Concentrations of ²²²RN, its short-lived daughters and ²¹²PB and their ratios under complex atmospheric conditions and topography

Atmospheric activity concentrations of ²¹²Pb and short-lived ²²²Rn daughters, together with meteorological elements, have been observed continuously at three sites at Kamisaibara Village in Japan. In addition, atmospheric activity concentration of ²²²Rn, equilibrium-equivalent concentration of ²²²Rn 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 ²²²Rn is almost the same as the atmospheric activity concentration of short-lived ²²²Rn daughters. The activity concentrations of ²¹²Pb and the short-lived ²²²Rn 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 ²¹²Pb and the short-lived ²²²Rn 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 ²²⁰Rn and ²²²Rn due to strong winds and snow cover. There were also cases in which the ratio of the atmospheric activity concentration of ²¹²Pb to that of the short-lived ²²²Rn daughters at night was equal to or less than the ratio in the daytime. This inverse trend, as in the period 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 ²²²Rn (the activity concentration of short-lived ²²²Rn daughters) to the activity concentration of ²²²Rn 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 ²²²Rn daughters to that of ²²²Rn during the three observation periods. These differences can be explained by the submergence of paddy fields.