A Theoretical Study of the Wet Removal of Atmospheric Pollutants. Part V: The Uptake, Redistribution, and Deposition of (NM4)4SO4 by a Convective Cloud Containing Ice

Abstract

The effects of an ice phase on the wet deposition of aerosol particles was studied by means of the authors? 2D cloud dynamics model with spectral microphysics applied to the Cooperative Convective Precipitation Experiment in Miles City, Montana, on 19 July 1981. The cloud macrostructure as well as the cloud microstructure simulated by the model was found to agree well with observations. Although no on-site observations were available with respect to the chemical composition of the cloud and rain water, the values predicted by the model compared well with typical nearby measurements. The following conclusions can be derived from the model computations: (1) In confirmation of the authors' previous findings, derived from a parcel model, it was found that inside mixed ice-water clouds the aerosol mass becomes redistributed in such a way that the main aerosol mass is always associated with the main water or ice mass. (2) Since riming was the dominant growth mechanism of the hydrometeors in the cloud considered, the main aerosol mass?originally associated with the cloud drops via nucleation scavenging?became part of the graupel by riming. (3) In confirmation of earlier results for ?warm? clouds, the scavenging efficiency of the cloud was found to be given within a few percent by the precipitation efficiency of the cloud system. (4) By purposely inhibiting ice nucleation but otherwise keeping all dynamic, thermodynamic, and microphysical input parameters the same, it could be shown that the changes in the microphysical structure of the cloud, which significantly altered both the time rainfall began and the rainfall duration, also significantly altered the wet deposition of chemical species. A careful consideration of the ice phase in cloud chemical modeling is therefore required.