Altitude-Dependent Distribution of Ambient Gamma Dose Rates in a

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Altitude-Dependent Distribution of Ambient Gamma Dose Rates in a Mountainous Area of Japan Caused by the Fukushima Nuclear Accident Mutsuo Hososhima† and Naoki Kaneyasu*,‡ †

Mibu High School, 1194 Fujii, Mibu-machi, Tochigi 321-0221, Japan National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan

‡

S Supporting Information *

ABSTRACT: Large amounts of airborne radionuclides were deposited over a wide area in eastern Japan, including mountainous regions, during the devastating Fukushima Daiichi nuclear power plant accident. Altitudinal distributions of ambient gamma dose rate in air were measured in a mountainous area at the northern rim of the Kanto Plain, Japan, using a portable instrument carried along the mountain trails. In the Nikko Mountain area, located 120 km north of Tokyo, the altitudinal distribution exhibited a maxima at ∼900−2 000 m above sea level (ASL). This area was not affected by precipitation until 2300 Japan Standard Time (JST) on March 15, 2011. By that time, a substantial amount of radionuclides had been transported from the damaged reactor, according to the numerical simulations using transport models. Meteorological sounding data indicated that the corresponding altitudes were within the cloud layer. A visual-range monitor deployed in an unmanned weather station at 1 292 m ASL also recorded low visibility on the afternoon of March 15. From these findings, it was deduced that the altitude-dependent radioactive contamination was caused by the cloud/fog deposition process of the radionuclides contained in aerosols acting as cloud condensation nuclei.

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INTRODUCTION The massive discharge of radionuclides from the Fukushima Dai-Ichi Nuclear Power Plant (FDNPP) accident in March 2011 contaminated a vast area in eastern Japan. Airborne monitoring and in situ surface soil samplings have mapped the horizontal distribution of deposited radionuclides over the land area.1,2 A number of numerical modeling studies were conducted and reproduced the transport and dispersion pathways, deposition areas, and deposited amounts of radionuclides. The most severely contaminated zone is a narrow strip northwest of FDNPP that was formed by wet-deposition of radionuclides from the evening to midnight on March 15, 2011, according to a consensus of numerical modeling studies.3−5 Prior to this severe contamination event, large amounts of radionuclides had been discharged into the atmosphere since early morning on March 15 and were transported south− southwest.6 This radioactive plume penetrated the Kanto Plain area from the northeast. When the plume passed over this area, the level of deposition was not significant because of the lack of precipitation.3,6 However, after a clockwise change in the direction over the Kanto Plain area, the radioactive plume was advected northward and contaminated the mountainous region at the northern rim of the Kanto Plain, including the Nikko © 2015 American Chemical Society

Mountain area in the Tochigi Prefecture. Radioactive contamination occurred not only on the ground surface but also in the freshwater of Lake Chuzenji-ko,7 located in the heart of the Nikko Mountain area, and consequently affected the fish in the lake. The deposition mechanism of radionuclides that contaminated this area has not been surveyed in detail, although the level of contamination of the Nikko Mountain area was relatively significant, as indicated by the ambient gamma dose rate in air (hereinafter, ambient GDR) mapped through airborne surveys.2 The reproduction of surface deposition in this area by the transport/dispersion modeling studies was not very successful. For example, model-simulated surface deposition of 137Cs was either underestimated8 or overestimated9 around the northern rim of the Kanto Plain (in the Tochigi and Gunma Prefectures). To investigate the transfer of radionuclides from the atmosphere to vegetation, soil, groundwater, lakes, rivers, and the extended ecosystem, the distribution of radioactive Received: Revised: Accepted: Published: 3341

October 3, 2014 February 13, 2015 February 23, 2015 February 23, 2015 DOI: 10.1021/es504838w Environ. Sci. Technol. 2015, 49, 3341−3348

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mountains shown in Figure 1. The Nikko Mountain area is located about 120 km north of the central part of Tokyo and

contamination in the mountainous region must be surveyed first. Airborne surveys are able to provide the horizontally averaged data from a diameter ranging between 300 and 1 500 m underneath the aircraft.1 To improve the horizontal resolution of ambient GDR measurements, handheld instruments installed on automobiles have been utilized. For example, a car-borne survey system called the Kyoto University Radiation Mapping system (KURAMA) has greatly improved the understanding of the horizontal distribution of ambient GDRs in the Tohoku and Kanto areas in Japan.10,11 The drawback of automobile surveys is that the vehicles are limited by the availability of passable roads. In mountainous areas, the highest spatial resolutions of ambient GDRs can be acquired by carrying gamma-ray detection devices and conducting measurements on foot. This method, however, is also restricted by the existing layout of the mountain trails. Therefore, it is inappropriate to consider the data obtained from a mountain to be representative of the surrounding mountainous area. In this study, a distinctive pattern of ambient GDRs in the vertical direction obtained by the on-foot survey of multiple mountains in the Nikko Mountain area is reported. The possible mechanism involved in such radioactive contamination is discussed using meteorological data.

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MATERIALS AND METHODS Instrumentation and Data Mapping. A portable type CsI (TI) scintillation gamma-ray detector (GammaRAE II R, RAE Systems Inc.) was operated along the mountain trails at a height of 1 m, and data was stored every 5 s. The horizontal position of the observer was pinpointed every second using a global positioning system (GPS) and recorded in a connected data logger. Measured ambient GDRs and GPS horizontal positions were combined and plotted on the digital map issued by the Geophysical Information Authority of Japan. The altitude of the measurement position was extracted from the digital map with a vertical resolution of 20 m. For the expression of data obtained where the mountain trail is horizontal, the measured values were arithmetically averaged. Nominal specifications of the instruments, as provided by the manufacturers, are listed in Table 1.

Figure 1. Map of the study area. FDNPP indicates the location of Fukushima Dai-ichi Nuclear Power Plant. On-foot measurement tails are indicated as white lines in the lower panel.

Table 1. Specifications of the Gamma-Ray Detectors Used in This Study instruments

GammaRAE II R

manufacturer detector sensitivity measurement range energy range accuracy

RAE Systems CsI (TI) scintillation 6000 cpm/(μSv/h) as 0.01 μSv/h−6 Sv/h

PDR-111

137

0.06−3.0 MeV ±20% as dose equivalent rate for 137Cs

Cs

Hitachi-Aloka Medical CsI (TI) scintillation not specified 0.001−19.99 μSv/h

160 km southwest of FDNPP, where there are forested mountains with summit heights ranging from 1 000 to 2 500 m ASL. Mount Nantai-san (2 486 m) and Mt. Nyoho-san (2 483 m) are the two major mountains in the middle of the Nikko Mountain area. Lake Chuzenji-ko (water level altitude of 1 269 m) lies on the southern slope of Mt. Nantai-san. Mount Nakimushi-yama (1 103 m), Kirifuri Highland (1 700 m), and Mt. Gassan (1 280 m) are located east of Lake Chuzenji-ko. Mount Bizen-Tateyama (1 272 m) belongs to a different mountain range (Ashio Mountain range) located about 10 km south of the Nikko Mountain area. Comparison of Measured Data by Different Instruments. The output of the gamma-ray detector used in this study (GammaRAE II R) was compared with those from another portable type CsI (TI) scintillation gamma-ray detector commonly used in Japan (Hitachi-Aloka Medical, PDR-111). Nominal specifications of the PDR-111 are listed in Table 1.

0.060−1.25 MeV ±15% as dose equivalent rate for 137Cs

Data correction for gamma-ray originating from cosmic rays at higher altitudes was not applied because of its low magnitude compared to those measured in this study. For example, ambient GDR between 2 000 and 3 720 m above sea level (ASL) at Mt. Fuji, Japan, was within the range of 0.03 and 0.05 μSv/h, on average.12 Survey Area. In spring and summer of 2012 and 2013 and the spring of 2014, measurements were made on foot in the 3342

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2011, respectively, at the same station. Thus, it was concluded that the measured ambient GDRs in the mountainous area substantially reflected the radiation from the gamma-ray emitting radionuclides deposited on the ground and vegetation. In view of the change in the altitudinal distribution pattern, it is apparent that the data obtained in 2012 and 2013 conserved the spatial pattern of major deposition of radionuclides that occurred in March 2011. Altitudinal Distributions of Ambient GDRs around the Nikko Mountain Area. The altitudinal profiles of ambient GDRs are shown in Figure 3 for Mt. Nakimushi-yama (measured on April 29, 2012), Kirihuri Highland (May 13, 2012), Mt. Gassan (August 9, 2012), the south face of Mt. Nyoho-san (June 9, 2013), the south and north faces of Mt. Nantai-san (July 28, 2012), and Mt. Bizen-Tateyama (June 30, 2013). The maximum ambient GDR was observed at ∼1 040− 1 080 m ASL at Mt. Nakimushi-yama, the Kirifuri Highland, Mt. Gassan, and Mt. Bizen-tateyama. On the southern face of Mt. Nyoho-san, the maximum was detected at 960 m, with plateau-like high values between 1 100 and 1 340 m. At Mt. Nantai-san, the maximum occurred at 1 480 m on the south face, while the ambient GDRs on the north face above 1 760 m were low and stable except a small increase at 2 200 m. Data were not available below the plotted altitude for Mt. Nantai-san because the mountain trail on the south face begins from the lakeshore of Chuzenji-ko and the north face trail begins from the roadside at an altitude of 1 750 m ASL. Proposed Mechanism of Altitude-Dependent Deposition. The mechanism anticipated to have caused the concentrated deposition in a particular altitudinal range was cloud deposition (or fog/occult deposition). Atmospheric aerosols often act as cloud condensation nuclei (CCN) causing cloud (fog) droplets to form on them. With cloud/fog deposition, these droplets, and with them the original aerosol particles, are intercepted by vegetation and deposited on the ground. It is known that the apparent deposition velocity of cloud/fog droplets is far greater than the dry deposition velocity of aerosols in the accumulation and Aitken mode size range. Such differences in the deposition velocities arise due to the differences in ruling mechanisms, i.e., the cloud/fog droplets deposit mainly via inertial impaction, interception, and gravitational settling, while the accumulation and Aitken mode aerosols deposit through Brownian diffusion. Cloud/fog deposition has been identified as one of the potential causes of forest decline in the high-alpine regions.15−18 Airborne radiocesium, one of the major gamma-ray emitting radionuclides deposited onto the land and marine environments after the FDNPP accident, exists in the aerosol phase. Kaneyasu et al.19 reported that sulfate aerosol is a potential carrier of airborne radiocesium following the analysis of sizesegregated aerosols. Sulfate aerosol is one of the representative species acting as CCN. Therefore, cloud or fog droplets activated from CCN, including radiocesium and other gammaray emitting radionuclides, have greater deposition velocities than those of nonactivated aerosols. This leads to the transfer of radionuclides from the atmosphere to the forest and ground surface being far more effective. Timing of the Radioactive Plume. Because no direct observations are available for the cloud deposition of radionuclides in the area during the early stage of the FDNPP accident, the occurrence of cloud deposition is examined using circumstantial data. To pursue this hypothesis,

This comparison was necessitated due to the lack of calibration details provided by the manufacturer of the GammaRAE II R. Parallel measurements of these instruments were conducted in the Kirifuri Highland in April 2014. Readout data of PDR111 were manually recorded at about 10 min intervals, and the corresponding GammaRAE II R data averaged over 1 min were collected. The readouts of the two instruments, shown in Figure 2, corresponded well with each other. The differences in

Figure 2. Comparison of ambient gamma-ray dose rate in air measured by the GammaRAE II R (RAE Systems Inc.) with those measured by PDR-111 (Hitachi-Aloka Medical) in the Kirifuri Highland, Nikko Mountain area, in April 2014. Error bars indicate the width of nominal accuracy of the data with regard to the 137Cs calibration reference. The linear regression line of the plots is included in the figure.

the absolute values were within the nominal accuracy of the GammaRAE II R, in the range between 0.2 and 0.5 μSv/h where most of the data resided. Thus, the ambient GDR read by the GammaRAE II R was not corrected. It is important to note that, in this study, there is less emphasis on the absolute values of ambient GDRs because the patterns or shapes of the altitudinal distribution are the central concern.

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RESULTS AND DISCUSSION Temporal Variations in the Altitudinal Distribution of Ambient GDRs from 2012 to 2014. To address the issue of whether the measurements made 1−2 years after the accident conserve the initial spatial patterns formed in 2011, multipleyear measurements were compared for a single mountain. Altitudinal distributions of ambient GDRs measured in 2012, 2013, and 2014 at Mt. Nakimushi-yama were plotted in Figure S1 (Supporting Information). For the smoothing of the plotted data, the locally weighted scatterplot smoothing (LOESS) algorithm13 was applied. In brief, large decreases in the ambient GDRs observed during the three years, presumably reflecting not only the radioactive decay but also the weathering effect, did not substantially alter the altitudinal distribution pattern of ambient GDRs. By the spring of 2012, the concentration of airborne radionuclides at the northern rim of the Kanto Plain had greatly decreased. For example, at the Takasaki monitoring station that is part of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) monitoring network, the activity concentrations of airborne 134Cs and 137Cs were 59 and 95 μBq/m3, respectively, on April 29, 2012.14 These values were 8.5 × 10−6 and 1.6 × 10−5 times the maximum values observed March 15, 3343

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Figure 3. Altitudinal distribution of ambient gamma dose rate in air in the mountains shown in Figure 1. Error bars in each plot indicate one standard deviation.

the timing of when the major radioactive contamination occurred in this area must first be examined. According to the numerical simulation by transport/ dispersion models, the deposition of radionuclides at the northern rim of the Kanto Plain occurred on March 15, 2011.20 At a monitoring post in Utsunomiya (Figure 1), the maximum ambient GDR of 1.32 μSv/h was observed at 1000 JST.21 At the Takasaki CTBT monitoring station, the maximum value recorded was 0.6 μSv/h at 1300 JST.22 In Nasu-machi, located 50 km northeast of Utsunomiya and at the southern end of the Naka-dori basin-like valley, the ambient GDRs reached a maximum value of 1.68 μSv/h at 1530 JST.21 This suggests that, in the southern part of the Naka-dori basin-like valley, the radioactive plume moved from southwest to northeast on the afternoon of March 15. Using the displacement velocity from Utsunomiya to Nasu-machi (9 km/h), the arrival time of the radioactive plume to Lake Chuzenji-ko was anticipated to be between 1400 and 1500 JST. Clouds and Precipitation on March 15, 2011. In Japan, precipitation is observed by rain gauges in the Automated

Meteorological Data Acquisition System (AMeDAS), which is a robotic network of meteorological instruments deployed in more than 1 300 sites nationwide. The Japan Meteorological Agency (JMA) operationally reports the radar/AMeDASanalyzed precipitation fields23,24 every 30 min with a horizontal resolution of 1 km. A recent numerical transport/dispersion study of the FDNPP radionuclides9 used this data set to calculate wet deposition instead of using model-produced precipitation fields. Because model-produced precipitation may not necessarily reproduce the location and timing of actual precipitation, adoption of a reanalysis precipitation field in the model may improve the reproduction of the contaminated areas observed. Figure 4 shows the radar-AMeDAS analysis precipitation rates from 1500 JST on March 15 to 0100 JST on March 16, 2011. As the transient cyclone passed over East Japan,8 the precipitation area moved from the northwest to the southeast and covered the Nikko Mountain by around 2300 JST on March 15, 2011. From the radar-AMeDAS analysis charts, the 3344

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Figure 4. Radar-AMeDAS analysis precipitation rates between 1500 JST on March 15 and 0100 JST on March 16, 2011. Open circles indicate the location of Lake Chuzenji-ko in the Nikko Mountain area.

This indicates that the cloud base existed at altitudes approximately between 1 500 and 700 m ASL during the day on March 15 over Tateno. Tateno is located in the central part of the Kanto Plain where the topography is different from the mountainous area at the northern rim of the Kanto Plain. As upper air data in the inland area, the height of the cloud base was extracted from the Meteorological Aerodrome Report (METAR) at Fukushima Airport, located in the southern part of the Fukushima Prefecture, 110 km northeast of the Nikko Mountain area. Figure 6 shows the time series of the reported cloud-base

Nikko Mountain area was evidently not simultaneously affected by the areal precipitation and the radioactive plume. The second stage of the analysis involved the examination of whether or not a cloud layer existed around the Nikko Mountain area at the same time the radioactive plume had reached the area (i.e., in the early afternoon of March 15). Figure 5 depicts the vertical profiles of potential temperatures

Figure 5. Vertical profiles of potential temperature (filled circles), equivalent potential temperature (filled triangles), relative humidity (RH; filled squares), and wind vectors (arrows) in Tateno (Tsukuba) on March 15, 2011, at (a) 0900 JST and (b) 2100 JST. Note that the virtual correction was applied to potential and equivalent potential temperatures.

Figure 6. Time-series of cloud-base height (m ASL) reported by METAR at Fukushima Airport on March 15, 2011. Error bars indicate the width of the minimum reported value (100 ft).

height over Fukushima Airport on March 15, 2011, measured by a ceilometer. The cloud-base height at around noon was ∼1 000 m ASL; it then descended, reaching 620 m ASL by 1700 JST. The cloud base then began to ascend up to ∼1 130 m ASL by 2200 JST. From the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA-Interim, horizontal resolution 0.25°, time interval 6 h) data,25 a layer corresponding to low-stratus cloud, in view of the liquid water

and the relative humidity (RH) from the upper-air sounding data measured by the radiosondes at the Tateno (in Tsukuba) aerological observatory at 0900 and 2100 JST on March 15, 2011. According to the equivalent potential temperature profiles, a relatively warm and wet layer was present above 800 m ASL, over the cold layer near the surface at 0900 JST, and it continued until 2100 JST. At 0900 JST, the RH was close to 98% between 1 500 and 2 000 m ASL, and the bottom of the high RH (>98%) height descended to 700 m ASL at 2100 JST. 3345

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at the time of contamination so that cloud/fog deposition is the likely explanation for the observed maximum. Related Studies. To the authors’ knowledge, only one measurement has been reported on fission product radionuclides in cloudwater. In France, Masson et al.26 found that the activity concentration of 134,137Cs in cloudwater on the summit of Puy de Dôme (1 465 m ASL) was 40 times higher after the Fukushima accident than during the preaccident period. In the Kanto Plain, the altitude-dependent deposition of acidic pollutants onto the forest was reported at Mt. Oyana (1 252 m ASL),27 i.e., the deposition was significantly greater above 890 m ASL. In most transport/dispersion models used for the reproduction of radioactive contamination by the Fukushima accident, the deposition processes adopted were the same as those traditionally used for dry and wet deposition of air pollutants.28−30 These wet-deposition processes employ an empirical parametrization called the scavenging coefficient. In a numerical simulation by Morino et al.,3 a wet-scavenging scheme based on the budget of contaminants between cloudwater and precipitation was applied for the reproduction of radioactive depositions. None of these studies, however, has considered the cloud/fog-deposition process. Even for the deposition of conventional air pollutants, such as sulfate, in the forest ecosystems, only a small number of numerical models have taken cloud/fog deposition into account.16,31 Recently, Katata et al.5 incorporated a simple fogwater-deposition scheme into their numerical transport/ dispersion model and qualitatively reproduced the deposition of 137 Cs in the mountainous areas of eastern Japan. In addition, Katata32 published a more comprehensive review on the modeling of fogwater deposition. The results of the present study and that of Katata et al.5 suggest that the adoption of such schemes in the deposition process is essential when dealing with radioactive fallout in humid and complex terrain areas, as is the case for the FDNPP accident in Japan. Another issue to be considered in the cloud/fog deposition of radionuclides is the edge effect,33−35 characteristically found at the edges of forests facing grasslands or bare grounds or on the ridges of mountains. Although the edge effect has been traditionally studied for the deposition of air pollutants onto the forests, similar effects were observed for the deposition of radionuclides in forested areas after the Kyshtym and Chernobyl accidents36 and the Sellafield accident.37 During the on-foot measurements along the mountain trails, the ambient GDRs were sometimes found to be significantly higher at the edges of the forest or on the col of the ridge where wind velocities were accelerated due to the venturi effect. This phenomenon is out of the scope of the present paper because it does not affect the aerial deposition pattern of radionuclides. Nevertheless, further research is expected on this deposition mechanism because it is vital when considering the effects on agricultural activities in the mountainous area, such as tea plantations and fruit farms.

content, was resolved over the study area (Mt. Nantai-san) from 0900 JST on March 15 (Figure 7). The cloud-base height

Figure 7. Time−height cross section of the liquid water content (g/ kg) in the atmosphere, analyzed in the ECMFW ERA-Interim data product (horizontal resolution 0.25° × 0.25°, time interval 6 h) at 36.75° N, 139.50° E, which is close to the location of Mt. Nantai-san (36.76° N, 139.48° E). The area in white corresponds to the ground surface in the data set.

was descending slowly until 1500 JST, after which it descended relatively steeply and touched down to the grid-averaged ground surface at midnight. The “ceilometers calibrated” cloudbase height over this location was expected to be about 950 m ASL at 1500 JST (Figure S2 of the Supporting Information). An automated visual-range monitor was operated at the OkuNikko meteorological station (1 292 m ASL) located on the eastern shore of Lake Chuzenji-ko. It reported a visual range of 1 km after 1800 JST (Figure 8). The

Figure 8. Visual range at the Oku-Nikko meteorological station (Japan Meteorological Agency) measured by an automated visual-range monitor on March 15, 2011. Note that the scale of the y-axis is different above and below the interruption.

precipitation rate measured by a rain gauge was 0.0 mm, and the RH was between 99 and 100% from 1200 to 2300 JST at the station. With the ECMWF reanalysis and the visual-range monitor record, it can be interpreted that the cloud base that had been descending gradually touched down to the height of Oku-Nikko meteorological station at ∼1200 JST, and the station remained engulfed by the cloud layer until 1700 JST. Although no direct measurement exists in the cloud-layer height over the Nikko Mountain area, the upper sounding measurement data, ECMFW reanalysis data, and visual-range monitor record indicate that the cloud layer existed over the area at altitude above ∼900 m ASL on the afternoon of March 15. Thus, there is good evidence that the layer of increased ambient GDR from ∼900 to 2 000 m ASL was in a cloud layer

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ASSOCIATED CONTENT

S Supporting Information *

Altitudinal distributions of GDRs measured at Mt. Nakimushiyama in 2012, 2013, and 2014 (Figure S1) and the height− longitude cross section of liquid water content in the atmosphere analyzed in the ECMWF ERA-Interim data product over the study area (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. 3346

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(11) Tanigaki, M. Development of a carborne survey system, KURAMA. In Radiation Monitoring and Dose Estimation of the Fukushima Nuclear Accident; Takahashi, S., Eds.; Springer: 2014; pp 67−77; http://link.springer.com/chapter/10.1007/978-4-431-545835_7 (accessed Sep 11, 2014). (12) Yajima, K.; Iwaoka, K.; Yasuda, H. Radiation survey along two trails in Mt. Fuji to investigate the radioactive contamination caused by TEPCO’s Fukushima Daiichi Nuclear Plant accident. In Radiation Monitoring and Dose Estimation of the Fukushima Nuclear Accident; Takahashi, S., Eds.; Springer: 2014, pp 59−66; http://link.springer. com/chapter/10.1007/978-4-431-54583-5_6 (accessed Sep 11, 2014). (13) Cleveland, W. S. Robust locally weighted regression and smoothing scatterplots. J. Am. Stat. Assoc. 1979, 74 (368), 829−836 DOI: 10.2307/2286407. (14) Center for the Promotion of Disarmament Non-Proliferation. Results on the observation of particulate radionuclides at the CTBT Takasaki monitoring station (January 10, 2013) (translated from the original page title written in Japanese); http://www.cpdnp.jp/pdf/ 130110Takasaki_report_Dec31.pdf (accessed Dec 4, 2014). (15) Lovett, G. M.; Reiners, W. A.; Olson, R. K. Cloud droplet deposition in subalpine balsam fir forestsHydrological and chemical inputs. Science 1982, 218 (4579), 1303−1304 DOI: 10.1126/ science.218.4579.1303. (16) Lovett, G. M. Rates and mechanisms of cloud water deposition to a subalpine balsam fir forest. Atmos. Environ. 1984, 18 (2), 361−371 DOI: 10.1016/0004-6981(84)90110-0. (17) Kalina, M. F.; Zambo, E.; Puxbaum, H. Assessment of wet, dry and occult deposition of sulfur and nitrogen at an alpine site. Environ. Sci. Pollut. Res. 1998, 53−58. (18) Kalina, M. F.; Stopper, S.; Zambo, E.; Puxbaum, H. Altitudedependent wet, dry and occult nitrogen deposition in an Alpine region (Achenkirch, Austria, 920 m−1758 m a.s.l.). Environ. Sci. Pollut. Res. 2002, 16−22. (19) Kaneyasu, N.; Ohashi, H.; Suzuki, F.; Okuda, T.; Ikemori, F. Sulfate aerosol as a potential transport medium of radiocesium from the Fukushima nuclear accident. Environ. Sci. Technol. 2012, 46 (11), 5720−5726 DOI: 10.1021/es204667h. (20) Katata, G.; Terada, H.; Nagai, H.; Chino, M. Numerical reconstruction of high dose rate zones due to the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 2012, 111, 2−12 DOI: 10.1016/j.jenvrad.2011.09.011. (21) Results on monitoring of the air dose rate in Tochigi Prefecture (translated from the original page title written in Japanese); http:// www.pref.tochigi.lg.jp/kinkyu/documents/20110312-18.pdf (accessed Dec 4, 2014). (22) Yonezawa, S.; Yamamoto, H. Measurement of man-made radionuclides in the atmosphere at the monitoring network for radionuclides to watch nuclear tests. Bunseki 2011, 8, 451−458 (in Japanese). (23) Makihara, Y. A method for improving radar estimates of precipitation by comparing data from radars and rain gauges. J. Meteorol. Soc. Jpn. 1996, 74 (4), 459−480. (24) Makihara, Y.; Uekiyo, N.; Tabata, A.; Abe, Y. Accuracy of RadarAMeDAS precipitation. IEICE Trans. Commun. 1996, E79B (6), 751− 762. (25) Dee, D. P.; et al. The ERA-Interim reanalysis: Configuration and performance of the data assimilation system. Q. J. R. Meteorol. Soc. 2011, 137 (656), 553−597 DOI: 10.1002/qj.828. (26) Masson, O.; de Vismes Ott, A.; Bourcier, L.; Paulat, P.; Ribeiro, M.; Pichon, J.-M.; Sellegri, K.; Gurriaran, R. 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. 2015, 151, 45−51 DOI: 10.1016/j.atmosres.2014.03.031. (27) Igawa, M.; Matsumura, K.; Okochi, H. High frequency and large deposition of acid fog on high elevation forest. Environ. Sci. Technol. 2002, 36 (1), 1−6 DOI: 10.1021/es0105358.

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-29-861-8365; fax: +81-29-861-8358; e-mail: kane. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. H. Ogata (Waseda University) for providing the PDR-111 gamma-ray survey meter; Dr. A. Kitamoto of the National Institute of Informatics, Japan, for providing the Radar-AMeDAS analysis charts; and Dr. S. Taguchi and Dr. Y. Takane (National Institute of Advanced Industrial Science and Technology) for producing the charts for the ERA-Interim reanalysis and wind vectors at Tateno.

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REFERENCES

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