A Theoretical Study of the Wet Removal of Atmospheric Pollutants. Part III: The Uptake, Redistribution, and Deposition of (NH4)2SO4 Particles by a Convective Cloud Using a Two-Dimensional Cloud Dynamics Model

Abstract

Our model for the scavenging of aerosol particles has been coupled with the two-dimensional form of the convective cloud model of Clark and Collaborators. The combined model was then used to simulate a convective warm cloud for the meteorological situation which existed at 1100 LST 12 July 1985 over Hawaii; assuming an aerosol size distribution of maritime number concentration and of mixed composition with (NH4)2SO4 as the soluble compound. A shallow model cloud developed 26 min after the onset of convection leading to moderate rain which began after 45 min and ended after 60 min. Various parameters which characterize the dynamics and micophysics of the cloud, as well as the scavenging mechanism taking place inside and below the cloud were computed during the cloud development. The computation showed that: 1) the scavenged aerosol mass became redistributed inside the cloud water as the cloud grew, whereby the main aerosol mass scavenged always remained associated with the main water mass in the cloud; 2) in-cloud scavenging of aerosol particles was mainly controlled by nucleation while impaction scavenging played a negligible role; 3) below-cloud scavenging, which is caused by impaction scavenging, contributed only 5% to the overall particle scavenging and contributed about 40% to the aerosol mass in the rain on the ground; and 4) the sulfur concentrations inside the rain water were found to be reasonable as compared to observations available in literature, considering that the present model does not yet include the effects of SO2 scavenging.