摘要:

Summit‐Greenland firn and ice core sections have been analyzed to determine the fluoride background level of high northern latitude precipitation and to investigate its temporal variations over various time periods. A continuous profile covering the last century and a discontinuous study of the last 27,000 years spanning the entire Holocene time period (0 to 11,500 years B.P.) and the late part of the last glaciation have been achieved. This first detailed study of the fluoride content of polar ice help us obtain a better understanding of the atmospheric cycle of inorganic fluoride in remote atmospheres. First, our data suggest that blown dust is among the nonvolcanic sources that contributed to the natural fluoride background level of this high‐latitude precipitation. Primary sea‐salt emissions represent an insignificant contribution. Our study also indicates that high‐latitude biomass burning debris which sometimes reaches the Greenland ice cap represents an additional weak source of fluoride for Greenland precipitation. Second, this work provides useful information for evaluating the impact of the volcanic activity on the fluoride budget of these high northern latitude regions. Indeed, our data demonstrate that although the natural background fluoride content of Greenland snow is very often sporadically disturbed by volcanic emissions from Iceland (numerous Hekla eruptions and the 1783 Laki eruption, for instance), located relatively close to the Greenland ice cap, large explosive eruptions (Tambora in 1815, for instance) only weakly disturb this background level. Our data suggest that even in the case of significant input within the stratosphere, fluoride is rapidly scavenged probably along with ash particles. Finally, our study of recent snow layers suggests that man‐made sources (mainly coal burning) represent an important contribution which has dominated this high northern hemisphere fluoride budget for the three last decades. Furthermore, over the last 10 years, a possible influence of the stratospheric reservoir builded up from the chlorofluorocarbons' degradation cannot be ruled out and in the future this growing contribution may represent a significant part of the anthropogenic fluoride input compared to the input derived from coa

ISSN:0148-0227    

DOI:10.1029/93JD02660

年代:1994

数据来源: WILEY

摘要:

An interactive two‐dimensional model of the troposphere, stratosphere, and mesosphere, in which dynamics, radiation, and chemistry are treated interactively, is used to investigate the anthropogenic changes in the steady state chemical composition of the atmosphere since preindustrial times and to assess the associated changes in radiative forcing on climate. The perturbations in the atmospheric oxidation capacity due to anthropogenic emissions of source gases are found to be significant. In the troposphere, an ozone increase of 80–120% at northern midlatitudes and a global decrease of 10–20% in the OH concentration since the preindustrial period are calculated. In the polar lower stratosphere of the southern hemisphere, an ozone depletion since preindustrial times reaching more than 60% during spring is calculated as a result of rapid catalytical destruction of ozone by chlorine radicals in the presence of polar stratospheric clouds. Particular attention is given to the induced changes in radiative forcing. These results stress the potentially important role of chemical feedbacks on climate and indicate that the direct forcing associated with increasing concentrations of greenhouse gases is enhanced by about 30% when these feedbacks are taken into account. On a global average basis, the greenhouse effect of tropospheric ozone represents approximately 17% of the total radiative perturbation. This forcing is characterized by a strong latitudinal dependence, peaking at midlatitudes in the northern hemisphere. The importance of indirect climate forcings by stratospheric ozone (including local cooling of the stratosphere) is confirmed. It is found that the net (solar + infrared) indirect effect of stratospheric ozone changes is to increase the chlorofluorocarbon direct radiative forcing. On the other hand, the change in the longwave forcing associated with water vapor increase in the stratosphere appears to play a minor

ISSN:0148-0227    

DOI:10.1029/93JD02987

年代:1994

数据来源: WILEY

摘要:

This paper presents a comparison of the water vapor distribution obtained from two general circulation models, the European Centre for Medium‐Range Weather Forecasts (ECMWF) model and the National Center for Atmospheric Reseach (NCAR) Community Climate Model (CCM), with satellite observations of total precipitable water (TPW) from SSM/I and upper tropospheric relative humidity (UTH) from GOES. Overall, both models are successful in capturing the primary features of the observed water vapor distribution and its seasonal variation. For the ECMWF model, however, a systematic moist bias in TPW is noted over well‐known stratocumulus regions in the eastern subtropical oceans. Comparison with radiosonde profiles suggests that this problem is attributable to difficulties in modeling the shallowness of the boundary layer and large vertical water vapor gradients which characterize these regions. In comparison, the CCM is more successful in capturing the low values of TPW in the stratocumulus regions, although it tends to exhibit a dry bias over the eastern half of the subtropical oceans and a corresponding moist bias in the western half. The CCM also significantly overestimates the daily variability of the moisture fields in convective regions, suggesting a problem in simulating the temporal nature of moisture transport by deep convection. Comparison of the monthly mean UTH distribution indicates generally larger discrepancies than were noted for TPW owing to the greater influence of large‐scale dynamical processes in determining the distribution of UTH. In particular, the ECMWF model exhibits a distinct dry bias along the ITCZ and a moist bias over the subtropical descending branches of the Hadley cell, suggesting an underprediction in the strength of the Hadley circulation. The CCM, on the other hand, demonstrates greater discrepancies in UTH than are observed for the ECMWF model, but none that are as clearly correlated with well‐known features of the large‐scale ci

ISSN:0148-0227    

DOI:10.1029/93JD02912

年代:1994

数据来源: WILEY

摘要:

Photolysis of vibrationally excited oxygen produced by ultraviolet photolysis of ozone in the upper stratosphere is incorporated into the Lawrence Livermore National Laboratory two‐dimensional zonally averaged chemical‐radiative‐transport model of the troposphere and stratosphere. The importance of this potential contributor of odd oxygen to the concentration of ozone is evaluated based on recent information on vibrational distributions of excited oxygen and on preliminary studies of energy transfer from the excited oxygen. When energy transfer rate constants similar to those of Toumi et al.(1991) are assumed, increases in model ozone concentrations of up to 4.0% in the upper stratosphere are found, and the model ozone concentrations are found to agree slightly better with measurements, including recent data from the Upper Atmosphere Research Satellite. However, the ozone increase is only 0.3% when the larger energy transfer rate constants indicated by recent experimental work are applied to the model. An ozone increase of 1% at 50 km requires energy transfer rate constants one‐twentieth those of the preliminary observations. As a result, vibrationally excited oxygen processes probably do not contribute enough ozone to be significant in models of the upper strat

ISSN:0148-0227    

DOI:10.1029/93JD02549

年代:1994

数据来源: WILEY

摘要:

OH multipass absorption is an absolute and spatially resolving method for the in situ monitoring of tropospheric OH. The basis of the method is differential optical absorption spectroscopy. The absorption path in ambient air is folded within a 6 m White cell type multiple reflection system with an open path setup. SO2, CH2O, and naphthalene are measured at 308 nm simultaneously with OH by means of the first harmonic of the laser radiation. NO2can be measured using the fundamental at 616 nm, respectively. The detection limit of the reported instrument is 2–5×105OH cm−3corresponding to a signal to noise ratio of better than one or two for a 1‐min monitoring time dependent on the boundary conditions. The performance of the method is demonstrated by tropospheric OH measurements achieved during a field campaign at the Taunus Observatory on the Kleiner Feldberg in the vicinity of Frankfur

ISSN:0148-0227    

DOI:10.1029/93JD02859

年代:1994

数据来源: WILEY

摘要:

Sulfate and methanesulfonic acid (MSA) concentrations in aerosol, surface snow, and snowpit samples have been measured at two sites on the Greenland Ice Sheet. Seasonal variations of the concentrations observed for these chemical species in the atmosphere are reproduced in the surface snow and preserved in the snowpit sequence. The amplitude of the variations over a year are smaller in the snow than in the air, but the ratios of the concentrations are comparable. The seasonal variations for sulfate are different at the altitude of the Ice Sheet compared to those observed at sea level, with low concentrations in winter and short episodes of elevated concentrations in spring. In contrast, the variations in concentrations of MSA are similar to those measured at sea level, with a first sequence of elevated concentrations in spring and another one during summer, and a winter low resulting from low biogenic production. The ratio MSA/sulfate clearly indicates the influence of high‐latitude sources for the summer maximum of MSA, but the large impact of anthropogenic sulfate precludes any conclusion for the spring maximum. The seasonal pattern observed for these species in a snowpit sampled according to stratigraphy indicates a deficit in the accumulation of winter snow at the summit of the Greenland Ice Sheet, in agreement with some direct observations. A deeper snowpit covering the years 1985–1992 indicates the consistency of the seasonal pattern for MSA over the years, which may be linked to transport and deposition proces

ISSN:0148-0227    

DOI:10.1029/93JD02913

年代:1994

数据来源: WILEY

摘要:

The heterogeneous chemistry of trichloroacetyl chloride was studied using two different techniques allowing direct measurements of the uptake rate by water droplets and of the homogeneous aqueous phase hydrolysis rate. The first technique used was the droplet train technique for the study of the gas/liquid interaction which yielded uptake coefficients lower than 10−3and estimates of the Henry's law constant H of 2 M atm−1and of the hydrolysis rate constant khof 500 s−1forpH<3.5. The second technique used was a stopped‐flow method for the homogeneous aqueous phase study. The results led to a hydrolysis rate constant of 150 s−1in pure water forpH between 4.8 and 5.8 and strongly catalyzed by hydrogen and hydroxide ions. The results show that trichloroacetyl chloride can react readily with water leading to short residence time in the a

ISSN:0148-0227    

DOI:10.1029/93JD02915

年代:1994

数据来源: WILEY

摘要:

Large amounts of SO2injected into the stratosphere by the El Chichón volcanic eruption greatly enhanced the sulfuric acid aerosol loading. According to laboratory studies, the increase in aerosol surface area would lead to a loss of ozone. Radiation measurements from the solar backscatter ultraviolet spectroradiometer are used to probe the absorbing and scattering properties of the stratosphere after the eruption. The backscattered radiation is enhanced for wavelengths greater than 290 nm, with a peak at about 302 nm. The enhancements associated with aerosol backscattering and ozone change can be separated. A decrease in column ozone of 8 to 30 Dobson units was deduced within the aerosol layer at an altitude between 20 and 30 km following the month of the eruption

ISSN:0148-0227    

DOI:10.1029/93JD02661

年代:1994

数据来源: WILEY

摘要:

It has been suggested that the zonal‐mean stratospheric zonal‐wind anomalies can propagate downward into the troposphere, producing considerable changes. On the other hand, volcanic aerosols are thought to produce a stronger polar vortex in the winter stratosphere. If these suggestions are correct, the influence of volcanic eruptions can be expected to produce changes in the northern hemisphere winter troposphere through stratospheric process. In the present study, this idea is examined by analyzing changes in the January circulation after three recent volcanic eruptions. Regional features of observed changes in tropospheric temperatures due to volcanic eruptions can be explained by the changes in the propagation of stationary waves associated with dipole‐type anomalies in the zonal‐mean zonal winds, which are similar to those observed in a previous work on the influence of polar ni

ISSN:0148-0227    

DOI:10.1029/93JD02731

年代:1994

数据来源: WILEY

摘要:

We present vertical and horizontal wave number spectra of density perturbations in the upper stratosphere (25–40 km) and the upper mesosphere (∼80–105 km) inferred from ∼45 hours of airborne Na/Rayleigh lidar observations in the vicinity of Hawaii. The mean density variance is (0.92%)2for the upper stratosphere and (6.1%)2and for the upper mesosphere. The mean vertical shear variance and Richardson's number for the upper mesosphere are (6.1%/km)2and 0.69, respectively. The observed vertical wave numbermand horizontal wave numberkspectra exhibit power law dependencies. The mean slopes of the vertical wave number spectra are −2.5 for the upper stratosphere and −3.1 for the upper mesosphere. The mean slope of the horizontal wave number spectra is −1.9 for the upper mesosphere. The mean characteristic vertical and horizontal wavelengths inferred from the spectra are 13.4 and 5810 km, respectively. In all cases the magnitudes of the upper stratospheremspectra are more than an order of magnitude smaller than those of the upper mesosphere at all observable scales. In the stratosphere themspectra exhibit significant energy at low wave numbers that are less than the values expected for the characteristic wave numbers. This suggests the presence of gravity wave sources capable of generating significant energy at long vertical scales. The source is believed to be geostrophic adjustment of the jet stream. A large‐amplitude, highly coherent, quasi‐monochromatic, short horizontal scale (λh∼ 16 km) oscillation in the Na layer was observed on the March 22 flight. The oscillation resembled short horizontal scale structures often observed in OH airglow images and was found to be consistent with a gravity

ISSN:0148-0227    

DOI:10.1029/93JD02927

年代:1994

数据来源: WILEY