Depositional behaviors of plutonium and thorium isotopes at Tsukuba and Mt. Haruna in Japan indicate the sources of atmospheric dust

Monthly plutonium and thorium depositions at Tsukuba (28 m asl) and Mt. Haruna (1370 m asl) were measured during 2006 and 2007 (Jan 2006–Dec 2007 at Tsukuba, Nov 2006–Dec 2007 at Mt. Haruna). The monthly 239,240Pu depositions ranged from 0.044 to 2.67 mBq m−2 at Tsukuba and from 0.05 to 0.9 mBq m−2 at Mt. Haruna during the measurement periods. Monthly 239,240Pu deposition did not differ markedly between the two sites except in April 2007. Seasonal pattern of monthly 239,240Pu depositions at both sites showed high in spring and low in summer, and typical of seasonal variations in northeastern Asia. Thorium deposition at Tsukuba was higher than that at Mt. Haruna except in May and June 2007. 230Th/232Th activity ratios were used to partition deposition samples into locally and remotely derived fractions. The results revealed that a major proportion of total 239,240Pu and Th deposits are derived from remote sources, especially in spring.