A theoretical investigation of sulfate formation in clouds

Abstract

The relative importance of chemical pathways that govern the atmospheric oxidation of SO 2 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 SO 2 predominates in the absence of clouds. In non-raining clouds, aqueous oxidation of SO 2 by H 2O 2 dominates, although gas-phase SO 2 oxidation also contributes notably to sulfate formation. In raining clouds, aqueous oxidation of SO 2 by H 2O 2 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 SO 2 by O 3. The contributions of other SO 2 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.