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 ob served at the high-alpine site Jungfraujoch (3450 m a.s.l.), Switzerla nd. Detailed meteorological analyses including back trajectories showe d a transport of the dust-loaded airmass from the Sahara desert over t he Eastern Atlantic to Central Europe. During this episode, in situ st udies of the aerosol revealed significantly different properties from the background conditions usually observed at the remote site Jungfrau joch. An increase in the coarse particle number concentration was acco mpanied by a depletion in the ultrafine particle concentration. This u nusual absence of ultrafine particles during the first 1.5 days of the episode could be explained by coagulation processes in the Saharan ai r mass. The distinct coarse particle mode was reflected in the total m ass concentration which was higher by a factor of 30 than the annual a verage 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 d ust characteristics'' and offered the unique possibility to study in s itu the scavenging phenomena. Scavenging ratios were enhanced for most components-particularly for NH4+ and SO42- - during this event compar ed to typical values found for snowfall events without Saharan dust. T his was most probably due to heavy riming. This single event contribut ed substantially to the annual Ca2+ and carbonate deposition at this s ite and could neutralize 50% of the 1991/1992 winter and spring deposi tion of NO3- and SO42-.