A theoretical investigation of sulfate formation in clouds

Abstract The relative importance of chemical pathways that govern the atmospheric oxidation of SO2 to sulfate is investigated by means of a mathematical model of cloud chemistry and transport. Mixing layer under clear-sky conditions, non-raining cumulus and stratus clouds, and raining cumulus and stratus clouds are simulated for summer daytime conditions. Gas-phase oxidation of SO2 predominates in the absence of clouds. In non-raining clouds, aqueous oxidation of SO2 by H2O2 dominates, although gas-phase SO2 oxidation also contributes notably to sulfate formation. In raining clouds, aqueous oxidation of SO2 by H2O2 is initially the primary pathway for sulfate formation, but subsequently the high pH values, caused by the wet removal of pollutants, favor the aqueous oxidation of SO2 by O3. The contributions of other SO2 oxidation pathways are relatively small for the conditions considered. For the scenarios simulated in this study, turbulent diffusion does not appear to significantly affect the vertical distribution of pollutant concentrations in clouds.

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