Comparative trends and seasonal variation of 7Be, 210Pb and 137Cs at two altitude sites in the central part of France

The atmospheric concentrations of 137Cs, 210Pb, and 7Be were measured over a three-year period at two research stations located less than 12 km apart and at different altitudes (puy de Dôme, 1465 m a.s.l. and Opme, 660 m a.s.l., France). Seasonal trends in all radionuclides were observed at both stations, with high concentration measured during the summer and low concentrations during the winter. The 210Pb concentrations at both stations were similar to each other. Higher concentrations of both 7Be and 137Cs were measured at puy de Dôme than at Opme. These observations can be explained by the stratospheric and upper tropospheric sources of 7Be and the long-range transportation of 137Cs at high altitudes. Air mass origins during sampling periods were classified into several groups by their route to the stations (marine, marine modified, continental and mediterranean). We observed that 7Be concentrations were constant regardless of the air mass origins, unlike 137Cs and 210Pb concentrations that increased when influenced by continental air masses. Higher 7Be concentrations were observed when air masses were arriving from the upper troposphere than from the boundary layer, the opposite was observed for 137Cs. The temporal trend in concentrations of 7Be shows good agreement with previous modelling studies suggesting that there is a good understanding of its sources and the atmospheric vertical mixing of this radionuclide. The sources and mixing of 210Pb, however, seem to be more complex than it appeared to be in previous modelling studies.