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

Abstract

A theoretical model is formulated which allows the processes that control the wet deposition of atmospheric pollutants to be included in cloud dynamic models. The model considers the condensation process and the collision-coalescence process which, coupled together, control the fate of atmospheric aerosol particles removed by clouds and precipitation through nucleation scavenging and impaction scavenging. The model was tested by substituting a simple parcel model for the dynamic framework. In this form the model was used to determine the time evolution of the aerosol particle mass scavenged by drops as well as the aerosol particle mass left unactivated in air as ?drop-interstitial? aerosol. In the present computation all aerosol particles are assumed to have the same composition. Our study shows for inside cloud scavenging: 1) collision and coalescencence causes among the various drop size categories a redistribution of the scavenged aerosol particles in such a manner that the main aerosol particle mass is always associated with the main water mass, thus ensuring that if a cloud reaches the precipitation stage it will also return to the ground the main aerosol particle mass scavenged by the cloud; 2) although the main aerosol particle mass is contained in the large drops, the mass mixing ratio of the captured aerosol in the cloud water is larger inside smaller drops than inside larger drops, implying that smaller drops are more contaminated than larger ones; 3) through nucleation scavenging the total number concentration of aerosol particles is predicted to become reduced by 48 to 94% depending on the composition of the particles, the reduction being mainly confined to aerosol particles larger than 0.1 ?m in radius. This implies that a drop interstitial aerosol exists that consists of a particle population reduced in number concentration by up to 94% and reduced in mass by several orders of magnitude, as compared to the particle concentration outside the cloud. 4) Although the aerosol particle mass scavenged by impaction scavenging cannot completely be neglected in accounting for the total amount of aerosol particle mass scavenged by clouded it is smaller by several orders of magnitude than the aerosol particle mass removed by nucleation scavenging.