A theoretical study of the wet removal of atmospheric pollutants. Part II: The uptake and redistribution of (NH4)2SO4 particles and SO2 gas simultaneously scavenged by growing cloud drops

Abstract

A theoretical model has been formulated which allows the processes which control the wet deposition of atmospheric aerosol particles and pollutant gases to be included in cloud dynamic models. The cloud considered in the model was allowed to grow by condensation and collision-coalescence, to remove aerosol particles by nucleation and impaction scavenging, and to remove pollutant gases by convective diffusion. The model was tested by using a simple air-parcel model as the dynamic framework. In this form the model was used to determine the fate of ammonium sulfate [(NH4)2SO4] particles and sulfur dioxide (SO2) gas as they became scavenged by cloud and precipitation drops. Special emphasis was placed on determining 1) the evolution with time of the mass of total sulfur as S(IV) and S(VI) inside the drops, 2) the evolution with time of the acidity of the cloud water as a function of various oxidation rates and as a function of drop size, 3) the relative importance of sulfur scavenging from SO2 as compared to sulfur scavenging from (NH4)2SO4 particles, and 4) the effect of cloud drop evaporation on the aerosol particle size distribution in the air.