Scavenging of atmospheric constituents in mixed phase clouds at the high-alpine site Jungfraujoch. Part III : Quantification of the removal of chemical species by precipitating snow

Abstract In order to study scavenging processes of chemical species in mixed phase clouds, in-cloud field experiments were conducted in October and November 1993 at the high-alpine research station Jung-fraujoch, Switzerland (3450 m asl). During eight snowfall events, interstitial aerosol particles and cloud droplets were collected by a filter pack technique. Simultaneously, snow samples were obtained for subsequent chemical analysis of various trace species like, e.g. NO - 3 and SO 2- 4 . In addition, the ice water content was measured. From these measurements, air volume based mass balances between the concentration of a chemical species in the aerosol and cloud phase on the one hand, and the ice phase on the other hand, were calculated. This is a new approach to directly link the different phases within the same air parcel. From the mass balances, apparent ice phase scavenging efficiencies for the trace species were determined. They varied from event to event, but were on the average rather low with the highest value of 0.43 for nitrate. Thus, most of the air-borne chemical mass remained in the air parcel after the precipitation event. A simple model was applied to estimate the amount of air processed by a cloud resulting in an imaginary “column” of air of about 6×10 3 m 3 needed as supply for the water and chemical trace species deposited per square metre per hour.