Excess 228Th in the airborne dust: An indicator of continental dust from the East Asian deserts

Thorium isotopes in the airborne dust collected at Tsukuba Science City in Japan from January 1981 to December 1982 show clear seasonal variations. Especially 228Th in the airborne particles have a typical maximum concentration between February and April, which is about an order of magnitude higher than during the remainder of the year. According to the result of size distribution of thorium isotope-bearing particles in the air, about 53% of 228Th is associated with the particles less than 1.6 μm aerodynamic diameter. These results reveal that 228Th in the airborne particles is present in fine soily matters originating from the East Asian deserts. (orig.)