Temporal variation of post-accident atmospheric 137Cs in an evacuated area of Fukushima Prefecture: Size-dependent behaviors of 137Cs-bearing particles

The concentrations of Cs-137 in the air, which were divided into coarse (>1.1 mu m phi) and fine (<1.1 mu m phi) fractions of particulate matter (PM), were measured from October 2012 to December 2014 in an area evacuated after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Total atmospheric Cs-137 concentrations showed a clear seasonal variation, with high concentrations during summer and autumn related to the dominant easterly wind blowing from the highly radioactivity contaminated area. This seasonal peak was dominated by Cs-137 in the coarse PM fraction. The Cs-137 specific activity (massic Cs-137 concentration) in the coarse PM was also found to increase significantly in summer, whereas that in the fine PM showed no variability during the year. These results show that coarse and fine Cs-137-bearing PM have different origins and behaviors in the resuspension process. The seasonal variation in atmospheric Cs-137 concentration was well correlated with the mean Cs-137 surface contamination (deposition density) around the observation site weighted by the frequency of wind direction, indicating that the atmospheric Cs-137 concentration in the observation site was explained by the distribution of the Cs-137 surface contamination and the frequency of different wind directions. We introduced a resuspension factor corrected for wind direction, consisting of the ratio of the atmospheric Cs-137 concentration to the weighted mean Cs-137 surface contamination, which evaluated the intensity of resuspension better than the conventional resuspension factor. This ratio ranged from 5.7 x 10(-11) to 8.6 x 10(-10) m(-1) and gradually decreased during the study period. (C) 2016 Elsevier Ltd. All rights reserved.