Effects of anthropogenic sulfate on cloud drop nucleation and optical properties

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Abstract
The characteristics of the cloud drop size distribution near cloud base are initially determined by the aerosol particles that serve as cloud condensation nuclei (CCN) and the updraft velocity. Changes in CCN concentrations can change cloud drop number thereby affecting cloud optical properties and the global radiation budget. The CCN concentration depends on the composition and size distribution of the aerosol particles. Chemical reactions of the emitted gaseous sulfur compounds due to human activities will alter, through gas-to-particle conversion, the aerosol size distribution, total number, and its chemical composition. It is important to assess these changes in order to estimate the effect of anthropogenic sulfur emissions on cloud drop number concentrations. Here, we assume that the aerosol size distribution is modified, with total number unchanged, by two processes: condensation of sulfuric acid vapor (H2SO4) on a prescribed pre-existing particle size distribution and aqueous-phase oxidation of SO2 followed by evaporation of the drops. We examine the relationship between the resulting anthropogenic sulfate-containing aerosol size distribution and cloud drop number concentrations. The results are used to estimate the possible change in cloud optical thickness and cloud albedo due to an increase of anthropogenic sulfate mass concentration. This work is aimed at improving the assessment of the effects of anthropogenic sulfate on cloud optical properties and the global radiation budget. DOI: 10.1034/j.1600-0889.47.issue5.4.x