The background aerosol size distribution in the free troposphere:: An analysis of the annual cycle at a high-alpine site

Measurements during free tropospheric (FT) and planetary boundary layer (PBL) conditions were conducted over an annual cycle at the Jungfraujoch high-Alpine research station (3454 m), Switzerland, in order to establish diurnal and seasonal cycles of the background continental aerosol over central Europe. Using a condensation nucleus counter (TSI 3025) and an optical particle counter (PMS Las-X) from June 1996 to May 1997, the following were determined: (1) accumulation mode lognormal parameters and (2) number concentrations for the nucleation (diameter d < 0.1 mu m), accumulation (0.1 less than or equal to d less than or equal to 1.0 mu m), and part of the coarse (1.0 < d less than or equal to 7.5 mu m, designated "coarse") modes. Lognormal parameters were found to be similar for FT and PBL conditions, and exhibited a weak seasonality in geometric median diameter d(GN) = 0.13 and 0.10 mu m, and standard deviation sigma(G) = 1.73 and 1.64 for summer and winter, respectively. Aerosol number concentrations in each mode exhibited a more pronounced seasonality, with FT concentrations being lower than those for PBL. Summer and winter FT median concentrations for the nucleation, accumulation, and "coarse" modes were 405 and 195 cm(-3), 114 and 26 cm(-3), and 0.052 and 0.014 cm(-3), respectively. These results provide tentative support of other long-term observations that the FT background aerosol mode appears to vary mainly in concentration rather than accumulation mode shape. Further analysis indicated that only the total concentration in each mode varied with weather type and a classification between that of a remote continental and polar aerosol model was found for the Jungfraujoch.