A climatology of 7Be at four high-altitude stations at the Alps and the Northern Apennines

The Be-7 activity concentrations measured from 1996 to 1998 at four high-altitude stations, Jungfraujoch-Switzerland, Zugspitze-Germany, Sonnblick-Austria and Mt. Cimone-Italy, were analyzed in combination with a set of, meteorological and atmospheric parameters such as the tropopause height, relative and specific humidity and also in conjunction with 3D back-trajectories in order to investigate the climatological features of Be-7. A frequency distribution analysis on Be-7 activity concentrations revealed the existence of two concentration classes around 1.5 and 6 mBq m(-3) and a transition class between the two modes of the distribution at 3-4 mBq m(-3). Cross-correlation analysis performed between Be-7 and a number of meteorological and atmospheric parameters at the first three stations showed a strong negative correlation with relative humidity (-0.56, -0.51, -0.41) indicating the importance of wet scavenging as a controlling mechanism. Also, the positive correlation with the height of 3-days back-trajectories and tropopause height ( + 0.49/ + 0.43, + 0.59/ + 0.36, + 0.44/ + 0.38) shows that downward transport from the upper or middle to lower troposphere within anticyclonic conditions plays also an important role. Trajectory statistics showed that low Be-7 concentrations typically originate from lower-altitude subtropical ocean areas, while high concentrations arrive from the north and high altitudes, as is characteristic for stratospheric intrusions. Although the Be-7 activity concentrations are highly episodic, the monthly means indicate an annual cycle with a late-summer maximum at all stations. The correlation coefficients calculated for monthly means of the Be-7 and atmospheric data suggest that the main predictor controlling the seasonality of the Be-7 concentrations is tropopause height ( + 0.76, + 0.56, + 0.60), reflecting more vertical transport from upper tropospheric levels into the lower troposphere during the warm season than during the cold season. (C) 2001 Elsevier Science Ltd. All rights reserved.