Factors controlling 7Be and 210Pb atmospheric deposition as revealed by sampling individual rain events in the region of Geneva, Switzerland

Bulk atmospheric deposition of 7Be and 210Pb has been measured at Versoix, close to Geneva, Switzerland. Collectors were continuously deployed from November 1997 through November 1998 for periods from 1 to 22 days depending on the frequency of rain. The activities of 7Be and 210Pb integrated over the sampling interval were moderately well correlated with rainfall (r2 of 0.66 and 0.55, respectively; p<0.001) and well correlated one with the other (r2 of 0.91; p<0.001). The 7Be/210Pb activity ratio is close to 13.5, except for samples collected in the periods of very low precipitation which have a distinctly lower ratio. A simple model accounting for rainfall, seasonal variations and reload of the local atmosphere after a rain event explains 90% of the variance of 7Be and 210Pb deposition. Concentrations of 210Pb and Ca++ in rain were correlated with transport time of air masses over the continent as indicated by reconstruction of air mass trajectories over three days.