The objective of this research is to determine what fraction of the aerosols and gases deposited in snowfall is due to liquid cloud water removal by the riming mechanism, whether by snow crystal riming or by high mountain-surface riming. This study will focus on how and where the various trace constituents of the atmospheric aerosol are distributed among the three water substance components within the cloud: ice, liquid water and water vapor. The results will be interpreted using acquired knowledge of the microphysical properties of the cloud at the study site. Estimates of the microphysical conditions within which the ice particles grew will be made from observed meteorological conditions and the use of a simple ice crystal trajectory and growth model. Knowledge of the atmospheric conditions which occur and careful on-site characterization of the cloud droplets, ice particles and aerosol particles should permit a better understanding of what scavenging and growth processes lead to the final observed ice crystal chemical composition. Key questions of atmospheric chemistry pertain to the mechanisms by which airborne aerosol particles and gases are scavenged by clouds and rain, and deposited at the earth's surface. Under freezing conditions a significant fraction of these scavenged materials is deposited in rime ice, which is the ice that results from freezing of supercooled fog or cloud droplets either on preformed snow crystals or on objects at the earth's surface. The scavenging of aerosols and gases under snowing conditions is believed to occur predominantly by collection in liquid droplets and subsequent deposition in the rime, rather than in the snow crystals. Data on the relative importance of competing deposition mechanisms will contribute to the understanding of the global cycling of the chemical elements in nature. In particular, this knowledge should help to understand the conditions governing the deposition of acidic materials at high mountain elevations. The recognition of forest damage at the highest elevations has raised questions about what mechanisms cause these effects to be so sharply delimited in areal extent. A detailed understanding of atmospheric deposition processes should help to sort out the roles of acids, oxidants, or other airborne constituents.
