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 campaigns in April 1992 and in October–November 1993. Cloud water and bulk snow crystal samples were concurrently collected during precipitation events, and were chromatographically analyzed for inorganic ions. The extent of riming was determined from the collection of individual snow crystals on Formvar replicas. Relatively low degrees of riming were observed during all the events. The rimed mass fraction (RMF) was being used as a tool to determine the role of riming with respect to other scavenging 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 concentration of aerosol particles and the equivalent potential temperature during an event. When the site could be considered as a “closed system”, meaning that there was no air mass change, and that all the physical processes were occurring in the air mass being sampled at the site, then a strong positive correlation between the RMF and the concentration ratios, R, of snow water to cloud water was found. In contrast, a lack of statistical correlation between the RMF and the ratios R was observed 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 RMF and R is expected, provided that the cloud water samples are representative of the air mass in which the snow crystals are generated. Further modification of this scheme provided a relationship for the assessment of the importance of riming as transfer mechanism. The riming process was found to be responsible for 82 (±17) % of Cl-, 49 (±31) % of NO-3, 77 (±20) % of SO2-4, 82 (±16) % of Na+, 48 (±45) % of NH+4 and 76 (±19) % of Mg2+ transported from the cloud water in the ice phase. The cloud microphysics and the extent of the precipitation layer above the sampling site are considered to be the limiting factors for the contribution of riming to the incorporation of the inorganic atmospheric impurities into snow. The impact of the latter factors is also highlighted from the comparison of the observations at Jungfraujoch with those from lower altitude sampling sites.