ABSTRACT

In this study, the potential natural sources of secondary radiocesium isotope (¹³⁴Cs and ¹³⁷Cs) emissions were investigated, with a focus on n-alkanes, a characteristic bioaerosol compound. Monitoring was performed to obtain a time series of aerosol, samples, from winter 2013 to summer 2014, and size-resolved aerosol samples in 2012 and 2014. Samples were collected from the area heavily contaminated after the Fukushima Daiichi Nuclear Power Plant accident in March 2011. A correlation analysis of radiocesium, n-alkanes, and black carbon concentrations was performed to identify the contributions of aerosols from biogenic and anthropogenic sources. Biogenic n-alkanes exhibited similar concentration ranges except for spring 2014. The continuous input of biogenic n-alkanes is characteristic of a sampling site surrounded by forest, where pollen dispersion increased the concentration of biogenic n-alkanes in spring 2014. On the other hand, anthropogenic n-alkane concentrations were significantly increased in spring and summer 2014 (>50 ng/m³), compared with those prior to winter 2014 (<20 ng/m³). This anthropogenic n-alkane increase represents the beginning of reconstruction near the area. The carbon preference index (CPI) clearly showed biogenic n-alkanes with coarse-sized particles (CPI > 3), and more anthropogenic n-alkanes were contained in fine particle aerosols. Our results showed that radiocesium and biogenic n-alkane concentrations in seasonal and size-resolved aerosol samples have a partially positive correlation, which supports the hypothesis that the secondary emissions of radionuclides occurred in the forested areas.