A STUDY OF AN OUTSTANDING SAHARAN DUST EVENT AT THE HIGH-ALPINE SITE JUNGFRAUJOCH, SWITZERLAND

From 20 until 23 March 1990 an outstanding Saharan dust episode was observed at the high-alpine site Jungfraujoch (3450 m a.s.l.), Switzerland. Detailed meteorological analyses including back trajectories showed a transport of the dust-loaded airmass from the Sahara desert over the Eastern Atlantic to Central Europe. During this episode, in situ studies of the aerosol revealed significantly different properties from the background conditions usually observed at the remote site Jungfraujoch. An increase in the coarse particle number concentration was accompanied by a depletion in the ultrafine particle concentration. This unusual absence of ultrafine particles during the first 1.5 days of the episode could be explained by coagulation processes in the Saharan air mass. The distinct coarse particle mode was reflected in the total mass concentration which was higher by a factor of 30 than the annual average at Jungfraujoch. The mineralogical and elemental composition of the dust point to a source region in North and West Africa which was corroborated by the trajectory calculations. The chemical composition of aerosol particles was characterized by high concentrations of soil and sea spray tracers. A snowfall event on 20 March showed ''Saharan dust characteristics'' and offered the unique possibility to study in situ the scavenging phenomena. Scavenging ratios were enhanced for most components-particularly for NH4+ and SO42- - during this event compared to typical values found for snowfall events without Saharan dust. This was most probably due to heavy riming. This single event contributed substantially to the annual Ca2+ and carbonate deposition at this site and could neutralize 50% of the 1991/1992 winter and spring deposition of NO3- and SO42-.