Atmospheric activity concentrations of Pb-212 and short-lived Rn-222 daughters, together with meteorological elements, have been observed continuously at three sites at Kamisaibara Village in Japan. In addition, atmospheric activity concentration of Rn-222, equilibrium-equivalent concentration of Rn-222 and conditions of the lower atmosphere were observed for three intensive observation periods at Akawase, one of the three sites in Kamisaibara Village. The equilibrium-equivalent concentration of Rn-222 is almost the same as the atmospheric activity concentration of short-lived Rn-222 daughters.
The activity concentrations of Pb-212 and the short-lived Rn-222 daughters and their ratio were low in the daytime owing to convective mixing, and high at night owing to the surface-based inversion during periods of no precipitation. Their variations have several patterns corresponding to the scale of the drainage wind or weak mixing.
Mechanical mixing due to strong winds through both day and night during the first and second observation periods made the atmospheric activity concentrations of Pb-212 and the short-lived Rn-222 daughters continuously low. However, their ratios were continuously high during the first period yet continuously low during the second period. This difference can be explained by the effect of extraction of Rn-220 and Rn-222 due to strong winds and snow cover. There were also cases in which the ratio of the atmospheric activity concentration of Pb-212 to that of the short-lived Rn-222 daughters at night was equal to or less than the ratio in the daytime. This inverse trend, as in the periods of no precipitation mentioned above, is considered to be due to near-neutral conditions on these nights.
We find a difference in the ratio of the equilibrium-equivalent concentration of Rn-222 (the activity concentration of short-lived Rn-222 daughters) to the activity concentration of Rn-222 during the first observation period and that during the second. The difference can be explained by snow cover on the ground. We also find differences among the ratios of the activity concentration of the short-lived Rn-222 daughters to that of Rn-222 during the three observation periods. These differences can be explained by the submergence of paddy fields.