Altitudinal characteristics of atmospheric deposition of aerosols in mountainous regions: Lessons from the Fukushima Daiichi Nuclear Power Station accident

Yukihisa Sanada; Genki Katata; Naoki Kaneyasu; Chika Nakanishi; Yoshimi Urabe; Yukiyasu Nishizawa

doi:10.1016/j.scitotenv.2017.08.246

Altitudinal characteristics of atmospheric deposition of aerosols in mountainous regions: Lessons from the Fukushima Daiichi Nuclear Power Station accident

Sci Total Environ. 2018 Mar 15:618:881-890. doi: 10.1016/j.scitotenv.2017.08.246. Epub 2017 Nov 6.

Authors

Yukihisa Sanada¹, Genki Katata², Naoki Kaneyasu³, Chika Nakanishi⁴, Yoshimi Urabe⁵, Yukiyasu Nishizawa⁶

Affiliations

¹ Fukushima Remote Monitoring Group, Fukushima Environmental Safety Center, Japan Atomic Energy Agency, 45-169 Sukakeba, Kaihama-aza, Haramachi-ku, Minami-soma 975-0036, Japan. Electronic address: sanada.yukihisa@jaea.go.jp.
² Institute for Global Change Adaptation Science (ICAS), Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan.
³ Atmospheric Environment Research Group, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.
⁴ Fukushima Remote Monitoring Group, Fukushima Environmental Safety Center, Japan Atomic Energy Agency, 45-169 Sukakeba, Kaihama-aza, Haramachi-ku, Minami-soma 975-0036, Japan.
⁵ NESI, Inc., 38 Shinko-cho, Hitachinaka, Ibaraki 312-0005, Japan.
⁶ OYO Corporation, 1-66-22 Miyahara-cho, Kita-ku, Sitama-shi, Saitama 331-0812, Japan.

PMID: 29102197
DOI: 10.1016/j.scitotenv.2017.08.246

Abstract

To understand the formation process of radiologically contaminated areas in eastern Japan caused by the Fukushima Daiichi Nuclear Power Station (FDNPS) accident, the deposition mechanisms over complex topography are the key factors to be investigated. To characterize the atmospheric deposition processes of radionuclides over complex mountainous topography, we investigated the altitudinal distributions of the radiocesium deposited during the accident. In five selected areas, altitudinal characteristics of the air dose rates observed using airborne surveys were analyzed. To examine the deposition mechanisms, we supplementarily used vertical profiles of radiocesium deposition in each area calculated in the latest atmospheric dispersion model. In southern Iwate, the vertical profile of the observed air dose rate was uniform regardless of altitude. In western Tochigi, the areas with the highest levels of contamination were characteristically distributed in the middle of the mountains, while in southern Fukushima, the areas with the highest contamination levels were enhanced near the summits of mountains. In central Fukushima, high air dose rates were limited to the bottoms of basin-like valley. In the region northwest of FDNPS, the air dose rate was the highest at the bottom of valley topography and decreased gradually with altitude. The simulation results showed that calculated wet deposition and observed vertical profiles of total deposition were similar in areas of southern Iwate and northwest of FDNPS qualitatively, suggesting that the dominant deposition mechanism was wet deposition. In contrast, the atmospheric dispersion model failed to reproduce either the timing of precipitation events or vertical profiles of radiocesium deposition in three other areas. Although it was difficult to elucidate the deposition mechanisms in these areas due to uncertainties of the present model results, potential mechanisms such as cloud water deposition were still proposed based on circumstantial evidences of limited meteorological data during the early stage of the accident.

Keywords: (137)Cs; Airborne survey; Altitudinal distribution; Deposition mechanism; Mountainous region.