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Volume 16, issue 20
Atmos. Chem. Phys., 16, 13149–13172, 2016
https://doi.org/10.5194/acp-16-13149-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Chem. Phys., 16, 13149–13172, 2016
https://doi.org/10.5194/acp-16-13149-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 27 Oct 2016

Research article | 27 Oct 2016

Long-term assessment of airborne radiocesium after the Fukushima nuclear accident: re-suspension from bare soil and forest ecosystems

Mizuo Kajino1,2, Masahide Ishizuka3, Yasuhito Igarashi1, Kazuyuki Kita4, Chisato Yoshikawa5, and Masaru Inatsu6 Mizuo Kajino et al.
  • 1Meteorological Research Institute (MRI), Japan Meteorological Agency (JMA), Tsukuba, Ibaraki, Japan
  • 2RIKEN Advanced Institute for Computational Science (AICS), Kobe, Hyogo, Japan
  • 3Faculty of Engineering, Kagawa University, Takamatsu, Kagawa, Japan
  • 4Faculty of Science, Ibaraki University, Mito, Ibaraki, Japan
  • 5Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka, Kanagawa, Japan
  • 6Faculty of Science, Hokkaido University, Sapporo, Hokkaido, Japan

Abstract. The long-term effect of 137Cs re-suspension from contaminated soil and forests due to the Fukushima nuclear accident has been quantitatively assessed by numerical simulation, a field experiment on dust emission flux in a contaminated area (town of Namie, Fukushima prefecture), and air concentration measurements inside (Namie) and outside (city of Tsukuba, Ibaraki prefecture) the contaminated area. In order to assess the long-term effect, the full year of 2013 was selected to study just after the start of the field experiments. The 137Cs concentrations at Namie and Tsukuba were approximately 10−1–1 and 10−2–10−1 mBq m−3, respectively. The observed monthly median concentration at Namie was 1 to 2 orders of magnitude larger than that at Tsukuba. This observed difference between the two sites was consistent with the simulated difference, indicating successful modeling of 137Cs re-suspension and atmospheric transport. The estimated re-suspension rate was approximately 10−6 day−1, which was significantly lower than the decreasing rate of the ambient gamma dose rate in Fukushima prefecture (10−4–10−3 day−1) as a result of radioactive decay, migration in the soil and biota, and decontamination. Consequently, re-suspension contributed negligibly in reducing ground radioactivity. The dust emission model could reproduce the air concentration of 137Cs in winter, whereas the summer air concentration was underestimated by 1 to 2 orders of magnitude. Re-suspension from forests at a constant rate of 10−7 h−1, multiplied by the green area fraction, could explain the air concentration of 137Cs at Namie and its seasonal variation. The simulated contribution of dust re-suspension to the air concentration was 0.7–0.9 in the cold season and 0.2–0.4 in the warm season at both sites; the remainder of the contribution was re-suspension from forest. The re-suspension mechanisms, especially through the forest ecosystems, remain unknown. This is the first study that provides a crude estimation of the long-term assessment of radiocesium re-suspension. Additional research activities should investigate the processes/mechanisms governing the re-suspension over the long term. This could be achieved through conducting additional field experiments and numerical simulations.

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The current study provides the first quantitative budget analysis of radiocesium re-suspended from ground surface contaminated by the Fukushima nuclear accident. It provides useful information to society since our simulation can be used for the long-term assessment of internal exposure to residents in Japan. It also discussed that the re-suspension from forest ecosystems could be a dominant source of suspended radiocesium in the warm season in Japan.
The current study provides the first quantitative budget analysis of radiocesium re-suspended...
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