Abstract
Natural radionuclides, i.e., thorium isotopes, in dry and wet deposition samples were measured at Tsukuba, Japan, as were their surface air concentrations, in order to understand dry and wet removal processes of the natural radionuclides. The dry deposition velocities of thorium isotopes, calculated from surface air concentrations and dry deposition fluxes, showed difference among isotopes and temporal variations. The dry deposition velocities of thorium isotopes were greater than that of the anthropogenic radionuclides. The wet deposition velocities of thorium isotopes for individual rainfall events varied largely. The wet deposition velocity is given as the product of washout ratio and rainfall rate. Typically it was found that the washout ratios of thorium isotopes show a temporal variation. In order to explain this phenomenon, we examined the relationship between the washout ratios and rainfall rate and found a negative correlation between washout ratios of thorium isotopes and rainfall rate. The result suggests that factors controlling the wet removal of the thorium isotopes for individual rainfall events is surface air concentrations, particle sizes and rainfall rate rather than precipitation amount, which is in agreement with previous understandings. As a result, fractionation among thorium isotopes in surface airborne particles is partly controlled by wet and dry removal processes.
