Soil dust and bioaerosols as potential sources for resuspended Cs occurring near the Fukushima Dai-ichi nuclear power plant.

One of the current pathways to radiation exposure, caused by the radionuclides discharged during the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, is the inhalation of resuspended Cs present in the air. Although wind-induced soil particle resuspension is recognized as a primary resuspension mechanism, studies regarding the aftermath of the FDNPP accident have suggested that bioaerosols can also be a potential source of atmospheric Cs in rural areas, although the quantitative impact on the atmospheric Cs concentration is still largely unknown. We propose a model for simulating the Cs resuspension as soil particles and bioaerosols in the form of fungal spores, which are regarded as a potential candidate for the source of Cs-bearing bioaerosol emission into the air. We apply the model to the difficult-to-return zone (DRZ) near the FDNPP to characterize the relative importance of the two resuspension mechanisms. Our model calculations show that soil particle resuspension is responsible for the surface-air Cs observed during winter-spring but could not account for the higher Cs concentrations observed in summer-autumn. Higher Cs concentrations are reproduced by the emission of Cs-bearing bioaerosols (fungal spores) that replenishes the low-level soil particle resuspension in summer-autumn. Our model results show that the accumulation of Cs in fungal spores and large emissions of spores characteristic of the rural environment are likely responsible for the presence of biogenic Cs in the air, although the former must be experimentally validated. These findings provide vital information for the assessment of the atmospheric Cs concentration in the DRZ, as applying the resuspension factor (m) from urban areas, where soil particle resuspension would dominate, can lead to a biased estimate of the surface-air Cs concentration. Moreover, the influence of bioaerosol Cs on the atmospheric Cs concentration would last longer, because undecontaminated forests commonly exist within the DRZ.