Progress in Earth andPlanetary Science

Tang et al.
Progress in Earth and Planetary Science(2022) 9:17
https://doi.org/10.1186/s40645-022-00475-6

Open Access

RESEARCH ARTICLE

Atmospheric resuspension of insoluble
radioactive cesium‑bearing particles foundin the difficult‑to‑return area in FukushimaPeng Tang1 , Kazuyuki Kita1*, Yasuhito Igarashi1,2*, Yukihiko Satou3, Koutarou Hatanaka1, Kouji Adachi4,
Takeshi Kinase5, Kazuhiko Ninomiya6 and Atsushi Shinohara6

Abstract
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 easternJapan. 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 first time,
provides the direct evidence for the resuspension of these insoluble CsMPs to the atmosphere from (1) proximity of137
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 CsMPsof which the ratios of Na/Si, K/Si and/or Cs/Si were smaller than those from the initially released CsMPs collected ateither long distance or near F1NPP, which can be ascribed to the slowly natural corrosion of CsMPs by the loss of thesmall amount of soluble contents in CsMPs, and (3) high CsMPs concentration of 10 granules/g in the surface soil ofour sampling site and high resuspension frequency of CsMPs in spring when predominant suspended particles weresoil dust. Specifically, 15 single CsMPs were successfully isolated from the aerosol filters collected by unmanned highvolume 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 137Cs radioactivity
was 0.35 ± 0.23 Bq/granule. The contribution rate of the resuspended CsMPs to the atmospheric radiocesium wasestimated from the ratio of 137Cs radioactivity of a single CsMP to that of the aerosol filter to be of 23.9 ± 15.3%. There
has been no considerable decreasing trend in the annual CsMP resuspension frequency.
Keywords: Resuspension, Radiocesium, Water-insoluble radiocesium-bearing microparticles (CsMPs), Fukushima
accident, Aerosol

Highlights*Correspondence: kazuyuki.kita.iu@vc.ibaraki.ac.jp; igarashi.
yasuhito.4e@kyoto-u.ac.jp
1
Graduate School of Science and Engineering, Ibaraki University, 2‑1‑1
Bunkyo, Mito, Ibaraki 310‑8512, Japan
Full list of author information is available at the end of the article

1. This study, for the first time, provides the direct
evidence of natural resuspension contribution of
the insoluble radiocesium-bearing microparticles(CsMPs) to the atmospheric radiocesium.

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Tang et al. Progress in Earth and Planetary Science(2022) 9:17

2. This study introduces the evaluating method ofresuspension contribution rate of CsMPs by the ratioof the radioactivity of 137Cs in a single CsMP to thatof aerosol filter.
3. This work reveals that there is no clear downward
trend in the annual frequency of CsMP resuspension
and a slightly higher frequency in the springtime isattributed to high wind gust speed and low moistureof topsoil, though the distribution of CsMP resuspension is observed in each season.

1 Introduction
1.1 Accident

The Fukushima Daiichi Nuclear Power Plant (F1NPP)
accident was triggered by the 9.0-magnitude TohokuOki earthquake and the subsequent tsunami on March
11, 2011 (Committee and on the Accident at the Fukushima Nuclear Power Station of Tokyo Electric Power
Company 2011; Fuchigami and Kasahara 2011), which
resulted in the complete loss of all power and over-heatand over-pressurization of Units 1–3 (there are 6 Units
in F1NPP and Units 4–6 had been “shut down” for regular maintenance prior to the earthquake (Tokyo ElectricPower Company Holdings 2011)). Over the subsequent
several days, despite the attempt to inject cooling water
and/or perform venting operations for Units 1–3 (starting at 10:40 JST on March 12 (Fuchigami and Kasahara2011), 10:15 JST on March 13 (Tokyo Electric Power
Company Holdings 2011) and 08:41 JST on March 12(Hatamura et al. 2014), respectively), hydrogen explosions could not be avoided at Units 1 and 3 (at 15:36 JST
on March 12 and 11:01 JST on March 14 (Committee andon the Accident at the Fukushima Nuclear Power Station of Tokyo Electric Power Company 2011)). Thus, alarge quantity of radioactive materials was released into
the atmosphere (Igarashi et al. 2015), the land (Yoshida
and Kanda 2012; Yoshida and Takahashi 2012), and thePacific Ocean (Hirose 2016). Specifically, the amount of
total atmospheric release of 137Cs was up to 15–21 PBq(1 PBq = ­1015 Bq), and 3–6 PBq was directly deposited
onto the land (Aoyama et al. 2020). The rest of the 137Cs
was distributed over the North Pacific, which was quantified to be 15–18 PBq (Aoyama et al. 2020). Moreover,
Steinhauser et al. (Steinhauser et al. 2014) reported thatthe highly contaminated area, with 137Cs levels exceeding
185 kBq ­m−2, spread approximately 1700 ­km2 in Fukushima Prefecture.
1.2 Radiocesium‑bearing microparticles

Water-insoluble radiocesium-bearing microparticles(hereafter CsMPs) (Adachi et al. 2013; Igarashi et al.
2019a) were released and deposited in the environment

Page 2 of 15

by the F1NNP accident. In recent several years, researchon the morphology, elemental compositions, classifications, and emission sources of CsMPs has been comprehensively reported (Adachi et al. 2013; Igarashi et al.
2019a; Satou et al. 2018; Snow et al. 2016). Specifically,
Adachi et al. (2013) first reported that spherical CsMPs
mainly contained O, Fe, and Zn as well as other traceelements. These CsMPs were referred to as the released
and deposited CsMPs in early stage of the F1NPP accident (Ishizuka et al. 2017). Based on the difference of sizeand structure as well as the difference in the isotopic ratioof 134Cs/137Cs, the CsMPs have been classified into type
A (possibly derived from Unit 2 or 3 with a size of lessthan 10 μm and 134Cs/137Cs ≈ 1.0) and type B (perhaps
released from Unit 1 with a size of more than 100 μm
and 134Cs/137Cs ≈ 0.9) (Igarashi et al. 2019a; Satou et al.
2018). There are also reports suggesting that type A
CsMPs were derived from Unit 2 (Hidaka 2019) or Unit
1 (Abe et al. 2014). Moreover, the glassy state of CsMPs
implied water insolubility [discussed in detail in references (Adachi et al. 2013; Niimura et al. 2015)) and comparatively persistent toxicity and hazards in the terrestrial
environment (Suzuki et al. 2020) (137Cs has a half-life of
30.2 years (Press 2015)]. The high-dose internal radiation
exposure by the inhalation of radioactive aerosols and theconsumption of food associated with radioactive material
is the most concerning health hazard (Yamamoto et al.
2019; Shannon 2012; Nakamura 2020). In order to assess
and reduce the risk of radiocesium inhalation (Higaki
et al. 2020) and the intake of food (Miura et al. 2021;
Nihei et al. 2018) contaminated by CsMPs, it is necessary
to understand its distribution and transport in the secondary processes in the environment.
1.3 Resuspension of radiocesium

Hirose (2013) found that the deposition of the early-stage
released radiocesium was significant from March to June
2011 and that an obvious decrease in radiocesium deposition rate occurred after July 2011. Hirose (2020) concluded that most of the F1NPP-derived 137Cs depositedon the ground surface remained in the surface soil layer
as a potential secondary source of atmospheric 137Cs, andTanaka et al. (2012) reported that more than 90% of theradiocesium was fixed within the top 5 cm of the profile. Although the F1NPP accident has passed for about
10 years, most of the 137Cs is thought to remain there(Matsuda et al. 2015), and thus the decontamination byremoval of topsoil was conducted in Fukushima Prefecture (Mishra et al. 2015; Evrard et al. 2019). Resuspensionof radiocesium refers to the secondary migration of thedeposited radiocesium into the atmosphere from surfaces and terrestrial ecosystems by winds (Ochiai et al.
2016), biomass burning (Igarashi et al. 2015), and otherTang et al. Progress in Earth and Planetary Science(2022) 9:17

different anthropogenic disturbances (Yamaguchi et al.
2012; Akimoto 2015), which can be found in the reports
about the F1NPP accident (Ochiai et al. 2016; Yamaguchi et al. 2012; Akimoto 2015; Kinase et al. 2018) andthe Chernobyl Nuclear Power Plant accident (Wagenpfeil and Tschiersch 2001; Garger et al. 1996; Yoschenko
et al. 2006). Regarding the issue, Igarashi et al. (2015)
emphasized, through their observation and data analysisof radiocesium in air over Tsukuba and Ibaraki, that “elucidating the secondary emission processes of the F1NPP
radionuclides remains an imminent scientific 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 ofpolluted garbage and open field burning. Although themain emission sources are not yet well understood, thiselucidation must be performed as soon as possible.” Significantly, this process is an uncontrollable route that cancause widespread secondary contamination (Nihei et al.
2018) in previous noncontaminated or decontaminatedareas. Although the radioactivity level of resuspension(Igarashi et al. 2015; Kinase et al. 2018) was a few ordersof magnitude lower than that of the early-stage released,
it would be a matter of public concern, especially for theevacuated people who lived in difficult-to-return areas.
Hirose (2013) suggested that the second increase inatmospheric radiocesium in the spring 2012 can be
attributed to the resuspension process. Moreover, seasonal variations of radiocesium in aerosol samples wereobserved in severely contaminated areas of Fukushima
Prefecture (Ishizuka et al. 2017; Kinase et al. 2018; Kita
et al. 2018; Igarashi et al. 2019b), being explained as theresult of radiocesium resuspension because the observed
seasonal dependence of the atmospheric radiocesium
concentration cannot be interpreted from direct ordelayed primary release from F1NPP accident site (Igarashi et al. 2019b). Specifically, Ishizuka et al. (2017)
measured and modeled the particle size distribution ofsuspended soil dust carrying radiocesium considering
different soil textures. Kinase et al. (2018) and Igarashi
et al. (Igarashi et al. 2019b) discussed possible resuspension-hosting sources and showed that soil particles(Kaneyasu et al. 2012) can be significant in the springtime and that bioaerosols such as pollens (Nakagawa
et al. 2018), spores (Igarashi et al. 2019b), and microorganisms (Kinase et al. 2018) can be a major host particle in summer and autumn. Kajino et al. (2016) modeled
the resuspension of 137Cs from bare soil and forest ecosystems and suggested a similar seasonal change of theresuspension host particle to above-mentioned studies, namely, the high radiocesium concentration can beobserved in warm seasons and the low can be obtained

Page 3 of 15

in cold seasons. They also estimated respective resuspension rates of 1–2 × ­10−6 ­day−1 (Kajino et al. 2016).
These results were helpful to estimate the inhalation dose
(EPA 1977; Garger and Talerko 2020). However, researchon the resuspension of radiocesium has still been insufficient, and both observational and theoretical modelstudies are necessary for a quantitative understanding.
Notably, these studies did not discuss the resuspension of
the CsMPs.
Higaki et al. (2014) reported that the main sources ofradiocesium 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 (Higaki et al. 2020) (the masks
were worn by the Fukushima residents in the spring of2013). The CsMPs were also isolated from Japanese spinach (Nihei et al. 2018). Although these results suggested
the possibilities of the resuspension of CsMPs from theground 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 first time, provides the direct evidenceof 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 aseverely polluted area to identify them as CsMPs by analyses of their morphology and elemental compositions,(2) describing the possibility of the natural resuspensionof CsMPs to the atmosphere and (3) evaluating the frequency of CsMP resuspension and its contribution ratio
to the whole atmospheric concentration of radiocesium.

2 Experiments and methods
2.1 Sample collection
2.1.1 Sampling of aerosols

Aerosol samples were collected at a site (37.6° N,
140.8° E, and altitude 438 m) located in a highly contaminated zone that was difficult for residents toreturn to (approximately 25 km northwest from the
F1NPP), as shown in Fig. 1. Details of the sampling site
are described elsewhere (Ishizuka et al. 2017; Kinase
et al. 2018; Igarashi et al. 2019b; Kajino et al. 2016).
Aerosol samples were collected on quartz-fiber filters(2500QAT–UP, Pall Corp., NY, USA or QR100, Advantech, Tokyo, Japan) with high-volume samplers (HVsamplers; HV–1000R/F, Sibata Scientific Technology,
Ltd., Saitama, Japan and 120SL, Kimoto Electric Co.
Ltd., Tennoji, Osaka, Japan) at a sampling air flow rateof 0.7 ­m3 ­min−1 from January 2015 to September 2019.

Tang et al. Progress in Earth and Planetary Science(2022) 9:17

Page 4 of 15

Fig. 1 Map of the sampling site (37.6° N, 140.8° E) in this study plotted over the surface contamination map for 137Cs obtained by the Japanese
government (Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) 2011) as of April 29, 2011 (Onda et al. 2015)

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 werealso collected separately on slotted quartz-fiber filters
using an HV sampler with a seven-stage cascade impactor (TE–230, Tisch Environmental Inc., OH, USA)with slotted quartz-fiber filters. The aerodynamic sizedistribution ranges with a collection efficiency exceeding 50% were sampled as follows: Q1 (> 10.3 μm), Q2(4.2–10.3 μm), Q3 (2.1–4.2 μm), Q4 (1.3–2.1 μm), Q5(0.69–1.3 μm), Q6 (0.39–0.69 μm), and Q7 (< 0.39 μm,
working as a backup filter) at a sampling air flow rateof 0.556 m­ 3/min. Note that the aerosol samples weresuccessively taken by 10 timer-controlled HV samplers
without anthropogenic interference, except for replacing the sampling filters over the course of a few hours
every 2–4 weeks. In all sampling processes, aerosol

filter samples were collected at a height of 1.2 m above
the ground. The sampling periods ranged from 3 days
to 4 weeks.
2.1.2 Collection of soil samples

About 200 g of surface soil samples was collected on June25, 2016, at the same site as the aerosol-sampling location
with a 10 × 10 ­cm2 area and 1.5 cm depth and were transported to the laboratory and stored at ambient temperature. They were homogenized in a plastic bag (Onda et al.
2015) for measurement of their radioactivity and stored
for further analysis. For extracting the CsMPs, approximately 2.5 g soil sample was randomly retrieved (theradioactivity was 173.6 ± 2.00 Bq/g and 166.2 ± 0.40 Bq/g
for 134Cs and 137Cs, respectively). The radioactivity datain this study were all corrected back to March 11, 2011,
14:46 JST.

Tang et al. Progress in Earth and Planetary Science(2022) 9:17

2.2 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 andisolate CsMPs from the aerosol filters. The isolation procedure of CsMPs from the aerosol filters and from thesoil samples was conducted as the similar dry process(without use of water) of Adachi et al. (2013) and Satouet al. (2016), respectively.
2.3 Measurement of gamma‑ray spectrometry and SEM
observation

The gamma-ray peak intensities at 605 and 662 keV wereused to identify and determine the radioactivities of134
Cs and 137Cs, respectively (Satou et al. 2016). Identification of the CsMPs was based on the morphology andelemental 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) equippedwith energy-dispersive X-ray spectroscopy (EDS: X-max
50 mm, Horiba Ltd., Kyoto, Japan) for aerosol CsMPs
and a field emission SEM (JSM-7800F, JEOL Ltd. Tokyo,
Japan) for visualizing soil CsMPs and water-immersed
aerosol CsMPs.

3 Results and discussion
3.1 Identification of resuspension of CsMPs
3.1.1 Aerosol CsMPs (A‑CsMPs)

Twenty highly radioactive granules were detected fromthe aerosol samples by using IP imaging, 15 of whichhave been successfully isolated, and the rest of which waslost during the isolation procedures. Their SEM observation images are shown in Fig. 2, and the detailed information regarding radioactivity and elemental compositions
is given in Table 1 and Additional file 1: Table S1. These
results indicate that all isolated granules have characteristics corresponding to those reported for type A CsMPs(Igarashi et al. 2019a; Satou et al. 2018) as described
below.
All CsMPs were of an almost spherical or distorted
spherical shape, and their diameter distribution ranged
from 1.2 to 3.0 μm with the same mean and median values of 1.8 μm. The 137Cs radioactivity of the single CsMP
varied from 0.13 to 0.87 Bq/granule, and the mean andmedian values of 137Cs radioactivity were 0.35 Bq/granuleand 0.36 Bq/granule, respectively. The 134Cs radioactivity was less than the detection limit in most cases due toits shorter physical half-life of about 2 years than that of137
Cs. The elemental mapping images (a) and EDS spectra (b) of #11-HNVF-300601-Q are given in Fig. 3. It is
obvious that Cs in the particle showed multiple peaks (CsPage 5 of 15

Lα and Cs Lβ at 4.42 and 4.55 keV), and O, Si, Na, S, Cl,
K, Ca, Ti, Fe, and Zn also coexisted within the particle
according to the elemental mapping images of major elements. Moreover, based on EDS analysis of 15 CsMPs, themain elemental components included Si (average atomic
mass fraction 36.0 ± 5.3 wt.%), O (50.1 ± 3.7 wt.%), Cs(1.0 ± 0.2 wt.%), Zn (2.7 ± 1.0 wt.%), Sn (0.2 ± 0.2 wt.%), S(0.1 ± 0.1 wt.%), Na (3.2 ± 3.3 wt.%), K (2.5 ± 2.2 wt.%), Fe(1.0 ± 0.3 wt.%), Cl (0.2 ± 0.1 wt.%), and some other traceelements in some individual particles, such as Ba, Al, P,
Ca, and Ti. The elemental composition of each CsMPs isgiven in Additional file 1: Table S1.
The water insolubility of radiocesium in CsMPs has
been reported by Adachi et al. (Adachi et al. 2013), andother reports also support this fact [CsMPs can be isolated from the wet-isolation method (Imoto et al. 2017;
Kurihara et al. 2020a) and can be detected in river water(Miura et al. 2018)]. 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 Additional
file 1: Fig. S1. There was no obvious detectable change
in their radiocesium activity and seeable change in morphology before and after the immersion. Therefore, we
conclude that CsMPs exhibit very low water-solubility
in short term. Regarding the long-term solubility, there
have been a few research (Miura et al. 2018; Okumura
et al. 2019). The morphology, the 137Cs radioactivity, theelemental compositions and distributions, and the water
insolubility of isolated CsMPs in this study were consistent with those of the reported type A CsMPs (Adachi
et al. 2013; Igarashi et al. 2019a; Satou et al. 2018; Abeet al. 2014). We can conclude that type A CsMPs aredetected 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. (2020)) and we refer tothese soil CsMPs as “S-CsMPs.” The 137Cs radioactivityof these S-CsMPs mostly ranged from 0.06 to 0.95 Bq/
granule, except for one that exhibited 6.70 Bq. Theirmean and median values were 0.29 and 0.22 Bq/granule,
respectively, and the sum of 137Cs radioactivity of these
S-CsMPs was 13.4 Bq. Six single granules from the groupof 24 S-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 Fig. 4
and Additional file 1: Table S2, respectively. They were
all spherical particles, and their diameters were 1.8 μm(S-CsMPs-1, 0.93 Bq/granule), 1.6 μm (S-CsMPs-2,

Tang et al. Progress in Earth and Planetary Science(2022) 9:17

Page 6 of 15

Fig. 2 Scanning electron microscopic images of 15 single A-CsMPs isolated from the aerosol filter samples

0.56 Bq/granule), and 1.8 μm (S-CsMPs-3, 0.75 Bq/
granule), with an average diameter of 1.7 ± 0.1 μm(0.75 ± 0.18 Bq/granule).

Moreover, the EDS spectra of an A-CsMP (#15-HNVA190919) and an S-CsMP (S-CsMPs-3) are shown in comparison in Fig. 5. These EDS spectra showed that both
A-CsMPs and S-CsMPs mainly consist of Si, O, Cs, Fe,

Tang et al. Progress in Earth and Planetary Science(2022) 9:17

Page 7 of 15

Table 1 Information of 15 single isolated insoluble Cs-bearing particles (The radioactivity is corrected back to 11/03/2011 14:46, JST)No.

Sample name137
Cs (Bq) in filter(T)137
Cs (Bq) per
granule (P)Resuspension contribution ratio of
CsMPs ( TP × 100%)D / μm1

NHVA-270504_Fri-Q–a

2.03

0.18

8.7%

1.2

2

NHVA-270504_Fri-Q-b

2.03

0.40

20.1%

2.0

3

NHVA-270618_Wed-Q–a

1.53

0.13

8.6%

1.7

4

NHVF-270730-J-Q3

2.44

0.38

15.7%

1.2

5

NHVA-271015_Mon-Q

1.77

0.36

20.5%

2.5

6

NHVA-271105_Tue-Q

1.61

0.13

8.3%

1.9

7

NHVF-280128-Q

0.84

0.17

20.6%

1.8

8

NHVB-280310-I-Q7

1.36

0.67

49.5%

1.4

9

NHVB-280429-I-Q7

1.91

0.52

27.0%

1.8

10

NHVF-300322-Q-b

1.16

0.17

15.0%

1.5

11

NHVF-300601-Q

1.73

0.87

50.6%

2.0

12

NHVF-300906-Q

3.55

0.18

5.0%

1.4

13

NHVA-190327-C-Q-b

0.65

0.17

25.6%

3.0

14

NHVA-190425-B-Q

1.36

0.60

43.9%

2.0
1.2

15

NHVA-190919-A-Q

0.90

0.36

39.2%

Mean

1.66

0.35

23.9%

1.8

Median

1.61

0.36

20.5%

1.8

Zn, K, Na, and Cl. Specifically, the average atomic massfraction of the main elemental compositions of thethree 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 eachother. These results imply that A-CsMPs were derived
from the resuspension of S-CsMPs.
3.2 Possibility of natural resuspension of CsMPs
into the atmosphere

The possibility of CsMP resuspension from the contaminated surface soil was suggested in studies (Ishizuka et al.
2017; Higaki et al. 2020; Nihei et al. 2018; Utsunomiya
et al. 2019). Specifically, CsMPs were detected in facemasks worn by Fukushima residents in the spring of 2013
(Higaki et al. 2020). The resuspended CsMPs were also
found on Japanese mustard spinach in Fukushima Prefecture cultivated from August to December 2017 (Nihei
et al. 2018). The resuspension of radiocesium caused bydecontamination work has also been reported by several
studies (Ikehara et al. 2020; Hosoda et al. 2019; Doi et al.
2018), suggesting a possibility of the resuspension of theCsMPs induced by decontamination work by the government. However, these studies have not discussed whether
the resuspension of CsMPs into the atmosphere canoccur naturally or not because CsMPs in these studies

were found in living areas or in association with humanactivities. In contrast, A-CsMPs of this study were foundin aerosol samples collected by unmanned-operated HVsamplers at a site located in an evacuated area that wasdifficult 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 andother remediation activities were carried out. If a significant number of radioactive pollutants, including CsMPs,
were transported to the sampling site, the atmosphericconcentration of 137Cs should have increased. However,a systematic increase in atmospheric 137Cs concentration was not detected during the sampling periods of
A-CsMPs, and no leakage events of radioactive pollutantsfrom the F1NPP were reported. These results imply a low
likelihood that A-CsMPs were derived from decontamination work at the F1NPP site and the severely contaminated areas. On the one hand, transport of contaminatedsoil 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 specifically, only one A-CsMP was foundafter June 2019, as shown in Table 2. Therefore, it seems
unlikely that A-CsMPs in this study were derived fromhuman activities.

Tang et al. Progress in Earth and Planetary Science(2022) 9:17

Page 8 of 15abFig. 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

Tang et al. Progress in Earth and Planetary Science(2022) 9:17

Page 9 of 15

Fig. 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)

lowering the Cs concentration with time lapse implies
mitigation of the anxieties for the hazard resulting fromthe resuspension of the CsMPs. Moreover, we observed
a typical suspension phenomenon of surface soil on April
22, 2021, in the playground of Ibaraki University, where
have similarly environmental circumstances as our sampling site. It can indirectly provide the natural resuspension condition of CsMPs by the wind gust of which speed
exceeding 4 m ­s−1, as shown in Additional file 1: Fig. S2and Additional file 2: Supporting video.
3.3 Resuspension frequencies of CsMPs

Fig. 5 Energy-dispersive X-ray spectrum of A-CsMPs (#15-HNVA-1909
19) and S-CsMPs-3. (Cu and C are derived from copper and carbon
tape substrates, which were used to confirm the existence of silicon
and the fixation of CsMPs)

Significantly, we can find that the atomic ratios of Na/
Si (≈ 0.086), K/Si (≈ 0.045), and/or Cs/Si (≈ 0.031) of ourisolated A-CsMPs are smaller than that of the CsMPs(~ 0.139 / ~ 0.055 / ~ 0.040) released in the early stage(approximately 6 months after the Fukushima accident) (Okumura et al. 2020), even lower than that of theCsMPs collected at far distance away from F1NNP (Cs/Si ≈ 0.208) (Utsunomiya et al. 2019), herein, the concentration of Cs has been corrected back to March 11,
2011, 14:46 JST, and assuming that there has been no
decrease in Si. This special characteristic of our isolated
A-CsMPs quantitatively exemplifies the high possibilityof A-CsMPs resulted from the resuspension of S-CsMPs.
This phenomenon may be attributed to the slow environmental erosion by the loss of the very small amount of
Na and K, even the loss of Cs, which is consistent withthe report of slow environmental weathering dissolvable CsMPs (Okumura et al. 2019). This information of

In this study, 165 aerosol filter samples collected over
4 years (2015, 2016, 2018, and 2019) were used todetect and isolate the CsMPs. Although they were not
all aerosol samples because of time and cost considerations, the sampling periods of the used samples(165 samples) covered more than 80% of the total sampling period: 290.5(days)/365(days) in 2015, 352.5/366
in 2016, 346.5/365 in 2018, and 225.5/274 in 2019, and1215/1461 = 0.83 overall. As mentioned above, 20 highly
radioactive granules were detected in these 165 samples,and 15 single A-CsMPs have been successfully obtained.
The detailed data for A-CsMPs and aerosol filters are
given in Tables 1, 2, and Additional file 1: 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 1.2 × ­10−5 granule
per unit sampling-air volume (­ m−3).
Furthermore, the yearly frequency was obtained
as follows: 2.1 × ­10−2 granule·day−1 in 2015,
8.5 × ­10−3 granule·day−1 in 2016, 8.7 × ­10−3 granule·day−1
in 2018, and 1.3 × ­10−2 granule·day−1 in 2019. There
was no considerable decreasing trend of these resuspension frequencies, which strongly suggests necessityof the long-term monitoring of resuspended CsMPs.
Alternatively, the seasonal frequency variation of resuspended A-CsMPs can be found in Table 2. The number

Tang et al. Progress in Earth and Planetary Science(2022) 9:17

Page 10 of 15

Table 2 Seasonal or yearly frequency variations of 15 single isolated insoluble CsMPs
Sampling year

Jan

Feb Mar Apr May Jun Jul

Aug Sep Oct Nov Dec Total Yearly frequency

0

0

0

0

2

1

1

0

0

2

0

0

6

2015

Number of CsMPs

Sampling period/days 1801414.5 31

30

31

31

30

31

29

31

290.5

2016

Number of CsMPs

1

0

1

1

0

0

0

0

0

0

0

0

3

Sampling period/days 31

29

31

30

31

30

31

31

30

31

30

17.5 352.5

2018

Number of CsMPs

0

0

1

0

1

0

0

1

0

0

0

0

3

Sampling period/days 12.5

28

31

30

31

30

31

31

30

31

30

31

346.5

2019

Number of CsMPs

0

0

1

1

0

0

0

1

0

0

0

0

3

Sampling period/days 23.5

030.5 30

31

30

31

31

18.5

0

0

0

225.5

2.1 × ­10−2 granule·day−1
8.5 × ­10−3 granule·day−1
8.7 × ­10−3 granule·day−1
1.3 × ­10−2 granule·day−1
Seasonal frequency

Dec.–Feb. (DJF)

Number of CsMPs

1

Sampling period/days 221.5
Mar.–May (MAM) Number of CsMPs

8

Sampling period/days 335
Jun.–Aug. (JJA)

Number of CsMPs

Sep.–Nov. (SON)

Number of CsMPs

4

Sampling period/days 368
2

Sampling period/days 290.5

and seasonal frequencies of A-CsMPs detected in four
seasons of four sampling years are as follows: (a) 1 and
4.5 × ­10−3 granule·day−1 in December–February (DJF),(b) 8 and 2.4 × ­10−2 granule·day−1 in March–May(MAM), (c) 4 and 1.1 × ­10−2 granule·day−1 in Jun–
August, and (d) 2 and 6.9 × ­10−3 granule·day−1 in September–November. The seasonal frequency tended
lower in DJF and higher in MAM. In DJF the surface soil
is mostly frozen or very wet, and snow often covers theground at the sampling site (Ishizuka et al. 2017), andthese conditions can inhibit the resuspension of CsMPsfrom the surface soil in this season. Kinase et al. (2018)
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 theground surface occurred in this season. It is reasonable
and possible that this dispersion process may cause theresuspension of CsMPs. Kinase et al. (2018) also foundthat the peak of the atmospheric radiocesium concentration was consistent with maximal average wind speed in
the springtime. Based on the above discussion, it would
be obvious that the atmospheric radiocesium concentration was associated with wind-driven suspended soil inthe springtime. This seasonality is consistent with thenatural resuspension of the CsMPs in this study.
Meteorological data, including the gust, the averagewind speeds (the wind speeds were sampled at a heightof approximately 10 m from the ground), the mean soil–
water content, and the relative humidity (RH), were
measured at the sampling site in 2015, when six CsMPs

4.5 × ­10−3 granule·day−1
2.4 × ­10−2 granule·day−1
1.1 × ­10−2 granule·day−1
6.9 × ­10−3 granule·day−1

were sampled. These data from May 2015 are plotted inFig. 6. Two CsMPs were collected when the wind gustspeed exceeded 4 m ­s−1 and the soil water content was
not notably high (significantly, during the sampling time,
wind gust speeds exceeding 4 m ­s−1 can be frequently
observed and sometimes even exceeded 8 m ­s−1, asshown in Fig. 6 and Additional file 1: Fig. S3). Furthermore, Wu et al. (1992) observed that particles (withdiameters ranging from 7.0 to 42.3 μm) could be suspended occasionally at wind speeds of 4 m ­s−1, with morethan half of the particles suspended within 2 s at a windspeed of 8 m ­s−1. Nicholson (1993) obtained a gentle
increasing trend for suspension of particles with a 4.1 μm
diameter along with increments of wind speed, and apositive correlation between P­ M2.5 and a wind speed
higher than 3 m ­s−1 was also found by Wang and Ogawa(2015). These characteristics of microparticle suspension
can indirectly explain the resuspension of CsMPs and thelow frequency of A-CsMP detection in this work. Additionally, Ishizuka et al. (2017) developed a size-resolved
suspension scheme for soil particles from the groundsurface to estimate and explain measured size distribution of resuspended radioactive particles at the samplingsite same as ours. They found that the particle size distributions of radioactive particles, adopting sandy loam
and silty loam as soil textures, were in good agreementwith that of the particles with diameters ranging around
1–2 μm in the size-resolved atmospheric radioactivity
concentration observation. Thus, natural resuspensionof CsMPs with diameter of 1–2 um by wind could beTang et al. Progress in Earth and Planetary Science(2022) 9:17

Fig. 6 Temporal variations of the average wind speed, the gustspeed, the mean soil–water content, and the relative humidity datacollected in May 2015 (The average wind speed and gust speed
mean an average wind speed was recorded per 30 min by the
sampler. Also, for Gust wind, the value was output only considering
data when it is 1 m/s or more)

possibly and reasonably observed when wind speed washigh and soil moisture was low. Especially, the large scaleof a SEM image of a representative aerosol filter sample
collected on May 16, 2016, as shown in Additional file 1:
Fig. S4, exhibits many microparticles from topsoil, which
can also be observed in the report of Kaneyasu et al.
(Kaneyasu et al. 2017). Obviously, it was almost impossible to find the location of a CsMP in a filter or soil
sample before isolation. However, these observations
confirmed the resuspension of CsMPs absorbing/attaching on surface soil by strong winds. Also, in Fig. 2, it canbe found that the isolated aerosol CsMPs (#9 NHVB280429-I-Q7 and #14 NHVA-190425-B-Q) were still
adsorbed on other large particles. In addition, in our isolated S-CsMPs, the phenomenon of S-CsMPs absorbing
other particles was not clear, but it was able to find in thereports of Satou et al. (2018) (samples of FT-13#01 and
NM-14#02). These results further supported the resuspension mechanism of CsMPs: they are readily absorbed/

Page 11 of 15

attached on topsoil particles and resuspended into air
by high wind speed. Therefore, we speculate that CsMP
resuspension needs both a strong wind of 4 m ­s−1 andincidental phenomena such as aggregate lumps formedby attachment/collisions of CsMPs with larger dust particles scattered by winds. Furthermore, according to SEM
observations of aerosol CsMPs and soil CsMPs, the possibility of soil dust working as the carriers of the resuspended CsMPs is high. Moreover, it is unable to exclude
CsMP resuspended by itself, obtained from the reasons:(1) the high CsMPs concentration of 10 granules/gramin the surface soil of our sampling site, (2) high resuspension frequency of CsMPs in spring when major suspended particles sampled there was soil dust, and (3) thereports of Ishizuka et al. (2017) that the particle size distributions of radioactive particles, adopting sandy loam
and silty loam as soil textures, were in good agreementwith that of the particles with diameters ranging around
1–2 μm in the size-resolved atmospheric radioactivity
concentration observation. However, the organic matterworks as the carrier of the resuspended CsMPs were notclear in this study. Additionally, because some holes thatwere presumably dug by wild boars were found aroundthe sampling site (within a few hundred meters), a possibility, that the dispersion of CsMPs occurred with suchanimal activities, cannot be ruled out. It is a great challenge to discuss more detailed information by random
appearance of wild animals in the sampling location, also,
this consideration is waiting for more observations.
3.4 Contribution of the resuspended CsMPs
to atmospheric radioactivity

Various studies (Ikehara et al. 2020, 2018) have shownthat significant levels of CsMPs were released into theenvironment (Furuki et al. 2017) and that a considerable part of the deposition of radiocesium should be
attributed to the released CsMPs (Imoto et al. 2017;
Utsunomiya et al. 2019). Based on the results and discussion above, CsMPs can be directly resuspended into
atmosphere. Herein, the contribution ratio of the resuspended CsMPs to atmospheric 137Cs can be defined as
the ratio of the radioactivity of single CsMPs (137Cs) tothat of the aerosol filters 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 themean and median values were 23.9% and 20.5%, respectively. The remainder of the resuspension hosts of atmospheric radiocesium can be attributed to bioaerosols andsoil particles bearing with low concentrations of radiocesium (Igarashi et al. 2019b; Kita et al. 2020) 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 filter by IP inspection

Tang et al. Progress in Earth and Planetary Science(2022) 9:17

Page 12 of 15

when the IP plate was exposed for tens of hours, as
shown in Additional file 1: Fig. S5. Because of the excessive amount of time and human resources required forthe extraction procedure, further extraction of CsMPs inthese filters has not been conducted. Moreover, the average contribution ratio of A-CsMPs, which is the ratioof total 137Cs radioactivity of 15 A-CsMPs to that of all
165 aerosol samples, was estimated to be about 2.0%, asshown in Additional file 1: Table S3. Correspondingly,
the ratio of total 137Cs 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 tothe total atmospheric radioactivity, therefore, was smaller
than that in the corresponding surface soil.

et al. 2019), and plutonium (Kurihara et al. 2020b) into
the environment, although they are at very low level, andCsMPs resuspension increases the possibility of unpredictable inhalation. If they are inhaled, they can easily
reach the deep respiratory system (Assessment N. C. f. E.,
NC 1996). Thus, more investigation is necessary regarding the risk assessment of even the single CsMP radiation
exposure (Suzuki et al. 2020) and long-term monitoringof these resuspended insoluble type A CsMPs.

4 Conclusions
This study reveals the occurrence of direct resuspensionof water-insoluble CsMPs, composed of silicate glass, tothe atmosphere without human interference. The fact canbe confirmed by (1) their successful extraction from aerosol samples collected by unmanned-operated samplers,(2) the nearly resembling characteristics of A-CsMPs andS-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(3) the seasonality of the A-CsMP detection. The sizes
and radioactivities of both A-CsMPs and S-CsMPs foundin 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 (witha diameter of less than ca. 10 μm) (Adachi et al. 2013;
Abe et al. 2014) sampled in primary emission from the
F1NPP accident in 2011. This can be used to exclude theprobability 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.
(Okumura et al. 2019). Therefore, the erosion of CsMPsin the natural environment should be a significant process to be clarified 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 absoluteactivity 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 foundfrom annual resuspension frequencies, suggesting its
persistence. The CsMPs can also work as the transferring
medium of fuel debris (Ochiai et al. 2018), trace uranium(Abe et al. 2014; Kurihara et al. 2020a), strontium (Zhang

Supplementary Information

Abbreviations
F1NPP: Fukushima Daiichi Nuclear Power Plant; CsMPs: Radiocesium-bearing
microparticles; A-CsMPs: Aerosol radiocesium-bearing microparticles; S-CsMPs:
Soil radiocesium-bearing microparticles; JAEA: Japan Atomic Energy Agency;
MRI: Meteorological Research Institute; TEPCO: Tokyo Electric Power Company.

The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​022-​00475-6.
Additional file 1. Supporting figures and tables.
Additional file 2. Supporting video: suspension of surface soil.
Acknowledgements
We thank Ms. N. Hayashi, Mr. K Minami, Ms. H. Li, and other students of theCollege of Science and College of Engineering, Ibaraki University, for the
maintenance of the sampling instruments, special installations in the field,
and data collection. We especially thank the local government of Namie andthe Fukushima prefectural government, who kindly provided the monitoring
opportunity. We also acknowledge Mr. K. Hama (Atox Co. Ltd) and Mr. K. Obara
for assistance with the activity measurements and separation of A-CsMPs.
Authors’ contributions
PT and KH analyzed the samples and summarized these data in this article.
PT prepared the manuscript. KK planned and carried out the measurements
as well as the samplings. YI is the leader of our project team. KK and YI helpedin writing the manuscript and provided important discussions for this article.
YS, KA, and TK supported the measurement of the aerosol and soil CsMPs by
SEM–EDS. KN and AS collected and measured radioactivity of the soil samples.
YS and KA also provided important suggestions for this article. All authors readand approved the final manuscript.
Authors’ information
PT received his Ph.D. from University of Fukui in 2020 and worked at Ibaraki
University as a postdoctoral researcher and wrote this article during thisperiod.
KK received his Ph.D. from Tokyo University in 1990 and worked at Tokyo
University until 2002 as a research assistant. He moved to Ibaraki University
in 2002. His studies focus on the atmospheric ­O3 and ­NO2, the effects ofblack carbon (BC) on global warming, and the resuspension processes ofradiocesium.
YI received his Ph.D. from the University of Tsukuba in 1987. From 1987 to
1991, he worked at the former National Institute of Radiological Sciences andmoved to the MRI in 1991. His current interests are atmospheric aerosols; theirprecursors, including Asian dust and P­ M2.5; their possible influences on climate
and general environmental changes and other related phenomena.
YS received his Ph.D. from University of Tsukuba in 2016, worked as researcher
in Collaborative Laboratories for Advanced Decommissioning Sciences, Japan
Atomic Energy Agency. His current interests are focused on the research of
F1NPP accident released radioactive particles.
KA received his Ph.D. from Kobe University in 2005, worked at Arizona State
University between 2005 and 2011 as a postdoctoral/faculty research associate, and is currently studying atmospheric aerosols at the MRI.

Tang et al. Progress in Earth and Planetary Science(2022) 9:17

TK received his Ph.D. from Ibaraki University in 2016. His interests are focusing
on BC aerosols and Environmental Transfer of Radionuclides from the Fukushima Daiichi NPP Accident.
KN received his Ph.D. from Osaka University in 2007 and worked at Osaka University in 2008. He moved to JAEA and worked as a research scientist between
2009 and 2012; then, he returned to Osaka University. His main research fieldis nuclear- and radiochemistry.
AS received his Ph.D. from Osaka University in 1985 and then worked at theJapan Society for the Promotion of Science. He moved to Nagoya University,where he worked between 1987 and 1998. Then, he moved to Kyoto University in 1998, before moving to Osaka University in 1999. His main research fieldis nuclear and radiochemistry.
Funding
This study was financially supported by Nuclear Energy Science & Technology and Human Resource Development Project (International Joint Research
Program of Nuclear Decommission of Japan-UK, 30I125) and partly by Grantsin-Aid for Scientific Research (B) (Nos. 20H04318 and 17H01873).
Availability of data and materials
The datasets supporting the conclusions of this study are available as additional files (Supporting information file and Supporting video of suspensionof surface soil) and are available upon request to the corresponding authors,
Kazuyuki Kita (E-mail: kazuyuki.kita.iu@vc.ibaraki.ac.jp) and Yasuhito Igarashi(E-mail: igarashi.yasuhito.4e@kyoto-u.ac.jp).

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Graduate School of Science and Engineering, Ibaraki University, 2‑1‑1 Bunkyo,
Mito, Ibaraki 310‑8512, Japan. 2 Institute for Integrated Radiation and Nuclear
Science, Kyoto University, 2‑chome, Asashiro‑Nishi, Kumatori, Sennan, Osaka
590‑0494, Japan. 3 Collaborative Laboratories for Advanced Decommissioning
Science (CLADS), Japan Atomic Energy Agency (JAEA), 790‑1 Otsuka, Motooka,
Tomioka, Fukushima 979‑1195, Japan. 4 Meteorological Research Institute(MRI), Japan Meteorological Agency (JMA), 1‑1 Nagamine, Tsukuba, Ibaraki
305‑0052, Japan. 5 Japan Agency for Marine‑Science and Technology, ResearchInstitute for Global Change (RIGC), Institute of Arctic Climate and Environment
Research (IACE), 3173‑25 Showa‑machi, Kanazawa, Yokohama, Kanagawa
236‑0001, Japan. 6 Graduate School of Science, Osaka University, 1‑1 Machikaneyama, Toyonaka, Osaka 560‑0043, Japan.
Received: 20 August 2021 Accepted: 28 February 2022

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