitations.The first question raised concerns the relative importance of convection. Clearly this is of great potential significance in the tropics. If the ERA-40 data set effectively smooth over the convective activity, then trajectories represent some average picture. Convection cannot be explicitly considered in the trajectories because it occurs typically at scale smaller than the grid, used for the calculation of the trajectories. However the winds fields of ERA-40 should represent the larger scale behaviour.To investigate the potential importance of convection for the African outflow into the Indian Ocean, we have used the Lifted Index, LI, and assumed that all negative LI are strongly convected. This is considered to be a worst case. Whilst some redistribution of the trajectories is observed, there is still a significant difference in the pattern observed in GOME columns of O3, NO2and HCHO and that predicted for the outflow from Africa.The second question addresses the sensitivity of the starting level. The transport differs for trajectories released at different altitudes. Above Africa in the region selected for study, the majority of air parcels, released at lower altitudes are transported to the west, whereas those released at higher altitudes (200 hPa above the ground) are transported to the east. The “release” altitude of the trajectories was chosen to be at the top of the planetary boundary layer and at the bottom of the free troposphere, because this altitude is the best representation of the height of the airborne measurements used to initialise the chemistry model.For the case that the fire index is non-zero within a grid box, trajectories are started on a 1° × 1° horizontal grid. At each starting position trajectories are released at 50, 100, 150 and 200 hPa above the ground pressure. The meteorological data are represented on model levels.As shown inFig. 2, the overall distribution of the calculated trajectory shows similar flow out of East Africa at all the pressure altitudes. The chemistry calculated along the trajectories follows the same behaviour.The observations of tropospheric trace columns leaving Africa as determined from the GOME observations in September 1997 appear to be significantly further north of the flow taking trajectories and an estimate of convection into account (Fig. 3). In this case the Lifted Index is used to determine instable air and all such air masses are convected.The trajectory density calculated for the trajectories, leaving biomass burning regions at pressures of 50, 100 and 200 hPa.Trajectory density calculations for trajectories leaving the biomass burning region including an estimate of convection.The combined trajectory density for outflow from Africa is extended northwards but beyond longitudes of 60 °E is further south than the GOME observations. Clearly more data and years need to be carefully analysed to see whether September 1997 is a special case or representative of a trend.The third question addresses the selection of trajectories. As described in the text, all trajectories, which enter the region selected for observation were calculated.The fourth question raised questions whether the ozone formed should follow the trajectory density. Our conclusion is based on the fact that the column amounts of O3, NO2and HCHO as observed from GOME follow similar but not identical paths in the outflow from east Africa. The NO2flow is shifted northwards.In summary we agree that convection is an important process in the tropics. We have investigated what impact convection might have and conclude that it is unlikely to explain all the difference between calculated and observed in the outflow from east Africa for September 1997. Similarly we have argued that it is unlikely that inaccuracies in the retrievals of the three trace gas columns from GOME data are the source of the difference. In the longer term the assimilation of GOME data into ERA-40 may be the best way to resolve the origin of discrepancies between our relatively simple approach and account optimally for convection.Dr Monksaddressed Professor Burrows and Jaeglé:(1) Does the distribution of clouds/smoke plumes and the potential changes with seasonetc. bias the emission estimates? In a sense are you sampling different parts of column NO2?(2) Could you develop a spatial/vertical sampling metric, that could be superimposed on the data?Professor Jaegléreplied:(1) Clouds and smoke plumes do have the potential of affecting the retrieval of NO2columns from GOME. We take into account both effects in our air mass factor (AMF) calculation, but these introduce uncertainties in our tropospheric NO2retrieval: 28% for cloud and 30% from aerosols.1We have rejected scenes where the cloud radiance fraction exceeds that from clear sky, corresponding to cloud cover>40%. As a result of this threshold, no NO2columns are available over regions with large cloud cover during some months. This is can be seen in Fig. 3 of the paper during January, where no NO2columns are available over parts of eastern Europe and east Asia.(2) The spatial and vertical sensitivity of our retrieval of tropospheric NO2columns to clouds and aerosols has been discussed in previous papers by Martinet al.1,21 R. V. Martin, D. J. Jacob, K. Chance, T. P. Kurosu, P. I. Palmer and M. J. Evans,J. Geophys. Res., 2003,108, DOI: 10.029/2003JD003453.2 R. V. Martin, K. Chance, D. J. Jacob, T. P. Kurosu, R. J. D. Spurr, E. Bucsela, J. F. Gleason, P. I. Palmer, I. Bey, A. M. Fiore, Q. B. Li, R. M. Yantosca and R. B. A. Koelemeijer,J. Geophys. Res., 2002,107, DOI: 10.1029/2001JD002622.Professor Burrowsreplied:(1) To a reasonable first approximation in nadir viewing ultraviolet and visible electromagnetic radiation are backscattered from the top of clouds and therefore GOME measurements contain negligible information about trace gases below the cloud top. Cloud top height and cloud cover are routinely evaluated for all GOME data by using the O2A-Band absorption. The cloud fraction reveals the African west coast over the Atlantic are frequently covered with clouds. The cloud top height data indicates that these clouds remain at altitudes of less than 2 km, thus significant information from GOME data is available between 2 km and the tropopause. The region of strong smoke from a biomass burning region is usually small compared to a GOME ground pixel.The use of ground scenes, having <10% cloud cover in our analysis minimises the impact of cloud. Our composites represent cloud free regions between clouds. As we have demonstrated in previous work NO2from convective uplifting and lightning can be observed above clouds. In our comparisons of model and measurement, we are comparing as best we can cloud free regions, so we consider that there is not a bias. The 50% back scattered radiation criterion, used by Jaegleet al. is a different selection criterion. As cloud has a higher reflectance than the surface, then we consider that the criteria for cloud screening are effectively probably very similar.Emissions are expected to be seasonally dependent and are. In our study we have focussed on the large ENSO event of 1997, which resulted in dry conditions in Africa in September 1997.(2) The number of individual data points used for each grid point is known. An intrinsic assumption is that the fire pattern and biogenic emissions were reasonably constant during September 1997. This is of course a simplification but not unreasonable considering the size of the GOME ground scene. An evaluation of GOME data reveals that the instrument was sampling all parts of the examined region equally during September 1997.Dr Chipperfieldasked: The two groups using GOME data employ different thresholds (10%, 50%) for the exclusion of data due to cloud contamination. Why is there this difference?Professor Jaegléresponded: Our AMF calculation to convert slant columns to tropospheric columns, enables quantitative retrieval of partly cloudy scenes. The AMF calculation accounts for cloud scattering using cloud fraction, cloud top pressure, and cloud optical thickness observed by GOME and retrieved with the GOMECAT algorithm. Thus our analysis does not need to be restricted to cloud-free or very low cloud cover areas. We do eliminate regions where clouds contribute to more than 50% of backscattered radiation, which corresponds to cloud cover >40%. At these high cloud covers our retrieval becomes more uncertain.Professor Burrowsresponded: The University of Bremen group has developed trace gas tropospheric column data products for effectively cloud free ground scenes, because clouds block the penetration of solar radiation in the troposphere. Data products can also be generated for cloudy scenes. The UB group considers that cloud coverage of larger than 10% within a ground scene of GOME impacts significantly on the determination of the tropospheric columns. The UB retrievals have been validated by comparison with ground based and aircraft data. The approach by Jaegléet al. to deal with cloudy ground scenes is similar but somewhat different. As clouds typically have a significantly higher spectral reflectance than land or ocean, then the 50% reflectance criteria results in a small effective cloud fraction in a ground scene.Dr Sarkarsaid:(1) Thanks for your presentation. Please could you highlight an alternate method (other than TEM) to resolve the total column amount into tropospheric and stratospheric components, especially to analyse Indonesian Forest fire?(2) In India we have approximately 15 ground-based stations operated by the Indian Space Research Organisation which have state-of-the-art equipment to analyse physical and chemical properties of aerosols and trace gases. Do you plan to compare your results with ground-based measurements?Professor Burrowsreplied:(1) From GOME or any nadir-sounding passive remote sensing instrument operating in the ultraviolet, visible and near IR spectral regions, the derivation of tropospheric trace gas columns depends on factors, which are dependent on either the molecule (e.g.absorption cross section or line strength, and the vertical distribution in the atmosphere) or radiative transfer issues (the relative importance of molecular, particle and surface scattering). The determination of the tropospheric columns of gases, having significant but variable amounts in the stratosphere or above, requires an accurate knowledge of the stratospheric or upper atmospheric amount. Thus as ∼90% of O3is in the stratosphere the retrieval of tropospheric O3represents the most difficult challenge. Fishman and co-workers at NASA Langley1and Hudson and Thompson,2at NASA GSFC/University of Maryland, the University of Bremen, and the University of Heidelberg pioneered the use of the residual approaches to derive tropospheric trace gas columns. In its simplest form this approach uses the difference between two measurements: one in a location having a clean and known tropospheric and stratospheric column, the other with a different location with a different tropospheric column. An effective assumption of longitudinally homogenous stratospheric columns is often made. This works well for the monthly composites in the tropics and sub tropics. As one moves to higher latitude variability in the lower stratosphere has to be explicitly taken into account.A related method to derive tropospheric O3columns from the measurements of a nadir-looking instrument is the convective-cloud-differential (CCD) method.3This assumes that high cloud are close to the tropopause and enables the stratospheric column to be separated from the tropospheric column. In the special case of O3, the variation of penetration depth a function of altitude, coupled with the temperature dependence of the to retrieve the tropospheric ozone (Hoogenet al.4and references therein). At present the radiometric calibration of GOME limits the application of this approach. Neural networks have also been used for the determination of profile information.5These methods effectively combine a variety of different measurements together and the information content and results are therefore of mixed origin.In addition models of the stratospheric or upper atmospheric burden, stratospheric have been combined with satellite measurements. For the stratospheric column of the trace gas is derived from a chemistry and transport model (e.g.ROSE/DLR or SLIMCAT). The total column is derived from satellite measurements. The difference between them represents the tropospheric amount. The CTMs take explicitly into account planetary wave activity and the dynamics of the stratosphere.6For retrieval of tropospheric NO2, an additional option is to use the penetration depth dependence of the spectral bands.7Finally for HCHO as the stratospheric column is small the total column is dominated by the tropospheric column.(2) The validation of data products derived from measurements made by instruments such as GOME or SCIAMACHY on satellite platforms is a critical activity within the entire project. Comparisons with ground based and aircraft borne measurements have been successfully undertaken. The University of Bremen team have been involved in ground based measurements around the world. For the validation of tropospheric columns there has been a focus on using European and American networks at this point in time for logistical reasons. Comparison with ground basedin situmeasurements requires care because the satellite instrumentation yield the tropospheric columns, whereas measurements in the lowermost troposphere do not include the column above the instrument. However in a variety of studies we have been able to show that in polluted situations the boundary layer column dominates the tropospheric column. The use of data from India for the validation of satellite measurements is an important potential future project.1 J. Fishman, C. E. Watson, J. C. Larsen and J. A. Logan,J. Geophys. Res., 1990,95, 3599–3617.2 R. D. Hudson and A. M. Thompson,J. Geophys. Res., 1998,103, 22129–22146.3 J. R. Ziemke, S. Chandra and P. K. Bhartia,J. Geophys. Res., 1998,103, 22115–22127.4 R. Hoogen, V. V. Rozanov and J. P. Burrows,J. Geophys. Res., 1999,104(D7), 8263–8280, DOI: 10.1029/1998JD100093.5 M. D. Müller, A. K. Kaifel, M. Weber, S. Tellmann, J. P. Burrows, D. Loyola,J. Geophys. Res., 2003,108(D16), 4497, DOI: 10.1029/2002JD002784.6 W. Thomas, F. Baier, T. Erbertseder and M. Käster,Tellus, Ser. B, 2003,55, 993–1006.7 A. Richter and J. P. Burrows,Adv. Space Res., 2002,29, 1673–1683.Professor Jaegléresponded:(1) Our method to separate the tropospheric and stratospheric NO2columns is similar to the tropospheric excess method (TEM). We determine the stratospheric component of the column by using GOME observations over clean Pacific regions where NO2is low. We then assume that this stratospheric component is longitudinally invariant and subtract it from the total columns. Finally, we correct for the assumption of zero tropospheric NO2over the Pacific by adding to the columns the modelled estimate for tropospheric NO2columns over the clean Pacific. New observations of NO2columns from SCIAMACHY have both nadir and limb measurements, allowing a better separation of tropospheric columns by nadir–limb subtraction under some circumstances.(2) We found that current biomass burning emission inventories over India and SE Asia are likely to be too high by ∼50%. This is consistent with previous studies based on satellite and aircraft observations of CO.1,2It is clear that direct surface observations of CO, NO2and other compounds emitted by biomass burning over India would be very valuable to further constrain biomass burning emissions in this region.1 P. I. Palmer, D. J. Jacob, D. B. A. Jones, C. L. Heald, R. M. Yantosca, J. A. Logan, G. W. Sachse and D. G. Streets,J. Geophys. Res., 2003,108, DOI: 10.1029/2003JD003397.2 C. L. Heald, D. J. Jacob, D. B. A. Jones, P. I. Palmer, J. A. Logan, D. G. Streets, G. W. Sachse, J. C. Gille, R. N. Hoffman and T. Nehrkorn,J. Geophys. Res., 2004,109, DOI: 10.1029/2004JD005185.Dr Stevensonasked Professor Burrows: Can the use of 10 day trajectories in the tropics be justified? Surely it is very likely that such trajectories will encounter convection (i.e.rapid vertical motion unresolved by the large-scale winds) and cannot be trusted? This seems almost guaranteed given the intimate link between lightning and convective clouds.Similarly, the use of a trajectory model (albeit one with multiple trajectories and detailed chemistry) precludes interactions along the trajectory, such as mixing of various sources, and crucial processes such as wash-out, and variations in photolysis rates as clouds are encountered. All these factors suggest that the model results should be viewed as only a first approximation. Use of a more interactive chemistry–transport model would have been preferable (although would still contain multiple approximations).With these points in mind, I find some of the conclusions of the paper unjustified. For example, the model overestimates NO2columns by a factor of two. This is ascribed to an overestimate of the lightning NOxsource. This may be true, but it could be that lack of convection (and hence lack of delivery of HOxto the upper troposphere) may significantly affect the NO2lifetime and resultant NOxconcentrations.Similarly, the apparent misplacement of the outflow plume over the Indian Ocean is put down to errors in the ERA-40 reanalysis fields. Transporting NO2at the wrong vertical level (due to the lack of convection) may contribute here (see also comments from Evans and Chipperfield).Professor Burrowsreplied: The justification of the use of trajectory models depends on the question being posed. We argue that the large number of 10 day trajectories (about 230 000) used in this study should depict reasonably well the large scale motion of the atmosphere. However the pattern of the GOME observations for September 1997, which have themselves a relatively large ground scene, are significantly shifted compared to the pattern determined from the trajectories coming off the coast of East Africa with or without chemistry.Convection is clearly an important process in the tropics and is probably best treated as a transition probability between two model layers in trajectory models. Similarly mixing of the air in the trajectory clearly does take place. In a simple test the trajectories were lifted up to an altitude of 300 hPa over areas where the Lifted Index indicated instable atmospheric conditions. The transport analysis resulted in an eastward transport to the Indian Ocean at latitudes extending over a wider range than in the absence of convection. However beyond longitudes of about 70 °E the plume turns southward and travels significantly further south than is indicated by the GOME observations.In a related study using the same GOME data and the MATCH-MPI 3D CTM model was used to investigate the pattern of behaviour over several years. This CTM accounts explicity for convection but uses NCEP rather than ERA-40 meteorological data, the general flow pattern off Africa is qualitatively in agreement with the trajectory analyses of this study. Thus a difference also exist here.For HCHO, which is produced primarily by the oxidation of volatile organic compounds from biogenic emissions and biomass/biofuel burning in the lowermost troposphere, the agreement in spatial pattern and magnitude between that predicted for the HCHO behaviour and that observed by GOME is reasonable. This probably indicates that the emission estimates of VOC and the production and loss chemistry of HCHO in the model is reasonable. In contrast the agreement for model and observations for NO2and the O3are poor. We have considered explicitly stratospheric–tropospheric exchange as a possible source of O3but have found this is of negligible significance for the region under study in September 1997. As an excess of NOxis likely to reduce the chain length for the photochemical production of O3, we therefore consider that the high NOxand low O3are connected. In the chemistry model clouds are taken into account by using the ISCCP cloud climatology for September 1997.1Photolysis frequencies are calculated as a function of height taking each cloud type, specifically into account. There is however no feedback mechanism concerning the formation or destruction of clouds. We do however explicitly consider loss on particles of soluble chemical species.In summary we recognise the importance of convection and the limitations of our trajectory model but consider that they are unlikely to explain the large scale differences between the flow pattern, observed off the coast of East Africa in the trace gases retrieved from GOME, and that predicted by the model.1 W. B. Rossow,International Satellite Cloud Climatology Project (ISCCP), Technical Document No. 737, World Meteorological Organization, Geneva, 1996.Dr Chipperfieldasked: By using trajectories the authors’ model will only have the large-scale (resolved) advection. It will miss the sub-grid processes of convection and turbulent mixing. What was the resolution of the analyses used to force the model? Can the authors discuss the possible impact of the neglect of the ‘sub-grid’ transport?Professor Burrowsreplied: The ERA-40-data used in this study have a spatial resolution of 1.5° × 1.5°, the vertical coordinate being ECMWF model levels. In order to minimise the problems arising from turbulent mixing within the boundary layer, all trajectories are released from 50 to 200 hPa above the ground (which corresponds 500 to 2000 m). This altitude range represents the transition region between the boundary layer and the free troposphere. The airborne measurements, which were used as initialisation data for the chemistry modelling were made at this altitude range. We therefore hope to have minimised the impact of turbulent mixing within the boundary layer on for example the spatial distribution of fire emissions.Convective processes lead to a redistribution of air masses as a function of altitude, characterised typically by small scale updraft and larger scale downdraft, which may be regarded as a vertical mixing process. Whilst recognising the potential importance of convection at sub grid scales, we consider that the large number of trajectories calculated represent a reasonable approximation to the mean monthly behaviour of the atmospheric dynamics in September 1997 for a snap shot at 10.30 am. As described above we have used the Lifted Index to assess the impact of convection. There is an impact but consider that convection does not explain the observed deviation of the location of the outflow off East Africa into the Indian Ocean in September 1997. Qualitatively similar results with respect to the position of the outflow have been obtained using the MATCH-MPI 3D model, which explicitly takes convection into account. As noted above this uses NCEP rather than ERA-40 meteorological data.Dr Chipperfieldsaid: Can the authors be more specific about the problems they find with the ECMWF ERA-40 data for their studies?Professor Burrowsreplied: The spatial distribution of trajectories and the amounts of tropospheric NO2, HCHO and O3columns calculated using a chemistry model along the trajectory for the September 1997 have been compared with those retrieved from GOME measurements. The main objective of the study was to investigate the overall budget for the different trace gases. In this context the agreement between modelled and observed HCHO is reasonable and its spatial distribution is relatively well captured for trajectories released over the Savannah and the tropical rain forest. The trajectory analysis suggests that the main outflow into the Indian Ocean is around 20 °S, which indicates a shift compared with GOME observations of O3, NO2and HCHO. As described above the use of Lifted Index to assess convection does extend the region of outflow close to the coast of Africa. In contrast the trajectory analysis of fire emissions from the Indonesian forest fires in September 1997, which were located mainly in the lowermost troposphere, yields a spatial distribution of trace gas columns similar to that derived from GOME measurements well [Ladstätter-Weißenmayeret al., 2005].Dr Sarkarcommented: I suggest you could include the convective term in your trajectory model. Particle-in-cell Lagrangian techniques (as suggested by Dr Stevenson) are quite unrealistic with respect to the uncertainties in the input data.Professor Burrowsresponded: In trajectory (Lagrangian) models convection is arguably best described by a transition probability between different layers. However temperature and humidity fields are only available on the grid representation of the input data. Thus processes at sub grid scale therefore are not readily taken into account. We are of the opinion that the overall larger scale mean monthly behaviour of atmospheric dynamics should be reasonably well captured by the trajectories.Dr Evansasked:(1) Given your chemistry transport model does not consider convection do you think that it is a suitable tool for diagnosing errors within the ECMWF winds from the satellite data?(2) How does your model treat the reactive conversion of N2O5to aerosol?Professor Burrowsreplied:(1) In short the intention was not to diagnose errors in ERA-40, but rather to use this data set, which we consider to be the best available. The analysis performed in this study comprises two steps. First the transport of a large number of trajectories is computed using the reanalysed analysed wind fields from the ERA-40 dataset are applied. Secondly the chemistry boxmodel BRAPHO is run on a large number of these trajectories. It is initialised with volume mixing ratio measured by airborne instruments during the TRACE-A campaign, and a parameterisation for lightning NOx. We argue that for the monthly mean situation, the large number of trajectories should capture the large scale pattern of flow. As noted in answers to similar questions from other participants, we have attempted to estimate the maximum impact of convection on the flow by use of the Lifted Index. Convection does have an impact but does not explain the difference in the large scale outflow from east Africa. Similarly comparisons with the MATCH-MPI 3D model, which uses NCEP data and attempts to take convection explicitly into account, also show differences in outflow from Africa. Whilst recognising the potential importance of convection, we consider that at least for September 1997, the average flow in the middle and upper troposphere as expected from ERA-40 appears to be somewhat shifted in latitude as compared to the retrievals of the trace gas columns having different lifetimes.(2) We recognise the potential importance of the night-time removal of N2O5. The chemistry used in this study is based on the Master Chemical Mechanism.1The MCM compilation uses a parameterisation for the reaction N2O5→ NA + NA where NA denotes liquid nitric acid, which then deposits. NA formed in the model is not converted back to any other nitrogen compound. This MCM parameterisation is therefore an effective reaction with a recommended mean rate coefficient. This approach has the disadvantage that the microphysical processes, such as hydrolysis of N2O5on particles (aerosols or clouds), are not considered explicitly. Similarly it may be weighted towards mid-latitude conditions. We therefore expect that this simple parameterisation will underestimate the conversion in areas with high aerosol loading and overestimate it in regions with low aerosol loading. In the tropics in our region of study, the relatively high temperatures in the lower troposphere and rate of photolysis of tropopsheric ozone are such that the loss of NOxby the three body reaction of OH with NO2is probably more important than the removal of N2O5at night. We therefore do not expect that an improved parameterisation of this reaction will explain the poor description of the amount of NO2. However in future studies an improved description of the microphysics such heterogeneous processes will represent a significant improvement of the model. At this time we attribute the poor description of NO2to be in our parameterisation of NOxfrom lightning, which probably overestimates NOx, in line with early results from recent studies of lightning in the tropics. Further work is clearly needed, utilising the knowledge gained from the latter studies and the recent measurements of NO3and N2O5made in the boundary layer and troposphere.1 S. M. Saunders, M. E. Jenkin and R. G. Derwent,Atmos. Chem. Phys., 2003,3, 161–180.Professor Cohenopened the discussion of Professor Jaeglé’s paper: What are the other consequences of high soil NOxemissions for (a) the N2O budget and (b) for HNO3deposition?Professor Jaegléreplied: Microbial soil processes emit both NOxand N2O. The ratio of NO:N2O emissions is dependent on soil environmental conditions, in particular soil moisture. Combining spatially and temporally varying estimates of this ratio with our top-down estimate for soil NOxemissions could be used to infer N2O soil emissions, providing useful constraints for this very poorly quantified source of N2O. One caveat is that soil NOxemissions can be lost by reactions on the vegetation canopy while N2O emissions are not—this would complicate the analysis. In answer to your second point, high soil NOxemissions imply larger HNO3and nitrate deposition. We plan on examining the consistency of our top-down estimate of soil NOxemissions with deposition observations at rural US National Atmospheric Deposition Program sites. In addition we will examine the effects of increased soil NOxemissions on our simulation of background levels of ozone in the summer.Dr Stevensonasked: Do you think the results you find would differ if you used a different chemistry–transport model?Professor Jaegléreplied: Information from the GEOS-CHEM chemical transport model directly affects our global inventory of NOxemissions at two stages: in the AMF calculation (we need to usea prioriinformation on the vertical distribution of NO2), and in the inversion to derive NOxemissions from tropospheric NO2columns (based on model-calculated NOxlifetimes and NOx/NO2ratios). The shape of the NO2vertical distribution over land is mostly determined by the simulated boundary layer depth, and potential errors in this parameter result in a 15% uncertainty in our AMF calculation. As for NOxchemistry, we estimated a 30% error on this factor based on comparisons to observed NOx/NOyratios. Thus overall, the top-down estimate of surface NOxemissions will be affected by the CTM used, but this influence is relatively small compared to other uncertainties. To illustrate this point, we can compare our results to the study of Müller and Stavrakou1who used an adjoint modelling technique based on the IMAGES CTM to derive optimised NOxand CO emissions inventories from GOME NO2observations for 1997 and surface CO observations. Their results are very similar to ours, in particular they also infer a large source of NOxfrom soils. This give us confidence that this result is independent of the inverse modelling technique and CTM model used and comes directly from the GOME NO2observations.1 J. F. Müller and T. Stavrakou,Atmos. Chem. Phys. Discuss., 2005,4, 7985.Professor Cohencommented: In response to Dr Stevenson’s question about whether the inverse model retrieval of NO2will be strongly dependent on which global model is used, I point out that the lifetime of NOxis only a few hours so large scale meteorological differences in global models cannot affect the inversion. Those differences are important for long lived species such as CO. For NO2systematic difference in OH/between models would matter.Dr Chipperfieldcommented: The author quoted comparisons between the GEOS-CHEM and IMAGES model for NO2and said that the agreement is good. What does this depend on and really test for a short-lived species such as this? Is it known how well the models compare for longer-lived species such as CO?Professor Jaegléreplied: With a short-lived species like NO2, which is mostly confined in the boundary layer, comparisons between global models is a test of both the emissions inventory and of NOxchemistry (NOxlifetime). For a longer-lived species such as CO the comparison is a little more difficult as, in addition to chemistry and emissions, one also needs to consider differences in transport.Dr Evansasked: How model dependent do you think the NO2columns are? Would you come to the same quantitative conclusions with a different model?Professor Jaegléresponded: The short answer is that the tropospheric NO2columns are relatively model insensitive, as I noted in my answer to Dr Monks. However, the tropospheric NO2columns will be affected by the actual retrieval algorithm used and most importantly its assumptions for cloud, aerosol scattering, and surface reflectivity. We have done our best to account for these effects, and assessed their impacts on the retrieved tropospheric NO2columns by detailed error analyses. The higher spatial resolution measurements of NO2from newer instruments such as SCIAMACHY and OMI, combined with more information on clouds, aerosols and surface reflectivity should result in more accurate retrievals and reduced uncertainties.Dr Stevensonsaid: You don’t consider the lightning NOxsource, and suggest it is only a small component of the GOME NO2column. This seems to be at odds with Professor Burrows’s paper, which uses the GOME NO2data to investigate the lightning NOxsource. How does your model deal with lightning? Are you using a different GOME NO2product?Professor Jaegléanswered: Our NOxemission inventory includes a 3.5 TgN year−1lightning source. The contribution of lightning NOxto column NO2over land in our model simulation is generally small. For example we conducted a simulation without lightning and found that over Africa lightning accounts for less than 0.25 × 1015molecules cm−2, much smaller than the observed enhancements in the GOME NO2columns1(∼2–3 × 1015molecules cm−2). This small sensitivity to lightning results from the fact that lightning NOxis preferentially deposited in the upper troposphere, where it will be present mostly as NO because of the low temperatures and the resulting slow rate for NO + O3. Our inversion procedure, where we relate NO2columns to surface NOxemissions, thus does take into account the small enhancement from lightning in the model. So, in other words, we take out the lightning influence using the model—assumed inventory, and examine the land surface sources only. Over the ocean—away from any large land NOxsources-lightning NOxwill stand out more clearly and is likely to account for part of the observed enhancements over the Atlantic presented in the previous paper.The main difference in the tropospheric GOME NO2columns used in our analysis and in the analysis presented in the previous paper lies in the retrieval used, and more specifically in the different treatments of scattering by clouds, aerosols, gases, and surface albedo. These differences have been discussed previously.2Briefly the main effects are that our approach results in lower tropospheric NO2columns over cloudy regions, oceans, and regions with high albedo, while we derive larger columns over biomass burning regions. In addition, we calculate an AMF for each scene using daily information from GEOS-CHEM.1 L. Jaeglé, R. V. Martin, K. Chance, L. Steinberger, T. P. Kurosu, D. J. Jacob, A. I. Modi, V. Yoboué, L. Sigha-Nkamdjou and C. Galy-Lacaux,J. Geophys. Res., 2004,109, DOI: 10.029/2004JD004787.2 R. V. Martin, K. Chance, D. J. Jacob, T. P. Kurosu, R. J. D. Spurr, E. Bucsela, J. F. Gleason, P. I. Palmer, I. Bey, A. M. Fiore, Q. B. Li, R. M. Yantosca and R. B. A. Koelemeijer,J. Geophys. Res., 2002,107, DOI: 10.1029/2001JD002622.Dr Monksasked:(1) Is there a fundamental inconsistency between the presented work, in that the Burrows paper suggests a requirement for less (modelled) NO2(see Table 1 in the paper)vs. the conclusion from Jaegléet al. that there is increased soil emissions of NO2.(2) With respect to the soil moisture/NOxpulse mechanism, is there an overpass sampling bias?Professor Jaegléreplied:(1) We use fundamentally different approaches and assumptions, making a direct comparison difficult. In our paper we use surface emissions combined with a Eulerian chemical transport model to calculate NO2columns and then use this as a framework for our inversion analysis to relate GOME NO2columns to surface NOxemissions. Because of the short lifetime of NOx(a few hours), we assume that NO2columns directly map onto local NOxemissions. The paper presented by Professor Burrows uses a trajectory model, which calculates excess NO2along the trajectory. One way to compare the two methods would be to examine the differences in oura prioriNOxemissions over Africa. During September 2000, our African NOxemissions add up to 0.52 TgN (0.36, 0.04, 0.07, 0.05 from biomass burning, biomass and fossil fuel combustion, soils and lightning). Table 1 of the paper gives the excess NO2emissions for September 1997, which account for 0.05 TgNO2. Not knowing background NO2emissions used in this paper, it is difficult for me to relate the excess NO2emissions to total emissions.(2) This is a good point. One expects rainfall to occur mostly in the afternoon, and thus GOME with its 10:30 am equatorial sampling time would miss the initial pulse from soils. Field experiment have shown that pulsing of soil NOxfollowing rain can last for several days, thus GOME would sample the NO2enhancement 1 or 2 days later.Professor Burrowsreplied:(1) The two studies both use GOME data, but the retrieval procedures for the derivation of trace gas tropospheric column amounts, whilst being similar in concept, are different in detail. For example the air mass factors are calculated using different models and assumptions. Overall the general agreement between the different retrieval approaches appears reasonable in areas of high tropospheric NO2. The focus of the two studies is significantly different. The University of Bremen study has selected the period of intense biomass burning and lightning during September 1997 for a case study. The results indicate that the model emission used for HCHO and its precursors yields a reasonable description of the HCHO behaviour. A reasonable parameterisation of lightning in line with other recent studies results in an overestimate of the column of NO2. The O3column calculated is lower than that observed. The University of Washington study has a different focus. We therefore do not consider that there is a fundamental inconsistency between the two studies.(2) At the University of Bremen study we have not investigated the potential soil humidity pulsing mechanism for the emission of NOx, which has been observed in forested region of California. However this mechanism is likely to be only of minor significance for our region of study in September 1997.Professor Cohenobserved:(1) During INTEX/ICARTT we measured NO2profiles from the NASA DC-8. Over the continental US we frequently observed half the NO2column in the upper troposphere.(2) However, so long as the lightning doesn’t perfectly covary with the model representation of soil NO emissions the model’s inadequacies with respect to lightning will not affect its retrieval of soil NO emissions.Dr Shallcrossopened the discussion of Dr Bloss’s paper; In your comparison between model and measurement you conclude that the variability in OH is driven by variability inj(O1D), does not variability in H2O also play a significant role? In the tropics one would expectj(O1D) to vary rather little and for H2O variability to dominate OH levels; does not this limit the usefulness of your method?Dr Blossresponded: Our method for determining the mean global [OH] is to correct the hemispheric global mean OH values from the GEOS-CHEM model using the observations at Mace Head, Ireland and Cape Grim, Tasmania. This approach factors all the OH production and loss mechanisms included in the GEOS-CHEM model into our final values, including the simulated levels ofj(O1D) and of water vapour—we do not use the relationship between OH andj(O1D) to determine the mean global [OH].We do find a strong correlation between monthly mean model OH and the monthly mean modelledj(O1D) for the mid-latitude Mace Head location (Fig. 6 of the paper), but not for the tropics (Fig. 7). OH production depends also upon the levels of ozone and water, together with temperature and pressure, while OH loss depends upon the levels of a range of co-reactants. The observed correlation at Mace Head indicates that the combined effect of these factors varies withj(O1D); it is perhaps most likely that in a marine location such as Mace Head the loss processes are reasonably constant annually, and we might expect the monthly averaged humidity and temperature to show similar trends toj(O1D), hence the dependence of OH upon these factors is obscured.Dr Brauerssaid: The paper by Blosset al. compares local measurement of OH with a global model. However, the main dependence of OH is caused by the solar radiation. Solar radiation, namely photolysis frequencies likej(O1D) are an external parameter to the chemical model with no feed-back. Therefore, it is suggested to correlate the measured and modelled data toj(O1D) and compare the slope of the regression. The slope—or the OH toj(O1D) ratio—is an indicator for the chemical environment.Dr Blossreplied: We agree that a comparison of the simulated gradient of the OHvs. j(O1D) against the observed (on various timescales) could be a useful diagnostic for model performance, especially in locations where the chemical environment (OH co-reactants, NOx) is constant—however such comparison of modelled and measured OH was not the focus of our paper.Professor Cohencommented: The linearity and strong correlation of OH withj(O1D) is to be expected and uninteresting. What is interesting is the variability of the relationship between OH and its sources.Dr Brauersreplied: Rohrer and co-workers presented an analysis of the OH-j(O1D) correlation for datasets recorded in various environments. Surprisingly, the variability of OH at each location is almost entirely explained by thej(O1D) variation and the precision of the OH measurent, even for a 5 year OH dataset recorded at Hohenpeissenberg. Each location can be characterized by its slope and exponent of the relation between OH andj(O1D). The abstract of their presentation can be viewed athttp://www.igaconference2004.co.nz/abstractPreview.asp?absID=334Therefore, the OH variabilty due to other parameters, sinks or sources, might only be visible from experimental data if the local OH–j(O1D) relationship is thoroughly analysed or when different locations are compared.Professor Berresheimsaid:(1) Following the comment by Dr Brauers, other OH field measurements including our long-term measurements at Hohenpeissenberg have shown that the OH–j(O1D) covariance is highly correlated but with distinctly different slopes for different air mass regimes. Thus the Mace Head data set alone cannot be extrapolated by the model to other regimes and/or latitudes as done in the paper by Blosset al. Error estimates should be given or discussed in the paper in this respect.(2) For the tropics there may still be a strong OH–j(O1D) relation. The authors should test the covariance on a hourly rather than only monthly basis.(3) A detailed justification for choosingj(O1D) as a better parameter of choice to plot against OH rather than usingP(OH) has been given in the previous papers by Ehhalt and Rohrer and in the MINOS paper by Berresheimet al. Blosset al. should cite these papers in that regard, as well as the now citable Abstract by Berresheimet al. on their long-term measurements presented at the EGU 2005 and the IGAC 2004.Dr Blossreplied: The Mace Head OH–j(O1D) relationship was not used to determine the global [OH]. We use the marine boundary layer datasets to scale the OH field calculated by the GEOS-CHEM 3-D chemical–transport model. As GEOS-CHEM incorporates a full global emission inventory and chemical scheme, the model will take account of differing air mass regimes, and hence different relationships between OH, and (for example)j(O1D), at other locations. The OH–j(O1D) relationship observed at Mace Head is included in the paper purely to illustrate the likely reasons behind the success of the model simulation of observed OH levels at this location.We have averaged the observed and modelled data on a monthly timescale to remove day-to-day and synoptic variability from the OH levels—this is necessary as advection data is not available for the specific periods/locations of some of the measurement campaigns used, hence the fine scale variation must be removed. We are thus unable to compare the model and measurements on an hourly timescale—and indeed such a comparison would not be appropriate when comparing measurements of a short-lived species such as OH with values calculated by a model with the temporal and spatial resolution of a global CTM. One of the points we wished to make in the concluding section of the paper is that observational data is sparse in the tropics.Our aim is not to suggest thatj(O1D) was a statistically better diagnostic of OH thanp(OH) but rather that the large variability in OH seen at Mace Head was attributable to a changing radiation field rather than a chemical field. We do not in fact make any mention ofp(OH), the primary rate of production of OH from O3photolysis/O(1D) + H2O. If we were seeking to determine the relationship between OH and a single driving factor,j(O1D) would be the obvious choice as advocated by Ehhalt and Rohrer1—however we must be cautious especially when considering different chemical environments—the data used by Ehhalt and Rohrer were obtained at a “rural relatively unpolluted site” in Germany; recent measurements performed in our group have demonstrated that the OH–jcorrelation can break down in more complex environments, as evidenced by elevated levels of OH observed in wintertime in cities due to alkene ozonolysis.21 D. H. Ehhalt and F. Rohrer,J. Geophys. Res., 2000,105, 3565.2 D. E. Heard, N. Carslaw, L. J. Carpenter, D. J. Creasey, J. R. Hopkins, A. C. Lewis, M. J. Pilling, P. W. Seakins,Geophys. Res. Lett., 2004,31, L18112, DOI: 10.1029/2004GL020544.Professor Planeasked: This paper reports mean OH concentrations for both hemispheres. What use are these numbers? Fig. 9 of the paper shows that the average is heavily weighted to the tropical free troposphere, so the concept of a global average seems a strangely old-fashioned concept.Dr Blossresponded: The concept of a mean global OH concentration is the simplest way to define the atmosphere’s capacity to remove most emitted species. While strictly the value can only be applied to determine the lifetime of a species removed solely by a temperature-invariant bimolecular reaction with OH, and which is uniformly distributed throughout the atmosphere, in practice mean global OH values can be used to estimate the removal rate of most species with lifetimes over 1–2 years (e.g.CH4, many HCFCs) and thus provides a useful tool to assess their likely atmospheric persistence and distribution.More quantitatively, the global mean OH average provides a key metric for assessing the performance of global chemistry-transport models. The approach to determining the global average used in this work, constraining the simulated OH field to point field measurements, differs from most previous determinations which have used measurements of tracer species such as methyl chloroform, and have to take account of their non-uniform distribution throughout the atmosphere. It is thus encouraging that good agreement is obtained between this study and values obtained elsewhere.Dr Coxasked: Are the values given for mean [OH] in the northern and southern hemispheres significantly different from each other? No error limits are given.Dr Blossreplied: The mean [OH] values obtained for the northern and southern hemispheres (0.91 and 1.03 × 106cm−3, respectively) are not significantly different in our current analysis—the uncertainties (derived from the uncertainty in the constraining field measurement data only and neglecting model factors) are 19 and 13%, respectively.Professor Ravishankaraobserved: I would like to note that the global average [OH] derived from methyl chloroform concentrations are “weighted” towards the regions where most of the methyl chloroform is destroyed in the atmosphere. This is because methyl chloroform does not carry information about the regions that do not contribute to its loss. This is in spite of the corrections made by the people who carry out such calculations.In your defense regarding the usefulness of a “global” average number for [OH]: yes, it is useful for calculating the lifetimes of molecules which are primarily lostviaOH reactions in the troposphere and whose lifetimes are in the order of a few years (greater than 2 and less than 20 or so). Further, the rate coefficient for their reaction with OH should have a temperature dependence that is similar to that for OH + CH3CCl3reaction. It so happens that there a quite a few of the CFC-substitutes fall into this category, as does methane. However, this concept should not be pushed beyond this issue because there are questions as to what an average concentration means! We cannot assume the reactivity of OH to be the same irrespective of where it is (i.e., the temperature and pressure of its location)! Therefore, if we are not careful, a global mean OH concentration becomes a meaningless concept.Dr Blossreplied: We agree that there are potentially difficulties in the use of tracer measurements to derive a truly global mean OH value—our method, which employs the global distribution obtained from a CTM, scaled by observations, incorporates contributions from all regions of the atmosphere (although in the current analysis the limited observational dataset may introduce similar limitations). Methyl chloroform data are not used in our analysis so the requirement for the reaction kinetics to be similar to those for reaction of CH3CCl3with OH is removed; however strictly speaking a new constraint, that the reaction kinetics be reasonably invariant with respect to pressure and temperature (or that their atmospheric mass-weighted averages are used), is introduced for use of our current (mass-weighted) mean [OH].Professor Jaegléasked:(1) Does the global model get OH right for the right reasons? In other words, have you compared the model simulations to local observations of H2O, CO,j(O1D), NOx, in addition to looking at OH?(2) Given the high variability of OH driven by its short lifetime, to what extent do surface observations give information on global OH levels?Dr Blossresponded: This is a constant worry for models. GEOS-CHEM has been extensively evaluated against observational datasets globally and we believe that it does sufficiently well to provide a useful platform for our subsequent analysis. However, the raft of negative feedbacks inherent in the atmospheric chemical system make it very easy to calculate the right OH levels for the wrong reasons. A particularly under-assessed area of the model is probably the photolysis rates.We agree that the surface observations do not provide an ideal constraint on OH throughout the rest of the atmosphere. We are in the processes of incorporating aircraft data (and other surface sites) into our analysis which will allow a more complete analysis of the process. The inclusion of more locations/dates will also increase our confidence that the model is correctly simulating the photochemistry which determines [OH].Dr Stevensonasked: You state [OH] in Fig. 1 is ‘global mean annual tropospheric mass-weighted OH’. But most of the studies in Fig. 1 derive OH from methyl chloroform—so surely these studies are weighted by where the MC + OH reaction takes place? They can say very little about OH in the upper troposphere or at the poles. But your global model results will include these regions. Wouldn’t it make more sense to compare MC weighted OH concentrations? Lawrenceet al.1compares different measures of [OH] which may be useful.1 M. G. Lawrence, P. Jöckel and R. von Kuhlmann,Atmos. Chem. Phys., 2001,1, 37–49.Dr Blossreplied: Our purpose in Fig. 1 of the paper was simply to compare the recent determinations of mean global [OH] to our value—the methyl chloroform (MCF) tracer studies are amongst the only available observational determinations of mean global [OH]. The studies referred to in Fig. 1 of the paper which utilise observations of methyl chloroform (MCF) to derive mean global [OH] values1–4incorporate modifications to the inversions used to account for the spatial distribution of MCF, thus mass-weighted values of mean [OH] are obtained; however we accept that this may present a difficulty, and in particular may lead to greater uncertainties over OH levels in the colder regions of the atmosphere which contribute relatively little to MCF removal. Our approach, in which the mean OH is derived from the global model field scaled to direct OH observations, does not suffer from this limitation. We agree that one could weight the OH distribution we obtain from our constrained model according to the distribution of the species one is interested in investigating the removal of, and further weight the removal kinetics to their atmospheric average according to the same distribution, and hence obtain a global mean [OH] specific to an individual species such as MCF or CH4which could be compared to the observed MCF/CH4lifetime.1. R. G. Prinn, R. F. Weiss, B. R. Miller, J. Huang, F. N. Alyea, D. M. Cunnold, P. J. Fraser, D. E. Hartley and P. G. Simmonds,Science, 1995,269, 187.2. M. Krol, P. J. van Leeuwen and J. Lelieveld,J. Geophys. Res., 1998,103, 10697.3. R. Prinn, G. J. Huang, R. F. Weiss, D. M. Cunnold, P. J. Fraser, P. G. Simmonds, A. McCulloch, C. Harth, P. Salameh, S. O. Doherty, R. H. J. Wang, L. Porter and B. R. Miller,Science, 2001,292, 1882.4. S. A. Montzka, C. M. Spivakovsky, J. H. Butler, J. W. Elkins, L. T. Lock and D. J. Mondeel,Science, 2000,288, 500.Dr Chipperfieldcommunicated: Given the large impact ofj(O1D) on OH in many parts of the atmosphere should we be putting more effort into measuring and modelling this quantity? This would remove some uncertainties in looking at the more complicated issue of OH concentration. In contrast to box model studies which may factor out the effect ofj(O1D) from the data to detect other effects, for global models (chemical transport model, CTM and general circulation model, GCM) which calculate chemistry in an unconstrained way it is important to check these controlling processes.Dr Blosscommunicated in response: In general while global atmospheric models have been extensively compared to observations of chemical parameters, much less effort is typically made to evaluate the radiation fields, which drive the photochemistry occurring. Agreement between model and measuredj(O1D) is an essential prerequisite to getting the modelled OH correct (for the right reasons) so greater use of and/or generation ofj(O1D) and other radiation data could provide a useful and relatively inexpensive test of model performance.Professor Heardcommented: The very high correlation between [OH] andj(O1D) calculated in this study for unpolluted environments has prompted Dr Chipperfield to suggest thatj(O1D) levels can be used to infer [OH], and therefore that field measurements of [OH] are less important. In defence of OH measurements some observations concerning the polluted environment are pertinent. During the PUMA field campaigns in 1999 and 2000, the Leeds FAGE conducted measurements of OH concentrations at an urban site in both summer and winter.1–3The results showed that despite more than a tenfold decrease inj(O1D) between summer and winter, the average noontime [OH] only decreased by a factor of two. A rate of production study using a constrained box-model indicated that the surprisingly high concentrations of OH in winter are maintained by the reaction HO2+ NO, with HO2being generated by photoylsis of carbonyls (in particular HCHO) and also by O3+ alkene reactions, as well as direct OH production from O3+ alkene reactions. The reaction O(1D) + H2O was shown to be only a minor source of OH in winter. Thus one must be careful with the generalisation regarding correlations between OH andj(O1D), especially when comparing seasonal variations in polluted regions. Although for the PUMA campaign the ratiosj(O1D)summer/j(O1D)winterand [OH]summer/[OH]winterare very different, there is still a significant correlation between [OH] andj(O1D) in winter, as HO2, the major source of OH, is formed predominantly from the photolysis of carbonyls, the rate of which will be closely correlated withj(O1D). The gradient of [OH]versusj(O1D) will be quite different though for summer and winter in the urban environment, and it would be difficult to infer [OH]a priorifrom any measured value ofj(O1D).1 D. E. Heard, N. Carslaw, L. J. Carpenter, D. J. Creasey, J. R. Hopkins, A. C. Lewis, M. J. Pilling and P. W. Seakins,Geophys. Res. Lett., 2004,31, L18112, DOI: 10.1029/2004GL020544.2 R. M. Harrisonet al.,Sci. Built Environ., 2005, in press.3 K. M. Emmerson, N. Carslaw, L. J. Carpenter, D. E. Heard, J. D. Lee and M. J. Pilling,J. Atmos. Chem., 2004, submitted.Dr Blossreplied: We agree: To define the relationship between [OH]ssandj(O1D)a priori, the levels of O3, H2O, OH co-reactants,Tandphave to be known—these (especially the co-reactant levels) are unlikely to be well constrained other than in certain remote marine locations. This relationship could be useful in certain carefully defined environments where we are confident that the radical chain length is short (i.e.OH is formed overwhelmingly through primary production rather than by reaction of NO + HO2etc.) and that the variability inj(O1D) dominates over variability in (e.g.) H2O and temperature in determining OH production, and over the variability in the OH loss rates. However we are not currently able to define thej(O1D)vs. OH relationshipa priorifor most atmospheric locations.We believe that the only way to confidently ascertain the concentration of this important species is to go out and measure it; however making such measurements is expensive and so we need to evaluate the optimal locations in both time and space. We probably have sufficient data in the extra-tropical marine boundary layer; in the future more observations from tropical marine regions, tropical forests, deserts and mega-cities will be necessary.Dr Chipperfieldcommented: These points are well made. The motivation behind the comment onj(O1D) was not that it is an alternative proxy to measuring OH in clean environments, but rather that given its importance in determining the absolute [OH] then effort should be made to make sure that models can accurately reproduce this controlling factor. This is important for global models (CTMs, CCMs) wherej(O1D) is calculated rather than constrained. For these comparisons observations ofj(O1D) are clearly necessary.Professor Dibbleopened the discussion of Dr Seisel’s paper:(1) In Fig. 6, is the equilibrium constant for multilayer or sub-monolayer conditions? To the extent that Fig. 6 reflects multilayer conditions, the ΔHads values for soot and mineral dust are not directly comparable, since the ΔHadsfor soot refers to submonolayer conditions.(2) Using your desorption rate constants for water molecules from mineral dust, I computed anA-factor of 200 s−1. I cannot understand what that number means. Can you offer an explanation?Dr Seiselreplied:(1) For the maximum coverage observed in this study we estimated an upper limit of 40 monolayers and a lower limit of 1 monolayer. Since these values refer to the maximum coverage we consider the equilibrium constants valid for approximately 1 formal monolayer. The activation energy determined from the desorption rate constants (Fig. 4) for sub-monolayer conditions agree with the adsorption enthalpy derived from the equilibrium constants. Therefore, we consider the adsorption enthalpy for water on soot and mineral dust as comparable.(2) TheA-factor (Fig. 4 of the paper) as well as the adsorption entropy (Fig. 6 of the paper) are determined from the intercept and therefore depend on the absolute value of the surface area. As long as we did not know the absolute surface area physically realisticA-factors or adsorption entropies cannot be calculated.Dr Baltenspergercommented: A word of caution concerning the atmospheric implications: in the atmosphere the soot particles will rapidly be coated by condensing secondary material. Thus, the soot aerosol will be internally mixed and their properties will be determined by this condensed material (which dominates in mass) rather than the soot particle. This is confirmed by our observations at the high Alpine site Jungfraujoch, where the scavenging of soot behaves exactly as the total submicrometre aerosol volume.Dr Seiselreplied: This work and the atmospheric implications drawn are related to the surface properties of the particles under study. In the case wheree.g.a soot particle is internally mixed with other compounds the surface properties of the aerosol will change and therefore our results may not longer be applicable to the “new” type of aerosol.Dr McFigganscommented: To extend the point raised by Dr Baltensperger, not only will soot be associated with organic material but also, given extensive field observations in multiple locations, inorganic components (sulphates, nitratesetc…) are invariably internally mixed with carbonaceous components at only modest distances from emission sources. The atmospheric relevance of the hydrophobic nature of soot is therefore limited to locations sufficiently close to source and still at temperature too high for organic condensation (still in vehicle exhaust, for example).Dr Ammannsaid: In this paper, several fundamental parameters describing adsorption of water to mineral dust and soot are determined. These are then related to macroscopic aspects of hydrophilicity of the related atmospheric aerosols. We have recently determined hygroscopic growth of mineral dust aerosol particles (Arizona Test Dust) using a hygroscopicity tandem DMA.1We have related the very small hygroscopic growth observed on these particles to the presence of water soluble salts, mainly sulphate. While the paper presented by Dr Seisel considered water uptake on the surface in the form of BET type adsorption on an ‘inert’ surface, we suggest that water uptake and possibly also the CCN activities will be mainly controlled by water soluble material.A second point, which relates to this with regard to both fundamental adsorption modes and also the macroscopic hygrophilicity, arises from the way the soot samples were taken. Samples collected on a plate in the hot wake of the flame at probably above 200 °C will not contain many of the organic compounds usually associated with soot. Many of these contain a number of hydrophilic functional groups that have therefore not been considered in the present study, but that might be crucial in the behaviour of soot in the real atmosphere.With regard to the adsorption energetics as derived from the present study, the authors already state that the thermodynamic analysis is affected by the fact that it is not clear to what depth underneath the sample surface water actually diffuses in. In addition, I would like to recall the presence of water soluble material mentioned in my previous comment. The driving force for water uptake would then be the dissolution of the soluble material and not multilayer adsorption. This would then lead to a quite different and very challenging thermodynamic analysis of a multicomponent system. Therefore, it might be better to consider pure and well defined materials for determining fundamental adsorption parameters, and to determine lumped parameters, such as net growth curves or water mass changes for authentic materials.1 A. Vlasenko, S. Sjögren, E. Weingartner, H. W. Gäggler and M. Ammann,Aerosol Sci. Technol., 2005,39, 452–460.Dr Seiselreplied: There are still open questions concerning the detailed interaction of water with theses kind of surfaces. In order to get more insight into the problem you mentioned, it is planned to extent this study to surface which are less complex and are therefore better defined. In addition, we intent to use materials which have different surface coatingse.g.organics or sulfuric acid. With these results we may then be able to parameterize the uptake of water on complex, realistic surfaces like mineral dust or soot and get insight in the influence of “reactive” compounds on the surface.Professor Rudichcommented: Dust particles collected during dust storms in Israel were observed to have organic coatings and sulfates on the surface.1 D. Rosenfeld, Y. Rudich and R. Lahav,Proc. Natl. Acad. Sci. USA, 2001,98, 5975–5980.2 A. H. Falkovich, G. Schkolnik, E. Ganor, Y. Rudich,J. Geophys. Res., 2004,109, DOI: 10.1029/2003JD003950.Dr H. Roscoecommented: When reading this paper, there seemed to be some confusion about mechanisms of ice nucleation, which was not relevant to the main body of this excellent lab study. I recognised the confusion well, as I had been similarly confused a year ago whilst preparing a proposal. Searching the literature, all the papers about ice nucleation were about freezing of ice, never about condensation from vapour to ice.“Theory predicts that homogeneous nucleation of ice from vapor should only occur for extreme supersaturations, never observed in natural conditions” (ref. 1).“A calibration curve was developed to allow us to convertthe freezing temperatures to a saturation ratio required for ice nucleation.” (ref. 2).“... ice forming nuclei (IFN)” (ref. 3).“In particular the effects of solutes and mechanical pressure onthe kinetics of the liquid-to-solid phase transition of supercooled water and aqueous solutions to icehave remained unresolved.” (ref. 4).”Here we provide evidence that at atmospheric pressures the conditions leading to theinitiation of freezing in pure waterare those for which the liquid compressibility and the corresponding density fluctuations reach maxima.” (ref. 5).The classic review asserts that condensation to ice is energetically forbidden except at temperatures much colder than found in the atmosphere. But this is only true of the smallest nucleation particles, we are all familiar with rime on fences, and modern cloud-probes which photograph particles show rimed crystals in the 10 μm range.So my question for the audience is, at what size is direct condensation to ice allowed at, say −35 °C—temperatures routine in the upper troposphere, and above the inversion in Antarctica? If this is as small as 1 or 0.5 μm then we have been ignoring a significant amount of nucleation, particularly on broken wind-blown snow in Antarctica, perhaps also at mid-latitudes.1 W. Szyrmer and I. Zawadzki,Bull. Am. Meteorol. Soc., 1997,78, 209.2 M. E. Wise, R. M. Garland and M. A. Tolbert,J. Geophys. Res., 2004,109(D19203), DOI: 10.1029/2003JD0043133 E. K. Bigg and C. Leck,J. Geophys. Res., 2001,106(D23), 32155.4 T. Koop, B. P. Luo, A. Tsias and Th. Peter,Nature, 2000,406, 611.5 M. B. Baker and M. Baker,Geophys. Res. Lett., 2004,31, L19102, DOI: 10.1029/2004GL020483.Dr Seiselresponded: Heterogeneous ice nucleation, especially the direct condensation to ice (deposition mode) does not only depend on the size of the ice nuclei. In order to be an efficient ice nuclei, the particle has to be insoluble in water. Moreover, the surface of the particle has to exhibit an ice-like structure in order to support the build-up of the ice lattice. Therefore, it is difficult to give a particle size at which direct condensation to ice will occur. In addition, atmospheric aerosol particles are often internally mixed, which may increase its water solubility and consequently lower its nucleation efficiency.Professor Zellneraddressed Dr H. Roscoe: The distinction between water and ice nucleation under sub-monolayer conditions is not sensible. The addition of one or several water molecules to a surface can impossibly determine whether an ice or liquid water layer is being formed, since the terms “ice” and “water” refer to macroscopic ensembles of molecules in certain structural environments. By implication, the comment made by Dr. Roscoe may be atmospherically very relevant but it is not pertinent to the present paper.Dr H. Roscoereplied: My comment was stimulated by statements in the Introduction of the present paper, that seemed to suggest that ice nucleation from vapour was possible in the atmosphere, whereas most texts would assert otherwise. It is clearly pertinent, if only because the Introduction takes up over a page. My point is that the textbook view demands re-examination because at the largest sizes it is certainly possible. Moreover it may also be possible to start the process on small diameter particles because, as Prof. Zellner rightly comments and the paper discusses, sub monolayer deposition cannot have the energy costs of bulk deposition at high curvature, which in a liquid are associated with surface tension forces. This is another reason why ice nucleation from vapour may demand re-examination. Naturally, a process which starts only in sub-monolayer conditions may have difficulty with growth rates, but this is something for modellers to consider.Professor Abbattcommented: Ice formation by deposition mode nucleation is a highly viable ice formation mechanism. In laboratory studies where particles are supported on a temperature controlled teflon substrates (from −10 to −60 °C), we have recently shown that a variety of dust particle types are highly efficient IN, nucleating ice at about 105% relative humidity with respect to ice whereas soot is highly inefficient. It may be that the ice nucleation proceeds initiallyviafor the adsorption of water to the particle surfaces, providing a nice connection to the results of Dr Seisel’s . paper.Dr Coeaddressed Dr H. Roscoe: Riming is the collection of liquid drops by an ice particle and subsequent adhesion by freezing. It is a growth process for ice particles of any size, nucleation of ice is the creation of new ice particles. The latter may occur by freezing of water droplets or by nucleation on a surface such as mineral dust. The former, homogeneous nucleation, is only significant at temperatures below −40 °C. The latter can occur at much warmer temperatures. Ice may grow by vapour diffusion and also by riming.Dr H. Roscoereplied: Dr Coe is quite correct, I made a mistake in my haste to formulate the comment. The process seen in the real atmosphere whereby ice forms on larger scales directly from vapour is the formation of hoar frost, which occurs well above −40 °C. The question then is, at what size of deposition surface does hoar frost demand a large supersaturation? (Size is relevant because surface curvature implies an energy cost when depositing a film, a cost which in a liquid film is associated with surface tension.)Dr McFiggansadded: The distinction between heterogeneous and homogeneous nucleation and riming is well established and treated in the literature. The uncertainty is not in the description but in the nature and abundance of ice forming nuclei (IN) (surfaces providing site for heterogeneous nucleation). Homogeneous nucleation, by definition, is nucleation from the vapour without a pre-existing nucleus. It is “fairly” well-described as a function of temperature and thus supersaturation over ice.Professor Planeasked Dr Seisel: When you calculate the uptake coefficients, do you use the BET or geometric surface areas of the sample? Since theγvalues that you measure are quite large, using the BET surface area would lead to an underestimate of the trueγ, whereas the geometric surface area and produces an upper limit.Dr Seiselreplied: For the calculation of the uptake coefficients the geometric (projected) surface area of the sample has always been used. We did not observe any dependence of the initial uptake coefficients on the mass or height of the sample and interpretated this finding with the absence of diffusion into the bulk.Dr Kingopened the discussion of Professor Rudich’s paper by presenting slides demonstrating the use of Raman microscopy to study the uptake of nitric, acetic and formic acid on calcium carbonate particles. The work confirmed that the atomic nitrogen signal seen in the STM studies was indeed nitrate (owing to the Raman N–O nitrate stretch), and that deliquescence of the products occurred below 30% RH. The reaction went to completion. The reaction of acetic acid with calcium carbonate produced long crystals of calcium acetate (confirmed by Raman spectroscopy) that did not deliquesce. The reaction of formic acid with calcium carbonate produced calcium formate that also did not deliquesce at 30% RH. No evidence of new crystal growth has been observed as in the formate case. Neither of the reactions stopped after a surface passivation of calcium carbonate crystals, but appeared to go to completion.Dr Coxcommented: I would like to report some laboratory measurements pertinent to the atmospheric transformation of CaCO3particles into Ca(NO3)2. Fig. 4 shows the uptake coefficient (γ) for N2O5onto a submicrometre CaCO3aerosol measure in a flow tube experiment, as a function of relative humidity (RH).γincreases with RH approaching 10−2at high RH. Nitric acid produced in the heterogeneous hydrolysis of N2O5is expected to react with CaCO3to form (soluble) Ca(NO3)2with release of CO2. This would provide one efficient route for the transformation implied in the analysis of the atmospheric samples.Uptake coefficient of N2O5onto CaCO3measured in aerosol flow tube coupled to a chemiluminescence NOxanalyser. [N2O5]0∼ 500 ppb. Aerosol produced by atomising a suspension of CaCO3in distilled water; surface areas measured by a DMA. Reactive uptake according to reactions: N2O5+ H2O ⇌ 2HNO3; 2HNO3+ CaCO3⇌ Ca(NO3)2+ CO2+ H2O.Professor Waynesaid: Since N2O5hydrolysis is largely a heterogeneous process itself, at least in the atmosphere, the CaCO3particle probably plays a role in the initial step as well as in the subsequent conversion to nitrate.Dr Coxresponded: That is exactly the process we envisage. Following uptake at the CaCO3particle surface, N2O5molecules are hydrolysed to HNO3. The uptake rate increases with relative humidity, which we believe reflects the increasing amount of surface adsorbed water available to support the hydrolysis reaction. CaNO3is more hygroscopic than CaCO3and the hydrolysis may speed up on partially reacted particles due to increased water content, although this effect would be limited by the known inhibiting effect of high [NO3−] on heterogeneous N2O5hydrolysis.Professor Zellnerasked:(1) Why does the surface reaction product not limit the extent of the bulk reaction? Although it is realized that formation of a gaseous CO2product will help the reaction to run to completion, there may still be a limitation in transport of nitric acid into the bulk of the particles. Such limitation could only be prevented if the primary surface product will deliquesce and therefore ease the transport of nitric acid through a liquid surface layer.(2) The deliquescence of inorganic salts is heavily affected by the presence of organic species. Why do you not see the effect of organics in the evolution of your aerosols?Professor Rudichreplied:(1) The product Ca(NO3)2is shown to deliquesce at relative humidity of about 12%. In addition, we presented evidence that the converted particles are liquid at ambient conditions. The liquid layer conceivably allows the fast transport of the reactants and CO2from/to the atmosphere from the unreacted part and hence allow for the reaction to proceed until completion.(2) The effect raised by Prof. Zellner is observed mostly for homogenously mixed organic/inorganic particles. In dust, adsorbed organics are found on the surface of the dust particle.1Therefore it is expected that such coating could delay the reaction of the bulk by slowing mass transfer. However, if the coating is incomplete, the reaction may still proceed and once the reaction starts, it will proceed. Our experiment cannot probe such process, although preliminary experiments in our lab suggest that a thick coating of humic substances on dust may delay the reaction slightly. We do not expect that the minute amount of possible adsorbed organics may than be enough to show a substantial effect on the deliquescence of Ca(NO3)2, and in that case, ESEM methods will not be able to probe such saddle effects, should they exist.1 A. H. Falkovich, G. Schkolnik, E. Ganor, Y. Rudich,J. Geophys. Res., 2004,109, DOI: 10.1029/2003JD003950.Professor Ravishankarasaid: Could you please comment on why you think that your dust particles also had organics? Do you have other evidence for the presence of organics in or with your dust particles? You say that they are mixed with other air masses with organics. (Just a note—mixing is not easy to deal with when you have trajectory calculations to figure out where the air came from. After all, trajectories assume no mixing!)Professor Rudichresponded: We have not claimed that the dust particles probed in this study had any organics on them. However, in previous field studies we showed evidence and discussed the adsorption of organics (mostly pollutants such as PAH and pesticides) on dust transported from North Africa to the Eastern Mediterranean.1At least in this region, the amount and type of organics spends on the dust trajectory: dust that passed of over the Mediterranean Sea contains less, and different types of organic species than dust transported over land and passes over agricultural and urban areas in the Nile Valley and in Southern Israel.1 A. H. Falkovich, G. Schkolnik, E. Ganor, Y. Rudich,J. Geophys. Res., 2004,109, DOI: 10.1029/2003JD003950.Dr Kingaddressed Professor Zellner: In the Raman microscope study of the reaction between HNO3acid and CaCO3in real time it was notable that the uptake of water began after the surface oxidation of CaCO3and then the reaction accelerated uptaking water and HNO3relatively quickly (reaction over in 5–10 min) until a solution of CaNO3was all that remained. There was no evidence of surface passivation.Dr Allanasked: Given that the processed particles are aqueous, it may be possible that they are collected more efficiently on the impactor substrate through the elimination of bounce. Is it possible that the statistics may be biased towards the processed particles?Professor Rudichreplied: It is possible. We have not quantified this process and hence we cannot comment on its possible effect.Professor Donahueasked: Do these data speak to parameterizations of rainout or washout of aerosol as a function of size?Professor Rudichreplied: The current study does not allow for such parameterization and further studies that specifically address these questions should be planned.Dr Baltenspergercommented: According to Fig. 1, RH was below 100% during rain events. From this, I conclude that your site was below the cloud during the rain event. If you had only below-cloud scavenging I would not expect highly preferential scavenging for processed particles.Professor Rudichresponded: The RH shown is from a meteorological station in the region and not from the site itself. During the rain event, the area was inside the cloud.Professor Herrmannsaid: During trajectory analysis the capability of HYSPLIT to identify precipitation periods could have been applied and might have led to further valuable information.Dr Kingopened the discussion of Professor Ziemann’s paper by presenting slides of the very recently published work by King, Thompson and Ward1demonstrating the use of laser Raman tweezers to hold a mixed droplet of oleic acid and synthetic sea-water at atmospheric pressure for 30 min, oxidize the oleic acid on the droplet with gas-phase ozone, follow the decay of oleic acid and the growth of the products (nonanoic acid and nonanal) with Raman spectroscopy. The growth of the droplet size as the droplet becomes more hydrophilic was monitored. The oxidation of the surface film of oleic acid turned a hydrophobic particle into a hydrophilic particle which then grew in size (data were presented for a particle that grew from 6.5 μm to 8 μm with a corresponding growth of nonanoic acid in the particle. Monitoring mixed droplets of oleic acid and synthetic sea-water in the size range of 1–7 μm demonstrated no size change in the absence of ozone.1 M. D. King, K. C. Thompson and A. D. Ward,J. Am. Chem. Soc., 2004,126, 16710–16711.Professor Ziemannreplied: This is an interesting study demonstrating a powerful new technique. The ability to monitor changes in particle compositionin situusing Raman spectroscopy offers important advantages over more destructive mass spectrometric methods that sample particles into a vacuum system for analysis. This is especially the case for particles containing water or other semivolatile species whose distribution between the gas and particle phases could be disturbed during sampling. This method will be useful for monitoring changes in functional groups as a particle reacts, but identifying individual compounds will be challenging and in most cases probably not possible because of the complex mixtures of products typically formed by organic oxidation.Can you distinguish between different organic acids with your Raman technique?Dr Kingreplied: With care, yes. It was possible to tell the difference between azelaic acid and nonanoic acid for instance.Dr McFiggansasked: Given Dr King’s assertion of surface and bulk products could someone please comment on the phase distribution of the products (gas, surface, bulk, solid, solute) as would be expected in real particles in the moist atmosphere?Professor Ziemannreplied: With regards to the products of oleic acid ozonolysis, one would expect nonanal and nonanoic acid to be present primarily in the gas phase, and the others in the particle phase. Because the particulate products are not very water soluble, they will probably be dissolved in the organic phase, which could be solid, liquid, or waxy, rather than in the aqueous phase. Products with carboxylic acid groups may act as surfactants and exist at the air–water or organic–water interface.Dr Kingadded: The question as I remember it was how to probe the phase distribution of whether of a product was at the surface or in the bulk and the answer is with scattering and reflectance techniques used in colloid science. Such experiments are in progress.Professor Rudichmade a general comment: Nonanal is the product with highest vapour pressure, it has been shown to evaporate from OA particles. In addition to the formed peroxides, people have monitored other high molecular weight products.1–51 T. Moise and Y. Rudich,J. Phys. Chem. A, 2002,106, 6469–6476.2 Y. Katrib, S. T. Martin, Y. Rudich, P. Davidovits, J. T. Jayne and D. R. Worsnop,Atmos. Chem. Phys., 2005,5, 275–291.3 Y. Rudich,Chem. Rev., 2003,103, 5097–5124.4 T. Thornberry and J. Abbatt,Phys. Chem. Chem. Phys., 2004,6, 84–93.5 Y. Katrib, S. T. Martin, H.-M. Hung, Y. Rudich, H. Zhang, J. G. Slowik, P. Davidovits, J. T. Jayne and D. R. Worsnop,J. Phys. Chem. A, 2004,108, 6686–6695.Professor Donahueasked: Oleic acid is typically present in organic aerosol in a small fraction with respect to saturated organic acids. In fact, the mole fraction of double bonds is quite small. What effect will these factors have on ozonolysis in real ambient organic particles?Professor Ziemannresponded: This could affect both the reaction products and kinetics. If the particle matrix is relatively nonpolar because saturated organic acids and other oxygenated compounds comprise only a small fraction of the particle organic mass, then stabilized Criegee intermediates (SCI) are expected to recombine to a significant extent with their aldehyde co-products to form secondary ozonides. If the matrix is relatively polar, however, then SCI become more solvated and the probability for reactions with saturated organic acids to form α-acyloxyalkyl hydroperoxides is enhanced. With respect to reaction kinetics, the presence of saturated organic acids can lead to a more waxy or liquid/solid matrix that reduces the diffusion of O3and oleic acid, thereby slowing down the rate of ozonolysis.Dr G. Smithasked:(1) Could the nonanoic acid product from oleic acid ozonolysis be evaporating during sampling into the vacuum chamber of your instrument?(2) The observed rate of oleic acid reaction depends on particle size. What size or size distribution of particles did you use?(3) Under the assumption that the reaction (O3+ oleic acid) is limited by O3diffusion (the so-called “square root” case), the rate of oleic acid reaction in the OA-DOS particles should be about three timesfasterthan for pure oleic acid. Your observations suggest that the reaction occurs at the surface of the particle instead of in a layer below the surface. Have you considered the analysis of your kinetic data in this regard?Professor Ziemannreplied:(1) I don’t think that nonanoic acid is evaporating from the particles during sampling. Experiments and calculations indicate that compounds that have sufficiently low volatility to be present in particles before they enter our thermal desorption particle beam mass spectrometer will not evaporate before they reach the detection region. Instead, I believe the nonanoic acid is evaporating in the environmental chamber where the particle residence times are on the order of minutes. We know from experience preparing nonanoic acid particle standards that they will completely evaporate within a few seconds.(2) Because the size dependence enters the kinetic expression as a particle volume/surface area ratio, we used a particle radius of 0.2 μm calculated from the ratio of the particle volume and surface area measured for the polydisperse aerosol using a scanning mobility particle sizer.(3) I have not considered other reaction mechanisms because I felt that the rather large scatter in my data would make it difficult to draw conclusions. The major point of the analysis was to demonstrate that the uptake coefficients for the pure oleic acid and oleic acid/DOS mixture were in reasonable agreement with literature data, and that the monocarboxylic acid matrix dramatically reduced the reaction rate.Professor Abbattasked: Do you have any evidence from your work of the chemistry that ozone might undergo with oleic acid substrates at very high ozone exposures,i.e.after the initial reaction of the carbon-carbon double bond is complete? I ask this because we have seen the CCN activity of oleic acid particles increase only after the ozone exposure is raised substantially beyond that needed to give rise to the particle size change attributable to loss of nonanal (cf. ref. 1).1 K. E. Broekhuizen, T. Thornberry, P. P. Kumar and J. P. D. Abbatt,J. Geophys. Res., 2004,109(D24206), DOI: 10.1029/2004JD005298).Professor Ziemannsaid: This is an interesting observation, and I do not have a clear answer. It is known that the rate of reaction of ozone with saturated hydrocarbons is very slow, so it seems unlikely that further oxidation of hydrocarbon chains is occurring. One possibility is that at high concentrations ozone might react with some of the peroxide products, such as secondary ozonides or α-acyloxyalkyl hydroperoxides, leading to decomposition. A major decomposition product would most likely be carboxylic acids, which could enhance CCN activity.Professor Ravishankaraopened a general discussion of the papers by Dr Seisel, Professor Rudich and Professor Ziemann: These are very nice works. It really helps us understand the complexity of the issues regarding the formation and reactivities of aerosols.Could the authors address how the information derived in their studies can be applied to the atmosphere? Also, how could one expand such work to make sure that the derived information is more useful for atmospheric modeling.Professor Ziemannanswered: The most important conclusion from my study is that the particle matrix can have a major effect on the reaction products and kinetics of oleic acid ozonolysis. The primary particle-phase products are organic peroxides, but the specific peroxides formed (e.g., secondary ozonides or α-acyloxyalkyl hydroperoxdes) depends on the polarity of the particle matrix and the concentrations of functional groups such as carboxyl, hydroxyl, and carbonyl that can react with stabilized Criegee intermediates. The phase of the particle affects the rate of ozonolysis by altering the rate of diffusion of ozone within the particle. Incorporating these results into atmospheric models would therefore be difficult, since it would require more knowledge of the organic composition and phases of real atmospheric particles than is currently available. Future studies should investigate the products and kinetics of heterogeneous alkene ozonolysis reactions with different alkene classes and matrices, approaching atmospheric complexity when possible, and seek to obtain more information on atmospheric particle composition and phase properties.Professor Rudichreplied: The specific study presented here shows beyond doubt that processing of Ca-containing mineral dust by HNO3or N2O5(as was shown by Dr Cox) occurs in the atmosphere by mixing of polluted air with natural aerosol particles. It now remains to quantify whether this process has implications regarding the possibility of such dust particles to substantially scavenge atmospheric nitric acid, and what are the resulting implications of the processed dust particles on atmospheric issues such as haze formation and cloud formation. We already demonstrated here that the processed dust participles are liquid at ambient relative humidity. Once these processes are quantified, it should be relatively easy to include them in models.Professor Herrmanncommented: Oleic acid was chosen as a proxy for the numerous fatty acids encountered in tropospheric particles. Detailed laboratory studies are now able to fully elucidate the reaction mechanism of such compounds in tropospheric particles as demonstrated by the study of Professor Ziemann.The question is how the enormous amount of information from such detailed studies might be incorporated into multiphase models. Maybe lumping of compounds will become necessary as it has been applied before in gas phase modelling,e.g.by Bill Stockwell.Professor Ziemannreplied: I agree that there will be a need to lump the heterogeneous chemistry of organic compounds as is done for gas-phase reactions. It may be possible to group reactions according to simple compound classes such as alkenes, alkanes, aromatics, and oxygenates reacting either with OH radicals, NO3radicals, or O3. The most difficult of these reactions to model may be the ozonolysis of alkenes since the products and kinetics are both strongly dependent on the particle matrix, which can be extremely complex and is not known for atmospheric particles. Reactions of compounds with OH radicals, which are the major atmospheric oxidant, may be simpler. Because the reaction is likely to occur very close to the surface, there may be no significant matrix effect on the kinetics. This could allow the initial oxidation kinetics to be modelled rather easily. Furthermore, because the reaction of OH radicals with most organic compounds will lead to the same initial product: an alkyl radical, the chemistry may be similar for many compounds. The challenge will be to determine the relative proportions of OH radical reactions that lead to volatile productsviadecomposition compared to those that add functional groups (e.g., hydroxyl, carbonyl, carboxyl, and nitrooxy) to the carbon chain. The former reactions will alter particle composition by volatilizing organic compounds, whereas the latter reactions will lead to products (and particles) that are more polar, less volatile, and more hygroscopic.Dr Seiselreplied: It will certainly not been possible to incorporate all information from detailed laboratory studies into atmospheric models. However, in order to lump together several compounds or make reasonable approximations, details on the reaction mechanism ase.g.the rate-limiting step have to be known. The goal of such detailed studies is therefore not to fill atmospheric models with an enormous amount of kinetic data. Rather, these detailed studies provide the necessary tool to simplify complex reaction mechanism.Dr McFiggansmade a general comment: It is evident that there is vast complexity at every process level and that, from field studies, vast numbers of individual organic components. It is also evident that the atmosphere averages/integrates the kinetic processes, chemical and thermodynamic properties as probed for various atmospheric implications.I don’t anticipate that a single number analogous to “mean global [OH]” would be a meaningful product to describe any aspect of atmospheric aerosol behaviour. However, for purposes of for example cloud activation, direct optical properties, phase partitioning, health effectsetc, it is necessary to reduce the complexity from the detailed studies of all possible systems. How can we strategically plan a meaningful reduction of the complexity as identified in this meeting towards useable products? If we do not do it as a community, parameterisation of the processes used ine.g.climate models will not be based on the physics and chemistry at a detailed rigorous level.Dr Kingmade a general comment: Atmospheric aerosol contains a high proportional of transition metals which are known to catalyse the decomposition of the ozonides formed from the reactions of ozone with unsaturated compounds such as oleic acid. Should studies of the oxidation of the organic alkenes in the atmosphere include these transition compounds?(N. B. The study of Kinget al.1oxidised oleic acid on synthetic seawater droplets containing many transition metals and a different ratios of products were seen).1 M. D. King, K. C. Thompson and A. D. Ward,J. Am. Chem. Soc., 2004,126, 16710–16711.Professor Ziemannreplied: I think that as knowledge is gained about the oxidation of alkenes in pure form and in simple organic mixtures it will be important to investigate more complex systems. Information is needed on organic mixtures more similar to real atmospheric particles and on the effects of species such as water, sulfuric acid, metals, and inorganic salts as well as UV light on the chemistry.Dr Baltenspergercommented: Of course, we all would like to knowwas die Welt/Im Innersten zusammenhält“what holds the Earth together in its innermost parts” (as Goethe says). However, in order to be able to do that we need to understand the fundamental processes. We just should be modest enough and not over-interpret our data. And we should talk to each other: since chemistry and physics talk to each other in a particle, we scientists should do as well.Dr Tuckcommented:(1) I strongly recommend that techniques, such as neutron analysis, that examine the surface layers of aerosols should be applied to real aerosols as well as idealized laboratory populations.(2) Many medical studies of the lung use surfaces of palmitic acid as a laboratory surrogate for the surface of lung tissue. This is interesting from the point of view of health effects, since it is very striking that the TOF-SIMS analysis of real aerosol surfaces done in Finland contain palmitic acid as one of only a very few prominent peaks.11 H. Tervahattu, J. Juhanoja, V. Vaida, A. F. Tuck, J. V. Niemi, K. Kupiainen, M. Kulmala and H. Vehkamaki,J. Geophys. Res., 2005,110(D6), DOI: 10.1029/2004JD005400, Art. no. D06207

ISSN:1359-6640    

DOI:10.1039/b507796f

出版商:RSC    

年代:2005

数据来源: RSC

摘要:

Chemical‐ionisation reaction time‐of‐flight mass spectrometry for real‐time analysis of atmospheric trace speciesR. S. Blake, A. M. Ellis, P. S. Monks, K. WillisandK. P. Wyche,University of Leicester, UKSolvation of O3and OH radical in bulk water and at the air‐water interface from molecular dynamics simulationsMartina Roeselová, John Vieceli, Nick Potter, Liem X. Dang, Bruce C. GarrettandDouglas J. Tobias,Academy of Sciences of the Czech Republic, Czech Republic, University of California, USAandPacific Northwest National Laboratory, USAMQDO comparative study of the dissociative behaviour of CF3Cl and CF2Cl2in the upper atmosphereE. Mayor, A. M. VelascoandI. Martín,Universidad de Valladolid, SpainCharacterisation of an instrument to measure the chemical lifetime of OH in the atmosphereK. A. Read, D. E. Self, T. Ingham, C. M. Moore, G. P. JohnsonandD. E. Heard,University of Leeds, UKDetection of atmospheric OH using chemiluminescent emission from Na(2PJ)D. E. Self, J. M. C. Plane, D. E. HeardandW. J. Bloss,University of Leeds, UKandUniversity of East Anglia, UKA hollow cathode PTR‐ToF mass spectrometer for atmospheric analysisC. J. Ennis, B. J. KeelyandL. J. Carpenter,University of York, UKNO3radical detection using broadband cavity‐enhanced absorption spectroscopyD. S. Venables, M. StaakandA. A. Ruth,University College, Cork, Republic of IrelandDevelopment of a LED based broadband cavity enhanced absorption spectrometer for atmospheric studiesJustin M. Langridge, Stephen M. BallandRoderic L. Jones,University of Cambridge, UKBroadband cavity ringdown spectroscopy for atmospheric applicationsA. J. L. Shillings, Mario Bitter, S. M. BallandR. L. Jones,University of Cambridge, UKPulsed quantum cascade laser spectroscopy of the natural atmosphereS. Wright, M. T. McCulloch, G. DuxburyandN. Langford,University of Strathclyde, UKDevelopment of a novel airborne differential absorption lidar system for the UK FAAM aircraftR. A. Robinson, T. D. Gardiner, B. Goody, P. T, Woods, S. Garcelon, T. RobertsandR. Jones,National Physical Laboratory, UKandUniversity of Cambridge, UKStructure‐activity relationships and correlations for some key reactions in atmospheric chemistryJozef Peeters, Sabine Vandenberk, Veerle Pultau, Werner Boullart, Jan Van Hoeymissen, Gaia FantechiandLuc Vereecken,University of Leuven, BelgiumAbsolute high‐resolution measurements and modelling of atmospheric transmission across the near‐infrared regionTom Gardiner, Marc Coleman, Sophie Casanova, Heather Pegrum, Keith ShineandMartin Milton,National Physical Laboratory, UKandUniversity of Reading, UKAbsorption spectra and photo chemistry of novel super greenhouse gasesWolfgang Eisfeld,Technical University of Munich, GermanySimulation chamber study of the oxidation of acetic acid by OH radicals. Detection of reaction products by CW‐CRDS in the near‐infrared rangeChrista Fittschen, Sabine Crumaire, Alexandre Thomas, Patrice CoddevilleandBernard Lemoine,Université des Sciences et Technologies de Lille, France, École des Mines de Douai, France,andPhLAM, FranceThe H/D fractionation in the photolysis of HCHO in natural sunlight studied at the E&cmb.b.line;u&cmb.b.line;ropean P&cmb.b.line;h&cmb.b.line;o&cmb.b.line;tor&cmb.b.line;e&cmb.b.line;actor facility (EUPHORE)Karen L. Feilberg, Barbara D’Anna, Matthew S. JohnsonandClaus J. Nielsen,University of Copenhagen, DenmarkandUniversity of Oslo, NorwayCharacterisation of the photolytic HONO‐source in the atmosphere simulation chamber SAPHIRTheo Brauers, Franz Rohrer, Birger Bohn, Andreas WahnerandJörg Kleffmann,Forschungszentrum Jülich, Germany and Bergische Universität Wuppertal, GermanyChemistry of NO2, N2O4, and N2O5at simulated stratospheric conditions – an aerosol chamber and modelling studyH. Saathoff, K.‐H. NaumannandR. Wagner,Institut für Meteorologie und Klimaforschung, GermanyInvestigation of background radical sources in the European Photoreactor (EUPHORE)Judit Zádor, Tamás Turányi, Klaus WirtzandMichael J. Pilling,Eötvös Lorand University, Hungary, CEAM, SpainandUniversity of Leeds, UKMeasurements and modelling of iodine species in coastal marine environmentsAlfonso Saiz‐Lopez, John M. C. PlaneandGordon McFiggans,University of East Anglia, UKandUniversity of Manchester, UKInfluence of the urban canopy on heterogeneous processing of pollutants at nightJochen Stutz, Andreas Geyer, Sebastian TrickandStephen C. Hurlock,University of California, USADaytime formation of HONO: a major source of OH radicals in a forestA, Hofzumahaus, J. Kleffmann, T. Gavriloaiei, F. Holland, R. Koppmann, L. Rupp, E. Schlosser, M. SieseandA. Wahner,Forschungszentrum Jülich, GermanyandBergische Universiät Wuppertal, GermanyPeroxy radicals in the UK summer heat wave of 2003M. J. Jacob, P. S. Monks, A. C. Lewis, J. F. Hamilton, J. F. Hopkins, B. J. Bandy, S. A. Penkett, K. M. EmmersonandN. Carslaw,University of Leicester, UK, University of York, UKandUniversity of East Anglia, UKNAMBLEX 2002 and TORCH 2003: OH and HO2measurements and modelling in clean and polluted airS. C. Smith, R. Sommariva, K. M. Emmerson, J. D. Lee, W. J. Bloss, T. Gravestock, G. P. Johnson, T. Ingham, D. E. Heard, M. J. PillingandN. Carslaw,University of Leeds, UKandUniversity of York, UKPreliminary comparisons of modelled and predicted spatial NOxand O3concentrations in an urban environmentD. J. Cryer, E. J. NichollandP. W. Seakins,University of Leeds, UKA new protocol for measurements of gas/particle partitioning of atmospheric oxidation productsBrice Temime‐Roussel, Robert HealyandJohn Wenger,University College Cork, Republic of IrelandObservations of the chemical evolution of the Sacramento urban plumeJ. G. Murphy, D. A. Day, P. A. Cleary, P. J. Wooldridge, R. C. Cohen, D. B. MilletandG. W. Schade,University of California, USA, University of Leeds, UK, Harvard University, USAandUniversity of Bremen, Germany“CHABLIS at CASLab” – a major overwinter atmospheric chemistry campaign in coastal AntarcticaA. E. Jones, S. J.‐B. Bauguitte, R. A. Salmon, E. W. Wolff, D. Ames, W. Bloss, P. Hamer, K. C. Clemitshaw, Z. Fleming, D. E. Heard, A. Jackson, J. Lee, A. Lewis, G. Mills, J. Plane, K. Read, A. Saiz‐Lopez, D. E. Shallcross, W. T. Sturges, S. WalkerandD. Worton,Natural Environment Research Council, UK, Imperial College of Science, technology and Medicine, UK, University of Leeds, UK, University of York, UK, University of East Anglia, UKandUniversity of Bristol, UKSeasonal variation of non‐methane hydrocarbons (NMHC) and dimethyl sulphide (DMS) in the Antarctic boundary layer: Early results from the CHABLIS experiment, Halley, AntarcticaK. A. Read, A. C. Lewis, R. A. Salmon, A. E. JonesandD. E. Heard,University of Leeds, UK, University of York, UKandBritish Atlantic Survey, UKHydrogen peroxide and formaldehyde in the Antarctic troposphereS. J. Walker, A. V. Jackson, M. J. Evans, J. B. McQuaid, R. Salmon, S. BauguitteandR. A. Salmon,University of Leeds, UKandBritish Atlantic Survey, UKMeasurements of OH and HO2at Halley Research Station, Antarctica: Preliminary resultsJ. D. Lee, W. J. Bloss, D. E. Heard, R. A. Salmon, S. BauguitteandA. E. Jones,University of York, UK, University of Leeds, UKandBritish Atlantic Survey, UKTropospheric ozone depletion events observed over the frozen Arctic OceanHans‐Werner Jacobi, Lars KaleschkeandAndreas Richter,Alfred Wegener Institute for Polar and Marine Research, GermanyandUniversity of Bremen, GermanyPartitioning of organic acids to a silica surface – a computational approachMartin D. KingandKatherine C. Thompson,Royal Holloway University of London, UKandBirkbeck College, University of London, UKNOxand OH radical production from mid‐latitude snowpacks: a field and modelling approachMartin D. King,Fleur N. Fisher and Julia Lee‐Taylor,Royal Holloway University of London, UK, King’s College London, UKandNational Centre for Atmospheric Research, UKKetone photolysis followed by the reaction of RCO with O2as a source of OH for kinetic studies of OH + ketone reactionsM. T. Baeza Romero, M. A. Blitz, D. E. Heard, M. J. Pilling, B. Price, andP. W. Seakins,University of Leeds, UKKinetics and photochemical study on the atmospheric fate of acetoneE. Farkas, Gg. Kovács, Krisztina Imrik, Gábor Vasvári, István Szilágyi, István Fejes, Ákos Bencsura, Sándor Dóbé, Tibor BércesandFerenc Márta,Hungarian Academy of Sciences, HungaryInfrared overtone spectroscopy and dissociation dynamics of peroxynitrous acid (HOONO)Ian M. Konen, Eunice X. J. Li, Ilana B. Pollack, Thomas A. StephensonandMarsha I. Lester,University of Pennsylvania, USATemperature dependence of the rate coefficients for the reactions between Cl atoms and pentanal, hexanal and heptanalCarlos A. Cuevas, Alberto Notario, Ernesto MartínezandJosé Albaladejo,Universidad de Castilla‐La Mancha, SpainAtmospheric chemical degradation of 2‐butanol and 2‐methyl‐2‐butanol initiated by OH radicalsElena Jiménez, Beatriz Lanza, Andrés GarzónandJosé Albaladejo,Universidad de Castilla‐La Mancha, SpainKinetics of the reaction of cyclohexoxy and related radicals with O2Theodore S. DibbleandLei Zhang,SUNY College of Environmental Science and Forestry, USAA kinetic study of the reactions of CI atoms with polycarbonyl compoundsC. S. Mills, A. J. Nalty, C. J. Tooze, C. E. Canosa‐MasandR. P. Wayne,University of Oxford, UKRate coefficients for the reaction HO + HOCH2C(O)CH3→ (Products), and comparison with reactions of other partially oxidised hydrocarbonsTerry J. Dillon, Abraham Horowitz, Luc Vereecken, Dirk HölscherandJohn N. Crowley,Max Planck Institute for Chemistry, GermanyTemperature dependence of the Cl atom reaction with deuterated methanesFrank Sauer, Stefan Bauerle, James B. BurkholderandA. R. Ravishankara,National Oceanic and Atmospheric Administration, USAGas‐phase rate coefficients for the reactions of NO3with (Z)‐pent‐2‐ene, (E)‐pent‐2‐ene, (Z)‐hex‐2‐ene, (E)‐hex‐2‐ene, (Z)‐hex‐3‐ene, (E)‐hex‐3‐ene and (E)‐3‐methylpent‐2‐ene at room temperatureC. Pfrang, R. S. Martin, A. Nalty, R. Waring, C. E. Canosa‐MasandR. P. Wayne,University of Oxford, UKandUniversity of Cambridge, UKA laboratory investigation under near‐tropospheric conditions of the reactions of alcohols with Cl atoms studied by FTIRBernabé BallesterosandJosé Albaladejo,Universidad de Castilla‐La Mancha, SpainThe reaction of NO3with peroxy radicals: an indirect source of OH radicals at nightS. Vaughan, C. E. Canosa‐Mas, C. Pfrang, R. P. WayneandD. E. Shallcross,University of Oxford, UKandUniversity of Bristol, UKA study of the kinetics of the reactions of IO with peroxy radicalsC. S. E. Bale, C. E. Canosa‐Mas, R. P. WayneandD. E. Shallcross,University of Oxford, UKandUniversity of Bristol, UKTemperature dependences of the reactions of O(3P) with chlorofluoroalkenesM. A. Teruel, Pablo M. Cometto, M. B. Blanco, R. A. TacconeandS. I. Lane,Universidad Nacional de Córdoba, ArgentinaRate constants for the reaction of chlorine atoms with haloethers at 298 K and atmospheric pressurePablo R. Dalmasso, Raúl A. Taccone, Jorge D. Nieto, Mariano A. TeruelandSilvia I. Lane,Universidad Nacional de Córdoba, ArgentinaStudy of the carbon‐13 and deuterium kinetic isotope effects in the OH and Cl reactions of CH3Cl and CH3BrAgnieszka A. Gola, Barbara D’Anna, Stig R. Sellevåg, Lihn Bache‐Andreassen, Claus J. Nielsen, Karen L. FeilbergandGunnar Nyman,Medical University of Wroclaw, Poland, University of Oslo, Norway, University of Copenhagen, DenmarkandGöteborg University, SwedenA recent investigation of the OH + CH3OCH3system including isotopic studiesTracy J. Still, Kenneth McKee, Mark A. BlitzandPaul W Seakins,University of Leeds, UKReaction kinetics of C2H2n+ OH from 200–400 KP. A. Cleary, M. T. Baeza Romero, M. A. Blitz, K. Hughs, P. W. Seakins, L. Wang, D. E. HeardandM. J. Pilling,University of Leeds, UKReaction of OH with acetaldehyde and deuterated acetaldehyde: Reexamination of the reaction mechanismTakahiro Yamada, Philip H. Taylor, Abdellatif GoumriandPaul Marshall,University of Dayton Research Institute, USAandUniversity of North Texas, USARationalisation of the trends in reactivity for peroxy radical self‐reactionTeresa Raventos Duran, Carl Percival, Dudley E. ShallcrossandZachary Solman,University of Manchester, UKInfrared remote sensing of organic compounds in the upper troposphereJ. J. Remedios, A. M. WaterfallandG. Allen,University of Leicester, UKDetermining tropospheric composition from satellite measurements. Stage I: chemical data assimilation in multiannual runs of the SLIMCAT 3D CTMLara Gunn, Martyn Chipperfield, Richard SiddansandBrian Kerridge,University of Leeds, UKandRutherford Appleton Laboratory, UKStratospheric age of air simulations with a 3D chemical transport modelBeatriz Monge Sanz, Martyn ChipperfieldandAdrian Simmons,University of Leeds, UKA 3D model study of the effect of new temperature‐dependent quantum yields for acetone photolysisS. R. Arnold, M. P. ChipperfieldandM. A. Blitz,University of Leeds, UKGlobal modeling of isoprene chemistry: the Mainz Isoprene Mechanism and the Unified ModelPaul Young, Guang ZengandJohn Pyle,University of Cambridge, UKThe role of peroxyacetyl nitrate in North Atlantic intercontinental transport eventsL. K. Whalley, M. J. Pilling, A. C. Lewis, Team ITOPandTeam ICARTT,University of Leeds, UKThe effect on tropospheric oxidant concentrations and climate on future sulphate aerosol predictionsColin E. JohnsonandJamie G. L. Rae,Hadley Centre for Climate Prediction and Research, UKCurrent and future deposition fluxes of nitrogen and sulphur over EuropeMichael Sanderson, William Collins, Colin JohnsonandRichard Derwent,Met Office, UKModelling stomatal ozone fluxes over HungaryIstván Lagzi, Róbert Mészáros, Ferenc Ács, Dalma Szinyei, Csilla Vincze, Alison Tomlin, Tamás TurányiandLászló Haszpra,Eötvös Löránd University, Hungary, University of Leeds, UKandHungarian Meterological Service, HungaryEstimation of methane fluxes from paddy fields and wetlands by the flux footprint techniqueUjjaini SarkarandRanjan Mukherjee,Jadavpur University, IndiaApplication of a non‐hydrostatic meteorological model to flow and dispersion of tracers in a street canyonS. Lock, N. S. Dixon, A. M. Gadian, J. McQuaid, P. K. SmolarkiewiczandA. Tomlin,University of Leeds, UKSimulating the formation of secondary organic aerosol (SOA) using the Master Chemical Mechanism (MCM)David Johnson, Stephen UtembeandMichael E. Jenkin,Imperial College London, UKLocal ozone production at TORCHJ. C. Stanton, M. J. PillingandA. C. LewisUniversity of Leeds, UKandUniversity of York, UKThe master chemical mechanism – version 3.1L. Whitehouse, A. R. Rickard, S. Pascoe, C. Bloss, S. M. Saunders, M. E. JenkinandM. J. Pilling,University of Leeds, UK, University of Western Australia, AustraliaandImperial College London, UKExamination of alumina as a model aerosol material for O3destruction using diffuse reflectance FTIR spectroscopyJohn M. RoscoeandJonathan P. D. Abbatt,Acadia University, CanadaandUniversity of Toronto, CanadaPhase transitions and hygroscopic growth of humic acid and mixed humic acid and ammonium sulphate aerosolsC. L. Badger, P. T. Griffiths, I. George, C. F. Braban, J. P. D. AbbattandR. A. Cox,University of Cambridge, UKNitric acid uptake to deliquescent sea salt aerosol: influence of organic aerosol constituentsK. Stemmler, A. VlasenkoandM. Ammann,Paul Scherrer Institute, SwitzerlandCharacterisation of aqueous aerosol droplets: studies of mass transfer dynamics and coagulationRebecca J. Hopkins, Chris R. Howle, Chris J. HomerandJonathan P. Reid,University of Bristol, UKStudying the uptake of NH3onto micron sized acidified water droplets using a laser induced fluorescence techniqueJariya Buajarern, Anna Frost, Chris R. Howle, Andrew J. Cotterill, Robert M. SayerandJonathan P. Reid,University of Bristol, UKKinetics of chlorine nitrate uptake on ice at temperatures of the upper troposphereMiguel FernandezandTony Cox,University of Cambridge, UKUptake of HNO3on aviation kerosene soot: thermodynamics and product studyRanajit K. Talukdar, Ekaterina E Loukhovitskaya, Olga PopovichevaandA. R. Ravishankara,NOAA, USAandMoscow State University, RussiaElectrochemical evidence of surface activity of organic components of the atmospheric aerosolsG. Szentes, E. Schmidt, Gy. KissandA. Marton,University of Veszprém, HungaryCarbon mass balance in secondary aerosol formation: a mass spectroscopic approachRalf Tillmann, Thomas F. Mentel, Astrid Kiendler‐ScharrandAndreas Wahner,ICG‐II Troposphäre, GermanyParticle substrate effects in organic aerosolsGeoffrey D. SmithandJohn D. Hearn,University of Georgia, USATowards a new instrument for determining size and composition of liquid‐phase aerosolsR. Symes, H. Meresman, R. GilhamandJ. P. Reid,University of Bristol, UKThe characterisation of a single aerosol droplet using optical tweezers and cavity enhanced Raman spectroscopyLaura Mitchem, Rebecca J. Hopkins, Nicola J. Howe, Andrew D. WardandJonathan P. Reid,University of Bristol, UKandRutherford Appleton Laboratory, UKOH and HO2radical behaviour and photochemical O3tendency in Tokyo during IMPACT IV in January/February 2004Yugo Kanaya, R. Cao, H. Akimoto, M. Fukuda, N. Takegawa, Y. Komazaki, Y. Yokouchi, M. KoikeandY. Kondo,Japan Agency for Marine‐Earth Science and Technology, Japan, University of Tokyo, JapanandNational Instiute for Environmental Studies, JapanChemical characterisation of aerosols during new particle formation in a boreal forest in FinlandJ. D. Allan, M. R. Alfarra, K. N. Bower, H. Coe, J. T. Jayne, D. R. Worsnop, P. P. Aalto, M. KulmalaandA. Laaksonen,University of Manchester, UK, NERC Centres for Atmospheric Science, UK, Aerodyne Research Inc., USA, University of Helsinki, FinlandandUniversity of Kuopio, FinlandOxidation studies of optically trapped aerosol droplets of seawaterAndrew D. Ward, Martin D. KingandKatherine C. Thompson,Rutherford Appleton Laboratory, UK, Royal Holloway University of London, UKandBirkbeck University of London, UKLaser tweezers Raman study of optically trapped aerosol droplets of seawater and oleic acid reacting with ozone: implications for cloud droplet propertiesMartin D. King, Katherine C. ThompsonandAndrew D. Ward,Royal Holloway University of London, UK, Birkbeck University of London, UKandRutherford Appleton Laboratory, UKKinetic model framework for aerosol and cloud surface chemistry and gas‐particle interactionsU. Pöschl, Y. RudichandM. Ammann,Technical University of Munich, Germany, Weizmann Institute of Science, IsraelandPaul Scherrer Institute, SwitzerlandADDEM – Aerosol Diameter Dependent Equlibrium ModelD. O. Topping, G. B. McFiggansandH. Coe,University of Manchester, UKModelling natural sources of PM10Mark Harrison, Alistair ManningandDerrick Ryall,Met Office, UKCoupling of marine aerosol, DMS and dust in a global model of aerosol processesKirsty Pringle, Dominick Spracklen, Ken Carslaw, Graham Mann, Martyn ChipperfieldandSat Ghosh,University of Leeds, UKTrapping of trace gases in growing ice crystalsBernd KärcherandMikhail Basko,Institut für Physik der Atmosphäre, GermanyandInstitute for Experimental and Theoretical Physics, RussiaMeasurements of peroxy radicals in air masses undergoing long‐range transport during ITOP with a new dual‐channel PERCAAlex E. Parker, Paul S. Monks, Mark J. Jacob, John Methven, Tim J. GreenandStuart A. Penkett,University of Leicester, UKLow pressure kinetic study of the Cl reaction with cyclooctane: atmospheric implicationsAlfonso Aranda, Yolanda Díaz de Mera, Lorena MoralesandIván Bravo,Universidad de Castilla‐La Mancha, SpainComparison of observations during the first Antarctic Match campaign with a 3D CTMWuhu Feng, M. P. Chipperfield, H. K. Roscoe, M. Rex, P. von der Gathen, F. Goutail, G. Konig‐Langlo, T. Deshler, B. Johnson, J. Easson, M. Yela, P. TaalasandK. Sato,University of Leeds, UKThe formation of organic films: Experimental and model resultsAstrid Kiendler‐Scharr, Thomas F. Mentel, Tatu AntillaandAndreas Wahner,Forschungszentrum Jülich, GermanyDetermination of particle size distribution of chloroacetates in the atmosphere of the Athens basinEvangelos B. Bakeas, Andreas Th. SouliotisandPanayotis A. SiskosNational and Kapodistrian University of Athens, GreecePhotochemistry of formaldehyde under tropospheric conditionsFrancis D. Pope, Carina A. Smith, Peter R. Davis, Dudley E. Shallcross, Michael N. R. AshfoldandAndrew J. Orr‐EwingUniversity of Bristol, UKMeasurements of organic nitrates and halocarbons in AntarcticaGraham Mills, Bill Sturges, Rhian Salmon, Stéphane BauguitteandAnna Jones,University of East Anglia, UK The Skinner Prize for the best poster was awarded to Stewart Vaughan from the University of Oxford, UK, for his poster on the reaction of NO3with peroxy radicals: an indirect source of OH radicals at night.

ISSN:1359-6640    

DOI:10.1039/b506852p

出版商:RSC    

年代:2005

数据来源: RSC

摘要:

Professor J. Abbatt,University of Toronto,CanadaMr L. Abraham,University of York,UKDr R. Alfarra,Paul Scherrer Institute,SwitzerlandDr J. Allan,University of Manchester,UKDr M. Ammann,Paul Scherrer Institute,SwitzerlandDr S. Arnold,University of Leeds,UKProfessor M. Ashfold,University of Bristol,UKDr S. Ashworth,University of East Anglia,UKDr L. Bache‐Andreassen,University of Oslo,NorwayMs C. Badger,University of Chemistry,UKDr M. Baeza Romero,University of Leeds,UKMiss C. Bale,University of Oxford,UKDr S. Ball,University of Cambridge,UKDr U. Baltensperger,Paul Scherrer Institute,SwitzerlandMiss J. Barnes,Cornwall College,UKDr C. Batchelor,Royal Society of Chemistry,UKDr S. Bauguitte,British Antarctic Survey,UKProfessor H. Berresheim,Deutscher Wetterdienst/BTU Cottbus,GermanyMr B. Blake,University of Leicester,UKDr M. Blitz,University of Lee,UKDr W. Bloss,University of Leeds,UKDr T. Brauers,Forschungszentrum Julich,GermanyMr I. Bravo,Universidad De Castilla La Mancha,UKMiss J. Buajarern,University of Bristol,UKDr J. Burkholder,NOAA Aeronomy Laboratory,USAProfessor J. Burrows,University of Bremen,GermanyDr C. Canosa‐Mas,University of Oxford,UKDr C. Cantrell,NCAR,USADr L. Carpenter,University of York,UKDr M. Chapman,Royal Society of Chemistry,UKDr M. Chipperfield,University of Leeds,UKDr P. Cleary,University of Leeds,UKDr H. Coe,Univeristy of Manchester,UKProfessor R. Cohen,University of California,USAF. Collingborne,Natural Environment Research Council,UKDr T. Cox,University of Cambridge,UKMiss B. Cronin,University of Bristol,UKMr J. Crosier,University of Manchester,UKDr J. Crowley,Max Planck Institute for Chemistry,GermanyMr D. Cryer,University of Leeds,UKMr C. Cuevas,University of Castilla La Mancha,SpainMr G. De Leeuw,TNO,The NetherlandsProfessor T. Dibble,SUNY College of Environmental Science and Forestry,USADr T. Dillon,Max Planck Institute of Chemistry,GermanyProfessor S. Dobe,Chemical Research Centre,HungaryDr R. Doherty,University of Edinburgh,UKProfessor N. Donahue,Carnegie Mellon University,USAProfessor J. Donaldson,University of Toronto,CanadaProfessor G. Duxbury,University of Strathclyde,UKDr W. Eisfeld,University of Munich,GermanyProfessor B. Ellison,University of Colorado,USADr C. Ennis,University of York,UKDr M. Evans,University of Leeds,UKMiss E. Farkas,Chemical Research Centre Budapest,HungaryMiss K. Feilberg,University of Copenhagen,DenmarkDr M. Fernandez,University of Cambridge,UKDr C. Fittschen,University of Lille,FranceDr B. Flake,European Office of Aerospace Research & Development,UKMr C. Floquet,University of Leeds,UKDr T. Gardiner,National Physical Laboratory,UKMiss K. Gawler,University College London,UKProfessor C. George,Universite Claude Bernard Lyon,FranceMr D. Glowacki,University of Leeds,UKDr A. Goddard,University of Leeds,UKDr P. Griffiths,University of Cambridge,UKMiss L. Gunn,University of Leeds,UKDr H. Harder,Max Planck Institute of Chemistry,GermanyDr M. Harrison,Meteorological Office,UKDr M. Heal,University of Edinburgh,UKMr R. Healy,University College Cork,Republic of IrelandProfessor D. Heard,University of Leeds,UKProfessor H. Herrmann,Leibniz Institut für Troposphärenforschung,GermanyDr E. Highwood,University of Reading,UKMiss V. Hilborne,London South Bank University,UKDr A. Hofzumahaus,Forschungszentrum Jülich,GermanyMiss R. Hopkins,University of Bristol,UKDr C. Howle,University of Bristol,UKDr R. Huie,REH Kinetics,USAProfessor A. Hynes,University of Miami,USADr Y. Iinuma,Leibniz Institut für Troposphärenforschung,GermanyDr T. Ingham,University of Leeds,UKMr M. Jacob,University of Leicester,UKDr H. Jacobi,Alfred Wegener Institute,GermanyProfessor L. Jaeglé,University of Washington,USADr M. Jenkin,Imperial College London,UKDr D. Johnson,Imperial College London,UKDr R. Jones,Cambridge University,UKDr B. Kärcher,Institute of Atmospheric Physics,GermanyDr Y. Kanaya,FRCGC/JAMSTEC,JapanDr A. Kiendler‐Scharr,Forschungszentrum Jülich,GermanyDr M. King,Royal Holloway, University of London,UKDr J. Kleffmann,Bergische Universität Wuppertal,GermanyDr C. Kolb,Aerodyne Research,UKMr G. Kovács,Chemical Research Centre,Budapest,HungaryDr I. Lagzi,Eotvos University,HungaryMr J. Langridge,University of Cambridge,UKDr J. Lee,University of York,UKDr G. Mann,University of Leeds,UKDr P. Marshall,University of North Texas,USADr D. Martin,University of Bristol,UKProfessor A. Marton,University of Veszprém,HungaryMr E. Mayor,Universidad de Valladolid,SpainDr G. McFiggans,University of Manchester,UKDr J. McQuaid,University of Leeds,UKDr T. Mentel,Forschungszentrum Jülich,GermanyMrs H. Meresman,University of Bristol,UKMr C. Mills,University of Oxford,UKDr G. Mills,University of East Anglia,UKDr L. Mitchem,University of Bristol,UKMs B. Monge,University of Leeds,UKDr P. Monks,University of Leicester,UKDr J. Mössinger,Nature,UKMs F. Nalden,Royal Society of Chemistry,UKMiss E. O’Brien,Royal Society of Chemistry,UKProfessor A. Orr‐Ewing,University of Bristol,UKMrs K. Parajuli,Leibniz Institut für Troposphärenforschung,GermanyMr A. Parker,University of Leicester,UKDr C. Percival,University of Manchester,UKMr C. Pfrang,University of Oxford,UKProfessor M. Pilling,University of Leeds,UKProfessor J. Plane,University of East Anglia,UKProfessor U. Platt,University of Heidelberg,GermanyDr F. Pope,California Institute of Technology,USAMr B. Price,University of Leeds,UKMiss K. Pringle,University of Leeds,UKProfessor J. Pyle,University of Cambridge,UKDr J. Rae,Meteorological Office,UKMiss T. Raventos‐Duran,University of Manchester,UKProfessor A. Ravishankara,NOAA Aeronomy Laboratory,USADr K. Read,University of Leeds,UKDr J. Reid,University of Bristol,UKDr J. Remedios,University of Leicester,UKMr R. Robinson,National Physical Laboratory,UKDr M. Roeselova,Czech Academy of Sciences,Czech RepublicDr H. Roscoe,British Antarctic Survey,UKProfessor J. Roscoe,Acadia University,CanadaDr D. Rowley,University College London,UKProfessor Y. Rudich,Weizmann Institute,IsraelDr H. Saathoff,Forschungszentrum Karlsruhe,GermanyMr A. Saiz‐Lopez,University of East Anglia,UKDr L. Salter,Cornwall College,UKDr M. Sanderson,Meteorological Office,UKDr U. Sarkar,Jadavpur University,IndiaDr P. Seakins,University of Leeds,UKDr S. Seisel,Universität Duisburg‐Essen,GermanyDr D. Self,University of Leeds,UKMr S. Sellevaag,University of Oslo,NorwayDr D. Shallcross,University of Bristol,UKMr A. Shillings,University of Cambridge,UKProfessor P. Siskos,University of Athens,GreeceMiss C. Smith,University of Bristol,UKDr G. Smith,University of Georgia,USAProfessor I. W. M. Smith,University of Birmingham,UKMiss S. C. Smith,University of Leeds,UKMiss J. Stanton,University of Leeds,UKDr K. Stemmler,Paul Scherrer Institute,SwitzerlandProfessor T. Stephenson,Swarthmore College,USADr D. Stevenson,University of Edinburgh,UKMiss T. Still,University of Leeds,UKProfessor J. Stutz,California State University,USADr C. Taatjes,Sandia National Laboratories,USADr R. Talukdar,NOAA Aeronomy Laboratory,UKDr C. Taylor,Royal Haskoning,UKMiss S. Taylor,University of Leeds,UKDr M. Teruel,Universidad Nacional de Cordoba,ArgentinaMr D. Topping,University of Manchester,UKDr A. Tuck,NOAA Aeronomy Laboratory,USAMr S. Vaughan,University of Oxford,UKDr D. Venables,University College Cork,Republic of IrelandDr L. Vereecken,K. U. Leuven,BelgiumMr E. Voehringer‐Martinez,University of Göttingen,GermanyProfessor A. Wahner,Forschungszentrum Jülich,GermanyMs S. Walker,University of Leeds,UKDr L. Wang,University of Leeds,UKDr A. Ward,CCLRC Rutherford Appleton Laboratory,UKProfessor R. Wayne,University of Oxford,UKDr J. Wenger,University College Cork,Republic of IrelandDr L. Whalley,University of Leeds,UKMr B. Wilson,Nottingham Trent University,UKMr S. Wright,University of Strathclyde,UKMr P. Young,University of Cambridge,UKMiss J. Zador,Eötvös University,HungaryProfessor R. Zellner,University of Duisburg‐Essen,GermanyProfessor P. Ziemann,University of California,Air Pollution Research Centre,USA

ISSN:1359-6640    

DOI:10.1039/b506853n

出版商:RSC    

年代:2005

数据来源: RSC

摘要:

Abbatt, J. P. D.,211, 258–260, 263, 383–385, 510, 515Alfarra, M. R.,265,341, 367Allan, J.,341, 366, 512Alwyne, E.,327Ammann, M.,195, 254, 262, 508Arnold, S., 129, 372Ashfold, M. N. R.,59, 249Baeza Romero, M. T.,73, 370Ball, S. M.,165Baltensperger, U., 253,265, 363–366, 368, 370, 508, 513, 516Bauer, D.,111Berresheim, H., 504Blitz, M. A.,73, 134, 139, 142Bloss, W. J., 143,425, 503–507Böge, O.,281Bower, K.,341Braesicke, P.27Brauers, Th., 133, 254, 503, 504Bui, T. P.,181Burrows, J. P., 242, 254,387, 491, 493, 495, 496, 498–500, 503Campuzano-Jost, P.,111Canagaratna, M. R.,327Chance, K.,407Chaudhuri, S. R.,227Chipperfield, M. P., 126–130, 132, 133, 491, 496, 499, 501, 506, 507Chuong, B.,297Coe, H., 258,265,341, 384, 511Cohen, R., 133, 258, 500, 503, 504Collins, B.,41Cox, R. A., 129, 131, 143, 144, 149, 255, 259, 372, 505, 511,519Cubison, M.,341Davis, P. R.,59Derwent, D.,41Derwent, R. G.,311Decesari, S.,341Dibble, T., 137, 508Dobé, S., 140Doherty, R.,41Dommen, J.,265Donahue, N. M., 148, 254,297, 364, 368–371, 374, 384, 513, 514Donaldson, D. J., 150,227, 256, 261–263D’Ottone, L.,111Duxbury, G., 136, 137, 144, 248, 374, 377Eisfeld, W., 138, 263Evans, M. J., 125,425, 500, 501Facchini, C.,341Fardy, M.,111Farkas, E., 142Fisseha, R.,265Friedl, R. R.,89Frieß, U.,153Fuzzi, S.,341Gao, R.-S.,181Gascho, A.,265George, C.,195, 252–256, 364, 367Glowacki, D., 256Gnauk, T.,281Graber, E. R.,453Gysel, M.,265Hamilton, J. F.,311Handley, S.,227Heard, D. E.,73, 129, 136, 142, 143, 150, 246, 249, 368, 381,425, 507Herndon, S. C.,327Herrmann, H., 126, 250, 252, 261,281, 363, 385, 513, 515Hopkins, J. R.,311Hovde, S. J.,181Huff Hartz, K. E.,297Hynes, A. J.,111, 146, 147, 149–151Ichkovich, A.,453Iedema, M. J.,453Iinuma, Y.,281, 366–368Ingham, T.,89, 143–145Jaeglé, L.,407, 495–497, 501, 502, 506Jayne, J. T.,327Jenkin, M. E.,311, 369, 371–374, 382, 384Johnson, C.,41Johnson, D., 382Jones, R. L.,165, 243, 244, 246, 247, 251Kalberer, M.,265Kärcher, B., 258King, M., 255, 511–514, 516Kleffmann, J.,195Knighton, B.,327Kolb, C. E., 145, 242, 248, 260,327, 375, 378–381Kovács, A., 140Ladstätter-Weißenmayer, A.,387Lamb, B. K.,327Laskin, A.,453Lee, J. D.,425Lewis, A. C.,311Lian, Y.,437Lloyd, S. A.,181McFiggans, G., 262,341, 363, 382–385, 508, 511, 513, 516McManus, J. B.,327Martin, R. V.,407Meyer-Arnek, J.,387Miao, Y.,281Mmereki, B. T.,227Monks, P. S., 129, 241, 244, 369, 371, 372, 379, 495, 502Nelson, D. D.,327Nyeki, S.,265Oh, M.,227Onasch, T. B.,327Orr-Ewing, A. J.,59, 133, 134, 136, 137Pandis, S. N.,297Pashkova, A.,437Paulsen, D.,265Pfrang, C., 493Pilling, M. J.,73,425Plane, J. M. C., 241, 246, 250, 251, 262, 365, 505, 511Platt, U.,153, 241–244, 248, 253Pope, F. D.,59Presto, A. A.,297Prevot, A. S. H.,265Price, B.,73Pyle, J. A.,27, 125, 126, 128, 131–133Ravishankara, A. R.,9, 125, 126, 128, 138, 139, 151, 253, 364, 366, 505, 512, 515Remedios, J. J., 131, 242, 491Richard, E. C.,181Richter, A.,387Robinson, A. L.,297Roscoe, H., 127, 509–511Rosenhorn, T.,297Rowley, D., 143Rudich, Y., 363–365, 385,453, 509, 512–515Sander, S. P.,89Sanderson, M.,41Sarkar, U., 244, 247, 496, 500Sax, M.,265Seakins, P. W.,73, 138, 139, 141, 377, 379Seisel, S.,437, 508–511, 516Self, D., 149Shallcross, D. E.,59, 128, 132,165, 244, 371, 380, 503Shorter, J. H.,327Sierau, B.,281Sinreich, R.,153Sjögren, S.,265Smith, C. A.,59Smith, G., 514Smith, I. W. M., 148, 251Sommariva, R.,425Stanier, C. O.,297Steinbacher, M.,265Steinberger, L.,407Stemmler, K.,195Stephenson, T., 145Stevenson, D.,41, 129–133, 247, 498, 501, 502, 506Strekowski, R. S.,195Stutz, J., 125, 242, 244Swartz, W. H.,181Taatjes, C. A., 137, 141, 142, 252Taraniuk, I.,453Thornberry, T.,211Topping, D.,341, 385Tuck, A. F., 125, 132,181, 248–252, 373, 517Ullerstam, M.,211Utembe, S. R.,311Wagner, T.,153Wang, L.,73, 135Wayne, R. P., 132, 144, 365, 370, 376, 511Weingartner, E.,265Wenger, J., 367, 373Williams, P.,341Wittrock, F.,387Worsnop, D. R.,327Zahniser, M. S.,327Zavala, M.,327Zellner, R., 127, 138, 139, 142, 259,437, 510, 512Zeng, G.,27Zenobi, R.,265Ziemann, P. J., 365, 367, 369,469, 513–516* The page numbers inboldtype indicate papers submitted for discussions.

ISSN:1359-6640    

DOI:10.1039/b506854c

出版商:RSC    

年代:2005

数据来源: RSC

ISSN:1359-6640    

DOI:10.1039/b506857f

出版商:RSC    

年代:2005

数据来源: RSC