Journal of Nuclear and Radiochemical Sciences, Vol. 15, No. 2, pp. 1-12, 2015

ArticlesA Database of Hourly Atmospheric Concentrations of Radiocesium (134Cs and 137Cs) in
Suspended Particulate Matter Collected in March 2011 at 99 Air Pollution Monitoring
Stations in Eastern JapanY. Oura,a M. Ebihara,a,*1 H. Tsuruta,b,*2,†1 T. Nakajima,b,†2 T. Ohara,c M. Ishimoto,dH. Sawahata,d Y. Katsumura,d and W. NittaeaDepartment of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji,
Tokyo 192-0397, Japanb
Atmosphere and Ocean Research Institute, University of Tokyo, Kashiwa, Chiba 277-8564, Japanc
National Institute for Environmental Studies, Tsukuba, Ibaraki 305-8506, Japand
Nuclear Professional School, University of Tokyo, Tokai, Ibaraki 319-1188, Japane
Japan Chemical Analysis Center, Inage, Chiba 263-0002, Japan
Received February 16, 2015; Accepted July 16, 2015
In this study, we compiled a comprehensive dataset of hourly atmospheric 134Cs and 137Cs radioactivity concentrations in suspended particulate matter (SPM; aerosols less than 10 µm in diameter) that was systematically collectedfrom March 12 to 23, 2011, in eastern Japan. In Japan, mass concentrations of SPM are measured using samples
automatically collected on filter tapes at the air quality monitoring stations managed by the local governments.
Most of the SPM monitoring stations in eastern Japan were operated during and/or after the Great East Japan
Earthquake and tsunami that occurred on March 11, 2011, which triggered severe accidents at the Fukushima Daiichi nuclear power station (FDNPS). And the used filter tapes were sent to Tokyo Metropolitan University by the
local governments through the Ministry of the Environment, Japan. The radionuclides in the hourly collected
SPM samples at 99 of the more than 400 SPM stations were measured using Ge detectors. Because SPMs werecollected hourly, the time series of the radiocesium concentration from March 12 to 23, 2011, has been describedin detail. During this time, several radioactive plumes were observed in eastern Japan. Thus, precise descriptionof these plumes (the peak time and the maximum radiocesium concentration in each plume at each SPM station)
was our primary concern in this study. To confirm that our data were consistent with data independently measured
by a research institution in Tokyo, we compared the hourly data by the institution with the hourly data for March
15-16 at an SPM station located near the institution. Although total suspended particulates were collected using ahigh-volume air sampler at the institution with a different sampling system, the data were highly consistent regarding the time series variations and radioactivity concentrations of 134Cs and 137Cs. This finding clearly indicates thatthe data presented in this study precisely reveal the 134Cs and 137Cs radioactivity concentrations in the atmosphere
during March 2011. In addition to the radioactivity measurements, identifying the time and date when individual
SPM samples were collected is equally important for producing a reliable database, and monthly reports of hourly
SPM mass concentrations and SPM recording charts were used for confirmation. Cross-contamination is possible
due to the apposition of radioactive materials from the SPM spot to the backside of the contacting tape. Thiscross-contamination is particularly likely for filter tapes made of polytetrafluoroethylene rather than glass fiber.

1. Introduction
Many estimates and evaluations have been made regarding
internal dose exposure rates from inhalation,1,2 atmosphericconcentrations and/or the deposition densities simulated byatmospheric transport models3,4 and the time-series release rateof radioactive materials just after the Fukushima Dai-ichi
nuclear power station (FDNPS) accident.5,6 In these studies,most of the data used were obtained from detailed deposition
density maps of several radioactive nuclides in the surface soil,
at local and regional scales by the surface or airborne measurements.7,8 Those estimates and evaluations have large uncertain*1Corresponding author 1. E-mail: ebihara-mitsuru@tmu.ac.jp
TEL: +81-42-677-2553; FAX: +81-42-677-2525* 2 Corresponding author 2. E-mail: haru2004@aori.u-tokyo.
ac.jp TEL: +81-3-6435-6784; FAX: +81-3-5777-1581
Current affiliation†1
H. Tsuruta: Remote Sensing Technology Center of Japan: 3-17-1,
Toranomon, Minato-ku, Tokyo 105-0001, Japan.
†2
T. Na kaji ma : Ea r t h Obser vat ion Resea rch Center, Japa nAerospace Exploration Agency: 2-1-1 Sengen, Tsukuba, Ibaraki
305-8505, Japan.

ties because few data are available regarding the timedependent concentrations of atmospheric radionuclides on a
local/regional scale. In addition, datasets containing the deposition density of radionuclides provide no information regarding their time-series variations because they indicate only thevalues that accumulated by precipitation. Therefore, estimatesof the internal radiation exposure from these data would beunderestimated if atmospheric radionuclides passed throughthe area and were not precipitated.
In Japan, air quality is automatically monitored at many stations operated by the local governments throughout theJapanese islands under the air pollution control law. In thismonitoring system, suspended particulate matter (SPM), which
is defined as aerosols less than 10 μm in diameter, is collected
on filter tape and its mass concentration is automatically measured using the β-ray attenuation method. Radioactive materials in atmospheric aerosols released by the FDNPS accidents
could be collected on filter tapes in the SPM monitors, whichwere continuously operated even after the Great East Japan
Earthquake and tsunami on March 11, 2011. By using the filter
tapes installed in the SPM monitors at the air quality monitoring sites in the Fukushima Prefecture and the surrounding© 2015 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 08/07/2015

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EbiharaJ. Nucl. Radiochem. Sci., Vol. 15, No. 2, 2015

areas, we have developed a new methodology how to determine
the spatio-temporal variation of radionuclides in atmosphericaerosols just after the accident, because there have been noreports of measurement methodologies for such variations.
Eventually, we successfully measured radionuclides in SPM
collected on filter tapes and published a paper that reported thespatio-temporal distribution of atmospheric 137 Cs in the
Fukushima Prefecture and the Tokyo metropolitan area fromMarch 12 to 23, 2011.9
The first aim of this paper is to summarize how to establish
methods for measuring radionuclides in atmospheric aerosols
collected on filter tapes installed in the SPM monitors in the
air quality monitoring networks. The second aim is to present
a dataset containing hourly atmospheric 134 Cs and 137Cs concentrations from 99 SPM monitoring sites for users who areconcerned with the transport of radioactive materials in theenvironment due to the FDNPS accident and their influenceson human health.
A brief description of the history of this study performed forthe first time is presented below. A test study regarding radionuclide measurements of used SPM filters was successfully
conducted between February and April, 2012, in cooperation
with the Ministry of Education, Culture, Sports, Science andTechnology (MEXT) and the Ministry of the Environment(MOE), Japan. In the test study, atmospheric radionuclides in
SPM samples at several sites were measured. A radioactivityof more than 1 Bq m-3 for 134Cs and 137Cs was determined when
conducting one-hour measurements with Ge detectors.
Unfortunately, 131I could not be detected because of its shorthalf-life of 8 days. After the test study, the SPM filter tapes
used in March 2011 at more than 400 SPM stations were collected at Tokyo Metropolitan University (TMU) from many
local governments in eastern Japan through the request of the
Ministry of the Environment, Japan. Then, TMU and theAtmosphere and Ocean Research Institute (AORI) of the
University of Tokyo began systematic measurements of radionuclides on the SPM filter tapes collected in the southern
region of Tohoku including the Fukushima Prefecture and theTokyo metropolitan area, for the periods of March 15 to 16 andMarch 20 to 23, 2011, when radioactive materials were widelytransported in the major plumes and caused severe environmental pollution in eastern Honshu (the main island of Japan).
The purpose of these measurements was to reveal when aradioactive plume reached each SPM station and to determine
how high the maximum atmospheric radionuclide concentrations were at their peak in the plume. Most of the data obtainedin FY 2012 are currently available to the public on the website.10 Following the measurements in FY 2012, the systematic
measurement of radionuclides of used SPM filter-tapes in many
SPM stations was succeeded in FY 2013, when the SPM samples (mostly from the same but slightly expanded localities asmentioned above) were measured by Japan Chemical Analysis
Center (JCAC) using the same methodology as that developedby TMU and AORI. The data obtained at JCAC are also open
to the public on the website.11
The SPM sample measurements at TMU were carried out in
FY 2012 and 2013 parallel to the measurements at JCAC andcontinued into FY 2014. These measurements included SPM
filter samples that were collected at selected SPM sites during
March 12 to 14 and March 17 to 19, 2011, when radioactive
materials were transported only to the southern region of
Tohoku.
In this paper, the data at 99 SPM stations from the local governments of Tokyo Metropolis, 8 prefectures (Yamagata,Miyagi, Fukushima, Ibaraki, Saitama, Chiba, Kanagawa,Niigata), and 13 cities (Sendai, Koriyama, Iwaki, Kawaguchi,
Tokorozawa, Kawagoe, Chiba, Kashiwa, Hachioji, Yokohama,Yokosuka, Fujisawa, and Sagamihara) are summarized andpresented in a unified format for wide use. The dataset

includes the previously mentioned data that are available on thewebsites10,11 and other data obtained by TMU and AORI which
are not opened yet. Although the experimental methods andresults presented in this study have been partially described
elsewhere,9,12 they are fully described in the following two sections.
2. Experimental methods
2.1. Collection of SPM samples. The monitoring sites of
the used SPM samples analyzed in this study are listed in Table
1 with the longitude and latitude of each site. A code number
is assigned for each monitoring station and is given in Table 1.
In addition, the material of the filter tapes (either glass fiber(GF) or polytetrafluoroethylene (PTFE)), the flow rate used for
SPM collection and the number of SPM samples collected oneach roll of filter tape for one round are indicated. The locations of the SPM monitoring stations are shown in Figure 1
with their code numbers.
The filter tape rolls used in March 2011 at more than 400
SPM monitoring stations in eastern Japan were collected at
TMU with the help of local governments and the MOE in 2012
and 2013. In this study, only portions of the SPM filter tapes
were used for the radiocesium determinations. These tape rolls
were usually stored in dark places by the individual local governments before being shipped to TMU.
An automatic SPM monitor is located at each air pollution
monitoring station. This monitor vacuums up the ambient air
through an inlet tube that is a few meters long at a flow rate of15, 16.7 or 18 liters per minute, depending on the instrument
model used. Atmospheric particulates with diameters of lessthan 10 μm are collected in the disk (11 mm or 16 mm in diameter) on a filter tape for one hour, and a mass concentration of
SPM is automatically measured using the β-ray attenuation
method. The SPM collected on the filter tape is usually visible
as a dark disk, which is called an SPM spot in this article. The
daily SPM collection begins at midnight (0:00) and ends 24 hlater at midnight (24:00). Before beginning collection for the
next day at 0:00, the tape is automatically wound forward by
the same length for each spot, with no sampling to distinguishthe change of collection date. This space with no SPM sample
is called a blank spot in this article. As shown in Table 1, twotypes of materials (PTFE and GF) of the filter tapes were used
depending on the model of the SPM monitoring instrument
installed at each station. Among these materials, GF was morecommonly used.
2.2. Identification of sampling date and time. Identificationof the sampling date and time is the most important step in thisstudy. Normally, a series of 24 SPM spots was collected for
one day between two blank spots. Because each spot and/or
each bunch of spots is not automatically marked, the sampling
date and time must be identified for each spot manually byreferring to the initial and/or final sampling date and time,
which was normally marked on the edge of the tape or its container box by the local government when setting up and/or
removing the tape. However, more than or fewer than 24 sampled spots were occasionally collected on the tape for one day.
This occurred when an instrument was subjected to maintenance or encountered trouble in operation or a planned or
unplanned electricity outage. In these cases, a monthly reportof the hourly SPM mass concentrations released by the local
government was helpful for identifying the sampling time.
Because no mass concentration values are reported when the
SPM collection temporarily stops, these missing values can bevery informative for identifying the collection time. A chart
paper was another important document for identifying the sampling date and time of the SPM spot. It usually draws an integrated mass concentration curve from zero to the maximum at

A Database of Hourly Atmospheric Concentrations of Radiocesium

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J. Nucl. Radiochem. Sci., Vol. 15, No. 2, 2015

TABLE 1: Sampling sites of SPM and filter-tapes analyzed in this study (Both of Station name and Site are written in
Japanese notation)

Code

Station name04-009Shichigo

04-01104-02304-02504-02804-02904-03004-03204-03604-037

YamadaShiroishi
Natori-jihai
Iwanuma
ShibataMarumori
Matsushima
Kokusetsu-nonodake
Tsukidate

06-00106-005

Yamagata-tokamachi
Yonezawa-kanaike

07-00107-00207-00307-00407-00507-00607-01207-01907-02007-03007-03107-03207-03307-03407-03507-03607-03707-03807-043

Furukawa
Minamimachi
Moriai
Sugitsumacho
Aizuwakamatsu
Asahi
DaishinAtagoshita
Ohara
ShirakawaHaramachi
Sukagawa
Kitakata
Soma
Nihonmatsu
Minami-aizu
Yabuki
Tanakura
Shinchi

08-009Tsuchiura hokenjyo
Tsuchiura-nakamuraminami
Koga hokenjyo
Shimotsuma
Toride shiyakusyo
Tsukuba-kouya
Kamisu-shimohataki
Kamisu-yokose
Moriya08-01008-01108-01508-02208-02308-03208-03608-041
11-00511-00911-02611-037
11-04111-042
11-04311-057
11-06011-06411-07111-07211-07311-07511-07711-080

Kawagoeshi-kawagoe
Kumagaya-koiduka-jihai
Tokorozawashi-higashitokorozawa
Kasukabe
KonosuKonosu-tenjin-jihai
FukayaWako
Kuki
MisatoOgawa
Higashi-chichibu
Honjyo-kodama
Yorii
Kankyokagaku Kokusaisenta
Kawaguchishi-shiba

1

Site

Longitude Latitude Tape

Miyagi Prefecture
Wakabayashi-ku,
140.94
38.24
Sendai-shi
Taihaku-ku, Sendai-shi 140.83
38.22
Shiroishi-shi
140.62
38.00
Natori-shi
140.8938.17
Iwamuma-shi
140.87
38.11
Shibata-machi
140.77
38.06
Marumori-machi
140.82
37.86
Matsushima-machi
141.07
38.39
Wakutani-machi
141.17
38.55
Kurihara-shi
141.02
38.74
Yamagata Prefecture
Yamgata-shi
140.34
38.25
Yonezawa-shi
140.11
37.92
Fukushima Prefecture
Fukushima-shi
140.4937.78
Fukushima-shi
140.4637.74
Fukushima-shi
140.4537.77
Fukushima-shi
140.47
37.75
Aizuwakamatsu-shi
139.92
37.49
Koriyama-shi
140.36
37.41
Koriyama-shi
140.34
37.39
Iwaki-shi
140.9
36.95
Iwaki-shi
140.8936.96
Shirakawa-shi
140.22
37.12
Minamisoma-shi
140.9537.64
Sukagawa-shi
140.37
37.29
Kitakata-shi
139.8737.66
Soma-shi
140.9237.80
Nihonmatsu-shi
140.46
37.59
Minamiaizu-machi
139.78
37.20
Yabuki-machi
140.3437.20
Tanakura-machi
140.38
37.02
Shinchi-machi
140.91
37.87
Ibaraki Prefecture
Tsuchiura-shi
140.1936.07
Tsuchiura-shi
Koga-shi
Shimotsuma-shi
Toride-shi
Tsukuba-shi
Kamisu-shi
Kamisu-shi
Moriya-shi
Kawagoe-shi
Kumagaya-shi

140.17

Number of spots per round2
Flow rate(L/min) March March March March
12-14 15-16 17-19 20-23

GF

16.7

-

13

-

11

GF
GF
GF
GF
GF
GF
GF
PTFE
GF

18
18
18
15
15
15
18
18
18

10
-

16
16
18
14
20
20
=
18

16
=
=
-

18
18
20
16
21
17
=
=
19

GF
GF

18
18

-

16
17

-

18
18

GF
PTFE
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
PTFE
GF
GF
GF
GF
GF

16.7
18
16.7
18
15
18
18
18
18
16.7
16.7
16.7
18
18
18
15
18
18
18

=
-

=
14
7
19
22
18
17
11
12
12
18
14
17
25
23
=
16

=
=
15
-

11
14
11
21
23
20
19
15
12
13
11
13
20
12
19
24
25
25
=

GF

18

-

17

-

19

36.04

GF

18

-

19

-

21

139.7236.20
139.9636.18
140.0535.91
140.0236.10
140.63
35.92
140.7
35.85
139.9835.95
Saitama Prefecture
139.4935.93
139.39
36.16

GF
GF
GF
GF
PTFE
PTFE
GF

18
18
18
18
16.7
16.7
16.7

-

13
16
15
15
12
=
23

-

16
18
16
17
12
12
24

GF
GF

18
18

-

21
20

-

15
22

Tokorozawa-shi

139.52

35.80

GF

18

-

22

-

17

Kasukabe-shi
Konosu-shi
Konosu-shi
Fukaya-shi
Wako-shi
Kuki-shiMisato-shi
Ogawa-machi
Higashichichibu-mura
Honjyo-shi
Yorii-machi

139.75139.52139.52
139.28139.62139.66139.88139.26139.19139.13139.19

35.97
36.07
36.06
36.19
35.78
36.06
35.84
36.06
36.01
36.19
36.12

GF
GF
GF
GF
GF
GF
GF
GF
GF
GF
GF

18
18
18
18
18
18
18
18
18
18
18

-

16
21
19
=
20
17
19
20
17
20
17

-

17
22/14a
20
=
21
18
21
22
19
21
18

Kazo-shi

139.56

36.08

GF

18

-

21

-

-

Kawaguchi-shi

139.69

35.83

GF

18

-

15

-

17

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EbiharaJ. Nucl. Radiochem. Sci., Vol. 15, No. 2, 2015

TABLE 1: Continued
1

Code

Station nameSite

12-004

Sanno shogakkou
Chishirodai-kita
shogakkou
TokeTateyama-kamegahara
Izumiya shogakkou
Kisarazu-chuo
Nodashi-noda
Mobara-takashi
Narita-karabe
Togane-horiage
Kashiwa-omuro
Katsuura-kobado
Abiko-kohokudai
Kamagaya-karuizawa
Kimitsu-kubo
Inzai-takabana
Sakae-ajikidai
Narita-nadoKatori-fuma
Ichinomiya-torami
Sodegaura-ozone

Inage-ku, Chiba-shi

12-00912-01912-04712-04812-04912-05712-06012-06112-06512-07012-07312-09312-09412-096
12-116
12-117
12-11812-12012-12312-13413-02313-02513-02713-02813-03113-05213-05613-05813-06513-06913-089HimonyaKannanadori-kakinoki
zaka
Kannanadori-matsubara
bashiNakaharakaido-minami
senzoku
Setagayaku-seijyo
Katsushikaku-mizumotokoen
Edogawaku-minamikasai
Hachiojishi-tatemachi
Omeshi-higashiome
Machidashi-nogaya
HachiojishiKawaguchimachi

Longitude Latitude Tape
Chiba Prefecture140.1435.66

Number of spots per round2
Flow rate(L/min) March March March March
12-14 15-16 17-19 20-23

GF

18

-

=

-

22/15b

GF

18

-

18

-

20

Midori-ku, Chiba-shi
140.26
35.53
GF
Tateyama-shi
139.8835.02 PTFE
Midori-ku, Chiba-shi
140.17
35.55
GF
Kisarazu-shi
139.93
35.38
GF
Noda-shi
139.87
35.95
GF
Mobara-shi
140.2935.43
GF
Narita-shi
140.3035.77
GF
Togane-shi
140.3735.56 PTFE
Kashiwa-shi
139.96
35.90
GF
Katsuura-shi
140.2735.18 PTFE
Abiko-shi
140.08
35.86
GF
Kamagaya-shi
140.02
35.79
GF
Kimitsu-shi
139.9035.33
GF
Inzai-shi
140.1335.79 PTFE
Sakae-machi
140.2535.85 PTFE
Narita-shi
140.4235.85 PTFE
Katori-shi
140.6135.79 PTFE
Ichinomiya-machi
140.3835.35 PTFE
Sodegaura-shi
140.02
35.42
GF
Tokyo Metropolitan Prefecture
Meguro-ku
139.6835.62 PTFE

16.7
18
18
18
18
18
18
18
18
18
15
15
18
18
18
18
18
18
15

-

13
14
19
16
16
17
17
13
18
16
16
19
16
16
16
16
14
14
13

-

15
14
=
18
17
18
18
=
20
16/13c
17
22
17
16
16
14
14
=
14

18

-

14

-

=

Wakaba-ku, Chiba-shi

140.1835.63

Meguro-ku

139.68

35.62

GF

15

-

15

-

17

Ota-ku139.71

35.60

PTFE

18

-

14

-

15

Ota-ku139.69

35.60

PTFE

18

-

14

-

14

Setagaya-ku

139.59

35.65

PTFE

18

-

14

-

15

Katsushika-ku

139.87

35.79

PTFE

18

-

13

-

14

Edogawa-ku
Hachioji-shi
Ome-shi
Machida-shi

139.87139.29139.28139.48

35.65
35.63
35.79
35.59

PTFE
GF
PTFE
PTFE

18
18
18
18

-

13
15
=
15

-

14
16
15
16

Hachioji-shi

139.28

35.70

GF

18

-

22/15d-16

Kanagawa Prefecture
14-008

Nakaku-honmoku

14-05014-06014-06414-065

Kurihama gyoseisenta
Shonandai bunkasenta
Odawara shiyakusyo
Odawara shiminkaikan

14-074

Tsukui14-07714-07814-07914-085

Misaki chugakkou
Hadano shiyakusho
Hadanoshi-honcho
Isehara shiyakusho

Naka-ku,
Yokohama-shi
Yokosuka-shi
Fujisawa-shi
Odawara-shi
Odawara-shi
Midori-ku,
Sagamihara-shi
Miura-shi
Hadano-shi
Hadano-shi
Isehara-shi

139.66

35.41

PTFE

18

-

13/15e-13139.71139.47139.15139.16

35.23
35.40
35.26
35.25

GF
GF
GF
PTFE

18
18
18
18

-

16
22
14

-

17
16
13
14

139.26

35.59

PTFE

18

-

15

-

-

139.62
35.14
GF
18
23
24
139.2235.37 PTFE
16.7
77139.2335.37 PTFE
16.7
12
12
139.3235.40 PTFE
16.7
12
12
Niigata Prefecture15-030
Muikamachi
Minamiuonuma-shi
138.87
37.07
GF
18
11
14
1. Filter tape materials (GF: glass fiber; PTFE: polytetrafluoroethylene).
2. -: not counted, =: missing. Two numbers separated by a slash indicate that the filter-tape rolls were changed. The change was performed at the following times and dates; a: 12:00 on March 23, b: 12:00 on March 23, c: 9:00 on March 22, d: 11:00 on March 15, e:
13:00 on March 15.

the end of the measurement for each hour. In addition, theoriginal SPM concentration for each hour is recorded on a chartwith the collection time in hours. Furthermore, if routine maintenance is performed or a sudden blackout occurs, the stoppingand starting time of the electricity is printed. Another advantage of using these documents was that, when a plume with highradioactive materials in SPM passed SPM monitoring stations,
irregular/sudden decrease to a very low/zero value for SPM
mass concentrations was frequently observed due to strong βray emission. By referring to these supporting materials, theidentification and validation of the sampling date and time
could be secured for almost all SPM spots in the database.

A Database of Hourly Atmospheric Concentrations of Radiocesium

J. Nucl. Radiochem. Sci., Vol. 15, No. 2, 2015

a

5

b

FDNPS
S. Tohoku
FDNPS

c
TMA

Figure 1. Sampling sites for the SPM samples in the air pollution monitoring network that were analyzed for 134Cs and 137Cs radioactivity concentrations. Two areas, i.e., S. Tohoku (southern Tohoku) and TMA (Tokyo metropolitan area), shown in Figure 1a, are expanded in Figures 1b and1c, respectively. The deposition density of 137Cs on the ground surface in eastern Japan from the Fourth Airborne Monitoring Survey by MEXT8
is shown in Figure 1a for information. The code numbers are indicated in Figures 1b and 1c for reference.

2.3. Gamma-ray spectrometry. The filter tape was cut inthe center between every two successive spots. A rectangularpiece of tape including an SPM spot was sandwiched between
weighing papers and was fixed on a thin plastic sheet by adhesive tape before subjecting to gamma-ray measurements. The
SPM samples were prepared so that they could be reused forother purposes after the gamma-ray measurements. The
gamma-ray measurements were performed at four different
facilities, TMU, the Nuclear Professional School of the
University of Tokyo (UT-NPS), the Japan Atomic Energy
Agency (JAEA), and JCAC. Each sample that included a blankspot was measured for 1 to 3 hours using a Ge semiconductor
detector. Several blank samples were measured for approximately 12 hours to monitor any possibility of contamination in
the SPM samples. An example of gamma- ray spectrum of an
SPM sample was shown in Figure 2.
The counting efficiencies of the Ge detectors for gammarays from 134Cs and 137Cs were determined by using homemade
standard source samples of the same size as the SPM spot samples, which included known 134Cs and 137Cs activities. Two setsof these standard samples were independently prepared by
TMU and JCAC. The typical detection limit was between
approximately 0.1 and 0.3 Bq m-3 for the one-hour measurement when using most of the detectors. However, one detector
had a higher detection limit of 1 to 2 Bq m-3.
3. Results and Discussions
3.1. Data validation
3.1.1. Consistency in the activity data obtained at four different facilities. As described in section 2.3, the standard
source samples of 134Cs and 137Cs were prepared separately at
TMU and JCAC. These samples were used first or second toquantify the radioactivity of 134Cs and 137Cs in each SPM sample at the four facilities (TMU, UT-NPS, JAEA and JCAC).

Figure 2. Gamma-ray spectrum of an SPM sampled at Kakinokizaka station (station code# 13-025) in Tokyo from 10:00 to 11:00 on
March 15, 2011. The SPM sample was fixed at a distance of 4 mmfrom a Ge detector (GMX-30200S, ORTEC). A net area of 662 keVgamma-ray from 137Cs counted 6973, corresponding to 50 Bq.

Thus, an inter-comparison of gamma-ray measurements was
conducted between TMU and JCAC. Figure 3 compares theradioactivity values of 134 Cs and 137Cs for the same set of 37
SPM spot samples determined at TMU and JCAC. Each facility used its own standard source sample and Ge detectors. The
134
Cs and 137Cs radioactivity in the 37 SPM samples spreads
from around the determination limit to 180 Bq m-3. As shownin Figure 3, excellent agreement was confirmed between TMU
and JCAC when determining the activity concentration rangeof 1 to 180 Bq m-3. Such consistency guarantees the high reliability of the preparation of standard source samples and the
gamma-ray spectrometry procedure at both facilities. For6EbiharaJ. Nucl. Radiochem. Sci., Vol. 15, No. 2, 2015

Figure 3. Comparison of 134Cs and 137Cs radioactivity concentrations (in Bq m-3) determined at TMU and JCAC for the same SPM samples. Thelines in the graphs show 1:1 relationships between TMU and JCAC rather than regression lines of measured values.

TABLE 2: Comparison of the sampling systems of atmospheric aerosols between the SPM monitoring
stations and TIRI
Items
SPM monitors
Inlet tube
a few meters long
Diameter of sampled aerosols< 10 µm
Diameter of sampled spot circle
11 mm or 16 mm
Material of filters*
GF or PTFE
15-18
Airflow rate (L m-3)
Sampling Interval (hour)1* GF: glass filter, PTFE: polytetrafluoroethylene.
134

Cs, data points smaller than 1 Bq m-3 are scattered approximately 1:1, but the values of TMU and JCAC are consistent
within 2σ error uncertainty. Thus, it can be concluded that thedata quality obtained at TMU and JCAC is equally high.
Calibration of the 134 Cs and 137Cs radioactivity at UT-NPS
and JAEA, where 134Cs and 137Cs standard source samples were
not used, was performed using some SPM samples with 134Cs
and 137Cs activity concentrations that were determined at TMU
by using their own standard source samples. Although theuncertainty of analytical results obtained at UT-NPS and JAEA
should have increased owing to the propagation of errors, thisincrement was very small because the SPM samples used forcalibration were measured long enough so that the errors due
to counting statistics became negligibly small.
3.1.2. Consistency in the activity data obtained by different
monitoring systems. In Japan, SPM samples have been collected at air pollution monitoring stations for several decades.
According to the Air Quality Standards set in Japan in 1973,
SPM is defined as suspended aerosols with diameters of lessthan 10 μm. Most monitoring stations continued to operatea f ter t he Great East Japa n Ea r t hqua ke a nd tsuna m i.
Meanwhile, several institutions began to independently measure radionuclides in the atmosphere soon after the FDNPS
accident in March 2011. Among them, the Tokyo Metropolitan
Industrial Technology Research Institute (TIRI) began measuring radionuclides, including 137Cs, in the total suspended particulates (TSPs) at Fukasawa, Setagaya-ku, Tokyo,13 so thataerosols even larger than 10 μm in diameter were also collected. TIRI directly collected TSPs on glass fiber filters (100mm φ) with no inlet tubes using a high-volume air sampler at aflow rate of 600 L min-1 for 1-3, 8, or 24 hours, depending on

Equipment at TIRI
direct sampling with no tube
all (usually < 30 µm)
100 mmGF
600
1-3, 8, 24

the gamma-ray dose rate at the sampling site. The gamma-raysof the TSPs were measured using a Ge detector for 1,000 to
20,000 seconds and the radionuclide concentrations were
promptly presented on the website. There are more than 50
SPM monitoring stations in the Tokyo Metropolis. Amongthem, the Kakinokizaka station (code #: 13-025) is locatedapproximately 1.6 km away from the TIRI sampling site, where
an SPM sampler using glass fiber filter tape was in operation in
March 2011. The sampling systems of atmospheric aerosols
used at the SPM monitoring stations and at TIRI are comparedin Table 2.
Figure 4 shows the time series of 137Cs concentrations inaerosols on the filters at Fukasawa13 and Kakinokizaka (fromthis work). As shown in Table 2, the SPM collection system at
Kakinokizaka was significantly different from that at
Fukasawa. Furthermore, the SPM samples collected at
Kakinokizaka were measured more than one year after collection. Nevertheless, excellent consistency was observed between
the two independent measurements. The time of the peak andits corresponding activity concentrations are nearly the same.
This finding clearly indicates that the SPM filter tape quantitatively preserves atmospheric radiocesium-containing materials
for more than a year, and that the SPM monitor could collectnearly all the atmospheric radiocesium in the particulate phasejust after the accident. In addition, this result suggests that thetime series variations of these radioactive nuclides in the atmosphere can be reproduced. Hence, hourly 134Cs and 137Cs activity values in the SPM samples can be used in further scientificstudies as reliable data for radiocesium concentrations in theatmosphere.9,12

A Database of Hourly Atmospheric Concentrations of Radiocesium102㻮㼄㼎㼌㼑 㼒 㼎㼌㼝 㼄㼎㼄㻩㼘 㼎 㼄 㼖㼄 㼚 㼄

101

100

10-1

137

Cs concentration / Bq m

-3㻫㼌㼐 㼒 㼑㼜㼄

10-2
3/15
00:00

3/15
6:00

3/15
12:00

3/15
18:00

3/16
00:00

3/16
6:00

3/16
12:00

3/16
18:00

Time at end of collection
Figure 4. Time series variation of atmospheric 137Cs concentrations
at three sites in Tokyo between March 15 and 16. The SPM samples
analyzed in this study were collected at the Himonya andKakinokizaka SPM monitoring stations which are shown in Figure
1c, and the samples from Fukasawa were collected by the Tokyo
Metropolitan Industrial Technology Research Institute (TIRI). These
three sites are located near each other. The concentrations below thedetection limit at Kakinokizaka (approximately 0.1 Bq m-3) are notshown. The data shown by an arrow for Himonya and Kakinokizaka
SPM stations are assumed false (see the text). At Fukasawa, the SPM
sampling durations used for the first three data points on the morningof March 15 were 0:00-7:12, 7:12-8:23, and 8:23-9:00 (JST). From
the next sample, a sampling time of exactly one hour was fixed. The
averaged 137Cs concentration in the first SPM sample was 1.8 Bq m-3,
according to Reference 13 (data not shown).

3.2. Cross-contamination. Although excellent agreement
can be confirmed in the time series variations of the 137Cs
activity concentrations between the two datasets (SPM and
TSPs) in Figure 4, an inconsistency can be noted. A small
peak appears at 1:00 on March 16 in the Kakinokizaka data,
while the 137Cs concentration by TIRI consistently decreased
with no such peak. In Figure 4, the 137Cs concentration variations of the SPM samples collected at the Himonya station(code #: 13-023) are also shown, which is located approximately 500 m south-southeast of the Kakinokizaka station (13025). The concentration variations of 137Cs at these two SPM
monitoring stations are generally consistent with each otherand with those determined by TIRI. However, the small peakat 1:00 on March 16 is considerably enhanced in the Himonya
data compared with that of the Kakinokizaka data (an arrow in
Figure 4). Because this peak was not observed at 1:00 at
Fukasawa, the peak was thought to be a ghost peak that
resulted from mechanical factors during the SPM filter tapesampling. When the data and the filter tape were carefully
inspected, cross-contamination was suspected to explain thisdiscrepancy.9,12 Because the filter tape was tightly wound, portions of the aerosols that included radioactive materials in the
SPM samples were potentially transferred to the backside of
the contacting tape. At this sampling time, we confirmed that
14 and 15 SPM spots were included in one round of the tapes
used at Himonya and Kakinokizaka, respectively. As a result,the peak top spot at 11:00 on March 15, when the maximum
concentrations of 52.3 and 55.6 Bq m -3 were measured at
Himonya and Kakinokizaka, respectively, faced the backsideof a blank spot between March 15-16 and the next SPM spot(collected at 1:00 on March 16) on the contacting tape just one
round after. In fact, a high 137Cs activity concentration (8.3 Bq
m-3, assuming that the same volume of air for an SPM spot
sample was vacuumed) was measured for the blank spot of
Himonya, whereas that of Kakinokizaka was very low (0.08
Bq m-3). The high concentration value at Himonya was lowered to 1.6 Bq m-3 by wiping the backside of the blank spot tape

J. Nucl. Radiochem. Sci., Vol. 15, No. 2, 20157

and could not be further lowered by additional wiping. Basedon these considerations and measurements, the peaks thatappeared at 1:00 on March 16 were erroneous and were causedby the transfer of a portion of the radioactive materials in the
SPM collected at 11:00 on March 15 onto the backside of thecontacting tape. Contributions to the backside were estimatedto be 15% for Himonya based on the geometry of the spot
arrangement and the 137Cs activity in the blank spot.
In this study, the contribution of cross-contamination (Cc in%) is defined as Cc=(Cb/(Cp+Cb))×100, where Cp and Cb (inBq m-3) are the highest concentrations at the peak time, and theconcentration of the sampled or blank spot corresponding with
one round after the peak spot, respectively. For the Kakinokizaka station, the contribution of cross-contamination was estimated to be 1.8%, because the highest concentration (Cp)occurring at a peak time of 11:00 on March 15 and the concentration (Cb) of the spot sampled at 1:00 on March 16, whichcorresponded to the 15th spot from the peak spot for one round,
were 55.6 and 1.0 Bq m-3, respectively. Here, it was assumed
that all the 137Cs radionuclides in the spot sampled at 1:00 on
March 16 were contributed by the sample at 11:00 on March
15. At Himonya, if all of the 137Cs in the blank spot (8.3 Bq m-3,
corresponding to Cb) were transferred from the spot with thehighest concentration (Cp, 52.3 Bq m-3) of 137Cs at a peak timeof 11:00, the contribution (Cc) was 14%, which is nearly equal
to the contribution of 15% provided by the above estimation.
The difference in the level of cross-contamination observedat the two sampling stations (Himonya and Kakinokizaka) canbe explained by the difference in the materials used for the filter tape. The filter tape used at Himonya was made of PTFE,
while that used at Kakinokizaka was glass fiber (GF). As
reported in the previous paper,9 there are several reasons why
the radioactive materials adhering to the PTFE filter tape aretransferred to the contacting tape more easily than those adhering to the GF tape; (i) the thickness of the PTFE filter isapproximately 30% of that of the GF filter, which allows thePTFE tape to be tightly wound, (ii) SPM can penetrate insideof the GF filter more deeply than the PTFE filter because of
the coarse structure of the GF, and (iii) the PTFE filter is easily
charged with static electricity. It should be noted that the transfer of radioactive materials underestimates and overestimates
the activity concentration of SPM samples around the peaktime in a plume and at the late time after one round, respectively.
We carefully investigated the contributions of cross-contamination (Cc) using the individually measured 137Cs concentrations at the SPM stations where the PTFE filter tape was used.
At this stage, the Cc was estimated to be less than 10% whenthe 137Cs concentration was higher than 70 Bq m-3 around thepeak time, except for one data that resulted in a Cc of 19% (at10:00 on March 16 at the station of 12-120). By contrast, theCc was less than 15% when the 137 Cs concentration was
between 20 and 70 Bq m-3. The reason why the contributions
for the lower peak concentrations (20-70 Bq m-3) were higher
than those for the higher peak concentrations (> 70 Bq m-3) has
not been clarified until now. Hence, the contribution of Cc was
independent of the magnitude of the high 137Cs concentration(> 20 Bq m-3) around the peak time. In fact, although the peak137
Cs concentrations at the two SPM stations were nearly equal,
the contributions were different from each other. On the contrary, the Cc for the SPM stations that used a GF filter tape wasless than 3% and 5% when the peak 137Cs concentration was
greater than 100 Bq m -3 and between 20 and 100 Bq m -3 ,respectively. These results clearly indicate that the Cc for the
SPM stations using GF was much smaller than that for the stations using PTFE.
Cross-contamination due to the transfer of radioactive materials was observed not only at the Himonya and Kakinokizaka
stations but also at other monitoring stations. Such cross-con-

8

EbiharaJ. Nucl. Radiochem. Sci., Vol. 15, No. 2, 2015

G1

(a)

T

G2

(b)

Figure 5. Typical autoradiographs (upper figure) obtained by using an imaging plate and photographs (bottom figure) of SPM samples. (a) A
blank sample between March 15 and 16 at the Himonya station (code #: 13-023, PTFE filter). Although there was not any visible SPM on theblank filter, some radioactive materials were detected by autoradiography. (b) An SPM sample collected from 20:00 to 21:00 on March 21 at the
Narita-Nado station (#12-118, PTFE filter). Although only one circular SPM was faintly visible in the photograph, one circular and two semicircular images were observed in the autoradiograph. Yellow circles are artificially drawn for better understanding. An image edged with circle T
is a radiograph of actually sampled spot, and images edged with circles G1 and G2 are radiographs of ghost spots attached on the backside of thefilter tape. Both of G1 and G2 are estimated to be portions of SPM sampled at 8:00 and 7:00 on March 21, respectively, when a plume passed thestation, considering that the total spot number was 14 per one round, as shown in Table 1.

tamination could be easily recognized by autoradiography
imaging with an imaging plate. Examples of these images andphotographs of the samples are shown in Figure 5. Figure 5 (a)shows an autoradiograph of a blank sample between March 15
and 16 at the Himonya station (code#: 13-023) describedabove, which should not have contained any SPM as shown in
the photograph. However, an approximate circle image and an
image of a small piece in the left side are clearly visible, which
imply that some radioactive materials were present on the blankspot tape and that these materials were transferred to the backsurface of the tape from the contacting spot having large
amounts of radioactive materials from a plume. Another caseis shown in Figure 5 (b), in which three images can be noticed,
i.e., one faint complete circle disk (T) and two clear incomplete
broken disks (G1 and G2). The complete disk is a real SPM
spot collected between 20:00 and 21:00 on March 21 at the
Narita-Nado station (code #: 12-118) with the use of PTFE filter tapes. Two incomplete discs were transferred, probably
from the spot at 6:00-7:00 and 7:00-8:00 on the same day
when high 137Cs activity concentrations of 148 and 57.2 Bq m-3
were measured due to a plume, respectively. Even when all theradionuclides of 137Cs concentrations (11.4 Bq m-3) in the sampled spot at 21:00 were supposed to be attached to the backsidefrom the surface of the spot sampled at 7:00, the contribution
to the backside was only 7.2%, which was much lower than that(14-15%) estimated for the Himonya station. From the IP
intensity, approximately 90% of the 137Cs activity at 21:00 was
estimated to be contributed from the transferred SPM. Thus,
the real 137Cs activity of this spot cannot be accurately obtained.

We are now preparing a technical report that describes theseimportant results in detail.14
In this paper, no additional information on the effect of crosscontamination as described for the Himonya and Narita-Nadostations is shown in the tables of Appendix A, because we havenot finished checking all the suspicious SPM samples by autoradiography. We have been judging one by one whether the137
Cs concentrations of a few sampled spots located one roundafter the previously sampled spots with the highest 137Cs concentrations in a plume, are higher due to cross contamination
than actual 137Cs concentrations which are estimated to bemuch lower. After completing this assessment, we will soon
publish a technical report and revise the dataset reported in thispaper, showing all the data which should not be used for further analysis due to overestimation.
3.3. Analytical data presentation. The numerical data of
the activity concentrations (in Bq m -3 ) are tabulated inAppendix A, Tables A1 through A99. Radioactivity values aredecay-corrected to the time when the collection of SPM samples ended. Each table corresponds to each sampling site. No
information for cross-contamination were made. The detection
limit is defined as 3σ of the total background counts (Cbk) at
the peak areas around 605 keV and 662 keV for 134Cs and 137Cs,
respectively, in gamma-ray spectrometry, where σ is a squareroot value of C bk . Examples of time series variations of theradioactive 137 Cs concentrations (in Bq m -3 ) discussed in
Section 3.2 are shown in Figure 6a to 6c. Time series variations of 137Cs concentrations at all SPM stations are shown inA Database of Hourly Atmospheric Concentrations of Radiocesium

J. Nucl. Radiochem. Sci., Vol. 15, No. 2, 2015

9

102

101

(a)

Concentration of137

Cs / Bq m-3㻮㼄㼑㼑㼄㼑㼄㼇 㼒㼕㼌㻐㼎㼄㼎㼌㼑㼒㼎㼌㼝㼄㼎㼄㻃 㻋 㻔 㻖 㻐㻓 㻕 㻘 㻌

100

10-1

10-23/15
0:00

3/16
0:00

3/17
0:00

3/20
0:00

3/21
0:00

3/22
0:00

3/23
0:00

3/24
0:00

Time at end of collection

102

101

(b)

Concentration of137

Cs / Bq m-3㻫㼌㼐 㼒㼑㼜㼄 㻋 㻔 㻖 㻐㻓 㻕 㻖 㻌

100

10-1

10-2

3/15
0:00

㼑㼒 㼗 㼐 㼈㼄㼖㼘 㼕㼈㼇

3/16
0:00

3/17
0:00

㼑㼒 㼗 㼐 㼈㼄㼖㼘 㼕㼈㼇

3/20
0:00

3/21
0:00

3/22
0:00

3/23
0:00

3/24
0:00

Time at end of collection

103

Concentration of137

Cs / Bq m-3㻱㼄㼕㼌㼗㼄㻐㼑㼄㼇 㼒 㻋 㻔 㻕 㻐㻔 㻔 㻛 㻌

102

(c)

101
100

10-1
10-23/15
0:00

3/16
0:00

3/17
0:00

3/20
0:00

3/21
0:00

3/22
0:00

3/23
0:00

3/24
0:00

Time at end of collection
Figure 6. Time series variation of atmospheric 137Cs concentration observed at (a) Kannanadori-kakinokizaka station (13-025) in Meguro-ku,
Tokyo (also Figure B75), (b) Himonya station (13-023) in Meguro-ku, Tokyo (also Figure B78), (c) Narita-nado station (12-118) in Narita-shi,
Chiba (also Figure B74), (d) Moriai station (07-003) in Fukushima-shi, Fukushima (also Figure B15) and (e) Minamimachi station (07-002) inFukushima-shi, Fukushima (also Figure B14). The data shown by an arrow for the Narita-nado station (c) is assumed false (see Section 3.2 andFigure 5 of the text).

10

EbiharaJ. Nucl. Radiochem. Sci., Vol. 15, No. 2, 2015103
Cs / Bq m-3㻰㼒㼕㼌㼄㼌 㻋 㻓 㻚 㻐㻓 㻓 㻖 㻌Concentration of

137

102

(d)

101
100
10-1
10-23/15
0:00

3/16
0:00

3/17
0:00

3/21
0:00

3/22
0:00

3/23
0:00

3/24
0:00

Time at end of collection

103
Cs / Bq m-3㻰㼌㼑㼄㼐 㼌㼐 㼄㼆㼋㼌㻃㻋 㻓 㻚 㻐㻓 㻓 㻕 㻌

102

(e)

101

137

Concentration of

3/20
0:00

100
10-1
10-2

3/15
0:00

3/16
0:00

3/17
0:00

3/20
0:00

3/21
0:00

3/22
0:00

3/23
0:00

3/24
0:00

Time at end of collection
Figure 6. Continued.

Appendix B, Figure B1 to B99. Although 134 Cs activity wasmeasured along with that of 137Cs, its variations are not shown
because its time series variations are essentially the same as
those for 137Cs, as implied by the nearly constant activity ratios(approximately 1) of 134 Cs/137Cs for most SPM samples analyzed. Some SPM samples collected at Haramachi (code #:07-031), Soma (07-034) and Shinchi (07-043) stations on
March 12, 2011 had obviously smaller ratios of approximately
0.9.15 The precise determination of 134Cs/137Cs activity ratios
will be reported elsewhere.
3.4. Some implications. In our first paper 9 we presented
the hourly spatial distribution of 137 Cs in the Fukushima
Prefecture and the Tokyo metropolitan area, which is locatedmore than 170 km southwest of FDNPS, just after the accident
by using hourly radiocesium concentration data from the SPM
filter samples collected at the 40 SPM stations from March 12
to 23, 2011. In the paper, we first revealed how the radioactive
materials released into the atmosphere following the accident
in March 2011 were transported in a local/regional scale. The
content of our study in the paper is summarized below.
1. Nine major plumes with 137Cs concentrations greater than
10 Bq m -3 were found between March 12 and 23. Of

these, five and four plumes were transported to the
Fukushima Prefecture and the Tokyo metropolitan area,
respectively. The transport of radioactive materials fromthe FDNPS to the northern part of Hamadori in the east
coast area of the Fukushima Prefecture, which wasobserved four times, could be firstly recognized by measuring radiocesium in the SPM samples.
2. Two plumes transported to the Tokyo metropolitan area
were observed on March 16 and 20, 2011 for the firsttime, in addition to the two previously recognized major
plumes on March 15 and 21, 2011.
3. The radiation dose rate measured at the monitoring posts
in Nakadori in the central part of the Fukushima
Prefecture did not increase, even when the plume passed
by on March 20, 2011, because the dose rate was alreadytoo high to detect a new plume due to ground-shine aftera large amount of radionuclides was deposited on the
grounds by precipitation.
4. Accordingly, it was firstly recognized that polluted air
masses with high 137Cs radioactivity (approximately 80
Bq m-3) were present in Nakadori for more than half a
day from the evening of March 20 to the morning ofMarch 21, 2011.

A Database of Hourly Atmospheric Concentrations of Radiocesium
137

5. An area with relatively high Cs deposition by precipitation was observed on the morning of March 21, 2011 in
the Tokyo metropolitan area, according to the airborne
measurements by the MEXT8 (Figure 1a). Within thearea, time-integrated atmospheric 137Cs concentrations atseveral SPM monitoring stations were also observed tobe high due to the transport of a plume on March 21,
2011. However, in the Fukushima Prefecture, such a correlation was not clear.
In this study, large amounts of data, including more than
twice the number of 40 SPM monitoring stations, have become
available for the first time. Thus, we are preparing to comprehensively describe the atmospheric transport of radioactive
materials in eastern Japan including discussions regarding thesource term. In addition to our research, we expect many
researchers to use the database presented in this study to produce new findings. In particular, we expect the uncertainty ofinternal dose rates from inhalation to decrease by the use ofhourly release rates from radioactive materials from FDNPS,
and of improved atmospheric transport models by validating
the simulated results.
3.5. Scientific perspectives related to measuring radioactive nuclides in SPM samples. Because more than four years
have passed, short-lived nuclides have decayed and can no longer be determined. However, in addition to radiocesium (134Cs
and 137Cs), which were targeted in this study, several radioactive nuclides can be measured in the SPM samples. For example, 90 Sr and 129I are representative, long-lived radionuclides.
Unlike radiocesium, these radioactive nuclides cannot be determined by measuring gamma-rays. Thus, extra work and time
are required for their measurement.
Because 90Sr and 129I are very important for assessing exposure dose due to internal exposure, the measurement of these
nuclides is important. In particular, 129I has been given considerable attention for estimating the internal exposure dose fromshort-lived radioiodine, 131I. Because the half-life of 131I isapproximately 8 days, 131 I decays quickly. Therefore, thespreading behaviors of 131I in the atmosphere from the nuclear
reactors are not sufficiently understood, because the data of
atmospheric 131I concentrations by direct measurements in eastern Japan just after the accident were very limited. The concentrations of 131I in the soil samples were measured in the
Fukushima Prefecture as part of a project aimed at drawing a
concentration map of radioactive nuclides that was led by
MEXT in June-July, 2011.16 However, because of the delayed
initiation of this project, 131I concentrations were observed only
in areas where heavy deposition of radioactive nuclides, including 131 I, occurred.17 Because 131 I and 129 I were fissiogenic
nuclides from 235U and released together into the environmentby the FDNPS accident, 129I can be used as a substitute for 131I.
Recently, the 129I concentrations in soil samples collected inJune-July 2011 were systematically determined using accelerator mass spectrometry (AMS). These results and a resulting
concentration map were uploaded on the website.18 Overall,these data can help to reveal how 129I is presently distributed
across a wider area than the area in which the 131I deposition
was directly measured by the MEXT project in June-July 2011.
However, several concerns exist regarding the use of 129I datathat were recently obtained from soil samples collected in 2011
to estimate the degree of internal exposure of 131I. One major
concern is whether it is possible to estimate the internal exposure dose from 131I based on the presently determined 129I datafrom soil samples. The 129I activity in soil samples is an integrated value of the 129I deposited on the ground from the atmosphere during the entire period, including the 129I deposition
before the Fukushima-nuclear-accident. If 129I can be measured
in the SPM samples used for this study, we will be able to estimate the internal exposure dose caused by 131I based on such

J. Nucl. Radiochem. Sci., Vol. 15, No. 2, 2015

11

129

I data more directly and reliably compared with estimations
based on the 129I deposition densities in the soil samples. To
evaluate the internal exposure dose from 131I, we must be able
to answer the following questions with “yes”: can 129 I be
detected in the SPM samples and can 129I be used in place of131
I in the SPM samples? From our preliminary study, bothquestions were answered with yes. Next, we can set up a reliable scheme for estimating the internal exposure dose from 131I
based on 129I measurements of the SPM samples used in thisstudy.
4. Guides for using the 137Cs radioactivity concentration
data
There are two important issues to consider carefully when
the data presented in this study are used as a primary dataset,
namely, cross-contamination and detection limit. As describedin Section 3.2, cross-contamination cannot be avoided in the
SPM samples analyzed in this study. Careful attention should
be given to the high concentration data in the major plumes
from the SPM samples collected on PTFE filter tape. Regarding
the detection limit, 3σ values are provided as a conventional
method for presenting radioactivity measurement data. The
detection limit depends on the experimental condition for
gamma-ray counting. The guidelines below should be considered when using the radiocesium concentration data presented
in this study.
(1) If there are more than one monitoring station located in anarrow area (2 to 3 km apart) and if both filter tapes ofPTFE and GF are used in these monitoring stations, thedata from the GF filter are more reliable, considering thehigher probability of cross-contamination with the PTFE
filter tape, as described in section 3.2. For example, fourmonitoring stations are located near in the Fukushima City
(code #: 07-001, 07-002, 07-003 and 07-004). The PTFE
filter tape was used only at the 07-002 station, and the GF
filter tape was used at the other three stations. Therefore,
the data from these three stations may be suitable as a primary data source. Consequently, in our previous paper9 thedata from these three stations with the use of GF were used.
As another example, at the two monitoring stations in Tokyo(Himonya and Kakinokizaka, code#: 13-023 and 13-025,
respectively), PTFE filter tape was used at station 13-023
and GF filter tape was used at station 13-025. Cross-contamination with these two stations was discussed in detailin Section 3.2, and the only data from station 13-025 wereused in our previous paper.9(2) At this stage, we introduce a simple method for determining the cross-contamination to users. For a given SPM station A where the PTFE filter tape was used, the 137 Cs
concentration in a sampled spot that is located one roundafter the previously sampled spot with a high 137Cs concentration around the peak time of a plume, might be considered unreasonably or irregularly high because these data
were thought to be affected by cross-contamination. Then,the data were compared with those measured simultaneously at another SPM station B, that was located near SPM
station A and where GF filter tape was used. In cases
where the 137Cs concentrations were nearly equal between
the two SPM stations, the effects of cross-contamination
can be neglected. However, when the 137Cs concentration at
A was higher than that at B and/or its temporal variation
was not similar to that at B, the data were judged to beaffected by cross-contamination and were not used in theprevious paper.9 In this case, because the 137Cs concentration at A around the peak time was underestimated, the137
Cs concentration derived for cross-contamination by calculating the contribution described in Section 3.2 is to be
added for further study and quantitative discussion.

12EbiharaJ. Nucl. Radiochem. Sci., Vol. 15, No. 2, 2015137

-3(3) The low Cs concentrations (approximately 1 Bq m or
lower) measured at a SPM station using the PTFE filter tapeafter high concentrations at the peak time, may have been
overestimated due to cross-contamination. This finding isevident when considering the data from the four samplingsites in the Fukushima City. Specifically, the 137Cs concentrations at station 07-002, which used PTFE filter tape,were systematically higher than those at the other three stations that used GF filter tape at 9:00 and thereafter on
March 21 (Figures 6d and 6e). Hence, comparisons
between the simulated results by atmospheric transportmodels and measured data should be made carefully whenthe 137Cs concentrations at the SPM stations with PTFE filter tapes are low (approximately 1 Bq m-3 or lower).
(4) Even when the concentration values are given by numericalvalues, it is acceptable to use such values as upper limits
when the values are near the detection limit. This method
is also acceptable when the concentration data are accompanied by large uncertainties (approximately 50% or higher
for relative uncertainties).
The data reported in this paper are from only 99 of approximately 400 SPM sites, and radioactivity measurements are
ongoing for the remaining samples. These data will be summarized and reported elsewhere and the 129I measurement
results will be reported in the near future.
Acknowledgments. We are grateful to the following individuals and organizations for their contributions to this project:
the local governments of Tokyo Metropolis, 8 prefectures(Yamagata, Miyagi, Fukushima, Ibaraki, Saitama, Chiba,
Kanagawa, Niigata), and 13 cities (Sendai, Koriyama, Iwaki,
Kawaguchi, Tokorozawa, Kawagoe, Chiba, Kashiwa, Hachioji,
Yokohama, Yokosuka, Fujisawa, and Sagamihara) for collecting, preserving and sending SPM samples; the Ministry of
Education, Culture, Sports, Science and Technology (MEXT)
and the Ministry of the Environment and the Nuclear
Regulation Authority for their financial support; K. Kumagai,
H. Takahashi, A. Tanaka, Y. Tanaka and Y. Yokozuka of Tokyo
Metropolitan University for their assistance in analyzing the
SPM samples; and K. Shiba and Y. Kusama in AORI at the
University of Tokyo for mapping Figure 1 and for carefully
treating and checking the data. In addition, we thank Prof. K.
Wakamatsu of Ehime University for his help in keeping the filter tapes in the early stage. This work was partly supported by
the “Interdisciplinary Study on Environmental Transfer ofRadionuclides from the Fukushima Daiichi NPP Accident”
project by JSPS KAKENHI (grant numbers 24110002,
24110008, and 24110009), the “Development of Seamless
Chemical Assimilation System and its Application for
Atmospheric Environmental Materials (SALSA)” project in
the Research Program on Climate Change Adaptation(RECCA) by MEXT, and the “Active evaluation of the environmental effects caused by greenhouse gases and short-lived
climate pollutants” project (S-12) in the Environment Research
and Technology Development Fund of the Ministry of theEnvironment, Japan. The radionuclide measurements in many
SPM samples were also partly supported by the “Measurements
and Distribution Map of Radionuclides by using the Filter
Tapes in SPM Monitors” project of the Ministry of theEnvironment in Japan (2012) and by the “Understanding of
atmospheric radionuclides concentrations by measuring radioactive materials in suspended particulate matter collected on
filter tapes” project of the Nuclear Regulation Authority(2013).

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Appendices A and B are available from the journal homepage.