SCAVENGING OF ATMOSPHERIC CONSTITUENTS IN MIXED-PHASE CLOUDS AT THE HIGH-ALPINE SITE JUNGFRAUJOCH - PART II - INFLUENCE OF RIMING ON THE SCAVENGING OF PARTICULATE AND GASEOUS CHEMICAL-SPECIES

The importance of riming as transfer mechanism of pollutants from the atmosphere into the snow in mixed phase clouds was investigated at the Jungfraujoch (3450 m a.s.l), during two intensive measurement campaig ns in April 1992 and in October-November 1993. Cloud water and bulk sn ow crystal samples were concurrently collected during precipitation ev ents, and were chromatographically analyzed for inorganic ions. The ex tent of riming was determined from the collection of individual snow c rystals on Formvar replicas. Relatively low degrees of riming were obs erved during all the events. The rimed mass fraction (RMF) was being u sed as a tool to determine the role of riming with respect to other sc avenging processes. No significant difference between the early-spring and the early-winter events was found. The precipitation events were classified on the basis of the temporal changes of the number concentr ation of aerosol particles and the equivalent potential temperature du ring an event. When the site could be considered as a ''closed system' ', meaning that there was no air mass change, and that all the physica l processes were occurring in the air mass being sampled at the site, then a strong positive correlation between the RMF and the concentrati on ratios, R, of snow water to cloud water was found. In contrast, a l ack of statistical correlation between the RMF and the ratios R was ob served when the sampling site was an ''open system'', meaning that it was affected by the entrainment of a new air mass. The application of a simple physical scheme revealed that a linear relationship between R MF and R is expected, provided that the cloud water samples are repres entative of the air mass in which the snow crystals are generated. Fur ther modification of this scheme provided a relationship for the asses sment of the importance of riming as transfer mechanism. The riming pr ocess was found to be responsible for 82 (+/- 17) % of Cl-, 49 (+/- 31 ) % of NO3-, 77 (+/- 20) % of SO42-, 82 (+/- 16) % of Na+, 48 (+/- 45) % of NH4+ and 76 (+/- 19) % of Mg2+ transported from the cloud water in the ice phase. The cloud microphysics and the extent of the precipi tation layer above the sampling site are considered to be the limiting factors for the contribution of riming to the incorporation of the in organic atmospheric impurities into snow. The impact of the latter fac tors is also highlighted from the comparison of the observations at Ju ngfraujoch with those from lower altitude sampling sites. (C) 1998 Els evier Science Ltd. All rights reserved.