A Theoretical Study of the Wet Removal of Atmospheric Pollutants. Part IV: The Uptake and Redistribution of Aerosol Particles through Nucleation and Impaction Scavenging by Growing Cloud Drops and Ice Particles

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Abstract
A theoretical model has been formulated which allows the study of the effects of an ice phase on the removal of atmospheric aerosol particles by nucleation and impaction scavenging in a convective cloud. This microphysical model—although in principle applicable to higher dimensional cloud dynamic models—was tested by using a simple parcel model with entrainment as the dynamic framework. The present model has been applied to and numerically evaluated for a convective cloud in which the cloud particles grow via vapor deposition, collision, and coalescence and riming. The computations were carried out for a rural-background aerosol of given particle size distribution. Two different chemical compositions of the aerosol particles and two different modes of ice initiation were considered. Our study shows for in-cloud scavenging (i) scavenging of aerosol particles by drop nucleation dominates impaction scavenging by drops as well as all other scavenging mechanisms; (ii) scavenging of aerosol particles by nucleation of snow crystals via drop freezing or any other nucleation mechanism dominates impaction scavenging by snow crystals; (iii) impaction scavenging of aerosol particles by snow crystals is, for the conditions studied, the least efficient scavenging mechanism; (iv) scavenging of aerosol particles by riming of graupel is an extremely efficient process due to the prominent scavenging of aerosol particles by nucleation of drops and the efficient uptake of drops by graupel; (v) the transfer of aerosol mass into the ice phase by riming of graupel and by freezing of drops dominates all other transfer mechanisms, (vi) inside mixed ice-water clouds the aerosol mass becomes redistributed in such a manner that the main aerosol mass is associated with the main graupel mass if riming is the dominant process of precipitation formation, and with the main water mass if collision and coalescence of drops in the dominant process of precipitation formation.

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