Evaluation of precipitation scavenging rates of background aerosol

An attempt was made to obtain information on the rates at which raining clouds cleanse the atmosphere, with natural radioactivity used as a tracer of naturally-occurring aerosols. In the atmosphere, the air concentrations of the radioactive Rn daughters would be expected to approach equilibrium with increasing height. In a theoretical study, Jacobi et al. (1959) showed that the short-lived first daughter of radon (218Po, 3 min half life) is in equilibrium with radon above about 20 m. Gamma-ray spectrometry was used to measure the isotopes of interest in successive samples of precipitation collected at ground level during nine rainfall occasions. Gamma-ray spectra of freshly collected rainfall were typically dominated by the photopeaks at 0.352 MeV (214Pb) and 0.609 MeV(214Bi). After these isotopes have decayed, the 212Pb photopeak at 0.323 MeV becomes apparent (212Pb is a thoron daughter). In the present work this feature was employed to allow accurate measurement of each isotope