Abstract
We have continued the monitoring of radioactivity deposition at the Meteorological Research Institute (MRI), Tsukuba, Japan. It is revealed that the anthropogenic radionuclides, released as the global fallout and deposited on the ground, will redistribute over the globe through the re-suspension processes. The 137Cs/90Sr activity ratio in the atmospheric deposition, free from the direct influence of nuclear tests and accidents, at the MRI during the 1990s was fairly low (average: 2.1). And these ratios were not in accordance with those in Japanese surface soils (average: 4-7). This finding suggests that the current radioactivity deposition contains local and remote resuspensions. The remote one should be aeolian dust which has been transported long distances from the continental arid and semi-arid area. In the present report, if there have been any changes in the deposition of 90Sr and 137Cs observed in the 2000s from the 1990s, is focused. The annual depositions of 90Sr and 137Cs from 1993 to 2004 were plotted and the apparent half decrease time was obtained. The apparent half decrease time for 90Sr and 137Cs was evaluated as about 20 and 10 year, respectively, which were not significantly different from the reported ones from the only 1990s' data. Although the Kosa was noteworthy during 3 consecutive years of 1999 to 2001 in the far East, no significant change was admitted. In addition to the above, the discussion on the 137Cs/90Sr ratios in the deposition and the comparison of the observation with the results of a global 3-D dust transport model will be given.
