Abstract
Abstract. Resuspension of 137Cs from the contaminated ground surface to the atmosphere is essential for understanding the environmental behaviors of 137Cs and estimating external and inhalation exposure of residents. Kajino et al. (2016) assessed the 137Cs resuspension flux from bare soil and forest ecosystems in eastern Japan in 2013 using a numerical simulation constrained by surface air concentration measurements. However, the simulation was found to underestimate the observed deposition amounts by 2 orders of magnitude. The reason for this underestimation is that the simulation assumed that resuspended 137Cs is carried by submicron aerosols, which have low deposition rates. Based on the observational indications that soil dust and bioaerosols are the major carriers of resuspended 137Cs, a new simulation is performed with higher deposition rates constrained by both surface concentrations and deposition amounts. In the new estimation, the total areal annual resuspension of 137Cs in 2013 is 25.7 TBq, which is equivalent to 0.96 % of the initial deposition (2.68 PBq). Due to the rapid deposition rates, the annual redeposition amount is also large at 10.6 TBq, approximately 40 % of the resuspended 137Cs. The resuspension rate through the atmosphere (0.96 % yr−1) seems slow, but it (2.6 × 10−5 d−1) may not be negligibly small compared to the actual decreasing trend of the ambient gamma dose rate obtained in Fukushima Prefecture after the radioactive decay of 137Cs plus 134Cs in 2013 is subtracted (1.0–7.9 × 10−4 d−1): resuspension can account for 1 %–10 % of the decreasing rate due to decontamination and natural decay through land surface processes. The current simulation underestimated the 137Cs deposition in Fukushima city in winter by more than an order of magnitude, indicating the presence of additional resuspension sources. The site of Fukushima city is surrounded by major roads. Heavy traffic on wet and muddy roads after snow removal operations could generate superlarge (approximately 100 µm in diameter) road dust or road salt particles, which are not included in the model but might contribute to the observed 137Cs at the site.
Highlights
More than 10 years have passed since the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident
Extensive studies have been performed far using field observations, laboratory experiments, and numerical simulations aiming at a full understanding of atmospheric dispersion and deposition of directly emitted radionuclides associated with the accident, which occurred in March 2011
The numerical simulations conducted by K16 were consistent with the findings described above: the surface concentrations in the mountainous forest area are low in winter and high in summer, the contributions of mineral dust are high in winter, and those of bioaerosols are high in summer
Summary
More than 10 years have passed since the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident. Tsuruta et al (2014) and Oura et al (2015) measured the hourly surface air activity concentrations of 137Cs at 99 stations in eastern Japan These two powerful spatiotemporal measurement datasets together with comprehensive emission scenarios provided by the Japan Atomic Energy Agency (e.g., Katata et al, 2015; Terada et al, 2020) enable us to identify transport and deposition events over the land surface in Japan (e.g., Tsuruta et al, 2014; Nakajima et al, 2017; Sekiyama and Iwasaki, 2018). These data were useful to validate the numerical simulation results provided by various regional-scale atmospheric models (Draxler et al, 2015; Leadbetter et al, 2015; Kitayama et al, 2018; Sato et al, 2018, 2020; Kajino et al, 2019; Goto et al, 2020) and were applied for other advanced numerical techniques, such as inverse modeling (Yumimoto et al, 2016; Li et al, 2019), ensemble forecasting (Sekiyama et al, 2021), and data assimilation (Sekiyama and Kajino, 2020)
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