Rain-induced bioecological resuspension of radiocaesium in a polluted forest in Japan

Abstract

It is the conventional understanding that rain removes aerosols from the atmosphere. However, the question of whether rain plays a role in releasing aerosols to the atmosphere has recently been posed by several researchers. In the present study, we show additional evidence for rain-induced aerosol emissions in a forest environment: the occurrence of radiocaesium-bearing aerosols in a Japanese forest due to rain. We carried out general radioactive aerosol observations in a typical mountainous village area within the exclusion zone in Fukushima Prefecture to determine the impacts and major drivers of the resuspension of radiocaesium originating from the nuclear accident in March 2011. We also conducted sampling according to the weather (with and without rain conditions) in a forest to clarify the sources of atmospheric radiocaesium in the polluted forest. We found that rain induces an increase in radiocaesium in the air in forests. With further investigations, we confirmed that the fungal spore sources of resuspended radiocaesium seemed to differ between rainy weather and nonrainy weather. Larger fungal particles (possibly macroconidia) are emitted during rainy conditions than during nonrainy weather, suggesting that splash generation by rain droplets is the major mechanism of the suspension of radiocaesium-bearing mould-like fungi. The present findings indicate that radiocaesium could be used as a tracer in such research fields as forest ecology, meteorology, climatology, public health and agriculture, in which fungal spores have significance.

Reassessment of the radiocesium resuspension flux from contaminated ground surfaces in East Japan

“…Igarashi et al (2019b) further investigated the possible sources of 137 Cs-rich bioaerosols in summer and suggested the substantial involvement of fungal spores. Atmospheric humidity plays a key role in the discharge of fungal spores, which is consistent with the findings of Kita et al (2020), who stated that the surface concentration of 137 Cs in mountainous forests became higher in the presence of precipitation in summer. Cedar pollen particles could contain a considerable amount of 137 Cs in the forest areas of Abukuma Highlands, but they are emitted from late February to early May, not during summer (Igarashi et al, 2019b).…”

“…However, Watanabe et al (2021) determined that the deposition amounts simulated by K16 were significantly underestimated. Based on recent cumulative knowledge, major resuspension aerosols could be supermicron-sized, such as soil dust (Ishizuka et al, 2017;Kinase et al, 2018) and bioaerosols (Kinase et al, 2018;Igarashi et al, 2019b;Kita et al, 2020;Minami et al, 2020;Igarashi, 2021). Lower possibilities of submicron particle involvement, such as that resulting from forest fires (Kinase et al, 2018) and epicuticular wax (Nakagawa et al, 2018), have been reported.…”

“…Soil dust and road dust emissions are relatively well-formulated, but bioaerosols are not. Substantial efforts have been made to understand the emission mechanisms and quantifications of bioaerosol emission fluxes (Igarashi et al, 2019b;Kita et al, 2020;Minami et al, 2020), but it is still difficult to establish a set of formulas that is applicable for various vegetation surfaces. Our hypothesis of the existence of superlarge particles is not proven at all.…”

Atmospheric Resuspension of Insoluble Radioactive Cesium Particles Found in the Difficult-to-Return Area in Fukushima

“…As given in Table 2, the resuspension contribution ratios of CsMPs ranged from 5.0 to 50.6%, and the mean and median values were 23.9% and 20.5%, respectively. The remainder of the resuspension hosts of atmospheric radiocesium can be attributed to bioaerosols and soil particles bearing with low concentrations of radiocesium 41,65 as well as low-level radioactive CsMPs. Although A-CsMPs were not detected by one-hour exposure by IP, the radioactive spot was detected in the aerosol lter by IP inspection when the IP plate was exposed for tens of hours, as shown in Figure S5.…”

Eight-year variations in atmospheric radiocesium in Fukushima City

“…They suggested that the fungal spores that accumulate radio-Cs may be significantly involved in resuspension in the forest in summer. Kita et al (2020) suggested that rain induced the emission of radio-Cs associated with fungal spores in the forest in summer. Minami et al (2020) combined aerosol flux measurements and a multilayer atmospheresoil-vegetation model and estimated that the bioaerosol emission flux was on the order of 10 -2 μg m -2 s -1 , which could account for the surface concentrations of 137 Cs in the forests in summer (Kajino et al, 2016;Kinase et al, 2018;Igarashi et al, 2019a).…”