Deposition behaviors of 210Pb, 7Be and thorium isotopes observed in Tsukuba and Nagasaki, Japan

210Pb, 7Be and thorium isotopes in deposition samples collected monthly in Nagasaki in 2000 and Tsukuba, Japan in 2000 and 2001 were determined. Monthly 210Pb depositions in Nagasaki and Tsukuba as did 7Be showed no consistent seasonal trend, whereas monthly 232Th deposition in Tsukuba and Nagasaki indicates a marked maximum in spring season (March–May), which corresponds to a seasonal cycle of soil dust fallout originating from the East Asian arid area. The 210Pb concentrations in rainwater calculated using the precipitation amount showed a typical seasonal change with high in winter and low in summer in two sites. The 7Be/210Pb ratios in the monthly deposition samples suggest that 7Be and 210Pb cannot be used as independent atmospheric tracers. On the other hand, thorium isotopes may be used as the atmospheric tracer of soil dusts originating from the East Asian desert and arid areas.