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Abstract. A two-dimensional model of sulfate aerosols has been developed. The model includes the sulfate precursor species H2S, CS2, DMS, OCS, and S02. Microphysical processes simulated are homogeneous nucleation, condensation and evaporation, coagulation, and sedimentation. Tropospheric aerosols are removed by washout processes and by surface deposition. We assume that all aerosols are strictly binary water-sulfuric acid solutions without solid cores. The main source of condensation nuclei for the stratosphere is new particle formation by homogeneous nucleation in the upper tropical troposphere. A signficant finding is that the stratospheric aerosol mass may be strongly influenced by deep convection in the troposphere. This process, which could transport gas-phase sulfate precursors into the upper troposphere and lead to elevated levels of S02 there, could potentially double the stratospheric aerosol mass relative to that due to OCS photooxidation alone. Our model is successful at reproducing the magnitude of stratospheric aerosol loading following the Mount Pinatubo eruption, but the calculated rate of decay of aerosols from the stratosphere is faster than that derived from observations.

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Numerous absorption lines of stratospheric sulfur dioxide (SO2) have been identified in solar occultation spectra recorded by the Atmospheric Trace Molecule Spectroscopy (ATMOS) Fourier transform spectrometer during the Atmospheric Laboratory for Applications and Science (ATLAS)‐1 shuttle mission (March 24‐April 2, 1992). Based on their analysis, a volume mixing ratio profile of SO2 increasing from (13 ± 4) p.p.t.v. (parts per 10−12 by volume) at 16 mbar (∼ 28 km) to 455 ± 90 p.p.t.v. at 0.63 mbar (∼ 52 km) has been measured with no significant profile differences between 20°N and 60°S latitude. The increase in the SO2 mixing ratios with altitude indicates the presence of a source of SO2 in the upper stratosphere. Profiles retrieved from ATMOS spectra recorded during shuttle flights in April‐May 1985 and April 1993 show similar vertical distributions but lower concentrations. Two‐dimensional model calculations with SO2 assumed as the end product of H2SO4 photolysis produce SO2 profiles consistent with the ATMOS measurements to within about a factor of 2.

Water vapor concentrations obtained by the Stratospheric Aerosol and Gas Experiment II (SAGE II) and collocated temperatures provided by the National Meteorological Center from 1986 to 1990 are used to deduce seasonally and zonally averaged acidity, density, and refractive index of stratospheric aerosols. It is found that the weight percentage of sulfuric acid in the aerosols increases from about 60 just above the tropopause to about 86 at 35 km. The density increases from about 1.55 to 1.85 g cm -3 between the same altitude limits. Some seasonal variations of composition and density are evident at high latitudes. The refractive indices at 1.02, 0.694, and 0.532/am increase, respectively, from about 1.425, 1.430, and 1.435 just above the tropopause to about 1.445, 1.455, and 1.458 at altitudes above 27 km, depending on the season and latitude. The aerosol properties presented can be used in models to study the effectiveness of heterogeneous chemistry, the mass loading of stratospheric aerosols, and the extinction and backscatter of aerosols at different wavelengths. Computed aerosol surface areas, rate coefficients for the heterogeneous reaction C1ONO2 + H20--> HOC1 + HNO3 and aerosol mass concentrations before and after the Pinatubo eruption in June 1991 are shown as sample applications. 3727 1981. Yue, G. K., M.P. McCormick, and W. P. Chu, Retrieval of composition and size distribution of stratospheric aerosols with the SAGE II satellite experiment, J. Atmos. Oceanic Technol., 3, 372-380, 1986.

In this paper the authors present a method that can be used to infer stratospheric aerosol composition and size distribution, based on the water vapor concentration and aerosol extinction measurements from the satellite instrument of the Stratospheric Aerosol and Gas Experiment (SAGE II) and the associated temperatures provided by the National Meteorological Center (NMC). To infer the chemical composition, the aerosols are assumed to be sulfuric acid‐water droplets. The equilibrium acid weight percentage and refractive index at the SAGE II aerosol wavelengths can then be estimated by using the SAGE II‐observed water vapor and the associated temperatures. To infer the aerosol size distribution, a modified Levenberg‐Marquardt algorithm is adopted for determining model size distribution parameters in the least squares sense, based on the SAGE II‐observed multiwavelength aerosol extinctions. Both single‐mode lognormal and modified gamma representations are employed in the analysis. One of the most significant conclusions from this analysis is a determination of the information content of the SAGE II multiwavelength aerosol extinctions with respect to stratospheric aerosol size distributions. It is concluded that the best aerosol size information is contained in the aerosol radius range between approximately 0.25 and 0.80 μm.

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The properties of the l‐μm volume extinction coefficient of two geographically different high‐altitude cloud systems have been examined for the posteruption period (1985–1990) of the April 1982 El Chichón volcanic event with emphasis on the effect of volcanic aerosols on clouds. These two high‐altitude cloud systems are the tropical clouds in the tropopause region observed by the Stratospheric Aerosol and Gas Experiment (SAGE) II and the polar stratospheric clouds (PSCs) sighted by the Stratospheric Aerosol Measurement (SAM) II. The results indicate that volcanic aerosols alter the frequency distributions of these high‐altitude clouds in such a manner that the occurrence of clouds having high extinction coefficients (6×10−3 – 2×10−2 km−1) is suppressed, while that of clouds having low extinction coefficients (2×10−3 – 6×10−2 km−1) is enhanced. This influence of the volcanic aerosols appears to be opposite to the increase in the extinction coefficient of optically thick clouds observed by the Earth Radiation Budget Experiment (ERBE) during the initial posteruption period of the June 1991 Pinatubo eruption. A plausible explanation of this difference, based on the Mie theory, is presented. The Mie calculation indicates that there are two possible types of response of cloud extinction coefficient to changes in aerosol concentration depending on the primary effective radius (re) of cloud systems observed by the instrument. These two types of response are separated by the cloud particle effective radius of about 0.8 μm. When re is smaller than 0.8 μm, the cloud extinction coefficient decreases in response to increases of aerosol concentration, and when re is greater than 0.8 μm, the opposite happens. As a consequence, the effective radius of most, if not all, of the high‐altitude clouds, measured by the SAGE series of satellite instruments must be less than about 0.8 μm. This mean cloud particle size implied by the satellite extinction‐coefficient data at a single wavelength (1 μm) is further substantiated by the particle size analysis based on cloud extinction coefficient at two wavelengths (0.525 and 1.02 μm) obtained by the SAGE II observations. Most of the radiation measured by ERBE is reflected by cloud systems comprised of particles having effective radii much greater than 1 μm. A reduction in the effective radius of these clouds due to volcanic aerosols is expected to increase their extinction‐coefficient values, opposite the effect observed by SAGE II and SAM II. This work further illustrates the capability of the solar occultation satellite sensor to provide particulate extinction‐coefficient measurements important to the study of the aerosol‐cloud interactions. Finally, the June 1991 Mount Pinatubo major eruption put 3 times more material into the stratosphere than that of the 1982 El Chichón volcanic event. It is important to examine the variations of the extinction coefficient of these two high‐altitude cloud systems for the posteruption years of the Pinatubo volcanic event for further evidence of the impac...