Open Access

Atmospheric radioactivity over Tsukuba, Japan: a summary of three years of observations after the FDNPP accident

  • Yasuhito Igarashi1Email author,
  • Mizuo Kajino1,
  • Yuji Zaizen1,
  • Kouji Adachi1 and
  • Masao Mikami1, 2
Progress in Earth and Planetary Science20152:44

DOI: 10.1186/s40645-015-0066-1

Received: 16 March 2015

Accepted: 22 October 2015

Published: 9 December 2015

Abstract

A severe accident occurred in March 2011 at the Fukushima Dai-ichi nuclear power plant (FDNPP) operated by the Tokyo Electric Power Company (TEPCO), causing serious environmental pollution over a wide range covering eastern Japan and the northwestern Pacific. This accident created a large mark in the atmospheric radionuclide chronological record at the Meteorological Research Institute (MRI). This paper reports the impacts from the FDNPP accident over approximately 3 years in Tsukuba, Ibaraki (approximately 170 km southwest from the accident site), as a typical example of the atmospheric pollution from the accident. The monthly atmospheric 90Sr and 137Cs depositional fluxes in March 2011 reached approximately 5 Bq/m2/month and 23 kBq/m2/month, respectively. They are 3–4 and 6–7 orders of magnitude higher, respectively, than before the accident. Sr-90 pollution was relatively insignificant compared to that of 137Cs. The 137Cs atmospheric concentration reached a maximum of 38 Bq/m3 during March 20–21, 2011. After that, the concentrations quickly decreased until fall 2011 when the decrease slowed. The pre-FDNPP accident 137Cs concentration levels were, at most, approximately 1 μBq/m3. The average level 3 years after the accident was approximately 12 μBq/m3 during 2014. The atmospheric data for the 3 years since the accident form a basis for considering temporal changes in the decreasing trends and re-suspension (secondary emission), supporting our understanding of radioCs’ atmospheric concentration and deposition. Information regarding our immediate monitoring, modeling, and data analysis approaches for pollution from the FDNPP accident is provided in the Appendices.

Keywords

Temporal change 90Sr 137Cs Atmospheric deposition Atmospheric concentration FDNPP accident

Background

We have conducted observational research on radionuclides in the environment for almost 60 years at the Meteorological Research Institute (MRI) in Japan, ever since the 1950s when the USA, Soviet Union, and others performed vigorous nuclear tests in the atmosphere. The atmosphere is the major medium into which radioactive materials were directly injected by the nuclear tests and accidents, and within it, transport, diffusion, and wet and dry removal of these materials occur. During the nuclear testing era, the major purpose of our research was to clarify the radioactive pollution situation and its major controlling factors in the atmosphere (Hirose et al. 1986; Katsuragi 1983; Miyake 1954; Miyake et al. 1963, 1975) and hydrosphere (Miyake et al. 1955, 1962, 1988). After the Chernobyl accident, the purpose of the research gradually shifted to obtaining more data about various processes in the atmosphere (Aoyama 1988; Aoyama et al. 1986, 1987, 1991, 2006; Hirose et al. 1993, 2001; Igarashi et al. 1996, 2003, 2009) and hydrosphere (Aoyama 1995, Aoyama and Hirose 2004; Hirose et al. 1999, Hirose and Aoyama 2003; Miyao et al. 2000). Of particular interest in this study, observation of monthly radionuclide deposition (atmospheric total deposition/radioactive fallout) for 90Sr (half-life, 28.8 years) and 137Cs (half-life, 30.2 years) had continued for 57 years as of April 2014, although the location of the observations moved from Koenji, Tokyo, to Tsukuba in 1980 when the science city was built (Katsuragi 1983). Both radionuclides are scientifically important because of their health and environmental impacts (e.g., see U.S. Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry ATSDR2004Cs 2004; U.S. Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry ATSDR2004Sr 2004). We continued collecting and analyzing atmospheric samples after the accident at Tokyo Electric Power Company’s (TEPCO) Fukushima Dai-ichi Nuclear Power Plant (FDNPP) in Ohkuma-machi and Futaba-machi, Fukushima prefecture (37.42 °N, 140.97 °E) in March 2011.

Many authors have attempted to determine the environmental impacts of the FDNPP accident, which have gradually come to light (e.g., Aoyama et al. 2012, 2013; Hirose 2012; Kusakabe et al. 2013; Masson et al. 2011; Masumoto et al. 2012; MEXT 2011a ; MEXT and USDOE 2011; Povinec et al. 2013a, b; Tsumune et al. 2013; Yamamoto et al. 2012; Yoshida and Kanda 2012; Yoshida and Takahashi 2012). We still need to study the following issues from an atmospheric science point of view (Igarashi 2009): (1) primary source terms including emissions inventory and temporal changes (e.g., Chino et al. 2011; Katata et al. 2012, b, 2014; Maki et al. 2013; Stohl et al. 2012; Terada et al. 2012; Winiarek et al. 2012), (2) transport and diffusion (e.g., Masson et al. 2011; Morino et al. 2011; Sekiyama et al. 2015; Stohl et al. 2012; Takemura et al. 2011; Tanaka 2013; Terada et al. 2012), and (3) dry and wet removal (e.g., Adachi et al. 2013; Hirose et al. 1993; Kristiansen et al. 2012), which governed radioactive surface contamination during the early phase of the accident. In addition, the physical and chemical properties of the radioactive materials (e.g., Adachi et al. 2013; Kaneyasu et al. 2012) are important factors that influence the second and third subjects to be investigated. Here, we summarize the observations, present a time series of the atmospheric impacts of the TEPCO FDNPP accident over approximately 3 years in Tsukuba, Ibaraki, Japan, and compare the levels to the situation before the accident as very basic scientific information (Igarashi, 2009). In addition, secondary emissions from contaminated surfaces to the atmosphere (re-suspension; Igarashi 2009) have become important during the later phases. Re-suspension comes from contaminated surfaces, terrestrial ecosystems, and open-field burning. These sources have undoubtedly supported atmospheric radionuclides but are not yet well understood and are thus considered briefly. Other information about the accident, related to our immediate monitoring and modeling endeavors and data analysis approaches to short-lived γ-emitters and 89Sr, is summarized in the Appendices.

Methods

Atmospheric deposition samples

The monthly atmospheric total deposition/atmospheric fallout has been sampled using a weathering-resistant plastic tray (area = 4 m2) installed on a cottage roof in an open field of the MRI in Tsukuba, Ibaraki (36.1 °N, 140.1 °E; approximately 170 km southwest of the FDNPP) since the 1980s. After April 2011, the sample size was reduced to two trays, each 1 m2, which we considered sufficient for the levels present after the FDNPP accident. The collected samples were evaporated and concentrated into a gross quantity with a rotary evaporator (Eyela NE-12) or an evaporating dish, and the samples were saved in a polyethylene safekeeping container. Each evaporated sample, packed in a cylindrical plastic container, was measured for γ-ray emitting radionuclides (134Cs and 137Cs) using a Ge semiconductor detector (coaxial-type from ORTEC EG&G or Eurisys) coupled with a computed spectrometric analyzer (Oxford-Tennelec Multiport or Seiko EG&G 92x). The precision, accuracy, and quality control of the measurements are described elsewhere (Otsuji-Hatori et al. 1996).

Part of the sample was then stored for future reanalysis. The remaining sample was added to concentrated nitric acid along with H2O2 and digested in a heating operation. Sr-90 was radiochemically recovered from the obtained sample solution, purified and finally fixed as Sr carbonate precipitate, an activity measurement source. After the source was left for several weeks to achieve 90Sr and 90Y radioequilibrium, its β-activity was measured using a low-background 2π gas-flow detector (Tennelec LB5100) with P10 gas (Otsuji-Hatori et al. 1996). Within several months after the FDNPP accident, 89Sr (half-life, 50.5 days) from the accident coexisted with 90Sr and affected the β-activity measurement. To remove the 89Sr influence, we occasionally repeated the Sr source measurement and evaluated the radioequilibrium between 90Sr and 90Y, as well as the decrease in 89Sr activity (see Appendix 2). When required, the influence of the 89Sr activity was subtracted from the β-activity counts to obtain the 90Sr activity. The activity was always decay-corrected mid-sampling. The detection limit for 90Sr was approximately 7.0 mBq/sample, approximately 3.5 mBq/m2 using a total of 30,000 s of measurement. For 137Cs, the limit was approximately 16.0 mBq/sample, approximately 8.0 mBq/m2 for an average of 120,000 s of measurement.

Atmospheric radioactive aerosols

Aerosol samples were collected weekly using a high-volume air sampler (HV; Sibata Scientific Technology Ltd., HV-1000 F) on a quartz fiber filter (Advantech QR100; 203 mm × 254 mm) (Igarashi et al. 1999a). During March 2011, the sampling frequency was intensified. The flow rate was set at 0.7 m3/min, and the daily sucked air volume was approximately 1000 m3. After collection, the filters were compressed into pellets using a hydraulic press device. They then underwent conventional γ-ray spectrometry with Ge detectors as described above. Current detection limits for 134Cs and 137Cs are approximately 9.0 mBq/sample (1.3 μBq/m3) and 10 mBq/sample (1.5 μBq/m3) for approximately 1,000,000 s measurements, respectively.

The filter samples collected before the radioactive plume arrived at Tsukuba were measured at the Kyoto University Research Reactor Institute to achieve lower detection limits and avoid contamination from the FDNPP accident. This was necessary because the Ge detectors, measurement environment, and experimental materials at the MRI were somehow contaminated by the radioactive plume’s passage on March 14–15 and 20–23, 2011 (see Appendix 1). To date, radioSr analysis has been performed on only a limited number of aerosol samples collected during March 2011. The results are presented in Appendix 2.

Results and discussion

Figures 1 and 2 depict the results of the atmospheric 90Sr and 137Cs deposition observations at the MRI for different durations. The temporal changes in monthly radionuclide depositions shown in Fig. 1 include those from the late 1950s to more recently available data, i.e., after the FDNPP accident. Figure 2 compares the amounts of atmospheric deposition after the FDNPP accident and from the late 2000s. Analyses of 90Sr and 137Cs deposition samples taken 6 and 8 months before the accident are ongoing to control for possible sample contamination at the MRI caused by the accident. Thus, these data are missing in Figs. 1 and 2.
Fig. 1

Sr-90 and 137Cs monthly deposition observed at the Meteorological Research Institute (MRI) from 1957 to 2014. Monthly deposition is expressed in millibecquerel per square meter on a logarithmic scale. Sr-90 and 137Cs analyses from deposition samples taken 6 and 8 months before the accident, respectively, are ongoing to avoid possible sample contamination at the MRI because of the accident. Thus, these data are missing not only in Fig. 1 but also in Fig. 2. The measurement uncertainty (1σ) is shown only for the data obtained after the FDNPP accident and is reasonably small compared to the analytical data. For comparison, uncertainty for the monthly data in 2010 is also given. The effects of atmospheric nuclear bomb tests have been recorded since 1957. Until the Partial Test Ban Treaty (PTBT) became effective in 1963, the USA, Soviet Union, and UK conducted atmospheric tests. France and China continued atmospheric testing until 1974 and 1980, respectively. Since 1981, all the nuclear bomb tests have shifted underground, so additional radioSr and Cs contamination should be negligible. However, the Chernobyl accident in 1986 also affected the time series. The simple summation of the deposition from 1957 to the time before the FDNPP accident (mid-2010) and decay-corrected summations for 90Sr and 137Cs can be compared to the FDNPP-derived deposition

Fig. 2

Monthly 90Sr and 137Cs deposition levels in pre- and post-accident periods. Partial enlargement of Fig. 1. The monthly deposition is expressed in millibecquerel per square meter on a logarithmic scale. The atmospheric depositions of 90Sr and 137Cs in 2013 observed at the MRI were a few orders of magnitude higher than those from 2005 to 2011 before the FDNPP accident. For 90Sr and 137Cs, monthly depositions during 2005 to 2010 were 0.5–19 mBq/m2/month and 1.2–97 mBq/m2/month, whereas they were 1–33 mBq/m2/month and 2–39 Bq/m2/month in 2013, respectively

Figure 3 depicts the temporal change in atmospheric activity concentrations of radioCs since March 2011. Before the FDNPP accident, it was difficult to detect 137Cs below about 1 μBq/m3 in the air (the global fallout background level).
Fig. 3

Temporal change in atmospheric radioCs concentrations at the MRI before and after the FDNPP accident (“Mar.-Aug. 2014”). Activity concentration is expressed in milli becquerel per cubic meter on a logarithmic scale. The measurement uncertainty (1σ) is shown. The maximum concentration of 38 Bq/m3 of 137Cs was observed during March 20–21, 2011. After that, the radioCs concentrations rapidly decreased until fall 2011 when the decrease slowed. The levels before (approximately 1 μBq/m3) and 3 years after the FDNPP accident (12 μBq/m3 from March to August 2014) are also compared. A difference of at least one order of magnitude is observed between the concentration level from March to August 2014 and the level before the FDNPP accident

Although there were small-scale Japanese nuclear accidents in the 1990s (Igarashi et al. 1999a, 2000; Komura et al. 2000), they did not cause significant marks in the present time series of monthly 90Sr and 137Cs depositions. The effects of the Chernobyl accident that occurred in 1986 were more evident for 137Cs than 90Sr (e.g., Aoyama et al. 1991) as illustrated in Fig. 1. However, the previous maximum 137Cs deposition was two orders of magnitude lower than those caused by the FDNPP accident. Thus, the impact of the FDNPP accident was more remarkable than any previous incident in our time series.

Temporal changes in monthly 137Cs atmospheric deposition

The monthly 137Cs deposition in March 2011, when the FDNPP accident occurred, was 23 ± 0.9 kBq/m2/month, which is six to seven orders of magnitude higher than the level before the Fukushima disaster (Figs. 1 and 2). Because the pollution source of the FDNPP accident is closer to the observation site (170 km) than it is to the weapons testing sites and Chernobyl (several thousand kilometers), the spatial representativeness of the MRI data (as an absolute value) is lower.

The cumulative 137Cs deposition at the MRI was 25.5 kBq/m2/year for the year 2011. The sum of the simple monthly 137Cs depositions from 1957 to mid-2010, the time before the Fukushima disaster, is approximately 7.0 kBq/m2 (this figure is thought to contain some error since the pre-1970s data did show individual undefined errors), as shown in Fig. 1. Considering the radioactive decay of the individual monthly 137Cs depositions, this past total contribution represents 2.3 kBq/m2. The FDNPP accident’s influence was over ten times larger than that of any past event. Almost the same amount of 134Cs (half-life, 2.1 years) was simultaneously deposited with the 137Cs; thus, the total cesium deposition came to more than 50 kBq/m2. This value agrees quite well with figures for the area around Tsukuba in observation mapping provided by the Ministry of Education, Culture, Sports, Science and Technology (MEXT 2011a).

Later, the deposition decreased rapidly, but the monthly 137Cs deposition in 2012 and 2013 ranged from 8–36 and 2–39 Bq/m2/month, respectively, where deposition during 2005–2010 had been in the range of 1.2–97 mBq/m2/month, i.e., three to four orders of magnitude higher. The deposition level at the end of 2013 was still as high as values registered when atmospheric nuclear tests were conducted by China in the 1970s to the early 1980s. The deposition rate slowly decreased in the following years.

Atmospheric concentrations of radioCs

Figure 3 displays the temporal change in the atmospheric radioCs activity concentrations at the MRI in Tsukuba since the FDNPP accident. The temporal trend shows an abrupt increase (peak) of several orders of magnitude, followed by a rather rapid concentration decrease over a short period (3 to 4 months after the FDNPP accident), with a smaller decreasing rate after. The highest 137Cs atmospheric concentrations (38 Bq/m3 in a 12 h sampling period) were registered on March 20–21, 2011, which slightly exceeded the limit stipulated by Japanese regulations and ordinances (30 Bq/m3). Although the pre-accident activity concentration level was not measured, it had been observed for a short period, from February to April 1997, which includes the time when the Power Reactor and Nuclear Fuel Development Corporation Tokai accident occurred (Igarashi et al. 1999a). The background level was approximately 1 μBq/m3 and did not decrease far below half that value (approximately 0.5 μBq/m3) until 2011. The decrease in monthly 137Cs deposition was small during the same period (Igarashi et al. 2003, 2009). Thus, the 137Cs activity concentration level registered during summer 2014 appears at least 10 times higher than that before the accident. During 2011 and 2012, small spikes were recorded from time to time (Fig. 4). In these cases, daily forward trajectory analysis suggested that the polluted air masses were transported from the accident site during the corresponding observation period as shown in the figure. In addition, relatively high concentrations were registered in the winter (Fig. 3). This phenomenon was noted at other places in northern and eastern Japan (Hirose 2013), so there is most likely a common explanation, as described in the literature.
Fig. 4

Atmospheric concentration increases observed during 2011 and 2012 and their air mass trajectories. Note that the activity concentration scale is linear. The forward air mass trajectory calculated by the NOAA’s HYSPLIT model is depicted for the radioCs activity concentration peaks, suggesting that the plume from the FDNPP site passed over the Tsukuba region. The shown trajectory cases are December 1, 2011 and April 5, 2012. The increases seem to be attributable to the transport of primary radioCs from the accident site

Temporal change in monthly 90Sr atmospheric deposition

In contrast to 137Cs, the monthly 90Sr deposition in March 2011 was 5.2 ± 0.1 Bq/m2/month. This was approximately 1/5000 the amount of 137Cs deposited in the same month. This deposition was 2–3 orders of magnitude larger than the level before the FDNPP disaster. The annual 90Sr deposition was 10.6 Bq/m2/year during 2011, approximately 1/2500 of the quantity of 137Cs deposited. The simple sum of the monthly 90Sr depositions from 1957 to mid-2010, before the Fukushima disaster, was approximately 2.7 kBq/m2, as shown in Fig. 1. Taking the radioactive decay of the individual monthly 90Sr depositions into account, the sum represents approximately 0.9 kBq/m2. The FDNPP accident’s impact on 90Sr was very small. The most extreme monthly 90Sr deposition, recorded during the global fallout era of May 1963 in Tokyo, was 170 Bq/m2/month. The FDNPP accident’s impact on the monthly 90Sr deposition was less than one-thirtieth of this maximum. Therefore, it is probable that 90Sr pollution over the Kanto Plain from the accident was relatively insignificant; the environmental and health impacts of 90Sr are relatively minor.

In addition, the 137Cs/90Sr activity ratio fluctuated between approximately 400 and 5000 (Fig. 5), except for some abnormal cases described below. This confirms that the degree of radioSr pollution is relatively insignificant compared to that of radioCs. However, it is still unknown why the 137Cs/90Sr activity ratio varied so widely despite the radionuclides having a common accident emission source, namely, the FDNPP accident. More discussion on the 137Cs/90Sr activity ratio is given in Appendix 2. The reason for the variability is worth studying in the future. The monthly 90Sr deposition recorded in 2012 was 10–31 mBq/m2/month, whereas during 2005–2010, it was 0.5–19 mBq/m2/month, a difference of up to two orders of magnitude.
Fig. 5

Activity ratio of 137Cs/90Sr in monthly depositions since March 2011 at the MRI. The temporal changes do not show a clear decreasing or increasing trend. The arrow shows the month during which an anomalous deposition of 90Sr was observed. Except for the anomaly, the 137Cs/90Sr activity ratio fluctuated from approximately 150 to 6700

A 90Sr deposition anomaly in October 2012

In October 2012, the monthly 90Sr deposition showed a peak of 145 ± 2 mBq/m2/month (see the arrow in Figs. 5 and 6), which is 1–2 orders of magnitude higher than any monthly 90Sr deposition registered that year, and its influence lasted a few months (Fig. 6). This small 90Sr event remains puzzling. By applying forward trajectory analysis and closely examining the precipitation over Tsukuba, we believe that the 90Sr may have come from the FDNPP and encountered precipitation on October 7 and 18–19, 2012. However, this increase was not accompanied by a radioCs deposition peak, and the major radionuclide emitted by the FDNPP accident is radioCs, which is inconsistent with FDNPP accident being the source of the October anomaly.
Fig. 6

Exponential fitting of the decreasing monthly 137Cs deposition trend since March 2011 at the MRI. The curve is composed of three exponential functions. These are attributable to the decreasing intensity of primary emission, tropospheric aerosol residence and re-suspension. The arrow shows the month during which an anomalous 90Sr deposition was observed. Possible causes are mentioned in the text

The Japanese Radioactivity Survey data on the Internet were checked, but no consistent data were evident for the corresponding period. In addition, no such anomaly was reported in Europe (Masson 2014, personal communication). Based on the timescale of this contamination, however, the source should be neither very local nor very small. This episode shows some similarities to the case in fall 1995 in Tsukuba (Igarashi et al. 1999b). We also assume unidentified, unreported incidents of burning and/or melting of industrial 90Sr sources in the Far East region as a possible explanation, such as the Algeciras (Spain) incident in 1998 with its 137Cs source of 3.7 TBq (Estevan 2003). Sr-90 is widely used in industrial applications, such as in thickness gauges, and its activity size ranges from 740 MBq to 3.7 GBq in Japan. Because 90Sr is a pure β-emitter, it is more difficult to determine the sources of its environmental pollution than it is for 137Cs.

Decrease in monthly 137Cs deposition after the FDNPP accident

Although researchers do not agree precisely on the FDNPP radioactivity emission inventory (Chino et al. 2011; Katata et al. 2012, 2012b, 2014; Maki et al. 2013; Stohl et al. 2012; Terada et al. 2012; Winiarek et al. 2012), if the 137Cs emission in March 2011 is assumed to be 10 PBq/month, the deposition/emission ratio (the monthly deposition at the MRI divided by the monthly emissions from TEPCO (2012)) would be approximately 10−12. If the MRI is included in the so-called “hot spot” area, the deposition could be approximately 100 kBq/m2 (five times larger). This would give a deposition/emission ratio of approximately 10−11. After March 2011, the ratio is calculated to be in the range of 10−10 to 10−9, which appears to be large, if the emission-deposition relation above is correct. We can presume that this excess deposition at the MRI, Tsukuba came from secondary emissions. Thus, Tsukuba can be regarded as representative of a typical suburban area in the Kanto Plain, and the relative trend of temporal changes there can be considered comparable to surface contamination levels for similar geographical domains. The temporal trends (holding time constant) may also be spatially representative, although this potential is limited.

To study the decreasing trend in monthly 137Cs atmospheric deposition caused by the FDNPP accident and to make future projections, a curve was fitted on the temporal trends using multiple components. A drawing software was employed, and the fitting operation was put through 100 iterations, each time changing the initial value so that the calculation results would converge, as shown in Fig. 6. A trinomial exponential function of the form a × (ek×t) was applied to fit the data (where a is a constant and k is an inverted time scale; Ln2/T1/2), and the individual half-times (T1, T2, and T3 in Fig. 6) were approximately 5.9 (±11 %) days, 16 (±18 %) days, and 1.1 (±32 %) years, respectively. The relative uncertainty is shown in parentheses. These appear to correspond to the time scale of (1) the reduction in the original FDNPP accident surge (primary emission source), (2) the tropospheric transportation and diffusion of the radioactive plume (equivalent to the removal of radioactive aerosols from the atmosphere), and (3) the emission intensity of re-suspension (secondary emission sources). We posit that some primary radiological release to the atmosphere continues because the FDNPP is not isolated from the neighboring environment (Hirose 2013; TEPCO 2012). The results, then, cannot be assumed to be completely free of primary release. However, the first and second terms can be reasonable estimates corresponding to the primary emission and tropospheric aerosol residence, respectively.

The second term is almost identical to figures obtained by other recent studies (e.g., Hirose 2012, 2013; Kristiansen et al. 2012). Hirose (2013) analyzed radioCs deposition data obtained during 2011–2012 from several places over the Kanto Plain and Fukushima prefecture, Japan. According to his report, “The apparent half-lives at Ichihara, Tokyo, Utsunomiya, Hitachinaka and Maebashi were 11.9, 10.6, 13.5, 11.5 and 12 d, respectively.” Hirose (2012) states that “the residence times of aerosols in the troposphere, which are in the range of 5–30 d, have been determined by natural and anthropogenic radionuclides, which depend on particle size and altitude (Ehhalt, 1973).” Hirose (2012) also argues “the temporal change of the Fukushima-derived 137Cs revealed that the apparent atmospheric residence time of the Fukushima-derived 137Cs in sites within 300 km from the Fukushima Dai-ichi NPP is about 10 d.” This long residence time might reflect the Fukushima radioactive plume’s circulation over the Northern Hemisphere, which takes about 20 days (Hernández-Ceballos et al. 2012). As shown in Fig. 8a in the Appendix 1, the third Fukushima plume’s arrival over the Kanto Plain was observed from March 28–31, 2011. It was well reconstructed by the aerosol transport model. Other observations over the Kanto Plain also revealed this transport event (e.g., Amano et al. 2012; Haba et al. 2012). However, we cannot clearly determine whether this concentration peak is due to delayed primary emission (e.g., Terada et al. 2012), hemispheric circulation, or a combination of both. This is because the current model simulation uses the emission inventory, which is also based on atmospheric monitoring results (e.g., Terada et al. 2012). Regarding this connection, Kristiansen et al. (2012) investigated the 131I and 137Cs removal times from the atmosphere using global-scale monitoring data. Their estimated 137Cs removal times were in the range of 10.0–13.9 days, which is closer to our present result. They also noted the difference from the typical values of 3–7 days obtained by aerosol model simulations, suggesting that the aerosol transportation models need improvement. We would like to add that the deposition results should be interpreted to reflect not only the surface air but also the air column up to at least the mixed layer. Therefore, the deposition may be affected by large-scale transportation, in contrast to indications obtained from the surface concentration only. For further reference, based on the monthly emission of radioCs until the end of 2011 estimated by TEPCO 2012, the primary emission decrease can be fitted using two exponential laws with half-time constants of 2.3 days (±2 %) and 48 days (±23 %).

The third term’s half-time of 1.1 years for the MRI data, despite its relatively large associated uncertainty, appears to reveal the total re-suspension of radioCs from contaminated surfaces. This value is too large to correspond to any primary releases from the FDNPP in the early phases. In addition, it agrees with the value for the re-suspension “descending trend” due to the Chernobyl accident reported by Garger et al. (2012), which was 300 days. It was possible to fit a two-term exponential curve to the present 137Cs data by fixing the 1.1-year half-time, obtaining a value of 7.8 days for the first term. When compared with the triple exponential (three-term) model, the fitting distance (defined by the ratio of the calculation to the observation) for the double exponential (two-term) model was larger for elapsed times of 2–12 months, although there were exceptions. The mean and standard deviation for the two- and three-term fit distances are 2.50 ± 2.02 and 1.54 ± 1.14, respectively. The medians are 1.82 and 1.09, respectively, suggesting that the three-term model fits better. Although we do not provide an illustration here, we found that fitting with three-term functions for the decrease in monthly 90Sr deposition after the disaster was also possible. Therefore, we preferred fitting with a trinomial exponential function to reproduce the deposition flux of radionuclides from the FDNPP accident. Again, the primary emissions of radioCs to the atmosphere are anticipated to continue at a non-negligible level (less than 7.2 GBq/month is assumed in TEPCO’s latest press release (in Japanese) at http://www.tepco.co.jp/life/custom/faq/images/d150129-j.pdf) because the FDNPP is not isolated from the surrounding environment (Hirose 2013). These delayed primary emissions of approximately 7 GBq are 6–7 orders of magnitude lower than the emissions in March 2011 (e.g., 15 PBq for 137Cs; NISA 2011). If the primary emission deposits were delayed in a fashion similar to those from March 2011, recent MRI records after the FDNPP accident would correspondingly be 6–7 orders of magnitude lower than the peak value caused by the accident (see Fig. 2). Therefore, we consider that the present decrease in the third term reflects secondary emission (re-suspension) trends over the Kanto Plain moderately well. In future, we plan to confirm this by applying different evaluation methods such as transport simulations or others.

Consideration of re-suspension and its persistence

Currently, there may be interest and concern about how long it will take for the atmospheric radionuclide deposition fluxes to return to pre-FDNPP accident levels (cf. Garger et al. 2012; Hatano and Hatano 2003). Although it seems slightly arbitrary, the monthly 137Cs depositions can be estimated if the fitted curve described above is extrapolated. The result of this extrapolation is illustrated in Fig. 7. This simple estimation shows that more than a decade will likely be required for the activity levels to return to pre-accident levels. Thus, re-suspension (secondary emission to the atmosphere; e.g., Igarashi 2009) must be scrutinized with long-term monitoring. Because it seems natural that radionuclide emission flux would be proportional to surface pollution density, there could be radioCs fluxes several orders of magnitude higher than those measured in Tsukuba in areas nearer the FDNPP site whose Cs surface pollution is several orders of magnitude higher than in Tsukuba. Therefore, elucidating the secondary emission processes of the FDNPP radionuclides remains an imminent scientific challenge, especially for heavily polluted areas. Secondary sources can include soil dust suspension from polluted earth surfaces, emissions from polluted vegetation and forests, and volatilization and release from combustion of polluted garbage and open field burning (e.g., Igarashi 2009). Although the main emission sources are not yet well understood, this elucidation must be performed as soon as possible.
Fig. 7

Future projection for monthly 137Cs deposition level using a trinomial exponential function. The present simple estimation shows that more than a decade would be necessary for the 137Cs atmospheric deposition level to return to pre-accident levels

Conclusions

The authors conducted atmospheric monitoring of airborne radioSr and Cs and their deposition at the MRI in Tsukuba, Japan. The monitoring period encompasses the FDNPP accident and the subsequent few years. The monthly 137Cs deposition at the MRI was (23 ± 0.9) × 103 Bq/m2/month in March 2011, which is 6–7 orders of magnitude higher than pre-accident levels. Almost equal amounts of 134Cs and 137Cs were deposited, causing surface pollution of more than 50 kBq/m2 in Tsukuba in 2011, in close agreement with the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT)’s airborne mapping. Deposition of 90Sr was 5.2 ± 0.1 Bq/m2/month in March 2011, which is less than 0.02 % of the total 137Cs deposition in that month. The level of 90Sr deposition was 3–4 orders of magnitude higher than pre-accident levels and did not reach the level registered during the 1960s after nuclear tests; the effects from 90Sr will not be as large as from radioCs. During 2013, the Fukushima fallout decreased by 3–4 orders from its magnitude at the time of the accident, yet some becquerel per square meter of monthly deposition continues. This corresponds to the level in the 1970s and early 1980s when China performed atmospheric nuclear tests. During 2013, the 137Cs concentration remained at a level of tens of micro becquerel per cubic meter. Because re-suspension (secondary emission) will continue over a long time, it is necessary to monitor its future trends and variability. An apparent decrease in atmospheric radioCs deposition was fitted by trinomial exponentials, giving information regarding the reducing trend of airborne radionuclide persistence through re-suspension into the atmosphere. Extrapolation of the decreasing rate suggests that it would take at least a decade for the activity to return to pre-disaster period levels. Further monitoring efforts are essential.

Abbreviations

ATSDR: 

US Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry

DOE: 

US Department of Energy

FDNPP: 

Fukushima Dai-ichi nuclear power plant

HYSPLIT: 

Hybrid Single Particle Lagrangian Integrated Trajectory Model

IAEA: 

International Atomic Energy Agency

JMA: 

Japan Meteorological Agency

KURRI: 

Kyoto University Research Reactor Institute

MANAL: 

Meso-regional objective analysis

MEXT: 

Ministry of Education, Culture, Sports, Science and Technology, Japan

MRI: 

Meteorological Research Institute, Japan

NASA: 

National Aeronautics and Space Administration, USA

NHM: 

The JMA/MRI non-hydrostatic meteorological model

NISA: 

The former Nuclear and Industrial Safety Agency, Japan Carriage Return

RAQM2: 

Regional Air Quality Model 2

SCJ: 

Science Council of Japan Carriage Return

TEPCO: 

Tokyo Electric Power Company

Declarations

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors’ Affiliations

(1)
Meteorological Research Institute
(2)
Present address: Japan Meteorological Business Support Center

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