Variation of atmospheric radon concentration with bimodal seasonality

Atmospheric radon (222Rn) concentration has been measured on the Oshika Peninsula, northeastern Japan, since July 2005 to assess fluctuations of natural gamma-ray dose rate. The radon concentration shows a typical diurnal cycle with a minimum in daytime and a maximum in nighttime generated by the different strength in atmospheric convection, which is well observed in the world. On seasonal time scale, monthly change of the daily minimums has a monomodal seasonality with a maximum in winter and a minimum in summer, while monthly changes of the daily averages and maximums exhibit a bimodal one with maximum both in winter and summer. The origins of air masses arriving to the measuring site characterized by the Asian monsoonal cycle appear to explain the monomodal seasonality rather than the bimodal one. In addition, the atmospheric stability is not strong enough to cause the radon increase in summer. These results suggest that the bimodal cycle is possibly decomposed of two phases: one is attributed to the seasonal difference in radon inflow by atmospheric circulation and the other in radon exhalation from the earth surface.