Possible change in Asian dust source suggested by atmospheric anthropogenic radionuclides during the 2000s Yasuhito Igarashi a, * , Yayoi Inomata a , Michio Aoyama a , Katsumi Hirose a , Hiroshi Takahashi a , Yoshihiro Shinoda a, 1 , Nobuo Sugimoto b , Atsushi Shimizu b , Masaru Chiba a a Meteorological Research Institute, Japan b National Institute of Environmental Studies, Japan article info Article history: Received 29 August 2008 Received in revised form 19 January 2009 Accepted 8 February 2009 Keywords: Asian dust Atmospheric deposition Anthropogenic radionuclides Climate change abstract Decades-long monitoring of anthropogenic radionuclides in the atmospheric deposition in Tsukuba, Japan suggests not only the substantial impacts of the Asian dust (Kosa) on the deposition but also the possible change of the Kosa source region, especially during springs of the 2000s. In order to know more about such change, 4 single wet deposition events occurred in the spring of 2007 were scrutinized. The largest anthropogenic radionuclides wet deposition was supplied by the April 2–4 event. It brought several tens % of the monthly depositions (April 2007) of the dust (residue) mass (4.5 g m 2 ) and anthropogenic radionulides ( 90 Sr: 16, 137 Cs: 97 and Pu: 3 mBq m 2 ). None of the events observed fulfilled both criteria of the specific activities and 90 Sr/ 137 Cs activity ratio to the Tsukuba soil; they did not exhibit local soil dust signature. The Kosa events in fact have extensive impacts on the atmospheric environment over Japan in spring season. Considering the elevated specific activities as well as greater 137 Cs/ 90 Sr activity ratio in the deposited dust, it is hypothesized that the dust source areas in Asian continent would be shifting from the arid zone to the desert-steppe zone suffering from desertification during the 2000s. This type of the Kosa may be called as the ‘new-regime Kosa’. Chemical observation in the far downwind region of the Kosa dust could allow us to know possible shift in the source regions. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction We have been continued the observation of anthropogenic radionuclides having large environmental impacts since the late1950s, at Geochemical Research Department, Meteorological Research Institute (MRI), Japan, so as to clarify the dynamics of the radionuclides in the atmosphere (e.g., Katsuragi, 1983; Hirose et al., 1987, 2003; Aoyama et al., 1991; Igarashi et al., 1996, 2003). Espe- cially, from the long-term record of the monthly deposition of 90 Sr, 137 Cs (Igarashi et al., 1996, 2001, 2005, 2006) and Pu (Hirose et al., 2003, 2007), we have pointed out the important role of the large- scale aeolian dust such as Kosa as the carrier of the anthropogenic radionuclides since the early 1990s (after the stratospheric fallout became negligible). Those radionuclides had been injected by the atmospheric nuclear tests, deposited over the globe during the late 1950s to early 1960s in large amount and have remained over the land surface (besides marine environment) for several decades. Aoyama et al. (2006) compiled the deposition, soil and water column inventory data and created the global 137 Cs fallout map. At the northern middle latitude, the total of cumulative 137 Cs depo- sition, being almost identical to the soil column inventory over the land, was in the range of several hundred to 10 thousand Bq m 2 at 1970. The soil 137 Cs inventory at present would be a little smaller than the half considering its physical decay (T 1/2 : about 30 years). As a result, the surface soil particles have been contaminated by anthropogenic radionuclides, and being suspended when the dust storm takes place. This means that long-lived anthropogenic radionuclides such as 90 Sr and 137 Cs are applicable as potential tracers in the aeolian dust research (Igarashi et al., 2001, 2005) as well as Pu (Hirose et al., 2003, 2007, 2008). Atmospheric tracer use of anthropogenic radionuclides was also studied by the global transport model studies (Lee et al., 2003, 2006). It is well-known that 137 Cs in soil has been applied to the soil erosion research (e.g. Ritchie et al., 1990). Moreover, the increase of the 137 Cs deposition at many sites over Japan during springs of the early 2000s, which is due to the frequent occurrence of Kosa phenomenon in East Asia since 2000. This becomes therefore an * Corresponding author. Fax: þ81 29 853 8728. E-mail address: yigarash@mri-jma.go.jp (Y. Igarashi). 1 Present affiliation: Kobe Marine Observatory, Japan Meteorological Agency, Japan. Contents lists available at ScienceDirect Atmospheric Environment journal homepage: www.elsevier.com/locate/atmosenv 1352-2310/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2009.02.018 Atmospheric Environment 43 (2009) 2971–2980