ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-11-2843-2011 Change of the Asian dust source region deduced from the relationship between anthropogenic radionuclides in surface soil and precipitation in Mongolia Igarashi Y. 1 Fujiwara H. 2 Jugder D. 3 Atmospheric Environment and Applied Meteorology Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan Soil Environment Division, National Institute for Agro-Environmental Sciences, 3-1 Kannondai, Tsukuba, Ibaraki 305-8604, Japan Institute of Meteorology and Hydrology, Ulaanbaatar 46, Mongolia 24 01 2011 11 1 2843 2871 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/11/2843/2011/acpd-11-2843-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/11/2843/2011/acpd-11-2843-2011.pdf

The Asian dust source region may be expanding primarily as a result of recent climate change, especially during the 2000s. This change was investigated by examining anthropogenic radionuclides contained in surface soil samples from Mongolia. Surface soil was globally labeled by radioactive fallout from nuclear testing during the late 1950s and early 1960s. There are no current direct sources for anthropogenic radionuclides in the air, so the radionuclides in the atmosphere are mainly carried by dust from wind-blown surface soil, that is, aeolian dust. Asian dust carries <sup>90</sup>Sr, <sup>137</sup>Cs, and other anthropogenic radionuclides; the heaviest deposition occurs in spring and has been recorded in Japan since the early 1990s. The composition of anthropogenic radionuclides in atmospheric depositions would be affected by a change in the dust source. Previous studies of atmospheric depositions at long-term monitoring sites (e.g. in Tsukuba, Japan) have detected changes in the <sup>137</sup>Cs/<sup>90</sup>Sr ratio and in the specific activity of the radionuclides. These changes in the composition of observed atmospheric depositions should be a reflection for a change in the climatic conditions of the dust source region. To investigate this dust source change, a field survey for radionuclides (<sup>90</sup>Sr and <sup>137</sup>Cs) in surface soil samples was conducted in September 2007 in the eastern and southern regions of Mongolia, where dust storms have occurred more frequently since 2000. It was found that specific activities of both radionuclides as well as the <sup>137</sup>Cs/<sup>90</sup>Sr ratio in the surface soil correlated well with annual average precipitation in the Mongolian desert-steppe zone. The higher specific activities and the higher <sup>137</sup>Cs/<sup>90</sup>Sr ratio were found in the grassland region with the greater precipitation. This finding suggests that the increased specific activities and the activity ratio detected in the atmospheric depositions in Japan during years of the frequent Asian dust transport event since 2000 should be a sign of grassland degradation.

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