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 area of the aerosol particles, was operated with a time resolution of 30 min. High correlation coefficients (r>0.8) were found between the epiphaniometer signal and other aerosol parameters, which could be attributed to a rather constant size distribution of the Jungfraujoch aerosol in the accumulation range (0.1m). Well-defined diurnal variations with a peak in the late afternoon were observed on many days during summer, which was not the case during winter. Comparison with black carbon and radon daughter measurements revealed that these diurnal 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 temperature profile indicated a stable atmosphere. First simulations with ALPTHERM, a new convection model which takes topography into account, were able to explain the observed aerosol patterns. This indicates that 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.¿ 1997 American Geophysical Union