Regression model analysis of the decreasing trend of cesium-137 concentration in the atmosphere since the Fukushima accident.

The decreasing trend of the atmospheric 137Cs concentration in two cities in Fukushima prefecture was analyzed by a regression model to clarify the relation between the parameter of the decrease in the model and the trend and to compare the trend with that after the Chernobyl accident. The 137Cs particle concentration measurements were conducted in urban Fukushima and rural Date sites from September 2012 to June 2015. The 137Cs particle concentrations were separated in two groups: particles of more than 1.1 µm aerodynamic diameters (coarse particles) and particles with aerodynamic diameter lower than 1.1 µm (fine particles). The averages of the measured concentrations were 0.1 mBq m-3 in Fukushima and Date sites. The measured concentrations were applied in the regression model which decomposed them into two components: trend and seasonal variation. The trend concentration included the parameters for the constant and the exponential decrease. The parameter for the constant was slightly different between the Fukushima and Date sites. The parameter for the exponential decrease was similar for all the cases, and much higher than the value of the physical radioactive decay except for the concentration in the fine particles at the Date site. The annual decreasing rates of the 137Cs concentration evaluated by the trend concentration ranged from 44 to 53% y-1 with average and standard deviation of 49 ± 8% y-1 for all the cases in 2013. In the other years, the decreasing rates also varied slightly for all cases. These indicated that the decreasing trend of the 137Cs concentration was nearly unchanged for the location and ground contamination level in the three years after the accident. The 137Cs activity per aerosol particle mass also decreased with the same trend as the 137Cs concentration in the atmosphere. The results indicated that the decreasing trend of the atmospheric 137Cs concentration was related with the reduction of the 137Cs concentration in resuspended particles.

Abundance of birds in Fukushima as judged from Chernobyl.

The effects of radiation on abundance of common birds in Fukushima can be assessed from the effects of radiation in Chernobyl. Abundance of birds was negatively related to radiation, with a significant difference between Fukushima and Chernobyl. Analysis of 14 species common to the two areas revealed a negative effect of radiation on abundance, differing between areas and species. The relationship between abundance and radiation was more strongly negative in Fukushima than in Chernobyl for the same 14 species, demonstrating a negative consequence of radiation for birds immediately after the accident on 11 March 2011 during the main breeding season in March-July, when individuals work close to their maximum sustainable level.