Variation in Atmospheric 137Cs and the Carriers in Aerosol Samples Obtained from a Heavily Contaminated Area of Fukushima Prefecture

Even a decade after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on 11 March 2011, fluctuations in atmospheric ¹³⁷Cs were still observed, and explanations for the fluctuations and their carriers remained elusive. In this study, small fluctuations within 0.0002 Bq∙m^-3 were still detected in aerosol samples obtained from January to April, and slightly higher levels of atmospheric ¹³⁷Cs were observed from May to September in a heavily contaminated area of Fukushima prefecture. Specifically, it is demonstrated that the ¹³⁷Cs carriers in the aerosol samples were a combination of carbon-containing particles and aluminum-containing particles (Al particles dominated, with the percentage being 68%) in early May, whereas the main ¹³⁷Cs carriers were carbonaceous particles, with the average percentage being 88% in September and at the end of May, using fluorescent upright microscope and scanning electron microscope equipped with an energy-dispersive X-ray spectrometer quantitatively. Additionally, small particles (less than 2 μm) and medium particles (2-8 μm) of carbonaceous particles had a higher level in the aerosol samples of May and September. Specifically, bacteria (1-1.8 μm) and spores (1.8-10 μm) had a linear relationship with the distribution of atmospheric ¹³⁷Cs in the aerosol samples of September. In addition, temperature and precipitation were the main impact factors affecting the distribution of ¹³⁷Cs and their carriers. This observation further suggests that there is still a need for long-term monitoring of atmospheric ¹³⁷Cs.