High 210Po atmospheric deposition flux in the subtropical coastal area of Japan

Bulk atmospheric deposition fluxes of 210Po and 210Pb were measured at three coastal regions of Japan, the Pacific Ocean coastal area of the Japanese mainland (Odawa Bay), the Chinese continental side of Japanese coastal area (Tsuyazaki), and an isolated island near Okinawa (Akajima). Wet and dry fallout collectors were continuously deployed from September 1997 through August 1998 for periods of 3 to 31 days depending on the frequency of precipitation events. Annual 210Pb deposition fluxes at Odawa Bay (35°N 139°E), Tsuyazaki (33°N 130°E) and Akajima (26°N 127°E) were 73.3 ± 8.0, 197 ± 35 and 78.5 ± 8.0 Bq m−2 y−1, respectively. Higher 210Pb deposition was observed at the Chinese continental side of Japanese coast than at the Pacific Ocean coastal site. The high 210Pb atmospheric flux at the Chinese continental side coast was thought to be attributable to 222Rn-rich air-mass transport from the Chinese continent during the winter monsoon. In contrast, the annual 210Po deposition fluxes at the three study sites were 13.0 ± 2.3 (Odawa Bay), 21.9 ± 4.4 (Tsuyazaki) and 58.4 ± 7.7 (Akajima) Bq m−2 y−1, respectively, indicating unusual high 210Po deposition at Akajima during winter. Anomalous unsupported 210Po input was observed during summer 1997, suggesting unknown source of 210Po at this area.