title: Regression model analysis of the decreasing trend of cesium-137 concentration in the atmosphere since the Fukushima accident.
abstract: The decreasing trend of the atmospheric137Cs 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. The137Cs particle concentration measurements were conducted in urban Fukushima and rural Date sites from September 2012 to June 2015. The137Cs 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-3in 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 the137Cs concentration evaluated by the trend concentration ranged from 44 to 53% y-1with average and standard deviation of 49 ± 8% y-1for 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 the137Cs concentration was nearly unchanged for the location and ground contamination level in the three years after the accident. The137Cs activity per aerosol particle mass also decreased with the same trend as the137Cs concentration in the atmosphere. The results indicated that the decreasing trend of the atmospheric137Cs concentration was related with the reduction of the137Cs concentration in resuspended particles.
authors.affiliation: Fukushima University, Kanayagawa 1, Fukushima-shi, Fukushima, Japan. Electronic address: r653@ipc.fukushima-u.ac.jp.
authors.forename: Kyo
authors.initials: K
authors.lastname: Kitayama
authors.affiliation: Fukushima University, Kanayagawa 1, Fukushima-shi, Fukushima, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Ohse
authors.affiliation: Fukushima University, Kanayagawa 1, Fukushima-shi, Fukushima, Japan.
authors.forename: Nagayoshi
authors.initials: N
authors.lastname: Shima
authors.affiliation: Fukushima University, Kanayagawa 1, Fukushima-shi, Fukushima, Japan.
authors.forename: Kencho
authors.initials: K
authors.lastname: Kawatsu
authors.affiliation: Fukushima University, Kanayagawa 1, Fukushima-shi, Fukushima, Japan.
authors.forename: Hirofumi
authors.initials: H
authors.lastname: Tsukada
mesh_terms: D000396:Air Pollutants, Radioactive / Q000032:analysis*; D002588:Cesium Radioisotopes / Q000032:analysis*; D062406:Fukushima Nuclear Accident; D008962:Models, Theoretical; D011834:Radiation Monitoring; D012044:Regression Analysis
chemical_list: D000396:Air Pollutants, Radioactive; D002588:Cesium Radioisotopes
keywords: Atmospheric concentration; Cesium-137; Decreasing trend; Fukushima; Regression model