title: Inverse modeling of the137Cs source term of the Fukushima Dai-ichi Nuclear Power Plant accident constrained by a deposition map monitored by aircraft.
abstract: The amount of137Cs released by the Fukushima Dai-ichi Nuclear Power Plant accident of 11 March 2011 was inversely estimated by integrating an atmospheric dispersion model, an a priori source term, and map of deposition recorded by aircraft. An a posteriori source term refined finer (hourly) variations comparing with the a priori term, and estimated137Cs released 11 March to 2 April to be 8.12 PBq. Although time series of the a posteriori source term was generally similar to those of the a priori source term, notable modifications were found in the periods when the a posteriori source term was well-constrained by the observations. Spatial pattern of137Cs deposition with the a posteriori source term showed better agreement with the137Cs deposition monitored by aircraft. The a posteriori source term increased137Cs deposition in the Naka-dori region (the central part of Fukushima Prefecture) by 32.9%, and considerably improved the underestimated a priori137Cs deposition. Observed values of deposition measured at 16 stations and surface atmospheric concentrations collected on a filter tape of suspended particulate matter were used for validation of the a posteriori results. A great improvement was found in surface atmospheric concentration on 15 March; the a posteriori source term reduced root mean square error, normalized mean error, and normalized mean bias by 13.4, 22.3, and 92.0% for the hourly values, respectively. However, limited improvements were observed in some periods and areas due to the difficulty in simulating accurate wind fields and the lack of the observational constraints.
authors.affiliation: Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki 3050052, Japan. Electronic address: yumimoto@mri-jma.go.jp.
authors.forename: Keiya
authors.initials: K
authors.lastname: Yumimoto
authors.affiliation: Center for Regional Environment Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 3058506, Japan.
authors.forename: Yu
authors.initials: Y
authors.lastname: Morino
authors.affiliation: Fukushima Project Office, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 3058506, Japan.
authors.forename: Toshimasa
authors.initials: T
authors.lastname: Ohara
authors.affiliation: Department of Chemistry, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 1920397, Japan.
authors.forename: Yasuji
authors.initials: Y
authors.lastname: Oura
authors.affiliation: Department of Chemistry, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 1920397, Japan.
authors.forename: Mitsuru
authors.initials: M
authors.lastname: Ebihara
authors.affiliation: Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 2778564, Japan.
authors.forename: Haruo
authors.initials: H
authors.lastname: Tsuruta
authors.affiliation: Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 2778564, Japan.
authors.forename: Teruyuki
authors.initials: T
authors.lastname: Nakajima
mesh_terms: D000396:Air Pollutants, Radioactive / Q000032:analysis*; D000401:Aircraft; D002588:Cesium Radioisotopes / Q000032:analysis*; D062406:Fukushima Nuclear Accident; D008962:Models, Theoretical; D055848:Nuclear Power Plants; D011834:Radiation Monitoring / Q000379:methods*
chemical_list: D000396:Air Pollutants, Radioactive; D002588:Cesium Radioisotopes
keywords: Atmospheric dispersion; Fukushima Daiichi Nuclear Power Plant accident; Inverse modeling; Source term; Surface deposition