Deposition of 137Cs in Rokkasho, Japan and its relation to Asian dust

Biweekly atmospheric depositions of 137Cs were measured in Rokkasho, Aomori, Japan from March 2000 to March 2006 to study recent 137Cs deposition. Although the deposition level was generally lower than the detectable limit, deposition samples collected in spring occasionally had measurable levels of 137Cs. The annual 137Cs deposition from 2001 to 2005 was 0.04–0.69 Bq m−2 with a mean value of 0.27 Bq m−2. Depositions of insoluble Al, Fe and Ti were strongly correlated with the 137Cs deposition, suggesting that suspension of soil particles was the main source of the recent 137Cs deposition. Asian dust events were coincident with some of the significant 137Cs depositions in spring. It was found that the ratios of 137Cs/Al and Fe/Al could be used as indices for discriminating Asian dust from suspension of the local surface soil. Backward trajectory analysis suggested southern Mongolian and northeastern China regions as sources of the significant 137Cs depositions.