Factors controlling beryllium-7 at Jungfraujoch in Switzerland

Be-7 activity concentrations were measured at the alpine research station at Jungfraujoch (JFJ), Switzerland, (07 degrees 59'E/46 degrees 32'N/3580 m asl) from 4 April 1996 to 1 January 1997 with a time resolution of 2 days using gamma spectroscopy. The data were analysed in relation to meteorological parameters (potential temperature at tropopause level, relative humidity, potential temperature at JFJ), and total ozone at Arosa in eastern Switzerland. Composite 500 hPa geopotential height maps were computed for days with high and low Be-7 activity concentrations. It was found that downward transport associated with an upper ridge and a high tropopause is a significant controlling mechanism for high Be-7 activity concentrations at JFJ. In addition, wet scavenging is also important. A multiple linear regression model using potential temperature at tropopause level and relative humidity at JFJ as predictors explained 60% of the variability in the Be-7 activity concentrations over the investigated period. The results indicate that the indirect influence of stratosphere-troposphere exchange (STE) to the Be-7 activity concentrations at JFJ may be more important than the direct one during the period under investigation. The vigorous downward transport of stratospheric air to JFJ may be considered as the direct influence from STE, whereas a multi-step transport process with a longer tropospheric age of the stratospheric air masses may be considered as the indirect one. The indirect influence of stratosphere-troposphere exchange is investigated in a case study in which the highest Be-7 concentration, occurring from 16 to 23 July at JFJ, is associated with stratosphere-troposphere transport above northern Europe and strong southward advection with subsequent subsidence.