Scholarly article on topic 'Weak size dependence of resuspended radiocesium adsorbed on soil particles collected after the Fukushima nuclear accident'

Weak size dependence of resuspended radiocesium adsorbed on soil particles collected after the Fukushima nuclear accident Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Naoki Kaneyasu, Hideo Ohashi, Fumie Suzuki, Tomoaki Okuda, Fumikazu Ikemori, et al.

Abstract Most studies of the properties of airborne radionuclides emitted from the Fukushima Daiichi Nuclear Power Plant have focused on the relatively early stages of the accident, and little is known about the characteristics of radiocesium in the long-term. In this study, we analyzed activity size distributions of airborne radiocesium collected over 5 months in Tsukuba, Japan. Radiocesium in the accumulation mode size range (0.1–2 μm in aerodynamic diameter) was overwhelming in the early aerosol samples and decreased with time, while that associated with coarse aerosols remained airborne. We examined the radiocesium adsorbed onto airborne soil particles, and found that the size dependence of 137Cs surface density adsorbed on soil particles was weak. That is, radiocesium was distributed homogeneously throughout the aerodynamic diameter range of 2.1–11 μm. This characteristic may be related to the reported structure of radiocesium-bearing soil particles collected from the ground, which consisted of an aggregate of specific clay minerals and other non-cesium adsorbing particles. The resuspension factors for the first two aerosol samples collected during late April and May 2011 were close to those in European cities in the months following the Chernobyl accident, despite different soil and weather conditions.

Academic research paper on topic "Weak size dependence of resuspended radiocesium adsorbed on soil particles collected after the Fukushima nuclear accident"

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Journal of Environmental Radioactivity

journal homepage: www.elsevier.com/locate/jenvrad

Weak size dependence of resuspended radiocesium adsorbed on soil particles collected after the Fukushima nuclear accident

Naoki Kaneyasu a *, Hideo Ohashi b1, Fumie Suzuki b, Tomoaki Okuda c, Fumikazu Ikemori d, Naofumi Akata e, Toshihiro Kogure f

a National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba 305-8569, Japan b Tokyo University of Marine Science and Technology, 4-5-7 Kounan, Minato-ku, Tokyo 108-8477, Japan c Keio University, 3-14-1 Hiyoshi, Kouhoku-ku, Yokohama 223-8522, Japan d Nagoya City Institute for Environmental Sciences, 5-16-8 Toyoda, Nagoya 457-0841, Japan e National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan

f Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan

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ARTICLE INFO

ABSTRACT

Article history: Received 9 December 2016 Received in revised form 20 February 2017 Accepted 2 March 2017

Keywords:

Fukushima nuclear accident

Radionuclides

Resuspension

Soil dust particles

Surface density

Most studies of the properties of airborne radionuclides emitted from the Fukushima Daiichi Nuclear Power Plant have focused on the relatively early stages of the accident, and little is known about the characteristics of radiocesium in the long-term. In this study, we analyzed activity size distributions of airborne radiocesium collected over 5 months in Tsukuba, Japan. Radiocesium in the accumulation mode size range (0.1—2 mm in aerodynamic diameter) was overwhelming in the early aerosol samples and decreased with time, while that associated with coarse aerosols remained airborne. We examined the radiocesium adsorbed onto airborne soil particles, and found that the size dependence of 137Cs surface density adsorbed on soil particles was weak. That is, radiocesium was distributed homogeneously throughout the aerodynamic diameter range of 2.1—11 mm. This characteristic may be related to the reported structure of radiocesium-bearing soil particles collected from the ground, which consisted of an aggregate of specific clay minerals and other non-cesium adsorbing particles. The resuspension factors for the first two aerosol samples collected during late April and May 2011 were close to those in European cities in the months following the Chernobyl accident, despite different soil and weather conditions.

© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND

license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Since the devastating accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011, a number of numerical modeling and observational studies have tried to map and simulate the activity of airborne radionuclides, their contribution to ground surface contamination via deposition processes, their migration through soil and riverine systems, and resultant gamma dose rates in air (Hirose, 2016). However, studies on the physicochemical properties of airborne radionuclides are limited. The physico-chemical properties of particles play critical roles in the deposition of radionuclides, and thereby may affect the location and intensity

* Corresponding author.

E-mail address: kane.n@aist.go.jp (N. Kaneyasu). 1 (Present address) EcoStudies, Toranomon 2-2-5, Minato-ku, Tokyo 105-0001, Japan.

of ground surface contamination (Adachi et al., 2013; Hososhima and Kaneyasu, 2015).

Several reports have been published on the physicochemical properties of radiocesium released from the FDNPP accident, based on measurements in Japan (Kaneyasu et al., 2012; Adachi et al., 2013; Doi et al., 2013; Miyamoto et al., 2014) and European countries (Masson et al., 2013). These studies have focused on the relatively early stages of the accident, when direct discharge from the FDNPP was massive. However, little information is available on the behavior of Fukushima-derived airborne radionuclides over a longer period.

In the months after the FDNPP accident, the initial massive emission of radionuclides from the damaged reactors decreased. For example, the estimated direct emission of 134Cs and 137Cs from the FDNPP in late June 2011 (109 Bq/h) was six orders of magnitude smaller than that in the maximum emission period on March 15, 2011 (2 x 1015 Bq/h) (TEPCO, 2011). At this stage of the accident,

http://dx.doi.org/10.1016/j.jenvrad.2017.03.001

0265-931X/© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

resuspension or secondary emission of radionuclides from areas where they were initially deposited became a public concern, although directly emitted radionuclides were still observed in areas close to the FDNPP.

Among secondary emission sources, wind-driven resuspension of contaminated soil particles was one of the primary factors controlling radionuclide deposition in Japan after the accident (Igarashi et al., 2011). For example, Akimoto (2015) pointed out that a time series of monthly fallout and airborne concentration measurements of radiocesium in Fukushima City, located 80 km northwest of the FDNPP, indicated high resuspension rates from January through May 2012. In the Kanto Plain, Japan, deposition of locally derived soil particles is typically observed from January to May, and this was assumed to be the cause of the increase in deposition of 137Cs from winter 2011 to spring 2012 (Hirose, 2013). In the 30 km exclusion zone around the Chernobyl Nuclear Power Plant (ChNPP), the size properties of resuspended radiocesium associated with soil particles were intensively investigated after the accident (Garger et al., 1998a, 1998b). However, since the FDNPP accident, only a few studies have been published on radionuclides in the coarse mode size range (aerodynamic diameter (Da) > 2 mm), presumably associated with airborne soil particles (Yamaguchi et al., 2012; Ochiai et al., 2016; Ishizuka et al., 2017), or dust particles from debris removal operations (Steinhauser et al., 2015). These studies have not explicitly examined the relationship between radioactivity in the coarse mode aerosols and their mass concentrations, which is essential for understanding the mobilization of secondary radioactive materials in the air, soil, and water.

In this paper, we characterized FDNPP-derived radiocesium adsorbed onto airborne soil particles. First, we examined the temporal trends in the activity size distribution of airborne radiocesium from late May through late September 2011 in Tsukuba, Japan. The authors previously reported two activity size distributions measured during the early stages of the accident and discussed potential carriers of radiocesium (Kaneyasu et al., 2012). In the present study, the changes in aerosol components that act as carriers were investigated using measurements taken over longer periods. By comparing the activity size and surface area size distributions of soil particles, we examined the size dependence of resuspended radiocesium adsorbed on soil particles.

2. Sampling and measurements

Aerosol sampling and analysis were conducted as described previously (Kaneyasu et al., 2012). In brief, size-resolved aerosol sampling was conducted using a low-pressure cascade impactor (12 stages with a backup filter, Tokyo Dylec LP-20) from the fourth-floor balcony of a building (15 m from the ground) in Tsukuba, a city located 170 km south of the FDNPP (Fig. 1). The cascade impactor was placed in the open air beneath an overhanging roof, about 3 m aloft. The 50% collection efficiency values for the impactor stages were 11, 7.8, 5.2, 3.5, 2.1,1.2, 0.7, 0.49, 0.3, 0.2, 0.12, and 0.06 mm in aerodynamic diameter. The cutoff diameter of particles by the inlet of cascade impactor (2.5 cm in diameter, 3 cm long) was assumed to be 30 mm based on a previous study that used the same impactor (Hitzenberger and Tohno, 2001). However, the value of 30 mm is rather vague and changes markedly with the wind velocity. Therefore, size data for Da > 11 mm were regarded as less reliable. As described in Kaneyasu et al. (2012), samples LPI-AIST-1 and -2 were collected between April 28 and May 26, 2011, and the subsequent samples, LPI-AIST-3 through -6, discussed in this study were collected until September 21, 2011. The sampling periods and impaction substrates used in this study are listed in Table 1. The effect of the use of different impaction substrates is discussed in Appendix A. The aerosol size distributions, particularly those in the

Fig. 1. Map of the study area. FDNPP indicates the location of Fukushima Dai-ichi Nuclear Power Plant, and AIST indicates the location where size-segregated aerosol sampling was conducted.

coarse mode size range, collected at a height of 15 m from the ground, are different from those at ground level, i.e., 1.5 m height. As Wagenpfeil et al. (1999) indicated, coarse soil particles, particularly those of Da > 10 mm, are difficult to transport at greater heights from the ground. Therefore, the fraction of particles in the coarse size range at the ground level was actually much larger than those measured in this study.

The radioactivity of cesium isotopes 134Cs and 137Cs in aerosol samples LPI-AIST-1 through -4 were determined by g-ray spec-trometry with a well-type germanium detector (GCW3523, CANBERRA). For samples LPI-AIST-5 and -6, we used coaxial-type germanium spectrometry (GEM20, Seiko EG&G Ortec). Because gray analysis of the LPI-AIST-5 and -6 aerosol samples was conducted 1.5—2 years after the accident, only 137Cs radioactivity was determined, as 134Cs activity had decayed to low counts. The uncertainty indicated by the error bars in the figures showing activity concentrations includes a 5% counting error in the g-ray measurements plus the nominal precision of the mass flowmeter (3%) used to calibrate the flowrate of the impactor.

After the g-ray analysis, the concentrations of ionic species in aerosols were determined by ion chromatography (DIONEX, IC-2000 and IC-1000) after extraction with ultra-pure water. Non-sea-salt sulfate (nss.SO24-) concentrations were calculated from measured SO4- and Na+ values, assuming the SO4-/Na+ ratio of sea salt particles to be the same as that of the surface seawater. Calcium (Ca) in the aerosols was determined using an x-ray fluorescence

Table 1

Summary of size-fractionated aerosol sampling in Tsukuba, Japan, after the Fukushima Daiichi nuclear accident.

Sample ID Sampling period Impaction substrate Sampled air volume (m3) AMAD (mm) 134Cs 137Cs

LPI-AIST-1* Apr. 28—May 12* Aluminum sheet* 495.9* 0.54* 0.53*

LPI-AIST-2* May 12—May 26* Quartz fiber filter* 495.1* 0.63* 0.63*

LPI-AIST-3 May 26—Jun. 9 Aluminum sheet 494.9 0.55 0.54

LPI-AIST-4 Jun. 9—Jul. 19 Quartz fiber filter 1415.1 0.87 0.87

LPI-AIST-5 Jul. 19—Aug. 21 Quartz fiber filter 1101.0 — 1.93

LPI-AIST-6 Aug. 21—Sep. 21 Quartz fiber filter 1095.7 — 1.43

Abbreviations: AMAD = activity median aerodynamic diameter."-": not quantified due to decay when measurements were conducted. *Data and the details of the LPI-AIST-1 and -2 are from Kaneyasu et al. (2012).

spectrometer (EDXL300, Rigaku).

The radioactivity of aerosol deposits on an impaction substrate was examined using the imaging plate (IP) method (BAS IP MS, Fuji Film). The IP was placed in the dark for about 8 days to expose it to radiation from the impaction substrate, and then scanned with an IP reader (FLA-7000, Fuji Film). A portion of the same aerosol sample was observed by scanning electron microscopy (SEM, Hitach-S4500).

collected in midsummer (July 21 through August 21, 2011) showed a distinct change in shape. The 137Cs radioactivity in the accumulation mode was greatly reduced, and additional modes of 3.5—5.2 and 7.8—11 mm size were recognizable in the coarse size range. The radioactivity of aerosols in sample LPI-AIST-6, collected from August 21 to September 21, 2011, was close to the detection limit over the entire size range; thus, it was difficult to characterize the shape of the size distribution.

3. Results and discussion

3.1. Changes in the activity size distributions of radiocesium through late September 2011

The activity size distribution of 137Cs in aerosol sample LPI-AIST-3, collected from May 26 through June 9, 2011 (Fig. 2), closely resembled the preceding two samples (LPI-AIST-1 and -2) presented in Kaneyasu et al. (2012), i.e., radioactivity of 137Cs was localized in the accumulation mode size range (Da < 2 mm). However, the total radioactivity decreased by one order of magnitude compared to the previous two aerosol samples (Fig. 3). Decay-corrected 134Cs had the same activity size distribution as 137Cs (Appendix B, Fig. A1). The activity size distribution of the next aerosol sample, LPI-AIST-4, collected from June 9 through July 19, 2011, exhibited slightly different characteristics. Although the mode of 137Cs radioactivity was in the 0.49—0.7 mm range, the size distribution generally shifted toward larger particles.

In contrast, the activity size distribution of LPI-AIST-5 aerosols

Fig. 2. Activity size distributions of 137Cs in LPI-AIST-3 to -6 aerosol samples collected in Tsukuba, Japan, from May 26 to September 21, 2011, at a height of 15 m from the ground. The error bars indicate the counting error in determining the exposure time for the g-ray spectrometry analysis plus the nominal precision of the mass flowmeter used to calibrate the flowrate of the impactor.

3.2. Changes in the carriers of radiocesium in air

In Kaneyasu et al. (2012), we noted that sulfate aerosols were the primary potential transport medium of radiocesium from the FDNPP accident during late April and May 2011. How long this sulfate-based transport continued was a subject to be addressed in future research.

In this study, we concluded that sulfate has not been the primary carrier of radiocesium since mid-July 2011. In Fig. 3, temporal changes in the activity median aerodynamic diameter (AMAD) of

Cs and mass median aerodynamic diameter (MMAD) of nss. SO4-are compared. The AMAD of 137Cs remained between 0.5 and 0.6 mm until June 9, 2011 (LPI-AIST-3 sample), in close agreement with the MMAD of nss.SO4- (Table 1). In LPI-AIST-4, the AMAD of

became larger, and the value separated from the MMAD of nss. SO24-. In midsummer of 2011 (LPI-AIST-5), when the radioactivity of 137Cs in the accumulation mode size range had fallen to a low level, the AMAD reached 1.9 mm, more than three times the MMAD of nss.SO42-. This suggests that changes in the dominant carrier of

Fig. 3. Total activity concentrations of radiocesium in LPI-AIST-1 to -6 aerosol samples (left Y-axis), activity median aerodynamic diameter (AMAD) of 137Cs, and mass median aerodynamic diameter (MMAD) of non-sea-salt sulfate (nss.SO24-) in the same aerosol samples (right Y-axis). Note that the AMAD of 137Cs and MMAD of nss.SO|" for the LPI-AIST-1 and -2 samples marked with * are from Kaneyasu et al. (2012).

radiocesium started to occur between LPI-AIST-3 and -4, despite their low radioactivity, and were completed between the LPI-AIST-4 and -5 aerosol sampling periods.

3.3. Surface area density of radiocesium on airborne soil particles

Because airborne soil particles are a potential carrier of radio-cesium in the coarse mode size range, the size distributions of particles collected in our measurements were examined. Mass size distributions of airborne soil particles were calculated using the calcium (Ca) content of the aerosol samples at each impactor stage. Generally, aluminum or silicon is used as an index of soil components. We selected Ca because the impaction substrates that collected the aerosol samples were either aluminum sheets or quartz-fiber (i.e., silica) filters, which would result in a large blank value for the analyses of aluminum or silicon. The Ca content within the size range of concern was assumed to be homogeneous based on the stable concentration ratio to other representative crustal elements, although this homogeneity was not directly confirmed.

To convert the measured Ca concentration to the mass concentration of soil particles, a reported Ca content (1.8%) in Tsukuba surface soil (Geological Survey of Japan, 2017) was applied over the entire size range. After conversion, the mass size distribution of soil particles was mostly in the coarse mode size range (Fig. 4). The total mass concentration of airborne soil particles, which corresponds to the area below the size distribution curves in Fig. 4, decreased over time from LPI-AIST-1 through -3 (from April 28 through June 6, 2011) and was stable in the later LPI-AIST-4 through -6 samples (from June 7 through September 21, 2011). The "dusty" air before June 7 corresponds to windy meteorological conditions in the Kanto Plain in spring (Fig. 5). LPI-AIST-4 and samples thereafter were collected in relatively "clean" air that coincided with the beginning of the rainy season in the middle of June, followed by calm conditions influenced by the North Pacific High during summer.

Radiocesium deposited on pre-existing soil particles is anticipated to be adsorbed on or near the particle surface, but not inside the material of the particle itself. If this is the case, the activity size distribution of resuspended radiocesium should be compared to the surface size distribution rather than the mass size distribution of

Fig. 5. Monthly precipitation (left Y-axis) and maximum wind velocities (left Y-axis) at Tsukuba (Tateno station) from March to September 2011.

airborne soil particles. To convert mass size distribution to surface size distribution, we assumed that soil particles were spherical with a density of 2.65 g/cm3 (Blake, 1979; Skopp, 2012).

The surface density of 137Cs was calculated as the ratio of radioactivity to soil surface area (Fig. 6). Note that the lower and upper size limits were 2.1 and 11 mm in aerodynamic diameter, respectively, for the surface density analysis (see Appendix C). As shown in Fig. 6, the size dependence of 137Cs surface density on soil particles was weak in most aerosol samples; in other words, the surface density of 137Cs on soil particles was homogenous throughout the entire size range considered. To the authors' knowledge, this is the first report on the size dependence of 137Cs surface density on airborne soil particles in the thoracic size fraction (i.e. collected with 50% collection efficiency at Da = 10 mm). The large fluctuation in the 137Cs surface density in LPI-AIST-5 aerosol (Fig. 6b) is caused by the irregularly large radioactivity measured at

Fig. 4. Mass size distributions of Ca in LPI-AIST-1 to -6 aerosol samples collected in Tsukuba, Japan, from April 28 to September 21, 2011, at a height of 15 m from the ground. Note that Ca size data of LPI-AIST-2 sample marked with * are from Kaneyasu et al. (2012).

impactor stages 3.5—5.2 and 7.8—11 mm in aerodynamic diameters. The cause of this relatively strong radioactivity is not obvious, and is discussed in section 3.4.

As for soil particles collected from the ground surface, measurements on the size dependence of surface radioactivity have been reported for several locations (e.g., Livens and Baxter, 1988; Garger et al., 1998a). These studies found that radionuclides were highly concentrated in the finest size fraction, i.e., clay-sized (<2 mm granulometric diameter) particles, which was attributed to the large internal surface area of clay minerals (Garger et al., 1998a). The long-accepted explanation for the mechanism of cesium adsorption on clay minerals states that radiocesium is specifically adsorbed at the frayed edge of layered mineral structures (Dolcater et al., 1968; Maes et al., 1985). In our measurements, we cannot distinguish the contribution of clay minerals to the adsorption of radiocesium from that of other aerosol components in the same size class. Nevertheless, it is unlikely that clay minerals <2 mm granulometric diameter were a major contributor to airborne radiocesium because the concentrations of soil particles were so small in that size range (see Fig. 4).

3.4. Possible form of airborne soil particles containing resuspended radiocesium

Combining our current findings with existing knowledge on radioactively contaminated soil particles, we hypothesize that radiocesium on airborne soil particles is due to the aggregation of sub-micrometer sized Cs-adsorbing clay minerals with soil particles in the coarse mode size range. Recent analysis of individual radioactive particles in litter soil collected in Fukushima showed that radioactive particles were composed of aggregates of fine minerals in which weathered biotite or aluminous smectite acted as

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-LPI-AIST-1 (Apr.28- -LPI-AIST-2 (May 12 -LPI-AIST-3 (May 26 -LPI-AIST-4 (Jun.9 - May 12) May 26) Jun.9) Jul.19)

-LPI-AIST-6 (Aug.21 Sep.21)

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Fig. 6. Surface density of 137C adsorbed on airborne soil particles in (a) aerosol samples LPI-AIST-1 to -4 and -6 and (b) in aerosol sample LPI-AIST-5. (Color in print and on Internet.)

radiocesium absorbers (Mukai et al., 2016). Although the aggregated particles reported in that study were far larger (about 40 x 80 mm) than the airborne soil particles referred to in Figs. 4 and 6, we suggest that similar aggregates of fine clay minerals and non-radioactive soil particles may explain our results; the natural breakdown of such aggregates (i.e., weathering) to form smaller (Da < 11 mm) soil particles may result in a relatively homogeneous surface density of radiocesium absorbers irrespective of size.

Among the aerosol samples, LPI-AIST-5 (Fig. 6) had exceptional fluctuations in 137Cs surface density values across the size range studied. Specifically, aerosols collected on an impactor stage with aerodynamic diameters of 3.5—5.2 mm had the highest surface density of radioactivity (115 Bq/m2). An IP autoradiograph of aerosol deposits (Fig. 7) on this impaction substrate showed that the intense radioactivity was concentrated in one spot. This spot of intense exposure was further observed by SEM (Fig. 8), but no specific clay minerals or spherical particles (Adachi et al., 2013) associated with the radioactivity were found. In the aerosol deposits collected at the 7.8—11 mm stage, no prominent exposure spot was recognized in the IP autoradiograph.

3.5. Cs associated with airborne soil particles collected in Fukushima City in July 2011

For comparison, we calculated the activity size distribution for aerosols collected in early July 2011 in Fukushima City, using data from Koizumi et al. (2012). The activity size distribution of 137Cs in Fukushima City had two prominent peaks, one in the accumulation mode size range and the other in the coarse mode size range (Fig. 9a). The coarse mode appeared to originate from resuspended radiocesium adsorbed on soil particles. During the sampling period, surface soil was removed from the playground of an elementary school adjacent to the sampling site (50 m apart), as part of decontamination operations. This artificial agitation of surface soil may have increased soil dust emission.

Assuming that reported aerosol mass concentrations in the coarse size range can be attributed solely to soil particles, surface densities of 137Cs on soil particles were calculated for Fukushima City (Fig. 9b). The surface density of 137Cs on soil particles from Fukushima City was uniform over the range of 3.5—11 mm in aerodynamic diameter, similar to results from Tsukuba. However, the surface density of small particles (2.1 —3.5 mm in aerodynamic diameter) was much smaller (87 Bq/m2) than that of larger

Fig. 7. (a) Photograph and (b) imaging plate autoradiograph of impaction substrate from the fourth stage (3.5—5.2 mm in aerodynamic diameter) of the LPI-AIST-5 aerosol sample collection.

Fig. 8. Representative scanning electron micrograph of an aerosol deposit on the fourth-stage (3.5—5.2 mm in aerodynamic diameter) collection substrate of the LPI-AIST-5 aerosol sample.

Fig. 9. (a) Activity size distributions of 137Cs in aerosol samples collected from July 2 through 6, 2011, and (b) surface density of 137C adsorbed on airborne soil particles in aerosol samples collected at Fukushima City, Japan. This figure was produced from the original data of Koizumi et al. (2011).

(3.5—11 mm) size ranges (233—260 Bq/m2), which was not observed in Tsukuba. At present, we have no information on the size dependence of resuspended radiocesium on soil particles in places other than Tsukuba and Fukushima City.

3.6. Resuspension factor of Cs

The resuspension factor, an indicator traditionally used to represent the resuspension status of radionuclides, is defined as:

concentration in air (Bq m 3) ground surface contamination (Bq m~

In calculating K in Tsukuba, we assumed that the resuspended 137Cs radioactivity adsorbed on soil particles was the sum of 137Cs radioactivity of aerosols with aerodynamic diameters greater than 2.1 mm (i.e., coarse particles). The ground surface contamination (17.3 kBq m~2) at Tsukuba was calculated as the arithmetic mean of four measurements taken at locations surrounding the sampling site (Fig. 1) reported by the Nuclear Regulation Authority, Japan(2017), i.e., 11 kBq/m2 (Matsushiro Park, 2.4 km northwest), 12 kBq/m2 (Arakawa-oki, 4.7 km southeast), 21 kBq/m2 (Tsukuba City office-Yatabe branch, 4.4 km southwest), and 25 kB/m2 (Shimo-hirooka, 4.5 km east), as of March 1, 2012.

As shown in Fig. 10, the calculated resuspension factors for 137Cs in the first two aerosol samples (1.6 x 10~8 m_1 and 1.8 x 10~8 m_1 for LPI-AIST-1 and -2, respectively) were about half that of the 1986 annual mean at Chernobyl City located 14 km south of ChNPP (Garger et al., 1997), and were comparable to the average value across 14 European cities (distances from ChNPP: 640—2200 km) during April—June 1986 (Garland and Pomeroy, 1994). The similarity of the measured 137Cs resuspension factors in our study and those recorded soon after the ChNPP accident is noteworthy because weather and ground surface conditions were quite different for the two events. At present, we cannot determine whether this similarity represents a universal property of K. Ochiai et al. (2016) measured the airborne concentration of and ground contamination with 137Cs at Namie, Fukushima Prefecture, 1.5 years after the FDNPP accident, and reported K in a range of 5.7 x 10_11 —8.6 x 10~10 m_1, which is smaller than our data for Tsukuba collected before September 2011.

Fig. 10. Resuspension factors of 137Cs in LPI-AIST-1 to -6 aerosol samples when 17.3 kBq/m2 was used as the average level of ground surface contamination with 137Cs in Tsukuba. The error bars indicate the calculated maximum and minimum values when 25 and 11 kBq/m2, respectively, were used as the ground surface 137Cs contamination levels. The resuspension factors at Chernobyl City in 1986 (after May) and 1987 (annual mean) and averaged over 14 European cities in 1986 (April—June) are indicated by horizontal lines.

After the end of May 2011 (LPI-AIST-3 and after), K values fell by about one order of magnitude (1.5—4.2 x 10-9 m-1). This dramatic change in K values reflects a change in the quantity of airborne soil particles (Fig. 4), but not in their quality (Fig. 6a), between the LPI-AIST-2 and -3 samples. In other words, the season of high resuspension for 2011 ended in late May in Tsukuba, as has been suggested by other studies for 2012 (Hirose, 2013; Akimoto, 2015). This seasonal cycle of high resuspension is anticipated to continue for decades, although the intensity will decrease over time due to the decay of radiocesium and weathering of the contaminated surface soil.

4. Conclusions

In Tsukuba, Japan, temporal changes in the activity size distributions of airborne radiocesium were measured over 5 months after the FDNPP accident, and analyzed with regard to radiocesium carriers in air. In the months relatively soon after the accident, radiocesium was localized in the accumulation mode size range of aerosols. The radiocesium in such fine aerosols decreased over time after the accident, while that associated with coarse aerosols remained airborne.

To examine the relationship with radiocesium, the mass size distributions of airborne soil particles were calculated from the Ca content in the aerosols, and were subsequently converted to surface area size distributions under the assumption that the soil particles were spherical. The size dependence of the 137Cs surface density on soil particles was calculated as the ratio of 137Cs radioactivity to the surface area of soil particles. This analysis indicated that the surface density of 137Cs adsorbed onto soil particles was almost uniform throughout the range of 2.1—11 mm in aerodynamic diameter. In other words, radiocesium was adsorbed homogeneously regardless of soil particle size.

The resuspension factor, K, a traditional index of the resuspension status of deposited substances on the ground, was calculated from the reported ground surface contamination in Tsukuba and the total activity concentrations of 137Cs in coarse aerosols (Da > 2.1 mm) obtained in this study. The resuspension factors of 137Cs measured before the end of May 2011 were similar to those in 14 European cities within the first 3 months following the ChNPP accident.

The results of this study are applicable to numerical models of radionuclide resuspension via windblown soil particles, and may be useful for improving estimations of the inhalation dose received by local residents. Such evaluations of health effects are necessary to assess the future habitability of highly contaminated areas, such as the restricted residence area extending northwest of the FDNPP.

Acknowledgments

We thank the Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Japan, for providing assistance and facilities for measuring aerosol radioactivity. The support of Dr. Kumiko Fukutsu and Dr. Yuji Yamada of the National Institute for Quantum and Radiological Science and Technology with the IP autoradiograph analysis is greatly appreciated.

Appendices. Supplementary data

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/jjenvrad.2017.03.001.

Conflict of interest

The authors declare that they have no conflicts of interest,

including financial, personal, or other relationships with other

people or organizations.

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