Atmospheric radioactivity of Cs-134/137 observed at Namie, Fukushima: seasonal variation and contribution of biological re-suspension.

Abstract:

Radionuclides emitted by the accident in Fukushima dai-ichi nuclear power plant (FNDPP) have been deposited on the soil, ocean and vegetation. Even after about 4 years since the FNDPP accident significant activities of the radionuclides have been observed over severely contaminated areas. Re-suspension of radioactive cesium from the soil and vegetation to the atmosphere has been one of significant paths for its diffusion after the accident. Although the quantitative understanding of the re-suspensions is important for the prediction of future transition of radionuclides, its mechanism, identification of aerosol species which bring radioactive cesium, and the resuspension flux have not been understood in Fukushima.

　We are continuously measuring atmospheric concentration of Cs-134/137 radioactivity at Tsushima, Namie-town, located about 30km northwest from FNDPP with high-volume air samplers. It showed clear seasonal variation: it increase from April to June, and decreased from September to December. In winter and spring, it was weakly but positively correlated with the surface wind speed. On the contrary, it did not depend on the wind speed in summer and autumn. It also has different diurnal variation: higher activities were observed in daytime in winter/spring, while the activities were obviously higher in nighttime in summer/autumn. The size distribution of aerosols contributing to the Cs-134/137 re-suspension has been measured using cascade impactors attached with high-volume air samplers, and it also shows different features in winter/spring and summer/autumn. These results indicate that the mechanism of the Cs-134/137 re-suspension is different with the season in Fukushima. Scanning electron microscope observation showed that most of suspended coarse particles were soil particles in spring and biogenic particles in autumn. Details on the Cs-134/137 re-suspension mechanisms revealed by our observations and contribution of biogenic emission will be presented in the meeting.