222Rn fluxes from the ground surface and 226Ra contents in soil were measured on Hachijo-jima Island, which is a solitary island in the Pacific Ocean located about 200 km to the south of the main island of Japan, to evaluate fractional contributions of the locally exhaled 222Rn and the long-range transported one to the surface air concentration measured on this island. Averages of 222Rn flux and 226Ra content in dry soil were evaluated to be 0.9 ± 0.4 mBq m-2 s-1 and 6.8 ± 0.2 Bq kg-1, respectively. These are considerably smaller than the respective values of 9.7 ± 0.8 mBq m-2 s-1 and 23.2 ± 0.4 Bq kg-1 measured at Nagoya as a reference. The lower value of the 226Ra content and the even lower 222Rn flux on this island can be attributed to the basaltic geology and the soil's coarse texture and high moisture, respectively. A simple model calculation assuming a typical nocturnal condition showed that the measured 222Rn flux would cause only a small increase in the surface air concentration by 0.035 to 0.072 Bq m-3 (relative contribution of 1 to 12%) in addition to the long-range transported 222Rn under a typical nocturnal condition. The contribution of the local flux would be smaller than that under nocturnal condition. This local 222Rn component is negligible as compared with the concentration of the long-range transported 222Rn (0.6 to 3.6 Bq m-3). It is, therefore, concluded that Hachijo-jima Island is suitable for measuring the long-range transported atmospheric 222Rn in East Asia region.
