Fogwater deposition of radiocesium in the forested mountains of East Japan during the Fukushima Daiichi Nuclear Power Plant accident: A key process in regional radioactive contamination

Atmospheric Environment 224 (2020) 117339

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Fogwater deposition of radiocesium in the forested mountains of East Japan during the Fukushima Daiichi Nuclear Power Plant accident: A key process in regional radioactive contamination Naohiro Imamura a, *, Genki Katata b, Mizuo Kajino c, d, Masahiro Kobayashi a, Yuko Itoh a, Akio Akama e a

Department of Forest Soils, Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki, 305-8687, Japan Institute for Global Change Adaptation Science (ICAS), Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki, 310-8512, Japan c Meteorological Research Institute, Japan Meteorological Agency (JMA), 1-1 Nagamine, Tsukuba, Ibaraki, 305-0052, Japan d Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8572, Japan e Center for Forest Restoration and Radioecology, Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki, 305-8687, Japan b

H I G H L I G H T S

� High concentrations of 137Cs activity in throughfall were most likely due to fogwater deposition. � Forested mountain areas were contaminated by fogwater deposition in East Japan. � Fogwater deposition may have a role in radiocesium cycling in forest ecosystems. A R T I C L E I N F O

A B S T R A C T

Keywords: Occult deposition Mountain forest Passive fogwater collector Radiocesium Fukushima Daiichi Nuclear Power Plant accident Throughfall

Because of limited environmental monitoring data, the regional-scale impact of the deposition of fogwater radiologically contaminated by the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident remains unclear. To redress this situation, we present an observational report of the radiocesium concentration in fogwater and its deposition in a Japanese forest during the early stages of the F1NPP accident (March 2011). The data were acquired by using a passive collector to capture fogwater above the forest canopy on a monthly basis. In addition, the radiocesium concentrations in monthly throughfall and stemflow were measured under the canopies of four tree species. The 137Cs activity concentration in fogwater during the observational period was 45.8 Bq L 1, which was twice as high as that present in bulk precipitation. The ratio of 137Cs in throughfall to that in bulk pre­ cipitation (TF/BP ratio) ranged from 1.0 to 2.5. The high TF/BP ratios may have been caused by the high radiocesium concentration in fogwater deposition. Based on this assumption, we assessed the TF/BP ratio ac­ cording to the 137Cs activity concentrations of throughfall and bulk precipitation measured in various moun­ tainous regions in East Japan. Our results reveal that the TF/BP ratio is high at some sites and that it increases with elevation. Sites with a high TF/BP ratio were almost entirely situated in areas of fogwater deposition, as predicted by an atmospheric dispersion model. In addition, sites with a high TF/BP ratio were above the cloud base at the time when plumes with high atmospheric 137Cs activity concentrations passed through the areas. Thus, these measurements of radiocesium in fogwater during the early stages of the F1NPP accident provide evidence that fogwater with high radioactive contamination was deposited in the forested mountain areas of East Japan. Given the major impact of fogwater deposition of radiocesium, its role should be considered carefully to better understand radiocesium cycling in forest ecosystems.

* Corresponding author. 1 Matsunosato, Tsukuba, Ibaraki, 305-8687, Japan. E-mail address: [email protected] (N. Imamura). https://doi.org/10.1016/j.atmosenv.2020.117339 Received 15 May 2019; Received in revised form 5 October 2019; Accepted 8 February 2020 Available online 12 February 2020 1352-2310/© 2020 Elsevier Ltd. All rights reserved.

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Fig. 1. (a), (b) Location of the study area in East Japan. Map with the 134Cs and 137Cs deposition levels on May 31, 2012, generated by using the website “Extension Site of Distribution Map of Radiation Dose, etc.” (prepared by MEXT, Japan). (c) Satellite image of broadleaved deciduous forest site (provided by Google Earth).

1. Introduction

radionuclide-contaminated fogwater is lacking. Gibb et al. (1997) re­ ported the velocities and scavenging coefficients of radionuclide parti­ cles deposited by fogwater, and Su and Huh (2006) measured 7Be and 210 Pb concentrations in cloud water from Taiwan. In addition, Livens et al. (1992) reported on 137Cs in orographic cloud water in England after the Chernobyl accident, and Bourcier et al. (2014) and Masson et al. (2015) measured the 137Cs background level in cloud water in France. Recent research on the F1NPP accident indicates that fogwater deposition (occult deposition) of radiocesium is a likely contributor to forest contamination in East Japan. Although many atmospheric dispersion models have heretofore considered only dry and wet depo­ sition, fogwater deposition may be one of the key radiocesium deposi­ tion mechanisms in forested areas (Katata et al., 2015). Kaneyasu et al. (2012) examined the bimodal distribution of the aerodynamic diameter

A huge amount of radiocesium (9–36 PBq) was emitted into the at­ mosphere during the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident of March 2011 (Chino et al., 2011; Stohl et al., 2011; Katata et al., 2012; Winiarek et al., 2012). The area contaminated by radio­ cesium deposition consists almost entirely of forest cover (Hashimoto et al., 2012), in which the uptake of radiocesium by trees is a concern (Imamura et al., 2017; Ohashi et al., 2017). Therefore, to better un­ derstand radiocesium cycling in forest ecosystems, it is important to understand the dominant radiocesium deposition processes during the early stages of a nuclear accident. In the period following the Chernobyl accident, the transfer factor of radiocesium from the ground to vegeta­ tion was relatively high at the site exposed to the radioactive cloud (Caput et al., 1990). However, research on deposition by 2

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of radiocesium aerosols and suggested the possibility of fogwater deposition following the F1NPP accident. On the basis of the altitudinal distribution of the air dose rate, Hososhima and Kaneyasu (2015) re­ ported that radionuclides were transported via fogwater in the Nikko mountains. In addition, Katata et al. (2015) studied the possibility of radiocesium deposition via fog events based on estimates of fogwater derived from visibility data for the mountains in Tochigi and Gunma Prefectures for March 15–16, 2011, when peak radiological contami­ nation occurred. In another work, Sanada et al. (2018) investigated how fogwater deposition affects radiocesium concentrations in the mountain areas in the western and northern parts of Tochigi and Gunma Pre­ fectures, respectively, and in the southern part of Fukushima Prefecture by comparing the altitudinal distributions of the air dose rate and the quantity of radiocesium deposited calculated numerically by the Worldwide Version of System for Prediction of Environmental Emer­ gency Dose Information (WSPEEDI-II). More recently, Kajino et al. (2019) produced simulations that indicate that fogwater deposition could account for 30%–80% of the total 137Cs deposition over the Tochigi and Gunma Prefectures in March 2011. Thus, the circumstantial evidence following the F1NPP accident and the specific models used by these studies strongly suggest that the spatial distribution of radio­ cesium is affected by fogwater deposition. Although these studies drew the same conclusions regarding the fogwater deposition of radiocesium, the limited meteorological data available and the lack of direct measurements of radiocesium in fog­ water during the early stages of the F1NPP accident have prevented the discussion from moving beyond the hypothesis and speculation stage. The only measurements of radiocesium in fogwater before and after the F1NPP accident were done by Masson et al. (2015) in France, which is far from Japan. Information is thus lacking on radiocesium cycling in mountain forest ecosystems covered by fog or cloud when plumes with high at­ mospheric 137Cs activity concentrations pass through. We hypothesize that high radioactive contamination by fogwater deposition in the forested mountains of East Japan may be revealed by studying direct measurements of radiocesium in fogwater made during the early stages of the F1NPP accident. This study thus (1) quantifies the chemical characteristics of fogwater in the study area in March 2011 and (2) apply these characteristics to data from previous studies to evaluate how fogwater deposition affected the radiocesium contamination in the forested area of East Japan after the F1NPP accident. The results help in elucidating radiocesium cycling in forest area contaminated by fogwater deposition of radiocesium.

Fig. 2. Cylindrical passive fogwater collector installed on the top of a 21-m-tall tower in the study area.

and 87 trees ha 1 (17.1%, 27.9%, and 25.1% of the total basal area), respectively. The 134Cs and 137Cs activity concentrations in the decid­ uous forest litter (dry weight basis) were updated on March 29, 2011, and were 1100 and 1270 Bq kg 1, respectively (The University of Tokyo Chichibu Forest, 2014). The air dose rate 1 m above the ground was 0.059 μSv h 1 on June 2, 2014 (The University of Tokyo Chichibu Forest, 2014). 2.2. Observational method Fogwater was sampled monthly by using a passive fogwater collector (ASRC Fog Collector, Field Pro, Inc., Tokyo, Japan) (Fig. 2; Falconer and Falconer, 1980) positioned 1 m above a 21-m tall observational tower in the deciduous forest (1276 m a.s.l.; Fig. 1(c)), which had a canopy height of 21 m. Evaporation was reduced by placing a ping-pong ball between the screen area (the fogwater-capture area) and the sample bottle. Fogwater was collected once a month from 2008 to 2012, originally to monitor the acid deposition onto the forest canopy (Imamura, 2010). The sampling period included the early phase of the F1NPP accident (March 2011). Bulk precipitation was sampled by using a bulk sampler at a meteorological station (35.94� N, 138.80� E; 1195 m a.s.l.; Fig. 1(c)). The bulk sampler used a 21-cm-diameter polyethylene funnel attached to a 10 L polyethylene bottle wrapped in aluminum foil. Nylon mesh fixed at the top of the funnel prevented contamination by large particles (e.g., dust). We assumed that the contribution of fogwater to bulk pre­ cipitation data would be negligible with this setup because, in principle, the contribution by fog entering the bulk sampler in open areas is small compared to the contribution of the throughfall sampler. Fogwater trapped by tree leaves accumulated and formed large droplets that subsequently fell into the funnel of throughfall samplers. Throughfall was sampled by using bulk samplers placed at three points below the canopies of the four tree species (F. crenata, F. japonica, T. sieboldii, and Cryptomeria japonica). The volume of water from throughfall was measured for F. crenata, F. japonica, T. sieboldii, and C. japonica by using five, three, six, and three collectors, respectively. The water volume of stemflow was measured for individual trees of each species by using stemflow collectors, which were constructed by using silicon sealant to fix plastic collars around the trunks to collect water into a hosepipe. All samples of fogwater, bulk precipitation, throughfall, and stemflow were gathered between February 24, 2011, and March 29, 2011. To prevent contamination, the bulk, throughfall, and stemflow bottles were lined with new plastic bags that were rinsed beforehand with deionized water. In addition, the bottle funnels and orifices were all cleaned with deionized water prior to installation. Details on the observation site are

2. Materials and methods 2.1. Study sites The study sites were located in a broadleaved deciduous forest (35.94� N, 138.80� E; 1264 m a.s.l.) and in a Japanese cedar plantation (35.94� N, 138.82� E; 1040 m a.s.l.) at The University of Tokyo Chichibu Forest (Fig. 1(b)). This area is situated approximately 260 km southwest of the F1NPP (Fig. 1(a)). The total radiocesium (134Cs and 137Cs) deposition was relatively low (<10,000 Bq m 2), as indicated by aerial survey monitoring conducted on May 31, 2012 (Ministry of Education, Culture, Sports, Science and Technology; MEXT, 2012). On the basis of the 134Cs/137Cs ratio in soils, Yamada (2013) suggested that radiocesium at this site came from the F1NPP accident. In a simulation, Morino et al. (2013) showed that radiocesium around the study area was likely deposited during the periods March 15–16 and 21–23, 2011. In partic­ ular, large amounts of 137Cs were deposited on the afternoon of March 15, 2011. The broadleaved deciduous forest in the study area is composed of Siebold’s beech (Fagus crenata), Japanese beech (F. japonica), and hemlock fir (Tsuga sieboldii). According to Sawada et al. (2006), the stand densities of F. crenata, F. japonica, and T. sieboldii were 54, 308, 3

4

36.20/140.14 36.01/140.13 36.28/140.17 36.32/140.20 36.51/140.31

1,260 130 210 355 330 380 310 210 25 160 400 220

No.13d No.7 No.8 No.1a No.1b No.1c No.1d No.2 No.6 No.3 No.4 No.5

35.64/139.47 35.65/139.27 36.17/140.18

1,260

35.98/139.34 35.94/138.82 35.94/138.80

850 800 200 230 230 60 1,040 1,260

Elevation (m) Hinoki cypress Japanese cedar Japanese cedar Japanese cedar Hinoki cypress Japanese cedar Japanese cedar Siebold’s beech Japanese beech Hemlock fir EBL Japanese cedar Hinoki cypress Japanese cedar DBL Japanese cedar Japanese cedar Japanese cedar Japanese cedar Japanese cedar Japanese cedar

Tree species

Ibaraki Airport

Camp Kasumigaura

Camp Tachikawa

Iruma Air Base

Camp Utsunomiya

Camp Somagahara

METAR site

36.18/140.42

37.03/140.19

35.71/139.40

35.84/139.41

36.51/139.87

36.43/138.95

Latitude/ Longitude

Tsuchiura-nakamura-minami

Machidashi-nogaya

Tokorozawa-higashitokorozawa

Koga hokenjyo

Honjyo-kodama

SPM siteb

36.04/140.17

35.59/139.48

35.80/139.52

36.20/139.72

36.19/139.13

Latitude/ Longitude

Sagamiharacyuou Tsuchiura

Tokorozawa

Koga

Fujioka

JMA sitec

TF/BP: throughfall and bulk precipitation observation; METAR: Aerodrome Routine Meteorological Report; SPM: suspended particulate matter network; JMA: Japan Meteorological Agency. a Site No.14 was studied by Kato et al. (2012), sites No.1–12 were studied by Itoh et al. (2015), and site No.13 was used in the current study. b The SPM site was from Oura et al. (2015). c The JMA site was from the Japan Meteorological Agency.

northern Ibaraki

southern Ibaraki

Tokyo

Saitama

36.60/139.24 36.79/138.99 37.13/138.77 36.38/139.73

Latitude/ Longitude

No.13c

No.10 No.11 No.12 No.14a No.14b No.9 No.13a No.13b

Gunma

Tochigi

TF/BP sitea

Prefecture

Table 1 Details of TF/BP, METAR, SPM, and JMA sites.

36.10/140.22

35.57/139.37

35.77/139.41

36.20/139.72

36.24/139.07

Latitude/ Longitude

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Fig. 3. Spatial distribution of cumulative fogwater deposi­ tion of 137Cs at 0:00 on April 1, 2011 based on an atmo­ spheric dispersion simulation (Katata et al., 2015). The red solid circle shows the location of the Fukushima Daiichi Nuclear Power Plant (F1NPP), and the red crosses show the observation sites of throughfall and bulk precipitation (TF/BP sites; Table 1). The bold numbers indicate the fog­ water deposition sites for other estimation methods. Orange circles show the Aerodrome Routine Meteorological Report (METAR) sites (Table 1), and black diamonds show sus­ pended particulate matter (SPM) network sites (a: Honjyo-kodama, b: Koga hokenjyo, c: Tokorozawa-higashi-tokorozawa, d: Machidashi-nogaya, e: Tsuchiura-nakamura-minami; Table 1). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

available in Imamura et al. (2012) and Imamura (2014). Measures of wind speed (at 5 m) and precipitation were recorded continuously at the meteorological station (Fig. 1(c)) by using an R.M. Young wind sensor (03001 Wind Sentry; Campbell Scientific, Inc., Logan, UT, USA) and a 0.5 mm tipping bucket rain gauge (sensitivity 0.5 mm, No. 34-T; OTA Keiki Seisakusho Co., Ltd., Tokyo, Japan), respectively.

throughfall was calculated for the three collectors under the canopies of each tree species, and the average throughfall for each tree species was retained. The amount of water collected from throughfall and stemflow was calculated by dividing the water volume by the surface area of the orifice of the bulk sampler and the canopy projection area, respectively (Imamura et al., 2012; Imamura, 2014).

2.3. Chemical analysis

2.4. Fogwater quantity

The 137Cs activity concentration in each sample was determined by using a pair of HPGe coaxial detector systems (GEM40P4-76 and GEMFX7025P4-ST; ORTEC, Oak Ridge, TN, USA) combined with spectral analysis software (DS-P1001 Gamma Station; Seiko EG&G, Inc., Tokyo, Japan). The measurement system was calibrated by using a standard gamma-ray source (MX033U8PP; Japan Radioisotope Association, Tokyo, Japan), and the reference standard material, IAEA-444, was used to verify the accuracy of the system. The measurement times were set to 1800–86400 s to obtain a137Cs activity concentration within a relative error of <10%. Radioactivity was decay-corrected to March 29, 2011. The volume-weighted mean 137Cs activity concentration of

The fogwater quantity (q, L) was calculated to estimate the temporal variation of fogwater deposition after the accident. Because no visibility data were available at the site, we used data from the surface meteo­ rological station at Karuizawa instead (Fig. 1(a)), which is the closest mountain site, although with a lower altitude. When the visibility (xvis, km) was between zero and 1, the hourly fogwater liquid water content (LWC: g m 3) was calculated from xvis by using the equation described in Stoelinga and Warner (1999): xvis ¼

ln(0.02) / 144.7 LWC

0.88

.

(1)

The fogwater quantity q is estimated from LWC by using (Ritter et al., 5

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Table 2 Amount of water (mm) and137Cs activity concentration (Bq L species from February 24 to March 29, 2011. Samples Water amount 137 Cs concentration

FG

– 45.8

BP 76.2 15.2

1

) in fogwater (FG), bulk precipitation (BP), throughfall (TF), and stemflow (SF) for the dominant tree

F. crenata

F. japonica

T. sieboldii

C. japonica

TF

SF

TF

SF

TF

SF

TF

SF

62.5 � 15.4 17.4 � 6.5

3.4 ND

46.0 � 3.1 14.7 � 3.0

1.8 ND

41.2 � 8.1 23.1 � 1.4

0.04 13.2

24.4 � 3.5 37.2 � 15.1

1.1 ND

The variation in throughfall and 137Cs activity concentration in throughfall are given by the standard deviation (n ¼ 5 for F. crenata, n ¼ 3 for F. japonica and C. japonica, and n ¼ 6 for T. sieboldii for throughfall and n ¼ 3 for all species for the 137Cs activity concentration). “ND” means “not detected” (i.e., below the detection limit).

Fig. 4. Wind speed, fogwater (fogwater droplet diameter: 15 μm), and precipitation as a function of time at the study area. The gray areas indicate the periods around the area (Morino et al., 2013).

z0 ¼ 0.0123h,

2008; Katata et al., 2010) q ¼ LWC(3.6 A Eobs U22 m),

¼ U5 m / [ln(5/z0) / ln(22/z0)],

(5)

2.5. Impact of fogwater deposition on a contaminated forest area in East Japan Because rainfall-borne 137Cs is initially intercepted and retained by the forest canopy, the ratio of 137Cs in throughfall to that in bulk pre­ cipitation (TF/BP ratio) is less than unity. However, the concentration of 137 Cs activity in fogwater is expected to exceeded that in rainfall (Bourcier et al., 2014; Masson et al., 2015), and the forest canopy effectively captures dry and fogwater 137Cs deposits (Shaw, 2007). As a result, the TF/BP ratio increases as a result of the contribution to throughfall of dry deposition and/or fogwater deposition. Therefore, the effect of fogwater on 137Cs deposition is reflected in the TF/BP ratio. On the basis of this hypothesis and by using data from prior studies (Kato et al., 2012; Itoh et al., 2015, Table 1), we measured the TF/BP ratio in March 2011 at 14 stations (TF/BP sites) to determine whether fogwater deposition of 137Cs occurred in the forested mountains of East Japan. A map of cumulative 137Cs deposition via fogwater at the end of March 2011, generated by using the WSPEEDI-II atmospheric dispersion model (Katata et al., 2015, Fig. 3), was used to evaluate the potential impact of fogwater deposition. Furthermore, to determine how the study

where Stk is the Stokes number (dimensionless). The fogwater droplet diameter was assumed to typically measure 5–30 μm (Klemm et al., 2005), and U22 m was estimated from the wind speed at a 5-m height by using a typical log-normal function (Katata et al., 2010): m

Cs input

where h is the average height (m) of the canopy.

(2)

where 3.6 is the unit conversion factor (s h 1 L g 1), A is the crosssectional area (m2) of the fog gauge, Eobs is the relative collection effi­ ciency (dimensionless), and U22 m is the average wind speed (m s 1) at the observation height (22 m) during fog events. Eobs was calculated by using the following equation based on the physical impaction theory (McComber and Touzot, 1981): � ðStk=ðStk þ 0:6ÞÞ2 ðStk � 0:08Þ Eobs ¼ (3) 0 ðStk < 0:08Þ

U22

137

(4)

where U5 m and U22 m are the wind speeds (m s 1) at heights of 5 and 22 m, respectively, and z0 is the roughness length (m), which is calculated as

6

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Table 3 137 Cs activity concentration in bulk precipitation (BP) and throughfall (TF) and the ratio of137Cs activity concentration in throughfall to that in bulk precipitation (TF/ BP ratio) at each observational site for throughfall and bulk precipitation (TF/BP sites). Prefecture

TF/BP site

BP activity concentration (Bq L 1)

TF activity concentration (Bq L 1)

TF/BP ratio

Reference

Gunma

No.10 No.11 No.12a No.14a No.14b No.9 No.13a No.13b No.13c No.13d No.7 No.8 No.1a No.1b No.1c No.1d No.2 No.6 No.3 No.4 No.5

153 16 15 256 256 38 15 15 15 15 36 34 195 195 195 195 172 289 93 76 69

152 � 20 (n ¼ 3) 12 (n ¼ 1) 2 (n ¼ 1) 28 (n ¼ 20) 28 (n ¼ 20) 15 (n ¼ 2) 37 � 15 (n ¼ 3) 17 � 7 (n ¼ 3) 15 � 3 (n ¼ 3) 23 � 1 (n ¼ 3) 25 � 7 (n ¼ 3) 8 � 1 (n ¼ 3) 186 � 58 (n ¼ 3) 156 � 29 (n ¼ 3) 186 � 18 (n ¼ 3) 78 � 4 (n ¼ 3) 98 � 16 (n ¼ 3) 113 � 11 (n ¼ 3) 34 � 1 (n ¼ 3) 64 � 12 (n ¼ 3) 33 � 3 (n ¼ 3)

0.99 � 0.13 0.73 0.13 0.11 0.11 0.39 2.45 � 0.99 1.14 � 0.43 0.97 � 0.20 1.52 � 0.09 0.68 � 0.19 0.23 � 0.02 0.95 � 0.30 0.80 � 0.15 0.95 � 0.09 0.40 � 0.02 0.57 � 0.09 0.39 � 0.04 0.37 � 0.01 0.84 � 0.16 0.49 � 0.04

Itoh et al. (2015) Itoh et al. (2015) Itoh et al. (2015) Kato et al. (2012) Kato et al. (2012) Itoh et al. (2015) This study This study This study This study Itoh et al. (2015) Itoh et al. (2015) Itoh et al. (2015) Itoh et al. (2015) Itoh et al. (2015) Itoh et al. (2015) Itoh et al. (2015) Itoh et al. (2015) Itoh et al. (2015) Itoh et al. (2015) Itoh et al. (2015)

Tochigi Saitama

Tokyo southern Ibaraki

northern Ibaraki

The variations in the TF activity concentration and in the TF/BP ratio are given by the standard deviation. a TF/BP site No.12 was located in Niigata Prefecture. However, this site was treated as being in the Gunma Prefecture.

sites were affected by fog and cloud, data on cloud-base height were gathered from Aerodrome Routine Meteorological Report (METAR) for several airports for days when the cloud cover exceeded five oktas (with few clouds, scattered clouds, broken clouds, and overcast conditions reported as 1–2, 3–4, 5–7, and 8 oktas by METAR, respectively; World Meteorological Organization, 2011). This overview was supported by the atmospheric 137Cs concentration, as measured by the SPM network (Oura et al., 2015, Table 1) and by reference to the amount of precipi­ tation at surface weather stations (Japan Meteorological Agency, JMA; Table 1). The locations of the TF/BP, METAR, SPM, and JMA sites differed between the prefectures (Table 1 and Fig. 3). Because of a lack of data from the same locations, we used data from the METAR, SPM, or JMA sites nearest to the TF/BP site in each prefecture (Fig. 3). This assumption justified for the current analysis by the large horizontal scale of the deposition events: the fogwater (or cloud water) depositions over the area occurred on March 15 and 21, 2011, and were associated with clouds in midlatitude cyclones (Hososhima and Kaneyasu, 2015; Sanada et al., 2018) and the with radioactive plumes with horizontal scales of approximately 50 km (see, for example, Figs. 3(c) and 6(c) of Tsuruta et al., 2014).

precipitation was collected. Thus, both fogwater and precipitation were collected during this period, and the wind speed was less than 2 m s 1. 3.2. Radiocesium concentrations in fogwater and throughfall Table 2 shows the 137Cs activity concentrations in fogwater, bulk precipitation, throughfall, and stemflow. The concentrations of 137Cs activity in fogwater and bulk precipitation during the observational period were 45.8 and 15.2 Bq L 1, respectively. Morino et al. (2013) calculated that radiocesium around the study area was deposited during the periods March 15–16 and 21–23, 2011. Therefore, the volume-weighted mean 137Cs activity concentrations in fogwater and bulk precipitation were estimated by using the fogwater quantity q and the amount of precipitation during these periods. The volume-weighted mean concentrations of 137Cs activity in fogwater and in bulk precipi­ tation during the periods March 15–16 and 20–23, 2011 were 65.3 and 33.0 Bq L 1, respectively. The 137Cs activity concentration in fogwater was 1.98 times that in the bulk precipitation. The 137Cs activity concentrations in throughfall for F. crenata, F. japonica, T. sieboldii, and C. japonica were 17.4, 14.7, 23.1, and 37.2

3. Results 3.1. Water input via fogwater Table 2 gives the amount of water in bulk precipitation, throughfall, and stemflow during the observational period. There was 76.2 mm of bulk precipitation during the observational period. The bulk precipita­ tion was separated into two categories: 32%–82% throughfall (24.2–62.5 mm) and 0.05%–4% stemflow (0.04–3.4 mm) for the sampled trees. The total collected fogwater over the entire fog period was 26 mL. Fig. 4 shows the wind speed, fogwater, and precipitation as function of time. For the period of March 15–16, 2011, 0.008–0.24 L was supplied as fogwater (with a droplet diameter of 5–30 μm). No precipitation was collected by the 0.5 mm tipping bucket rain gauge during this period. During the period March 21–23, 2011, the fogwater amount was 0.015–0.44 L (fogwater droplet diameter: 5–30 μm), and 35 mm of

Fig. 5. The ratio of 137Cs activity concentration in throughfall to that in bulk precipitation (TF/BP ratio) as a function of elevation obtained in this study and in previous studies (Kato et al., 2012; Itoh et al., 2015) in March 2011. The variation in the TF/BP ratio is given by the standard deviation (Table 3). 7

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Fig. 6. Cloud-base height at the Aerodrome Routine Meteorological Report (METAR) site, atmospheric 137Cs activity concentration at a suspended particulate matter (SPM) network site, amount of precipitation at a Japan Meteorological Agency (JMA) site, and elevation of observation sites for throughfall and bulk precipitation (TF/BP sites) on March 15 and 16, 2011. The location and details of these sites are provided in Fig. 3 and Table 1, respectively.

Bq L 1, respectively (Table 2). The concentrations for F. crenata and F. japonica were similar to the 137Cs activity concentration in the bulk precipitation, whereas the 137Cs activity concentration in T. sieboldii was significantly higher than that in the bulk precipitation. The 137Cs ac­ tivity concentration in C. japonica varied significantly. The 137Cs activity concentration in stemflow could only be determined for T. sieboldii (13.2 Bq L 1). The TF/BP ratios were 1.14, 0.97, 1.52, and 2.45 for F. crenata, F. japonica, T. sieboldii, and C. japonica, respectively.

(2015) (No.1a–12) were 0.11 and 0.13–0.99. The TF/BP ratios obtained in the present study were 0.97–2.45 (No.13a–13d). High TF/BP ratios were obtained at sites No.1, 4, 10, 11, and 13 (Table 3). In addition, the TF/BP ratio increased sharply with elevation (p < 0.001, Fig. 5). Fig. 3 shows large areas of fogwater deposition calculated by the atmospheric dispersion model. Sites with high TF/BP ratio (No.1, 4, 10, 11, and 13) mostly overlapped areas of fogwater deposition, as predicted by the atmospheric dispersion model. Fig. 6 shows the cloud-base height and atmospheric 137Cs activity concentration observed in East Japan on March 15 and 16, 2011. At all sites, the atmospheric 137Cs activity concentration peaked at around 06:00–12:00 on March 15 (Fig. 6). At that time, the cloud-base height was approximately 800 m at Gunma Prefecture (Fig. 6(a)), which is similar to the elevations of observational sites No.10 and 11. At Saitama Prefecture, the cloud-base height was approximately 1000 m (Fig. 6(c)).

3.3. Comprehensive analysis of throughfall, fogwater deposition map, and cloud-base height Table 3 shows the TF/BP ratios in March 2011 from this study and from previous studies (Kato et al., 2012; Itoh et al., 2015). The TF/BP ratios obtained by Kato et al. (2012) (No.14a, and 14b) and by Itoh et al. 8

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concentration in East Japan during the period March 20–23, 2011. At­ mospheric concentrations of 137Cs activity were high from about 12:00 on March 20 to 12:00 on March 21, 2011 at Gunma and Tochigi Pre­ fectures (Fig. 7(a) and (b)), and from 20:00 on March 20 to 12:00 on March 21, 2011 at Saitama Prefecture and Tokyo Metropolis (Fig. 7(c) and (d)). In addition, in the northern and southern parts of Ibaraki Prefecture, the atmospheric 137Cs activity concentration was high at about 12:00–15:00 on March 20 and 6:00–12:00 on March 21, 2011 (Fig. 7(e) and (f)). When the atmospheric 137Cs activity concentration was high, the cloud base was above the observational sites at Gunma and Tochigi Prefectures and Tokyo Metropolis (Fig. 7(a), (b), and 7(d)). By contrast, at Saitama Prefecture, sites No.13a and 13b–13d were above the cloud base (200–600 m; Fig. 7(c)). In addition, in the northern and southern parts of Ibaraki Prefecture, sites No.1a–1d and No.4 were above the cloud base (200–300 m; Fig. 7(e) and 7(f)). 4. Discussion 4.1. Chemical characteristics of fogwater during the early stage of the F1NPP accident The monthly mean radiocesium concentration in the water captured by the passive fog collector was approximately twice that in bulk pre­ cipitation. Fogwater generally contains a higher concentration of chemical species than rainfall. Igawa et al. (1998) reported dissolved-ion þ þ 2þ 2þ 2 concentrations (Hþ, NHþ 4 , Na , K , Mg , Ca , Cl , NO3 , and SO4 ) in fogwater were 3–50 times greater than those in bulk precipitation in Japan. In addition, the dissolved-ion concentrations in fogwater (Hþ, þ þ 2þ 2þ NHþ 4 , Na , K , Mg , Ca , and NO3 ) were 5–17 times greater than those found in precipitation in Costa Rica (Clark et al., 1998). These high concentrations in fogwater are attributed to the smaller volume of water per condensation nucleus compared with rain droplets (Hough, 1987), the higher scavenging rate of aerosol particles and gases over long residence periods, and the larger surface area of clouds (Su and Huh, 2006). Bourcier et al. (2014) reported that the 137Cs activity concen­ tration in fogwater (0.32 � 0.06 mBq L 1) was 4.5 � 2.9 times greater than that in bulk precipitation in the background level at the Puy de ^me atmospheric research station (1465 m) in France. Masson et al. Do (2015) reported that the 137Cs concentration in fogwater (0.25 mBq L 1) was approximately 3.3 times greater than that in bulk precipitation ^me site. By (0.075 mBq L 1) before the F1NPP accident at the Puy de Do contrast, in England, Livens et al. (1992)measured a lower 137Cs activity concentration in orographic cloud water compared with that in pre­ cipitation and suggested that the concentration of 137Cs particles in the boundary layer from which orographic clouds are mainly formed was less than that in the higher levels from which the rain was falling. When using a passive collector, fog is difficult to measure separately from wind-driven rain under conditions of high wind speed (Frumau et al., 2011). Although water samples were not contaminated by wind-driven rain in fog-only periods such as March 15–16, 2011 (Fig. 4), observa­ tions may be uncertain when both rain and fog appeared in the period March 21–23, 2011. However, any contamination should be relatively small because the wind speed was rather low (<2 m s 1; Fig. 4). In addition, dry deposition of 137Cs into the passive collector is another factor that may increase the 137Cs concentration in the samples, but this effect should have been small at our study site because it was located far from the F1NPP plant. In addition, Terada et al. (2012) calculated that, at this site, wet deposition contributed a dominant fraction (90%–95%) of the total deposition accumulated from March 12, 2011 to May 1, 2011. Therefore, the high 137Cs concentration in water samples may be attributed to the significant fogwater deposition at our study site. The higher 137Cs activity concentrations in throughfall compared with those in bulk precipitation indicate that the 137Cs input via dry and/or fogwater deposition was higher than that due to the loss of 137Cs by canopy interception of rainfall. At the observational site during the accident, the 137Cs concentration in fogwater was higher than that in

Fig. 7. Same as Fig. 6, but for March 20–23, 2011. The gray band indicates time when no data were available for the cloud base height.

Therefore, the elevation of site No.13a was similar to the cloud-base height, and sites No.13b–13d were above the cloud-base height. By contrast, the cloud-base height was 800 m at Tochigi Prefecture (Fig. 6 (b)), 1000 m at Tokyo Metropolis (Fig. 6(d)), 900 m in the southern part of Ibaraki Prefecture (Fig. 6(e)), and 700–800 m in the northern part of Ibaraki Prefecture (Fig. 6(f)), so these observational sites (i.e., No.1–8 and 14) were below the cloud-base height (Fig. 6(b)–(d)–6(f)). Fig. 7 shows the cloud-base height and atmospheric 137Cs activity 9

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rainfall (Table 2). By contrast, dry deposition is expected to have a little effect at the site (Terada et al., 2012). Therefore, we assume that, at our study site, the high TF/BP ratio for 137Cs activity concentration was mainly caused by fogwater deposition as opposed to dry deposition.

observation site. METAR data were provided by the Japan Meteorology Agency. This work was supported by JSPS KAKENHI (Grant Number JP22780139]. Appendix A. Supplementary data

4.2. How much of the forest area was contaminated by fogwater deposition?

Supplementary data to this article can be found online at https://doi. org/10.1016/j.atmosenv.2020.117339.

Upon applying the TF/BP ratio to previous data on the 137Cs activity concentrations in throughfall and bulk precipitation in East Japan (Fig. 3), we find that sites No.1, 4, 10, 11, and 13 had high TF/BP values (Table 3) and that the TF/BP ratio clearly increased with elevation (Fig. 5). These results suggest that the forested mountain areas in East Japan were contaminated by fogwater deposition of 137Cs, whereas some sites may have received deposits of 137Cs by both dry and fogwater deposition. These results are consistent with the vertical distribution of the air dose rate in mountain areas reported in previous studies (Hososhima and Kaneyasu, 2015; Sanada et al., 2018) and with the areas of high fogwater deposition (calculated by using the atmospheric dispersion model) (Katata et al., 2015). Sanada et al. (2018) reported that the air dose rate increased at 300 m and reached its highest value at 700–1000 m a.s.l. in the mountainous areas in the western and northern parts of Tochigi and Gunma prefectures. In addition, the TF/BP sites above 300 m (No.1a–1d, 4, 10, 11, and 13a–13d) overlapped almost entirely the area of fogwater deposition predicted by our atmospheric dispersion model (Katata et al., 2015, Fig. 3). The impact of fogwater deposition is also supported by data on cloud-base height. According to the cloud-base height measured during high concentrations of 137Cs in the atmosphere (March 15, 2011), the elevations of No.13a (1040 m) and No.13b–13d (1260 m) in Saitama Prefecture and of No.10 (850 m) and No.11 (800 m) in Gunma Prefec­ ture were similar to or less than the cloud-base height (Fig. 6). Addi­ tionally, on March 21, 2011, sites No.13a (1040 m) and No.13b–13d (1260 m) of Saitama Prefecture, No.1a–1d (310–380 m) of south Ibaraki Prefecture, and No.4 (400 m) of north Ibaraki Prefecture were all below the cloud base (Fig. 7). Taken together, these results suggest that, in East Japan, the forested mountain areas over 800 m a.s.l. were likely contaminated by deposition of 137Cs from fogwater on March 15, 2011 and March 21, 2011 above 310 m.

References Bourcier, L., Masson, O., Laj, P., Paulat, P., Pichon, J.M., Chausse, P., Gurriaran, R., Sellegri, K., 2014. 7Be, 210Pb and 137Cs concentrations in cloud water. J. Environ. Radioact. 128, 15–19. https://doi.org/10.1016/j.jenvrad.2013.10.020. Caput, C., Camus, H., Belot, Y., 1990. Observations on the behaviour of radiocesium in permanent pastures after the Chernobyl accident. In: Desmet, G., Nassimbeni, P., Belli, M. (Eds.), Transfer of Radionuclides in Natural and Semi-natsural Environments. Elsevier Applied Science, London (UK), pp. 283–291. Chino, M., Nakayama, H., Nagai, H., Terada, H., Katata, G., Yamazawa, H., 2011. Preliminary estimation of release amounts of 131I and 137Cs accidently discharged from the Fukushima Daiichi nuclear power plant into the atmosphere. J. Nucl. Sci. Technol. 48, 1129–1134. https://doi.org/10.1080/18811248.2011.9711799. Clark, K.L., Nadkarni, N.M., Schaefer, D., Gholz, H.L., 1998. Atmospheric deposition and net retention of ions by the canopy in a tropical montane forest, Monteverde, Costa Rica. J. Trop. Ecol. 14, 27–45. https://doi.org/10.1017/S0266467498000030. Falconer, R.E., Falconer, P.D., 1980. Determination of cloud water acidity at a mountain observatory in the Adirondack mountains of New York State. J. Geophys. Res. 85, 7465–7470. https://doi.org/10.1029/JC085iC12p07465. Frumau, K.F.A., Burkard, R., Schmid, S., Bruijnzeel, L.A.S., Tob� on, C., Calvo-Alvarado, J. C., 2011. A comparison of the performance of three types of passive fog gauges under conditions of wind-driven fog and precipitation. Hydrol. Process. 25, 374–383. https://doi.org/10.1002/hyp.7884. Gibb, R., Carson, P., Thompson, W., 1997. The effect of fog on radionuclide deposition velocities. In: Donnelly, J.V., Oliva, A. (Eds.), CNS Proceedings of the 1997 CNA/ CNS Annual Conference on Powering Canada’s Future. Canadian Nuclear Society, Toronto (Canada), p. 1122. Hashimoto, S., Ugawa, S., Nanko, K., Shichi, K., 2012. The total amounts of radioactively contaminated materials in forests in Fukushima, Japan. Sci. Rep. 2, 416. https://doi. org/10.1038/srep00416. Hososhima, M., Kaneyasu, N., 2015. Altitude-dependent distribution of ambient gamma dose rates in a mountainous area of Japan caused by the Fukushima nuclear accident. Environ. Sci. Technol. 49, 3341–3348. https://doi.org/10.1021/ es504838w. Hough, A.M., 1987. A computer modeling study of the chemistry occurring during cloud formation over hills. Atmos. Environ. 21, 1073–1095. https://doi.org/10.1016/ 0004-6981(87)90235-6. Igawa, M., Tsutsumi, Y., Mori, T., Okochi, H., 1998. Fogwater chemistry at a mountainside forest and the estimation of the air pollutant deposition via fog droplets based on the atmospheric quality at the mountain base. Environ. Sci. Technol. 32, 1566–1572. https://doi.org/10.1021/es970213x. Imamura, N., 2010. A Study of Nutrients Cycling between Atmosphere and Forest for Wide Area Monitoring. The University of Tokyo. Imamura, N., 2014. Study on Atmospheric Inputs of Dissolved Ions to Temperate Forests on the Kanto Plain. The University of Tokyo, Japan. https://doi.org/10.15083/ 00006511. Imamura, N., Tanaka, N., Ohte, N., Yamamoto, H., 2012. Nutrient transfer with rainfall in the canopies of a broad-leaved deciduous forest in Okuchichibu. J. Japanese For. Soc. 94, 74–83. https://doi.org/10.4005/jjfs.94.74. Imamura, N., Komatsu, M., Ohashi, S., Hashimoto, S., Kajimoto, T., Kaneko, S., Takano, T., 2017. Temporal changes in the radiocesium distribution in forests over the five years after the Fukushima Daiichi Nuclear Power Plant accident. Sci. Rep. 7, 8179. https://doi.org/10.1038/s41598-017-08261-x. Itoh, Y., Imaya, A., Kobayashi, M., 2015. Initial radiocesium deposition on forest ecosystems surrounding the Tokyo metropolitan area due to the Fukushima Daiichi Nuclear Power Plant accident. Hydrol. Res. Lett. 9, 1–7. https://doi.org/10.3178/ hrl.9.1. Japan Meteorological Agency. n.d. No Title [WWW Document]. http://www.data.jma. go.jp/obd/stats/etrn/index.php. accessed 3.22.17. Kajino, M., Sekiyama, T.T., Igarashi, Y., Katata, G., Sawada, M., Adachi, K., Zaizen, Y., Tsuruta, H., Nakajima, T., 2019. Deposition and dispersion of radio-cesium released due to the Fukushima nuclear accident : sensitivity to meteorological models and physical modules. J. Geophys. Res. Atmos. 124, 1823–1845. https://doi.org/ 10.1029/2018JD028998. Kaneyasu, N., Ohashi, H., Suzuki, F., Okuda, T., Ikemori, F., 2012. Sulfate aerosol as a potential transport medium of radiocesium from the fukushima nuclear accident. Environ. Sci. Technol. 46, 5720–5726. https://doi.org/10.1021/es204667h. Katata, G., Nagai, H., Kajino, M., Ueda, H., Hozumi, Y., 2010. Numerical study of fog deposition on vegetation for atmosphere-land interactions in semi-arid and arid regions. Agric. For. Meteorol. 150, 340–353. https://doi.org/10.1016/j. agrformet.2009.11.016. Katata, G., Ota, M., Terada, H., Chino, M., Nagai, H., 2012. Atmospheric discharge and dispersion of radionuclides during the Fukushima Dai-ichi Nuclear Power Plant accident. Part I: source term estimation and local-scale atmospheric dispersion in

5. Conclusion Direct measurements of 137Cs in fogwater provide evidence indi­ cating that fogwater with high levels of radioactive contamination deposited 137Cs in the forested mountain areas of East Japan during the early stages of the F1NPP accident. The fact that fogwater often lingers for a long wet period over canopies relative to rainfall (e.g., Katata et al., 2010) may enhance the direct uptake of radiocesium at the leaf surface (Nonaka and Hirono, 2011). During the F1NPP accident, the potential for foliar uptake of radiocesium at Mt. Tsukuba was reported (Nishikiori et al., 2015). The specific characteristics of the fogwater deposition process should thus be considered to better understand radiocesium cycling in forest ecosystems following nuclear accidents. Declaration of competing interest 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. Acknowledgments We thank the technical support staff of The University of Tokyo Chichibu Forest for their assistance with our observations. We also thank Dr. Nobuaki Tanaka of The University of Tokyo for setting up the 10

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early phase of the accident. J. Environ. Radioact. 109, 103–113. https://doi.org/ 10.1016/j.jenvrad.2012.02.006. Katata, G., Chino, M., Kobayashi, T., Terada, H., Ota, M., Nagai, H., Kajino, M., Draxler, R., Hort, M.C., Malo, A., Torii, T., Sanada, Y., 2015. Detailed source term estimation of the atmospheric release for the Fukushima Daiichi Nuclear Power Station accident by coupling simulations of an atmospheric dispersion model with an improved deposition scheme and oceanic dispersion model. Atmos. Chem. Phys. 15, 1029–1070. https://doi.org/10.5194/acp-15-1029-2015. Kato, H., Onda, Y., Gomi, T., 2012. Interception of the Fukushima reactor accidentderived 137Cs, 134Cs and 131I by coniferous forest canopies. Geophys. Res. Lett. 39, 1–6. https://doi.org/10.1029/2012GL052928. Klemm, O., Wrzesinsky, T., Scheer, C., 2005. Fog water flux at a canopy top: direct measurement versus one-dimensional model. Atmos. Environ. 39, 5375–5386. https://doi.org/10.1016/j.atmosenv.2005.05.041. Livens, F.R., Fowler, D., Horrill, A.D., 1992. Wet and dry deposition of 131I, 134Cs and 137 Cs at an upland site in Northern England. J. Environ. Radioact. 16, 243–254. https://doi.org/10.1016/0265-931X(92)90003-C. Masson, O., de Vismes Ott, A., Bourcier, L., Paulat, P., Ribeiro, M., Pichon, J.M., Sellegri, K., Gurriaran, R., 2015. Change of radioactive cesium (137Cs and 134Cs) content in cloud water at an elevated site in France, before and after the Fukushima nuclear accident: comparison with radioactivity in rainwater and in aerosol particles. Atmos. Res. 151, 45–51. https://doi.org/10.1016/j.atmosres.2014.03.031. MEXT, 2012. Extension Site of Distribution Map of Radiation Dose, etc.,/GSI Maps. WWW Document. http://ramap.jmc.or.jp/map/eng/. accessed 12.13.16. McComber, P., Touzot, G., 1981. Calculation of the impingement of cloud droplets in a cylinder by the finite-element method. J. Atmos. Sci. 38, 1027–1036. https://doi. org/10.1175/1520-0469(1981)038<1027:COTIOC>2.0.CO;2. Morino, Y., Ohara, T., Watanabe, M., Hayashi, S., Nishizawa, M., 2013. Episode analysis of deposition of radiocesium from the fukushima Daiichi nuclear power plant accident. Environ. Sci. Technol. 47, 2314–2322. https://doi.org/10.1021/ es304620x. Nishikiori, T., Watanabe, M., Koshikawa, M.K., Takamatsu, T., Ishii, Y., Ito, S., Takenaka, A., Watanabe, K., Hayashi, S., 2015. Uptake and translocation of radiocesium in cedar leaves following the Fukushima nuclear accident. Sci. Total Environ. 502, 611–616. https://doi.org/10.1016/j.scitotenv.2014.09.063. Nonaka, K., Hirono, Y., 2011. Absorption and transfer of cesium in tea plants during the growing period of second flush. Tea Res. J. 112, 55–59. Ohashi, S., Kuroda, K., Takano, T., Suzuki, Y., Fujiwara, T., Abe, H., Kagawa, A., Sugiyama, M., Kubojima, Y., Zhang, C., Yamamoto, K., 2017. Temporal trends in 137 Cs concentrations in the bark, sapwood, heartwood, and whole wood of four tree species in Japanese forests from 2011 to 2016. J. Environ. Radioact. 178–179, 335–342. https://doi.org/10.1016/j.jenvrad.2017.09.008. Oura, Y., Ebihara, M., Tsuruta, H., Nakajima, T., Ohara, T., Ishimoto, M., Sawahata, H., Katsumura, Y., Nitta, W., 2015. A 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 Japan. J. Nucl. Radiochem. Sci. 15, 15–26. https://doi.org/10.4236/cweee.2013.22B011. Ritter, A., Regalado, C.M., Aschan, G., 2008. Fog water collection in a subtropical elfin laurel forest of the Garajonay National Park (Canary Islands): a combined approach

using artificial fog catchers and a physically based impaction model. J. Hydrometeorol. 9, 920–935. https://doi.org/10.1175/2008JHM992.1. Sanada, Y., Katata, G., Kaneyasu, N., Nakanishi, C., Urabe, Y., Nishizawa, Y., 2018. Altitudinal characteristics of atmospheric deposition of aerosols in mountainous regions: lessons from the Fukushima Daiichi Nuclear Power Station accident. Sci. Total Environ. 618, 881–890. https://doi.org/10.1016/j.scitotenv.2017.08.246. Sawada, H., Oomura, K., Sibano, S., Fujiwara, A., Kaji, M., 2006. Enumeration Data (1994-2005) for a Long-Term Ecological Research Plot at Tokyo University Forest in Chichibu, vol.45. Misk Information, Tokyo Univ, pp. 71–218. Shaw, G., 2007. Radionuclides in forest ecosystems. In: Shaw, G. (Ed.), Radioactivity in the Terrestrial Environment. Elsevier, pp. 127–155. Stoelinga, M.T., Warner, T.T., 1999. Nonhydrostatic, mesobeta-scale model simulations of cloud ceiling and visibility for an east coast winter precipitation event. J. Appl. Meteorol. 38, 385–404. https://doi.org/10.1175/1520-0450(1999)038<0385: NMSMSO>2.0.CO;2. Stohl, A., Seibert, P., Wotawa, G., Arnold, D., Burkhart, J.F., Eckhardt, S., Tapia, C., Vargas, A., Yasunari, T.J., 2011. Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition. Atmos. Chem. Phys. Discuss. 11, 2313–2343. https://doi.org/10.5194/acpd-11-28319-2011. Su, C.C., Huh, C.A., 2006. Measurements of 7Be and 210Pb in cloudwaters: toward a better understanding of aerosol transport and scavenging. Geophys. Res. Lett. 33, 33–36. https://doi.org/10.1029/2005GL025042. Terada, H., Katata, G., Chino, M., Nagai, H., 2012. Atmospheric discharge and dispersion of radionuclides during the Fukushima Dai-ichi Nuclear Power Plant accident. Part II: verification of the source term and analysis of regional-scale atmospheric dispersion. J. Environ. Radioact. 112, 141–154. https://doi.org/10.1016/j. jenvrad.2012.05.023. The University of Tokyo Chichibu Forest, 2014. Impact study of radionuclides at the university of Tokyo Chichibu forest. WWW Document. http://chichi-en.blogspot.co m/2014/07/blog-post_4286.html. Tsuruta, H., Oura, Y., Ebihara, M., Ohara, T., Nakajima, T., 2014. First retrieval of hourly atmospheric radionuclides just after the Fukushima accident by analyzing filtertapes of operational air pollution monitoring stations. Sci. Rep. 4, 6717. https://doi. org/10.1038/srep06717. Winiarek, V., Bocquet, M., Saunier, O., Mathieu, A., 2012. Estimation of errors in the inverse modeling of accidental release of atmospheric pollutant: application to the reconstruction of the cesium-137 and iodine-131 source terms from the Fukushima Daiichi power plant. J. Geophys. Res. Atmos. 117, 1–16. https://doi.org/10.1029/ 2011JD016932. World Meteorological Organization, 2011. Manual on Codes - International Codes, Volume I.1, Annex II to the WMO Technical Regulations: Part A-Alphanumeric Codes. WMO Publ, p. 306. Yamada, T., 2013. Mushrooms: radioactive contamination of widespread mushrooms in Japan. In: Nakanishi, T.M., Tanoi, K. (Eds.), Agricultural Implications of the Fukushima Nuclear Accident. Springer Japan, Tokyo, pp. 163–176. https://doi.org/ 10.1007/978-4-431-54328-2_15.

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