AEROSOL CLIMATOLOGY AT THE HIGH-ALPINE SITE JUNGFRAUJOCH, SWITZERLAND

Continuous aerosol measurements have been performed at the high-alpine site Jungfraujoch (3450 m above sea level) since 1988 by means of an epiphaniometer. The instrument, which determines the Fuchs surface are a of the aerosol particles, was operated with a time resolution of 30 min. High correlation coefficients (r > 0.8) were found between the ep iphaniometer signal and other aerosol parameters, which could be attri buted to a rather constant size distribution of the Jungfraujoch aeros ol in the accumulation range (0.1 < d < 1 mu m). Well-defined diurnal variations with a peak in the late afternoon were observed on many day s during summer, which was not the case during winter. Comparison with black carbon and radon daughter measurements revealed that these diur nal variations are due to vertical transport processes. A statistical analysis showed that the fraction of days with a well-defined diurnal pattern increased with decreasing stability of the atmosphere; however , late afternoon peaks also occurred during days when the potential te mperature profile indicated a stable atmosphere. First simulations wit h ALPTHERM, a new convection model which takes topography into account , were able to explain the observed aerosol patterns. This indicates t hat slope winds over a certain catchment area are responsible for the transport to this high-elevation site. The distinct seasonal variation with summer values, which are about a factor of 10 higher than winter values, could therefore be attributed to seasonally varying transport processes, due to the seasonal variation of radiation. The data show that even sites at very high elevation cannot be assumed to be in the free troposphere all the time.