Abstract
At the Meteorological Research Institute (MRI, Tsukuba, Japan), we found that during the 1990s, the 137Cs/90Sr activity ratio resulting from atmospheric deposition and free from the direct influence of nuclear tests and accidents, was fairly low (average: 2.1). These values were not in accordance with those from Japanese surface soils (average: 4-7). This finding suggests that atmospheric deposition is currently a mixture of local and remote components. The remote component may be an aeolian dust which has been transported long distances from arid areas. The major component is likely to be Asian dust. It was further hypothesized from observations and model results that remote sources may exist other than Asian dust. To obtain more information, Saharan dust deposited in Monaco in 2002 and suspended dust collected in the Taklamakan area in 2001 were analyzed for 90Sr and 137Cs. The Taklamakan dust exhibited a 137Cs/90Sr ratio of about 4, within the range for atmospheric deposition recorded at the MRI, while the Saharan dust exhibited a higher ratio of about 13. Although the present Sahara datum was negative for our 'hyper-range transport' hypothesis, the seasonal change in the 137Cs/90Sr ratio in the atmospheric deposition implies an aeolian dust source other than Asian dust.
