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The observation network of atmospheric radon-222 (222Rn) concentration established in East Asia region is introduced, and the characteristics of the observations at two continental sites Beijing and Seoul and three remote sites Cape Ochiishi, Hachijo Is. and Hateruma Is. are discussed in this paper. Higher levels of 222Rn concentrations with typical diurnal variation with early morning maxima were observed on the continent, and lower levels with no diurnal variation at remote islands. Seasonal variations with summer minima and winter maxima were commonly obtained at all five observatories, and they suggested contribution of 222Rn originated from the continent to atmospheric 222Rn over the remote islands isolated in the ocean. A backward trajectory analysis showed clear relationship between variation in wind field and in 222Rn concentration at Hachijo Is., and proved availability of the observation for analysis of atmospheric transport in East Asia.

A three-dimensional Eulerian numerical model for 222Rn and its decay products coupled with a mesoscale meteorological model has been developed and applied to the reproduction of the daily and monthly variations of 222Rn concentration, the monthly variation of 210Pb deposition in Japan and the temporal variation of gamma dose rate after the cold front passage in the coastal area of the Japan Sea for the verification of model capability. The results are as follows: (1) The model reproduced the monthly variation of surface 222Rn concentration in remote islands, but underestimated inland concentration due to a coarse vertical resolution near the surface of the model. (2) The model reproduced the seasonal variation of the observed and the long-term yearly averaged 210Pb depositions as long as precipitations are predicted precisely. (3) The model reproduced the rise of gamma dose rate in precipitation accompanied by the cold front passage. In particular, 222Rn decay products in melted snow and graupel contributed the rise of gamma dose rate.