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

The deposition of insoluble radiocesium bearing microparticles (CsMPs), which were released from the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident in March 2011, has resulted in the widespread contamination of eastern Japan. Obviously, these deposited insoluble CsMPs may become the secondary contamination sources by atmospheric migration or other environmental transferring process, however, the understanding of the transport mechanism remains non-elucidation, and the relevant evidence has not been directly provided. This study, for the rst time, provides the direct evidence for the resuspension of these insoluble CsMPs to the atmosphere from 1) proximity of 137 Cs radioactivity and resemblance of the morphology and the elemental compositions of CsMPs in the samples of soil and aerosol derived from the same sampling site, 2) the special characteristics of the resuspended CsMPs of which the ratios of Na/Si, K/Si and/or Cs/Si were smaller than those from the initially released CsMPs collected at either long distance or near F1NPP, which can be ascribed to the slowly natural corrosion of CsMPs by the loss of the small amount of soluble contents in CsMPs and 3) high CsMPs concentration of 10 granules/gram in the surface soil of our sampling site and the observed resuspension of surface soil dust at wind gust speed higher than 4 m s -1 . Specically, fteen single CsMPs were successfully isolated from the aerosol lters collected by unmanned high-volume air samplers at a severely polluted area in Fukushima Prefecture, about 25 km away from F1NPP, from January 2015 to September 2019. The mean diameter of these CsMPs was 1.8 ± 0.5 μm, and the average 137 Cs radioactivity was 0.35 ± 0.23 Bq/granule. The contribution rate of the resuspended CsMPs to the atmospheric radiocesium was estimated from the ratio of 137 Cs radioactivity of a single CsMP to that of the aerosol lter to be of 23.9 ± 15.3%. There has been no considerable decreasing trend in the annual CsMP resuspension frequency. and S-CsMPs mainly consist of Si, O, Cs, Fe, Zn, K, Na and Cl. Specically, the average atomic mass fraction of the main elemental compositions of the three S-CsMPs were evaluated, given as following; Si (6.4 ± 6.7 wt.%), O (29.2 ± 4.7 wt.%), Cs (1.7 ± 2.48 wt.%), Zn (3.5 ± 4.6 wt.%), Fe (1.9 ± 2.4), Na (1.0 ± 0.7 wt.%), K (0.3 ± 0.4 wt.%) and Cl (0.4 ± 0.6 wt.%). The radiocesium activities, morphologies, and elemental compositions of A-CsMPs and S-CsMPs were almost consistent with each other. These results imply that A-CsMPs were derived from the resuspension of S-CsMPs. of the in Specically, CsMPs were detected in face masks worn by Fukushima in the The resuspended CsMPs were also found on Japanese spinach in Fukushima Prefecture from to 2017. The resuspension of radiocesium caused by decontamination work also reported by several studies, 55, 56, 57 suggesting a possibility of the resuspension of the CsMPs induced by decontamination work by the government. However, these studies have not discussed whether the resuspension of CsMPs into the atmosphere can occur naturally or not because CsMPs in these studies were found in living areas or in association with human activities. In contrast, A-CsMPs of this study were found in aerosol samples collected by unmanned-operated HV samplers at a site located in an evacuated area that was dicult for residents to return to, indicating the possibility that A-CsMPs were derived from naturally resuspended S-CsMPs, without anthropogenic activities. Furthermore, it is possible that the A-CsMPs were transported from F1NPP or areas where decontamination and other remediation activities were carried out. If a signicant number of radioactive pollutants, including CsMPs, were transported to the sampling site, the atmospheric concentration of 137 Cs should have increased. However, a systematic increase in atmospheric 137 Cs concentration was not detected during the sampling periods of A-CsMPs, and no leakage events of radioactive pollutants from the F1NPP were reported during the periods. These results imply a low likelihood that A-CsMPs were derived from decontamination work at the F1NPP and the severely contaminated areas. On the one hand, transport of contaminated soil by dump trucks has increased along a road near the sampling site since 2018, and decontamination activities in areas adjacent to the sampling sites (within 500 m range) were also conducted between June and December 2019. On the other hand, there was no increasing trend in the detected numbers of A-CsMPs in 2015, 2016, 2018, and 2019. More specically, only one A-CsMP was found after June 2019,

radiocesium in air over Tsukuba, Ibaraki, that "elucidating the secondary emission processes of the FDNPP radionuclides remains an imminent scienti c challenge, especially for heavily polluted areas. Secondary sources can include soil dust suspension from polluted earth surfaces, emissions from polluted vegetation and forests, and volatilization and release from combustion of polluted garbage and open eld burning. Although the main emission sources are not yet well understood, this elucidation must be performed as soon as possible." Signi cantly, this process is an uncontrollable route that can cause widespread secondary contamination 26 in previous noncontaminated or decontaminated areas. Although the radioactivity level of resuspension 5,36 was a few orders of magnitude lower than that of the early-stage released, it would be a matter of public concern, especially for the evacuated people who lived in di cult-to-return areas.
Hirose 27 suggested that the second increase in atmospheric radiocesium in the spring 2012 can be attributed to the resuspension process. Moreover, seasonal variations of radiocesium in aerosol samples were observed in severely contaminated areas of Fukushima Prefecture, 15,36,40,41 being explained as the result of radiocesium resuspension because the observed seasonal dependence of the atmospheric radiocesium concentration cannot be interpreted from direct or delayed primary release from F1NPP accident. 41 Speci cally, Ishizuka et al. 15 measured and modeled the particle size distribution of suspended soil dust carrying radiocesium considering different soil textures. Kinase et al. 36 and Igarashi et al. 41 discussed possible resuspension-hosting sources and showed that soil particles 42 can be signi cant in the springtime and that bioaerosols such as pollens, 43,44 spores, 41 and microorganisms 36 can be a major host particle in summer and autumn. Kajino et al. 45 modeled the resuspension of 137 Cs from bare soil and forest ecosystems and suggested a similar seasonal change of the resuspension host particle to above-mentioned studies, namely, the high radiocesium concentration can be observed in warm seasons and the low can be obtained in cold seasons. They also estimated respective resuspension rates of 1 × 10 − 6 day − 1 and 2 × 10 − 6 day − 1 . 45 These results were helpful to estimate the inhalation dose. 46,47 However, research on the resuspension of radiocesium has still been insu cient, and both observational and theoretical model studies are necessary for a quantitative understanding. Notably, these studies did not discuss the resuspension of the CsMPs.
Higaki et al. 48 reported that the main sources of radiocesium adhered to non-woven fabric face masks, which were worn both outdoors and indoors by 68 residents living in eastern Japan in the spring of 2012, were fugitive dust and CsMPs 24 (the masks were worn by the Fukushima residents in the spring of 2013). The CsMPs were also isolated from Japanese spinach. 26 Although these results suggested the possibilities of the resuspension of CsMPs from the ground to the atmosphere, there has been no evidence to indicate that it occurred naturally (without anthropogenic interferences) and no discussion about the special features of the collected CsMPs to demonstrate whether they are derived from resuspension or other reasons.
This study, for the rst time, provides the direct evidence of resuspension of the CsMPs in the monitoring duration at our sampling site, and explains the transport variables and mechanism of the CsMPs. Therefore, this study aims at 1) detection and isolation of highly radioactive granules from aerosol and soil samples collected at a severely polluted area to identify them as CsMPs by analyses of their morphology and elemental compositions, 2) describing the possibility of the natural resuspension of CsMPs to the atmosphere and 3) evaluating the frequency of CsMP resuspension and its contribution ratio to the whole atmospheric concentration of radiocesium.  (37.6° N, 140.8° E and altitude 438 m) located in a highly contaminated zone that was di cult for residents to return to (approximately 25 km northwest from the F1NPP), as shown in Figure 1. Details of the sampling site are described elsewhere. 15,36,41,45 Aerosol samples were collected on quartz-ber lters (QR100, Advantech, Tokyo, Japan or 2500QAT-UP, Pall Corp., NY, USA) with high volume samplers (HV samplers; 120SL, Kimoto Electric Co. Ltd., Tennoji, Osaka, Japan and HV-1000R/F, Sibata Scienti c Technology, Ltd., Saitama, Japan) at a sampling air ow rate of 0.7 m 3 min -1 from January 2015 to September 2019. We used the aerosol samples collected in 2015, 2016, 2018, and 2019 because the aerosol sampling was interrupted for several months in 2017. Additionally, aerosol samples with different aerodynamic diameters were also collected separately on slotted quartz-ber lters using an HV sampler with a seven-stage cascade impactor (TE-230, Tisch Environmental Inc., OH, USA) with slotted quartz-ber lters. The aerodynamic size-distribution ranges with a collection e ciency exceeding 50% were sampled as follows: Q1 (> 10.

Detection and Isolation of Highly Radioactive Granules
An imaging plate (IP) system (CR × 25P portable computed radiography, GE Measurement & Control, Massachusetts, USA) and a micromanipulator (Axis Pro; Micro Support Corp., Shizuoka, Japan) were used to detect and isolate CsMPs from the aerosol lters. The isolation procedure of CsMPs from the aerosol lters and from the soil samples was conducted as the similar dry process (without use of water) of Adachi et al. 11 and Satou et al. 50 , respectively.

Measurement of Gamma-Ray Spectrometry and SEM Observation
The gamma-ray peak intensities at 605 and 662 keV were used to identify and determine the radioactivities of 134 Cs and 137 Cs, respectively. 50 Identi cation of the CsMPs was based on the morphology and elemental compositions of the single highly radioactive particle. The observation of the morphology and elemental compositions was conducted with an SEM (SU3500, Hitachi High-Technologies Co., Tokyo, Japan) equipped with energy-dispersive X-ray spectroscopy (EDS: X-max 50 mm, Horiba Ltd., Kyoto, Japan) for aerosol CsMPs and a eld emission SEM (JSM-7800F, JEOL Ltd. Tokyo, Japan) for visualizing soil CsMPs and water-immersed aerosol CsMPs. , and the detailed information regarding radioactivity and elemental compositions is given in Tables 1 and S1. These results indicate that all isolated granules have characteristics corresponding to those reported for type A CsMPs 12, 13 as described below.

Results
All CsMPs were of an almost spherical or distorted spherical shape, and their diameter distribution ranged from 1.  Table S1.
The water insolubility of radiocesium in CsMPs has been reported by Adachi et al., 11 and other reports also support this fact (CsMPs can be isolated from the wet-isolation method 51,52 and can be detected in river water 53 ). The water insolubility of the CsMPs was also checked by immersion of the isolated A-CsMPs into water for 24 h (#11-NHVF-300601-Q) and 48 h (#12-NHVF-300906-Q), which is shown in Figure S2. There were no obvious detectable changes in their radiocesium activity and morphology before and after the immersion. So, we conclude that CsMPs exhibit very low watersolubility in short-term. Regarding the long-term solubility, there have been a few research. 53,54 The morphology, the 137 Cs radioactivity, the elemental compositions and distributions, and the water insolubility of isolated CsMPs in this study were consistent with those of the reported type A CsMPs [11][12][13]17 . We can conclude that type A CsMPs are detected in aerosol samples and refer to aerosol CsMPs as "A-CsMPs".
3.1.2 Comparison of CsMPs Isolated from Aerosols and Soil. There are 24 CsMPs detected in soil sample (namely, ten granules per gram, which was approximately consistent with the results of Ikehara et al. 55 ). The 137 Cs radioactivity of these CsMPs mostly ranged from 0.06 to 0.95 Bq/granule, except for one that exhibited 6.70 Bq. Their mean and median values were 0.29 and 0.22 Bq/granule, respectively, and the sum of 137 Cs radioactivity of these CsMPs was 13.4 Bq. Six single granules from the group of 24 CsMPs have thus far been successfully extracted from the soil samples, and three of them have been observed and analyzed by SEM-EDS. The SEM images and their elemental compositions are given in Figure 4 and Table S2, respectively. They were all spherical particles, and their diameters were 1.

Possibility of Natural Resuspension of CsMPs into the Atmosphere
The possibility of CsMP resuspension from the contaminated surface soil was suggested in studies. Speci cally, CsMPs were detected in face masks worn by Fukushima residents in the spring of 2013. 24 The resuspended CsMPs were also found on Japanese mustard spinach in Fukushima Prefecture cultivated from August to December 2017. 26 The resuspension of radiocesium caused by decontamination work has also been reported by several studies, 55,56,57 suggesting a possibility of the resuspension of the CsMPs induced by decontamination work by the government. However, these studies have not discussed whether the resuspension of CsMPs into the atmosphere can occur naturally or not because CsMPs in these studies were found in living areas or in association with human activities. In contrast, A-CsMPs of this study were found in aerosol samples collected by unmanned-operated HV samplers at a site located in an evacuated area that was di cult for residents to return to, indicating the possibility that A-CsMPs were derived from naturally resuspended S-CsMPs, without anthropogenic activities. Furthermore, it is possible that the A-CsMPs were transported from F1NPP or areas where decontamination and other remediation activities were carried out.  Table 2. Therefore, it is unlikely that A-CsMPs in this study was derived from human activities.
Signi cantly, we can nd that the atomic ratios of Na/Si (≈ 0.  Tables 1, 2, and Table S3. If all 20 CsMPs were A-CsMPs, the sampling probability of A-CsMPs at the sampling site can be estimated as 1.6 × 10 −2 granule per sampling day or 1.  36 showed that soil mineral particles were major coarse aerosols at our sampling site in the springtime (MAM), indicating that the suspension of soil particles from the ground surface occurred in this season. It is reasonable and possible that this dispersion process may cause the resuspension of CsMPs. Kinase et al. 36 also found that the peak of the atmospheric radiocesium concentration was consistent with maximal average wind speed in the springtime. Based on the above discussion, it obviously that the atmospheric radiocesium concentration was associated with wind-driven suspended soil in the springtime. This seasonality is consistent with the natural resuspension of the CsMPs in this study.
Similar to the dispersion of soil dust, natural resuspension of CsMPs by wind could occur when wind speed was high and soil moisture was low, which can be con rmed as discussed below. Meteorological data, including the gust, the average wind speeds, the mean soil-water-content, and the relative humidity (RH), were measured at the sampling site in 2015, when six CsMPs were sampled, as shown in Figure S4. These data from May 2015 are plotted in Figure 6. Two CsMPs were collected when the wind gust speed exceeded 4 m s -1 and the soil water content was not notably high.
Signi cantly, wind gust speeds exceeding 4 m s -1 can be frequently observed and sometimes even exceeded 8 m s -1 . These results imply the likelihood of CsMP resuspension, although dispersion of submicron dust by winds is less likely than that of larger particles. However, Wu et al. 60 observed that particles (with diameters ranging from 7.0 to 42.3 μm) could be suspended occasionally at wind speeds of 4 m s -1 , with more than half of the particles suspended within 2 seconds at a wind speed of 8 m s -1 . Nicholson 61 obtained a gentle increasing trend for suspension of particles with a 4.1 μm diameter along with increments of wind speed, and a positive correlation between PM 2.5 and a wind speed higher than 3 m s -1 was also found by Wang et al. 62 These characteristics of microparticle suspension can indirectly explain the resuspension of CsMPs and the low frequency of A-CsMP detection in this work. Furthermore, Ishizuka et al. 15 showed that the condition of higher wind speed was favorable for dispersion of soil mineral particles, including ne soil particles, in the springtime. We speculate that CsMP resuspension needs both a strong wind and incidental phenomena such as aggregate lumps formed by attachment/collisions of CsMPs with larger dust particles scattered by winds. Moreover, because some holes that were presumably dug by wild boars were found around the sampling site (within a few hundred meters), a possibility, that the dispersion of CsMPs occurred with such animal activities, cannot be ruled out.

Contribution of the resuspended CsMPs to Atmospheric Radioactivity
Various studies 55,63 have shown that signi cant levels of CsMPs were released into the environment 64 and that a considerable part of the deposition of radiocesium should be attributed to the released CsMPs. 51,59 Based on the results and discussion above, CsMPs can be directly resuspended into atmosphere. Herein, the contribution ratio of the resuspended CsMPs to atmospheric 137 Cs can be de ned as the ratio of the radioactivity of single CsMPs ( 137 Cs) to that of the aerosol lters from which the A-CsMPs was obtained. 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. Because of the excessive amount of time and human resources required for the extraction procedure, further extraction of CsMPs in these lters has not been conducted. Moreover, the average contribution ratio of A-CsMPs, which is the ratio of total 137 Cs radioactivity of 15 A-CsMPs to that of all 165 aerosol samples, was estimated to be about 2.0%, as shown in Table S3. Correspondingly, the ratio of total 137 Cs radioactivity of S-CsMPs to that of the surface soil sample used in this study was about 8.0% (=13.40 Bq/166.2 Bq); the contribution of A-CsMPs to the total atmospheric radioactivity, therefore, was smaller than that in the corresponding surface soil.

Conclusions
This study reveals that the direct resuspension of water insoluble CsMPs, composed of silicate glass, to the atmosphere without human interference. The fact can be con rmed by i) their successful extraction from aerosol samples collected by unmanned-operated samplers, ii) the nearly resembling characteristics of A-CsMPs and S-CsMPs, which were extracted from the respective aerosol and surface soil samples collected at the same sampling site in a heavily contaminated area of F1NPP and iii) the seasonality of the A-CsMP detection. The sizes and radioactivities of both A-CsMPs and S-CsMPs found in the present study were situated within the distributions of the reported type A CsMPs, but they were generally smaller than those reported for type A CsMPs (with a diameter of less than ca. 10 μm) 11,17 sampled in primary emission from the F1NPP accident in 2011. This can be used to exclude the probability that A-CsMPs may be derived from the primary release and may be explained by surface-deposited CsMPs subjected to their very slow dissolution rates in the natural environment, as suggested by Okumura et al. 54 Therefore, the erosion of CsMPs in the natural environment should be a signi cant process to be clari ed by future studies.
This study also reveals the frequency of CsMP resuspension and its contribution to atmospheric radiocesium, although this can vary with the location depending on the amount of radiocesium and CsMP deposition (surface contamination extent). The resuspension of insoluble type A CsMPs is indeed infrequent, and their absolute activity remains at the sub-Bq level. On the other hand, the detection of type A CsMPs has been continued up to 2019 and no obvious decreasing trend has been found from annual resuspension frequencies, suggesting its persistence. The CsMPs can also work as the transferring medium of fuel debris, 66 trace uranium, 17, 52 strontium, 67 and plutonium 68 into the environment, although they are at very low level, and CsMPs resuspension increases the possibility of unpredictable inhalation. If they are inhaled, they can easily reach the deep respiratory system. 69 Thus, more investigation is necessary regarding the risk assessment of even the single CsMP radiation exposure 19 and long-term monitoring of these resuspended insoluble type A CsMPs.   Figure 2 Scanning electron microscopic images of 15 single A-CsMPs isolated from the aerosol-lter-samples are shown for easy comparison and understanding (the original SEM images were given in Figure S1).

Figure 3
Elemental mapping images (a) and EDS spectra (b) of #11-NHVF-300601-Q. The Cs in the particle shows multiple peaks and O, Si, Na, S, Cl, K, Ca, Ti, Fe and Zn are coexistent within the particle according to the elemental mapping of main elements within the area.

Figure 4
Scanning electron microscopic images of 3 single S-CsMPs isolated from the soil-samples (The average diameter of all spherical S-CsMPs was 1.7±0.1 μm, which is conspicuously close to that of A-CsMPs).