摘要:

Total NOywas determined on the Pacific Exploratory Mission‐West A using two separate instruments, one operated by Georgia Institute of Technology and one by Nagoya University. The two data sets exhibited significant disagreement, with no systematic correlation, and differences often a factor of 2 or more. Additionally, regardless of data set chosen, the total NOywas much greater than the sum of its components measured separately. A panel was convened by NASA in July 1993 to examine these conflicting datasets and the question of “missing NOy” on this mision. This paper reports the conclusions and recommendations from the

ISSN:0148-0227    

DOI:10.1029/95JD02276

年代:1996

数据来源: WILEY

摘要:

On the basis of measurements taken during the NASA Global Tropospheric Experiment (GTE) Pacific Exploratory Mission‐West A (PEM‐West A), photostationary state model calculations were carried out for approximately 1300 three‐minute sample runs. The objective of this study was to look at a subset of this processed data to assess the level of agreement between observed ratios of NO2to NO and those estimated using current photochemical theory. This filtered data subset consisted of 562 NO2‐NO data pairs. The comparison between observations and predictions was based on the use of the photochemical test ratio (NO2)expt/(NO2)calc, designated here as Re/ Rc. Although the expected median value for this test ratio was unity, for the PEM‐West A data set it was found to be 3.36. The value of the ratio Re/Rcshowed a general trend of increasing magnitude with increasing altitude and decreasing latitude. Attempts to understand the sizable discrepancy between observation and prediction (especially for the high‐altitude and low‐latitude data) were explored in the context of two hypotheses: (1) incomplete model chemistry and (2) interferences in the measurement of NO2. Efforts to quantify the levels of HO2, CH3O2, RO2, and/or ClOxneeded to correct the Re/Rcdiscrepancy led to major inconsistencies in the predicted levels of other chemical species. Bromine and iodine chemistries were also investigated with results requiring Brxand Ixradical levels well in excess of what would seem reasonable given our current understanding of the source strengths for these elements. This suggests that incompleteness in the model's chemistry was unlikely the major cause of the discrepancy. The second hypothesis, involving interference in the measurement of NO2, now appears to be the most likely explanation for the largest component of the deviation in Re/Rcfrom unity. For example, the disagreement between (NO2)exptand (NO2)calcwas found to be a strong function of the NOx/NOyratio. Also, the magnitude of the discrepancy between (NO2)exptand (NO2)calcfell within the possible limits defined by other reactive nitrogen species (e.g., ΔNOy) available to generate the interference. These results suggest that the further development of a new direct measurement technique for NO2, involving a wall collision‐free inlet system, should be considered a high priority. We should also continue, however, to examine the chemical basis of current photochemical models to assess whether yet untested mechanisms might not provide an explanation for th

ISSN:0148-0227    

DOI:10.1029/95JD02201

年代:1996

数据来源: WILEY

摘要:

A three‐dimensional mesoscale transport/photochemical model is used to study the transport and photochemical transformation of trace species over eastern Asia and western Pacific for the period from September 20 to October 6, 1991, of the Pacific Exploratory Mission‐West A experiment. The influence of emissions from the continental boundary layer that was evident in the observed trace species distributions in the lower troposphere over the ocean is well simulated by the model. In the upper troposphere, species such as O3, NOy(total reactive nitrogen species), and SO2which have a significant source in the stratosphere are also simulated well in the model, suggesting that the upper tropospheric abundances of these species are strongly influenced by stratospheric fluxes and upper tropospheric sources. In the case of SO2the stratospheric flux is identified to be mostly from the Mount Pinatubo eruption. Concentrations in the upper troposphere for species such as CO and hydrocarbons, which are emitted in the continental boundary layer and have a sink in the troposphere, are significantly underestimated by the model. Two factors have been identified to contribute significantly to the underestimate: one is emissions upwind of the model domain (eastern Asia and western Pacific); the other is that vertical transport is underestimated in the model. Model results are also grouped by back trajectories to study the contrast between compositions of marine and continental air masses. The model‐calculated altitude profiles of trace species in continental and marine air masses are found to be qualitatively consistent with observations. However, the difference in the median values of trace species between continental air and marine air is about twice as large for the observed values as for model results. This suggests that the model underestimates the outflow fluxes of trace species from the Asian continent and the Pacific rim countries to the ocean. Observed altitude profiles for species like CO and hydrocarbons show a negative gradient in continental air and a positive gradient in marine air. A mechanism which may be responsible for the altitude gradients is pro

ISSN:0148-0227    

DOI:10.1029/95JD02277

年代:1996

数据来源: WILEY

摘要:

A useful application of the hydrocarbon measurements collected during the Pacific Exploratory Mission (PEM‐West A) is as markers or indices of atmospheric processing. Traditionally, ratios of particular hydrocarbons have been interpreted as photochemical indices, since much of the effect due to atmospheric transport is assumed to cancel by using ratios. However, an ever increasing body of observatonial and theoretical evidence suggests that turbulent mixing associated with atmospheric transport influences certain hydrocarbon ratios significantly. In this study a three‐dimensional mesoscale photochemical model is used to study the interaction of photochemistry and atmospheric mixing on select hydrocarbons. In terms of correlations and functional relationships between various alkanes, the model results and PEM‐West A hydrocarbon observations share many similar characteristics as well as explainable differences. When the three‐dimensional model is applied to inert tracers, hydrocarbon ratios andother relationships exactly follow those expected by simple dilution with model‐imposed “background air,” and the three‐dimensional results for reactive hydrocarbons are quite consistent with a combined influence of photochemistry and simple dilution. Analogous to these model results, relationships between various hydrocarbons collected during the PEM‐West A experiment appear to be consistent with this simplified picture of photochemistry and dilution affecting individual air masses. When hydrocarbons are chosen that have negligible contributions to clean background air, unambiguous determinations of the relative contributions to photochemistry and dilution can be estimated from the hydrocarbon samples. Both the three‐dimensional model results and the observations imply an average characteristic lifetime for dilution with background air roughly equivalent to the photochemical lifetime of butane for the western Pacific lower troposphere. Moreover, the dominance of OH as the primary photochemical oxidant downwind of anthropogenic source regions can be inferred from correlations between the highly reactive alkane ratios. By incorporating back‐trajectory information within the three‐dimensional model analysis, a correspondence between time and a particular hydrocarbon or hydrocarbon ratio can be determined, and the influence of atmospheric mixing or photochemistry can be quantified. Results of the three‐dimensional model study are compared and applied to the PEM‐West A hydrocarbon dataset, yielding a practical methodology for determining average OH concentrations and atmospheric mixing rates from the hydrocarbon measurements. Aircraft data taken below 2 km during wall flights east of Japan imply a diurnal average OH concentration of ∼3 × 106cm−3. The characteristic time for dilution with background air is estimated to be ∼2.5 days for the two

ISSN:0148-0227    

DOI:10.1029/95JD02733

年代:1996

数据来源: WILEY

摘要:

This study examines the influence of photochemical processes on ozone distributions in the western North Pacific. The analysis is based on data generated during NASA's western Pacific Exploratory Mission (PEM‐West A) during the fall of 1991. Ozone trends were best described in terms of two geographical domains: the western North Pacific rim (WNPR) and the western tropical North Pacific (WTNP). For both geographical regions, ozone photochemical destruction, D(O3), decreased more rapidly with altitude than did photochemical formation, F(O3). Thus the ozone tendency, P(O3), was typically found to be negative for z7 km) parcels influenced by deep convection/lightning. Significant negative P(O3) values were found when encountering clean marine BL air or relatively clean lower free‐tropospheric air. Photochemical destruction and formation fluxes for the Pacific rim region were found to exceed average values cited for marine dry deposition and stratospheric injection in the northern hemisphere by nearly a factor of 6. This region was also found to be in near balance with respect to column‐integrated O3photochemical production and destruction. By contrast, for the tropical regime column‐integrated O3showed photochemical destruction exceeding production by nearly 80%. Both transport of O3rich midlatitude air into the tropics as well as very high‐altitude (10–17 km) photochemical O3production were proposed as possible additional sources that might explain this estimated deficit. Results from this study further suggest that during the fall time period, deep convection over Asia and Malaysia/Indonesia provided a significant source of high‐altitude NOxto the western Pacific. Given that the high‐altitude NOxlifetime is estimated at between 3 and 9 days, one would predict that this source added significantly to high altitude photochemical O3formation over large areas of the western Pacific. When viewed in terms of strong seasonal westerly flow, its influence would potentially span a large

ISSN:0148-0227    

DOI:10.1029/95JD02755

年代:1996

数据来源: WILEY

摘要:

A new analysis of tropospheric iodine chemistry suggests that under certain conditions this chemistry could have a significant impact on the rate of destruction of tropospheric ozone. In addition, it suggests that modest shifts could result in the critical radical ratio HO2/OH. This analysis is based on the first ever observations of CH3I in the middle and upper free troposphere as recorded during the NASA Pacific Exploratory Mission in the western Pacific. Improved evaluations of several critical gas kinetic and photochemical rate coefficients have also been used. Three iodine source scenarios were explored in arriving at the above conclusions. These include: (1) the assumption that the release of CH3I from the marine environment was the only iodine source with boundary layer levels reflecting a low‐productivity source region, (2) same as scenario 1 but with an additional marine iodine source in the form of higher molecular weight iodocarbons, and (3) source scenario 2 but with the release of all iodocarbons occurring in a region of high biological productivity. Based on one‐dimensional model simulations, these three source scenarios resulted in estimated Ix(Ix= I + IO + HI + HOI + 2I2O2+ INOx) yields for the upper troposphere of 0.5, 1.5, and 7 parts per trillion by volume (pptv), respectively. Of these, only at the 1.5 and 7 pptv level were meaningful enhancements in O3destruction estimated. Total column O3destruction for these cases averaged 6 and 30%, respectively. At present we believe the 1.5 pptv Ixsource scenario to be more typical of the tropical marine environment; however, for specific regions of the Pacific (i.e., marine upwelling regions) and for specific seasons of the year, much higher levels might be experienced. Even so, significant uncertainties still remain in the proposed iodine chemistry. In particular, much uncertainty remains in the magnitude of the marine iodine source. In addition, several rate coefficients for gas phase processes need further investigating, as does the efficiency for removal of iodine due to aerosol scavenging proces

ISSN:0148-0227    

DOI:10.1029/95JD02727

年代:1996

数据来源: WILEY