Estimate of radon-222 concentrations in rainclouds from radioactivity of rainwater observed at ground level

A time-independent model is presented which predicts the short-lived radon daughter activity of rainwater at ground level for unit concentration of radon (²²²Rn) in cloud. The model incorporates the physical processes of diffusive attachment of radon daughters and impact collection for cloud droplets by raindrops. It also includes consideration of cloud droplet and raindrop size spectra. It is concluded from the results from the model that the specific activity of rainwater is not affected very much by aerosol concentration, liquid water content, average radius of raindrops and height of cloud base but is affected markedly by rainfall rate. The radon concentrations in the rainclouds are estimated from the comparison between calculation and near-ground observations carried out for a period of almost 3 years. The result shows that the monthly geometric mean concentration ranges from 2 to 6 pCi·m^?3 at Nagoya.