Beryllium‐10 and beryllium‐7 in precipitation in Dübendorf (440 m) and at Jungfraujoch (3580 m), Switzerland (1998–2005)

Abstract

Beryllium‐10 and beryllium‐7 have been measured in monthly precipitation samples at a high (Jungfraujoch, 46.32°N 7.59°E, 3580 m) and a low (Dübendorf, 47.25°N 8.27°E, 440 m) altitude station in Switzerland from 1998 till today. The average deposition fluxes of 7Be are 380 ± 30 atoms m−2 s−1 in Dübendorf and 320 ± 20 atoms m−2 s−1 at Jungfraujoch, and the fluxes of 10Be are 810 ± 70 atoms m−2 s−1 in Dübendorf and 810 ± 60 atoms m−2 s−1 at Jungfraujoch. The 10Be/7Be ratio measured at Jungfraujoch (2.4 ± 0.1) is higher than in Dübendorf (2.1 ± 0.1), which is probably caused by a greater share of stratospheric air at Jungfraujoch in which the 7Be content is reduced because of radioactive decay. The ratios correspond to a mean atmospheric residence time of ∼110–120 d. The deposition fluxes show a seasonal change with a summer maximum and a winter minimum. The 10Be/7Be ratio exhibits a maximum in the first half of the year at Jungfraujoch in accordance with seasonal stratosphere‐troposphere exchange (STE) variations. In Dübendorf the maximum occurs 1 month later. An analysis with a two‐box model shows that the calculated variability due to seasonally varying STE explains only one third of the observed variability in the 10Be/7Be ratio. The rest of the variability is caused by local effects, such as the precipitation rate and the larger than average STE variability at the midlatitudes. According to the model calculations, the global mean 10Be/7Be ratio only varies between 1.1 and 1.5 with a seasonally varying STE. The larger measured values in the midlatitudes point to a latitudinal dependence of the 10Be/7Be ratio.