Abstract
A large quantity of radionuclides was released by the Fukushima Daiichi Nuclear Power Plant accident in March 2011, and those deposited on ground and vegetation could return to the atmosphere through resuspension processes. Although the resuspension has been proposed to occur with wind blow, biomass burning, ecosystem activities, etc., the dominant process in contaminated areas of Fukushima is not fully understood. We have examined the resuspension process of radiocesium (134,137Cs) based on long-term measurements of the atmospheric concentration of radiocesium activity (the radiocesium concentration) at four sites in the contaminated areas of Fukushima as well as the aerosol characteristic observations by scanning electron microscopy (SEM) and the measurement of the biomass burning tracer, levoglucosan.The radiocesium concentrations at all sites showed a similar seasonal variation: low from winter to early spring and high from late spring to early autumn. In late spring, they showed positive peaks that coincided with the wind speed peaks. However, in summer and autumn, they were correlated positively with atmospheric temperature but negatively with wind speed. These results differed from previous studies based on data at urban sites. The difference of radiocesium concentrations at two sites, which are located within a 1 km range but have different degrees of surface contamination, was large from winter to late spring and small in summer and autumn, indicating that resuspension occurs locally and/or that atmospheric radiocesium was not well mixed in winter/spring, and it was opposite in summer/autumn. These results suggest that the resuspension processes and the host particles of the radiocesium resuspension changed seasonally. The SEM analyses showed that the dominant coarse particles in summer and autumn were organic ones, such as pollen, spores, and microorganisms. Biological activities in forest ecosystems can contribute considerably to the radiocesium resuspension in these seasons. During winter and spring, soil, mineral, and vegetation debris were predominant coarse particles in the atmosphere, and the radiocesium resuspension in these seasons can be attributed to the wind blow of these particles. Any proofs that biomass burning had a significant impact on atmospheric radiocesium were not found in the present study.
Highlights
In March 2011, abundant and various radionuclides were released into the atmosphere (e.g., Chino et al 2011) as a result of the nuclear accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) caused by the Great East Japan Earthquake (Holt et al 2012), and their amounts and diffusion have been of public concern because of their health impacts (Report: Working Group on Risk Management of Low-dose Radiation Exposure 2011)
Leaks of radiocesium from the FDNPP site significantly affected the radiocesium concentrations in 2011 and 2012, when spikes occurred in the radiocesium concentrations monitored at the Meteorological Research Institute (MRI), Tsukuba, and that atmospheric radiocesium was mainly supplied by resuspension after this period
To understand the resuspension process of radiocesium in areas heavily contaminated by the FDNPP accident, Fig. 12 The normalized particle count of the bioaerosols using scanning electron microscopy (SEM) and atmospheric concentrations of 137Cs activity sampled at the GD site during summer (July and August 2014) and winter (December 2014 and January 2015)
Summary
In March 2011, abundant and various radionuclides were released into the atmosphere (e.g., Chino et al 2011) as a result of the nuclear accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) caused by the Great East Japan Earthquake (Holt et al 2012), and their amounts and diffusion have been of public concern because of their health impacts (Report: Working Group on Risk Management of Low-dose Radiation Exposure 2011). The Rscale values approached unity, as shown, in these seasons, which can be attributed to longer bioaerosol residence times, which would result in well-mixed atmospheric 137Cs. In this study, the sampling sites are located in low mountainous areas and are surrounded by forests, and it is natural that the increases of bioaerosols significantly contribute to resuspension, unlike in urban areas.
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