7Be, 210Pb and 137Cs concentrations in cloud water

The 7Be and 210Pb average activity concentrations in cloud water found were 1.9 ± 0.11 mBq m−3 air and 140 ± 10 μBq m−3 air, respectively. For 137Cs, the average concentration was 0.14 ± 0.02 μBq m−3 air. This very low-level is representative of the long term post-accidental background level. Indeed, for the studied period, the last accidental 137Cs release was that of Chernobyl accident, in April 1986. To our knowledge this is the first data about 137Cs reference level determination in cloud water. The comparison between cloud water and rain water concentrations showed a ratio cloud/rain ranging between 3.4 and 8.1, in agreement with previous studies performed on inorganic compounds. Scavenging efficiencies of aerosols by cloud droplets were also calculated with the additional aerosol concentrations routinely measured at the station and were quite low (0.13-0.40) compared to what has previously been observed for inorganic soluble ions.