Factors and processes controlling the concentration of the cosmogenic radionuclide ~7Be at high-altitude Alpine stations

摘要

Three years (1996-1998) of activity measurements of the cosmogenic radionuclide ~7Be at three high-altitude Alpine stations -Jungfraujoch, Switzerland; Zugspitze, Germany; and Sonnblick, Austria - were analysed in combination with a set of meteorological and atmospheric parameters, in order to investigate the factors and processes that control the concentration levels and the variability of ~7Be. The ~7Be monthly means indicate an annual cycle with a late-summer maximum, mainly driven by tropopause height as revealed by the high correlation coefficients calculated between monthly means of ~7Be and tropopause height (+0.76, +0.56, +0.60), reflecting more vertical transport from upper tropospheric levels into the lower troposphere during the warm season. Cross-correlation analysis performed between ~7Be and the meteorological and atmospheric parameters for the whole period and for each month separately, showed a strong negative correlation with relative humidity and a strong positive correlation with tropopause height, indicating that wet scavenging and downward transport from the upper/middle to lower troposphere within anticyclonic conditions are the main controlling mechanisms throughout the year. Correlation coefficients between ~7Be and tropopause height for different classes of relative humidity showed that the correlation is significant mainly for high relative humidity conditions and that during intense downward transport conditions the correlation is partly destroyed, indicating the connection of tropopause folding events, upper-level troughs or cut-off lows with STE events. Superposed-epoch analysis on days with low relative humidity, as is indicative for stratospheric intrusions, reveals an increase of ~7Be to 10-12 mBq m~(-3) and a synoptic pattern of low tropopause height 2 to 3 days before.