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

In order to study scavenging processes of chemical species in mixed ph ase clouds, in-cloud field experiments were conducted in October and N ovember 1993 at the high-alpine research station Jungfraujoch, Switzer land (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 a nalysis of various trace species like, e.g. NO3- and SO42-. In additio n, the ice water content was measured. From these measurements, air vo lume based mass balances between the concentration of a chemical speci es in the aerosol and cloud phase on the one hand, and the ice phase o n the other hand, were calculated. This is a new approach to directly link the different phases within the same air parcel. From the mass ba lances, apparent ice phase scavenging efficiencies for the trace speci es were determined. They varied From event to event, but were on the a verage rather low with the highest value of 0.43 for nitrate. Thus, mo st of the air-borne chemical mass remained in the air parcel after the precipitation event. A simple model was applied to estimate the amoun t of air processed by a cloud resulting in an imaginary ''column'' of air of about 6 x 10(3) m(3) needed as supply for the water and chemica l trace species deposited per square metre per hour. (C) 1998 Elsevier Science Ltd. All rights reserved.