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.