Abe et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-020-00403-6(2021) 8:13

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open AccessWidespread distribution of radiocesiumbearing microparticles over the greater
Kanto Region resulting from the Fukushima
nuclear accident
Yoshinari Abe1,2* , Seika Onozaki1, Izumi Nakai1, Kouji Adachi3, Yasuhito Igarashi4,5, Yasuji Oura6,
Mitsuru Ebihara6,7, Takafumi Miyasaka3,8,9, Hisashi Nakamura8, Keisuke Sueki10, Haruo Tsuruta11 andYuichi Moriguchi12

Abstract
The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011 emitted a considerable amount ofradioactive materials. This study isolated radiocesium-bearing microparticles (CsMPs), a form of radioactive materials
emitted from the FDNPP at the early stage of the accident, from aerosols collected hourly on filter tapes at sevenmonitoring stations at the greater Kanto Region, including the Tokyo metropolitan area, on 15 March 2011. The
aerosols had a spherical shape ~ 1 μm in diameter with activity of less than 1 Bq of 137Cs per particle. Their physicaland chemical characteristics, including radioactivity ratio 134Cs/137Cs as well as chemical composition and state, areessentially the same as previously reported CsMPs. This study demonstrated that air parcels containing CsMPs
emitted from the FDNPP were widespread over the greater Kanto Region, more than 250 km away from the
FDNPP, during the daytime of 15 March. Trajectory analysis indicated that these particles were emitted from thereactor No. 2 of FDNPP between 14 March evening and 15 March early morning. The information obtained on thewidespread distribution of CsMPs can be useful for assessing the actual impacts of radioactive contamination fromthe FDNPP accident on the environment and human health.
Keywords: Fukushima Daiichi Nuclear Power Plant accident, Radiocesium-bearing microparticle, Suspended
particulate matter, Synchrotron radiation X-ray analysis, Trajectory analysis

1 Introduction
Considerable amounts of radioactive materials were released into the environment following the Fukushima
Daiichi Nuclear Power Plant (FDNPP) accident causedby tsunamis associated with the Tohoku Earthquake on
11 March 2011 (e.g., MEXT 2020; Steinhauser et al.
2014). To reveal the time evolution of atmospheric* Correspondence: y.abe@mail.dendai.ac.jp
1
Department of Applied Chemistry, Faculty of Science, Tokyo University of
Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
2
Division of Material Science and Engineering, Graduate School ofEngineering, Tokyo Denki University, 5 Senju Asahi-cho, Adachi-ku, Tokyo
120-8551, Japan
Full list of author information is available at the end of the article

radionuclide concentrations immediately after the accident, Tsuruta et al. (2014, 2018) and Oura et al. (2015)
investigated suspended particulate matter (SPM) collected on filter-tape at automated air pollution monitoring stations across Eastern Japan. More than 400
stations in Eastern Japan were in operation during theinitial period of the FDNPP accident. Analyzing atmospheric radiocesium concentrations at 99 of those SPM
monitoring stations, these studies revealed that radioactive materials emitted into the air from the FDNPP
were transported over Eastern Japan via several major
plumes during 12–23 March (Tsuruta et al. 2014, 2018).

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Abe et al. Progress in Earth and Planetary Science(2021) 8:13

As a form of radioactive materials emitted from the
FDNPP at the early stage of the accident, radiocesiumbearing microparticles (CsMPs) have been investigatedby numerous researchers in recent years (Igarashi et al.
2019). They were solid particles and not easily dissolved
into water, which first discovered by Adachi et al. (2013)
from aerosols collected at the Meteorological ResearchInstitute (MRI) in Tsukuba, 170 km south-southwest of
FDNPP, during 2110 JST 14 March and 0910 JST 15
March 2011. After the discovery of the CsMP, manystudies focused on their physical and chemical characteristics as well as their environmental distributions(Igarashi et al. 2019). While several types of CsMPs havebeen reported, almost CsMPs examined previously were
nearly spherical, several μm in diameter, and had relatively high specific radioactivity, i.e., ~ 1 Bq per particle
as 137Cs (Adachi et al. 2013; Furuki et al. 2017). Theirmain matrices were silicate glass (Satou et al. 2016) with
Fe and Zn (Adachi et al. 2013; Abe et al. 2014), and trace
amounts of various heavy elements associated with nuclear fuel and fission products (FPs) were detected (Abeet al. 2014). Physical and chemical characterizations ofradioactive materials, including their water solubility,
shape, size, and chemical composition and state, are critical factors that determine their behavior within the environment and human body. CsMPs and otherparticulate radioactive materials have previously been
identified in various environmental samples collected in
Fukushima Prefecture as follows: soils collected aroundthe FDNPP (e.g., Satou et al. 2016, 2018, Ono et al.
2017, Furuki et al. 2017, Martin et al. 2019, 2020), a
non-woven fabric cloth laid on an agricultural field(Yamaguchi et al. 2016), and river sediments (Miuraet al. 2018). Similar radioactive microparticles were also
found on masks worn during cleaning work in residential areas near the FDNPP 5 years after the accident(Higaki et al. 2017). As mentioned above, these CsMPs
found previously consist primarily of silicate glass. It isthus concerned that they have a long-term impact on
the environment compared to water-soluble radioactive
materials, whereas some recent investigations indicate avery slow rate of dissolution of CsMP into pure-water orseawater (Okumura et al. 2019; Suetake et al. 2019).
Meanwhile, some CsMPs containing chloride as a watersoluble compound had been recently found from aerosols collected near the FDNPP after the hydrogen explosion of the reactor No. 1 of the FDNPP (Onozaki et al.
2019). Comprehensive investigations of CsMPs withvarious physical/chemical properties are therefore vitally
important in accurately assessing the impacts of radioactive contamination from the FDNPP accident on theenvironment and human health.
Among the major polluted plumes identified by Tsuruta et al. (2014), the second plume (P2) carried thePage 2 of 10

aerosols collected at the MRI on 14 and 15 March 2011
(Adachi et al. 2013). After being observed at SPM monitoring stations in the vicinity of the MRI (Tsukuba) during the morning of 15 March, P2 was then spread into
the Kanto Plain, including the Tokyo metropolitan area,
one of the most populated areas in the world. The
southward spread of P2 was due to low-level northerlies
associated with an eastward-moving low-pressure system
located south of the Kanto Region (Takemura et al.
2011). Hypothesizing that CsMPs were the major carrierof radioactive Cs in P2, this study aims to verify thewidespread distribution of CsMP in P2 over the greater
Kanto Region (in and around the Kanto Region) on 15
March, focusing on how far CsMPs were transported
southwest from the FDNPP by the local wind system.
We examined radioactive aerosol particles from the
SPM filter-tape samples collected hourly on 15 March atseveral stations in the greater Kanto Region to compare
their physical/chemical characteristics with those ofpreviously reported CsMPs isolated from various
environmental samples. To investigate detailed chemical
characteristics of the radioactive aerosols isolated fromthe SPM filter-tape samples, we applied multiple X-ray
analytical techniques using a synchrotron radiation
microbeam (SR-μ) X-ray in a nondestructive manner.
The SR-μ-X-ray analysis is an analytical technique thatis commonly used for microscopic chemical
characterization of various materials in the nondestructive manner and is quite suitable for CsMPs as demonstrated by several previous studies (Abe et al. 2014; Onoet al. 2017; Onozaki et al. 2019; Miura et al. 2020; Kurihara et al. 2020a). This study has implications for the
impacts of radioactive materials on the environment andhuman health as well as the reactor condition during theearly stage of the accident.

2 Methods/experimental
2.1 SPM filter-tape samples and separation method ofparticles

The SPM monitors within the Japan air pollution monitoring network are routinely operated by local governments (prefectures and municipalities designated byordinance). Particulates less than 10 μm in diameter
were automatically collected on the filter tape installedin the SPM monitors as a sample spot (11 or 16 mm indiameter) for 1 h at a flow rate of 15.0, 16.7, or 18.0 l/
min. The filter tape was made of glass fiber or polytetrafluoroethylene. Detailed information of the SPM filtertape samples has been described in previous studies(Tsuruta et al. 2014, 2018; Oura et al. 2015). Seven
pieces of the filters sampled at seven monitoring stations
designated as A, B, C, D, E, F, and G within the greater
Kanto Region (see Table 1 and Fig. 1) were used in thisstudy. Each piece was selected which had the highest

Abe et al. Progress in Earth and Planetary Science(2021) 8:13

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Table 1 Information of seven monitoring stations (A~G) and SPM filter-tape samples investigated in the present study
Cs/Bq m−3

StationLatitudeLongitude

Distance from FDNPP

Sampling time (JST)

137

A

35.85

140.25

189 km

15 March 0800–0900

22.1

B

36.19

139.13218 km

15 March 1200–1300

59.1

C

35.78

139.62229 km

15 March 1000–1100

81.9

D

35.65

139.59236 km

15 March 1000–1100

29.4

E

35.37

139.22280 km

15 March 1400–1500

29.3

F

36.33

138.44261 km

15 March 1600–1700

22.1

G

36.41

138.24273 km

15 March 1600–1700

32.8

137

Cs concentration on March 15 at each station under
the direct influence of P2. We cut these filters into fouror eight portions in collecting radioactive particles. An
imaging plate (GE Measurement and Control, CR × 25P
computed radiography scanner) and micromanipulator(AP-xy-01; Micro Support Corp.) were used to detectand separate radioactive particles from the filters. The
amount and distribution of radioactive particles on thefilter was observed in a process similar to previous studies (Adachi et al. 2013; Abe et al. 2014). Prior to the SR
experiments, a low-vacuum scanning electron

microscope (SEM; SU 3500; Hitachi High-Technologies)
was used to observe the shapes of individual particles
isolated from the filters. The radioactivity of 134Cs and137
Cs in each particle was determined by using a Ge
semiconductor detector (GC4018; CANBERRA) coupledwith a multichannel analyzer (Lynx Digital Signal
Analyzer; CANBERRA). The gamma-ray spectrum wascollected for more than 400,000 s per particle. Twostandard radioactive sources, 134Cs standard by Japan
Radioisotope Association and 137Cs standard by Amersham plc, were measured in the same manner toFig. 1 Locations of monitoring stations A~G in the greater Kanto Region and radioactive particles isolated in this study. Coordinates andcollection time of the filters are shown for each station. White dashed lines on the map indicate distance from the FDNPP. Eight CsMPs wereisolated from these seven filters as displayed by SEM images around the map. They are nearly spherical in shape with a diameter of 1.3 μm(particle A1), 1.3 μm (B1), 1.4 μm (C1), 1.5 μm (D1), 1.2 μm (D2), 1.3 μm (E1), 0.9 μm (F1), and 1.8 μm (G1). The scale bars under the SEM images
show a length of 1 μm

Abe et al. Progress in Earth and Planetary Science(2021) 8:13

calibrate radioactivity of the particles. Absolute value
and statistical error of radioactivity of 134Cs and 137Cs inindividual particles were calculated as decay-corrected
data at the time of 1446 JST 11 March 2011. After theseanalyses, the radioactive particles on carbon tape fragments were removed and then placed on a flat Kapton
tape with a plastic holder for the SR-μ-X-ray analyses.
2.2 Synchrotron radiation X-ray analyses of CsMPs

The SR experiments using an X-ray microbeam werecarried out at the BL37XU (Terada et al. 2004, 2010), a
hard X-ray undulator beamline at SPring-8, located at
the Japan Synchrotron Radiation Research Institute(JASRI). The sample was placed on an automatic XY
stage in the experimental hatch. Monochromatic X-rays
were obtained with a Si (111) double crystal monochromator, and the X-ray microbeam with the size of ~1 μm (V) × ~1 μm (H) was produced by focusing
Kirkpatrick−Baez mirrors. We applied three X-ray analytical techniques: SR-μ-X-ray fluorescence (XRF) analysis,
SR-μ-X-ray absorption near edge structure (XANES) analysis, and SR-μ-X-ray powder diffraction (XRD) analysis.
The measurement conditions of the SR-experiments were
the same as in our previous investigations (Abe et al.
2014; Ono et al. 2017; Onozaki et al. 2019).
The SR-μ-XRF analysis was carried out using 37.5 keV
X-rays that enable to excite K-edges of Cs (36.0 keV)
and Ba (37.4 keV). Three types of energy-dispersive Xray detectors were used depending on the beamtime: aSi (Li) detector for the beamtimes until 2017, and eightelements silicon drift detectors or a Ge semiconductor
detector for the beamtimes in 2018 and 2019. The SR-μXRF spectrum was measured for 200 s in live time persample. The intensity of each spectrum was normalized
to that of the Thomson scattering peak detected at 37.5
keV. The SR-μ-XANES spectra of the particles and thereference samples were measured in fluorescence mode
for the following absorption edges: Fe-K edge (7111 eV),
Zn-K edge (9661 eV), Mo-K edge (20,000 eV), and Sn-K
edge (29,200 eV). The intensity of Kα line of each target
elements in individual particles was scanned with ameasurement step of ~ 1 eV/step, integration times of
3.0~10.0 s/step, and an energy range of ~ 100 eV fromthe lower to the higher energy sides of the absorption
edge. To normalize the Kα intensity of the target elements, an intensity of the incident X-ray beam (I0) wasmonitored using an ionization chamber. Reference materials (powders of metals, typical oxides, sulfides, silicates,and synthesized glass samples) containing each target elements were also measured as a same manner. In the
SR-μ-XRD analysis, the X-ray diffraction patterns of thesamples were measured with a Debye−Scherrer opticalsystem using a two-dimensional detector (CMOS flat
panel) placed 200 mm behind the sample. The energy ofPage 4 of 10

the incident X-ray was set to 15.0 keV with an exposure
time of 440 ms and an integration of 100 times/sample.
2.3 Trajectory analysis for the radioactive plume transport

A meteorological trajectory analysis was conducted toevaluate air parcel positions every 10 min based on windfields from the Japan Meteorological Agency mesoscalemodel objective analysis. The analysis has a horizontal
resolution of 0.0625 × 0.05° with 50 vertical levels up to
21,800 m. Three-hourly analysis data of wind on modellevels were used, as well as 3-hourly analysis and 1hourly forecast data of 10 m surface wind. The threedimensional wind data cannot resolve small-scale turbulence that is most vigorous within the mixed layer. Airparcels were initially placed at 50 m intervals from 50 m
to 1000 m above the surface. For each parcel, the trajectory calculation was terminated if it hit the surface.

3 Results and discussion
3.1 Physical/chemical characteristics of CsMPs isolated
from SPM filter-tape

In total, eight CsMPs were successfully isolated fromseven pieces of SPM filter-tape samples. As illustrated inFig. 1, the eight CsMPs were labelled as A1, B1, C1, D1,
D2, E1, F1, and G1 in corresponding to the seven SPM
monitoring stations. The particles were spherical withdiameters of ~ 1 μm, and their radioactivity was lessthan 1 Bq of 137Cs per particle (Fig. 2). The radioactivity
ratios between 134Cs and 137Cs (134Cs/137Cs) were ~ 1.0,
suggesting that these particles were emitted from either
reactor No. 2 or 3 of FDNPP (Nishihara et al. 2010) (seeadditional data Table S1 online). As first pointed out by
Satou et al. (2018), CsMPs from the FDNPP can be categorized into two major types, type A and type B, basedon the 134Cs/137Cs ratio of individual particles. Type A
particles are characterized by 134Cs/137Cs ratio of ~ 1.0,
in contrast to type B particles with 134Cs/137Cs ratio of ~0.9.
As a result of the SR-μ-XRF analyses of individual particles, A1~G1 were found to have qualitatively-similar
chemical compositions. The SR-μ-XRF spectra of four
representative particles (A1, B1, C1, and E1) are shownin Fig. 3 with that of a type A CsMP collected at the
MRI in a previous study (Abe et al. 2014). Note that allthese spectra were measured using the same Si (Li)detector in several beamtimes. The XRF analysis using a
monochromatic SR-μ-X-ray with high-energy (37.5 keV)for the excitation can detect trace amounts of heavy elements within individual particles, although lighter elements (such as Si) that are major components of theparticle could not be detected in the spectrum. In
addition to sharp K-line peaks of Cs which had been
identified by the gamma-ray spectroscopy, the following
eight heavy elements were detected in all particles: Fe,Abe et al. Progress in Earth and Planetary Science(2021) 8:13

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Fig. 2 Radioactivity of 137Cs and 134Cs/137Cs activity ratio of individual particles. The radioactivity of 137Cs (± 1 × standard deviation, σ) of eightparticles (A1~G1) are as follows: 0.471 ± 0.016 Bq (particle A1), 0.426 ± 0.015 Bq (B1), 0.546 ± 0.019 Bq (C1), 0.425 ± 0.014 Bq (D1), 0.318 ± 0.011
Bq (D2), 0.156 ± 0.008 Bq (E1), 0.317 ± 0.011 Bq (F1), and 0.665 ± 0.023 Bq (G1). The three dashed lines show estimates of the 134Cs/137Cs ratio of
nuclear fuel in FDNPP reactors No. 1, 2, and 3, respectively, at the time of the accident, as calculated using ORIGEN2 Code (Nishihara et al. 2010).
Activity values shown here were decay-corrected as of 11 March 2011

Zn, Rb, Mo, Sn, Sb, Te, and Ba. Several trace elements
specific to certain particles were also found: Zr fromseven particles except for particle B1; Nb from particle
C1; Ag from particle F1, Cd from particles C1 and G1;
Pb from particles A1, B1, C1, E1, F1, and G1; and U
from particles A1, B1, and F1.
The SR-μ-XANES analysis examined the chemical
states of four metal elements (Fe, Zn, Mo, and Sn) andindicated that these elements exist as cations in silicate
glass with high oxidation numbers (see additional data
Fig. S1 online). SR-μ-XRD analysis of individual particles
showed no diffraction peaks caused by crystal structure
for any of the particles (see additional data Fig. S2 online), confirming that these particles have glass bodies.

As discussed above, physical/chemical characteristics
obtained for the eight CsMPs (A1~G1) collected at theseven SPM monitoring stations in the greater Kanto Region are essentially the same as those of type A CsMPs
found in the previous studies (e.g., Abe et al. 2014; Igarashi et al. 2019). As first reported by Utsunomiya et al.
(2019), it is already-known fact that air parcels containing type A CsMPs passed over Tokyo City at some pointon 15 March. Our results strongly support their pioneering report. At the same time, this paper first demonstrated that CsMPs emitted from the FDNPP werewidespread over the greater Kanto Region, includingWest side of Tokyo metropolitan area, during the daytime of 15 March with a temporal resolution of an hour.

Fig. 3 SR-μ-XRF spectra of four representative CsMPs (A1, B1, C1, and E1) isolated from the SPM filter-tape samples, showing the heavy elemental
composition of individual particles. The intensity of each spectrum is displayed on a logarithmic scale and shifted in a longitudinal direction. The
spectrum of MRI-A shows datum obtained for one of type A CsMPs (particle A) collected at the MRI in the previous study (Abe et al. 2014)

(2021) 8:13

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3.2 Transport pathway of CsMP from FDNPP to greater
Kanto Region

Kanto Region on 15 March were emitted from the
FDNPP between the evening of 14 March and earlymorning 15 March. After being released into the mixed
layer, these polluted air parcels were advected southward
by low-level northerlies and later by northeasterlies. The
parcels very likely underwent vigorous mixing down tonear-surface levels, within the mixed layer above the stations. The earliest emission time is estimated for theparcels above station E, as the trajectories detoured far
offshore under the northwesterlies shortly after emitted.
Our trajectory analysis implies that individual aerosol
particles that were emitted locally from the FDNPP subsequently spread widely over the greater Kanto Region
within a relatively short period of time (within 18 h at
the longest) under time-varying flow conditions. Basedon the results of our backward trajectory analysis, we
calculated forward trajectories for air parcels over the
FDNPP starting at 0100 JST 15 March 2011 and showed
them on the map with the seven monitoring stations
A~G (Fig. 5). The air parcels situated above the FDNPP
at that time moved southward and then widespread overthe greater Kanto Region within a day.

Abe et al. Progress in Earth and Planetary Science

To estimate the emission time of the CsMPs and identify
their transport pathway(s) from the FDNPP to thegreater Kanto Region, we conducted trajectory analysis
of air parcels that passed over the seven monitoring
stations A~G. As an example, Fig. 4 shows backward
trajectories for air parcels situated at different heights
over station B starting at 1200 JST 15 March, where the
SPM aerosols including particle B1 were collected during 1200–1300 JST. The trajectories are color-coded to
distinguish parcel heights at station B in Fig. 4a and alsocolor-coded by parcel height at each time step in Fig. 4b.
Figure 4a, b suggests that the air parcels situated at 150–
600 m above station B at 1200 JST were likely to belocated below the 500 m level within 20 km range fromthe FDNPP sometime between 0040 and 0330 JST on
the same day. The vertical profile of potential
temperature at station B (Fig. 4c) indicates that the atmospheric mixed layer was as deep as 800 m above thesurface, suggesting that air parcels within the layer
should have experienced vigorous turbulent mixing andthus be well-mixed down to near-surface levels. Likewise, air parcels from the FDNPP were released into thenight-time mixed layer. Advected by low-level northerlies, these air parcels then travelled along the coast
mostly within the marine mixed layer (Fig. 4b), which
was probably well developed in the early morning given
the cool offshore northerlies over the relatively warm
ocean.
Results of our backward trajectory analysis for theseven monitoring stations are summarized in Table 2.
Although the estimated emission times have ranges thatspan several hours, we concluded that polluted air parcels containing CsMPs that passed over the greater

3.3 Possible source of CsMPs distributed over greater
Kanto Region

The chemical composition of particles emitted primarily
from the FDNPP could reflect the reactor condition during the early stages of the accident. As indicated by Okumura et al. (2019), we cannot ignore the possibility that
CsMP collected from soils years after the accident wasaltered physically or chemically in the environment even
if they are hardly soluble into the water. Unlike suchfield samples, there would be little change in physicaland chemical properties of the CsMPs as they were sampled on the SPM filter-tape shortly after emission. The

Fig. 4 Backward trajectories calculated for air parcels placed above station B at 1200 JST 15 March 2011. The trajectories are color-coded (a) to
distinguish heights (m) of the parcels at station B and (b) according to their heights at each time step. Small and large dots on the trajectories
indicate intervals of every hour and 3 h, respectively, after the calculation start time. c Vertical profile of potential temperature (K) above station B
at the trajectory start timeAbe et al. Progress in Earth and Planetary Science(2021) 8:13

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Table 2 Summary of the backward trajectory analysis for seven monitoring stations (A~G)
StationStarting time(JST) ofbackward
trajectories

Depth ofatmospheric
mixed layer
above the station

Estimated paths of air parcels between the station and 20 km range from FDNPP
Height above
the station

Height above 20
km range from FDNPP

Estimated emission time(JST) from FDNPP

A

15 March 0800

~500 m

750-800 m

0-600 m

15 March 0000-0220

B

15 March 1200

~800 m

150-600 m

0-500 m

15 March 0040-0330

C

15 March 1100

~900 m

800-900 m

100-700 m

15 March 0210-0340

D

15 March 1100

~900 m

850-900 m

50-650 m

15 March 0120-0310

E

15 March 1400

~700 m

750-850 m

100-650 m

14 March 2000- 15 March 0130

F

15 March 1600

~200 m50-650 m

50-550 m

15 March 0040-0340

G

15 March 1600

~200 m50-300 m

100-300 m

14 March 2330- 15 March 0200

chemical compositions of the CsMPs discussed in thisstudy are thus likely to be preserved except for radioactive
decay effects. Therefore, our analysis offers important information regarding raw materials and the generation processof CsMP in the reactor. All elements identified in theCsMPs can be associated with materials in the FDNPP. As
a result of the nuclear fission reaction of 235U, the FPs
yielded 11 elements (Rb, Zr, Nb, Mo, Ag, Cd, Sn, Sb, Te,
Cs, and Ba) (Crouch 1977, Burns et al. 2012, Yamamoto2012). It is also possible that Zr and Sn in the CsMPs originate to Zr−Sn alloy used for fuel cladding within the reactors. On the other hand, vaporous elements, such as Rb(e.g., the boiling points of RbOH and RbI are about 1660 K
and 1577 K under 1 atm, respectively) and Cs (e.g., the boiling points of CsOH and CsI are about 1263 K and 1553 K
under 1 atm, respectively), in the CsMPs were richer thanthe original composition of FPs derived from U fuel. Therefore, condensation of these elements by vaporization in thereactor pressure vessel (RPV) could happen during the particle generation process. Moreover, the amounts of Cs in
the particles would be higher than those of Mo evenFig. 5 Forward trajectories calculated for air parcels placed above
the FDNPP at 0100 JST 15 March 2011. The trajectories are colorcoded to distinguish heights (m) of the parcels at FDNPP. Small andlarge dots on the trajectories indicate intervals of every hour and 3 hsince the calculation start time, respectively

considering the difference in excitation efficiency frommonochromatic X-rays (37.5 keV). Although Cs2MoO4 wassuggested as one possible source for the vapor phases carrying Cs at high temperature (Kissane and Drosik 2006;
Gouello et al. 2013; Do et al. 2018), our result indicates thatthe volatilization and condensation of Cs2MoO4 were notpredominant processes of the generating of the CsMPs. Do
et al. (2018) have pointed out that CsOH is the predominant cesium species when the damaged fuel temperature ishigher than 2000 K at higher steam pressures, but
Cs2MoO4 would become more important at lower
temperature. In this connection, Imoto et al. (2017) reported the presence of nanoparticles of CsFeSi2O6 within
CsMP and pointed out that the material could be formedby the CsOH chemisorption onto Si-bearing stainlesssteel. Iron, Cr, Mn, Ni, and Mo could originate from stainless steel, which composed RPV and FDNPP buildings.
Regarding a possible source for Zn, we previously identified an additive agent of primary cooling water (Abe et al.
2014), but a thin plating of steel and inorganic paint on
RPV and buildings could be other potential sources (Itouet al. 2018). Lead metal and Pb-containing materials arecommonly used for shielding of radiation. To make amore detailed interpretation of the generation process of
the CsMPs, it is indispensable to carry out further scientific investigation using other analytical methods to reveal
more quantitative chemical composition and isotopic features of individual particles, such as secondary ion mass
spectrometry (Imoto et al. 2017; Kurihara et al. 2020a;
Kurihara et al. 2020b).
As described above, type A CsMPs including eightparticles investigated in this study have been thought tobe originated to either reactors No. 2 or No. 3 of the
FDNPP based on their 134Cs/137Cs ratio: there are two
opinions about from which the type A CsMPs were
emitted (e.g., Igarashi et al. 2019; Ikehara et al. 2020).
We therefore consider the source of the type A CsMPs
in relationship to the accident progress, making use ofhigh time-resolution of the SPM filter-tape samples. Our
trajectory analysis suggests that the emission time for theAbe et al. Progress in Earth and Planetary Science(2021) 8:13

type A CsMPs isolated from the greater Kanto Region was
between midnight and early morning 15 March, althoughemission time could have been as early as the evening of
14 March for parcels that reached station E (Table 2).
Consistent with these estimated emission times, pressure
inside the RPV in reactor No. 2 decreased after the usageof a safety relief valve around 1903 JST on 14 March,followed by three sharp RPV pressure peaks around 2100
and 2300 JST 14 March and 0100 JST 15 March (TEPCO2015). Around 0300 JST on 15 March, the pressure inside
the primary containment vessel of reactor No. 2 exceeded
its designed value (TEPCO 2015). In contrast, no incident
was reported for reactor No. 3 during the same period except for a hydrogen explosion at 1101 JST 14 March. We
therefore hypothesized that the incident(s) in reactor No.
2 were the most likely cause of the type A CsMPs, rather
than reactor No. 3. Our hypothesis is strongly supported
by recent investigation of isotopic ratios of U and Cs in
CsMPs (Kurihara et al. 2020b).

4 Conclusions
Eight CsMPs were isolated successfully from aerosol particles collected hourly on filter tapes at seven monitoring
stations in the greater Kanto Region, including the Tokyo
metropolitan area, on 15 March 2011. Our finding demonstrates clearly that air parcels containing CsMPs emitted from the FDNPP were widespread over the greater
Kanto Region, farther than 250 km away from the FDNPP,
during the daytime of 15 March. Detailed physical andchemical properties of individual CsMPs were investigatedby SR-μ-X-ray analyses. As a result, it was concluded thatthe incident(s) in reactor No. 2 of FDNPP were the most
likely cause of CsMPs distributed over the greater Kanto
Region. Our trajectory analysis also suggests that air parcels containing the CsMPs as water-insoluble microparticles with radionuclides likely passed over the greater
Kanto Region including Tokyo metropolitan area on 15
March. Some of those particles could have been depositedon the ground or suspended in the near-surface air, although most of them were transported to the ocean. Further investigation is necessary to estimate theenvironmental and health impacts from the CsMPs thattravelled into the metropolitan area. Information regarding widespread distribution of CsMPs can be useful toward calculating an inhalation dose of radionuclides
during the early stage of the accident.
5 Supplementary Information
The online version contains supplementary material available at https://doi.
org/10.1186/s40645-020-00403-6.
Additional file 1: Table S1. 134Cs and 137Cs radioactivities andCs/137Cs activity ratio of eight particles (A1~G1). Figure S1. SR-μXANES spectra of four representative CsMPs (A1, B1, C1, and E1) isolated
134

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from the SPM filter and reference materials. (a) Fe-K edge, (b) Zn-K edge,(c) Mo-K edge, and (d) Sn-K edge. Figure S2. SR-μ-XRD patterns of four
representative radioactive particles (A1, B1, C1, and E1) isolated from the
SPM filter and reference material. (a) Particle A1, (b) particle B1, (c) particle
C1, (d) particle E1, and (e) silicon powder (NIST SRM 640c). In contrast to
sharp diffraction peaks detected in silicon powder, no obvious peaks
were detected in four radioactive particles.
Abbreviations
FDNPP: Fukushima Daiichi Nuclear Power Plant; SPM: Suspended particulatematter; CsMP: Radiocesium-bearing microparticle; JST: Japan Standard Time;
MRI: Meteorological Research Institute; FP: Fission product; SR-μ-Xray: Synchrotron radiation microbeam X-ray; JASRI: Japan Synchrotron
Radiation Research Institute; XRF: X-ray fluorescence; XANES: X-ray absorption
near edge structure; XRD: X-ray powder diffraction; SEM: Scanning electron
microscope; RPV: Reactor pressure vessel
Acknowledgements
We thank all local governments for allowing us to investigate the SPM filtertape samples. We are deeply grateful to Dr. Y Terada, Dr. K Nitta, and Dr. O
Sekizawa (SPring-8/JASRI) for their assistances in the SR experiment at
BL37XU, SPring-8. The synchrotron radiation experiments were performed
with the approval of the SPring-8 Program Advisory Committee (proposal
numbers 2016A1705, 2016B1811, 2017A1719, 2017B1757, 2018A1704,
2018B1748, and 2019B1308).
Authors’ contributions
YA performed the SR measurements, data analysis, and wrote themanuscript. SO performed the separation of particles and SR measurements.
IN and YI conceived the study idea. KA conducted the SEM analysis. YO andME controlled the SPM filter-tape samples. TM and HN performed the trajectory analysis. KS conducted the gamma-ray spectrometry. HT and YM helped
to devise the study idea and assisted with cooperation and coordination
with local governments. All authors read and approved the final manuscript.
Funding
This work was supported by MEXT/JSPS KAKENHI Grants (24110003,
15H00978, 19H05702), in collaboration with projects of Environment
Research and Technology Development Fund of the Environmental
Restoration and Conservation Agency, Japan (JPMEERF20155001,
JPMEERF20181002, and JPMEERF20192004). This work was also supported by
Joint Research on Risk Evaluation and Management in Nuclear Technology(Japan-U.K.), the Collaborative Laboratories for Advanced Decommissioning
Science, JAEA (JPJA18F18072860).
Availability of data and materials
Data sharing not applicable to this article as no datasets were generated oranalyzed during the current study. Please contact corresponding author fordata requests.
Competing interests
The authors declare that they have no known competing financial interests
or personal relationships that could have appeared to influence the work
reported in this paper.
Author details
1
Department of Applied Chemistry, Faculty of Science, Tokyo University of
Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. 2Division ofMaterial Science and Engineering, Graduate School of Engineering, Tokyo
Denki University, 5 Senju Asahi-cho, Adachi-ku, Tokyo 120-8551, Japan.
3
Meteorological Research Institute, 1-1 Nagamine, Tsukuba-shi, Ibaraki
305-0052, Japan. 4Institute for Integrated Radiation and Nuclear Science,
Kyoto University, 2-chome, Asashiro-Nishi, Kumatori-cho, Sennan-gun, Osaka
590-0494, Japan. 5College of Science, Ibaraki University, 2-1-1 Bunkyo,
Mito-shi, Ibaraki 310-8215, Japan. 6Graduate School of Science, Tokyo
Metropolitan University, 1-1 Minami-Osawa, Hachioji-shi, Tokyo 192-0397,
Japan. 7Department of Earth Sciences, School of Education and Integrated
Arts & Sciences, Waseda University, 1-6-1 Nishi-waseda, Shinjuku-ku, Tokyo
169-8050, Japan. 8Research Center for Advanced Science and Technology,
The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan.

Abe et al. Progress in Earth and Planetary Science(2021) 8:13

9
Japan Meteorological Business Support Center, 1-1 Nagamine, Tsukuba-shi,
Ibaraki 305-0052, Japan. 10Faculty of Pure and Applied Sciences, University ofTsukuba, 1-1-1 Tennodai, Tsukuba-shi, Ibaraki 305-8571, Japan. 11Remote
Sensing Technology Center of Japan, 3-17-1 Toranomon, Minato-ku, Tokyo
105-0001, Japan. 12Department of Urban Engineering, The University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.

Received: 23 September 2020 Accepted: 21 December 2020

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