Detection of radioactive 35S at Fukushima and other Japanese sites

Antra Priyadarshi,1 Jason Hill-Falkenthal,1 Mark H. Thiemens,1 Naohiro Yoshida,2

Sakae Toyoda,3 Keita Yamada,2 Arata Mukotaka,2 Ayako Fujii,3 Mitsuo Uematsu,4

Shiro Hatakeyama,5 Izumi Noguchi,6 Yukihiro Nojiri,7 and Hiroshi Tanimoto7

Received 16 July 2012; revised 29 October 2012; accepted 25 November 2012; published 30 January 2013.

[1] The Fukushima nuclear power plant was severely damaged by an earthquake andconcomitant tsunami during March 2011. An effect of this disaster was secondaryformation of radioactive 35S via the 35Cl(n,p)35S reaction, when neutrons from the partiallymelted reactor cores activated the coolant sea water. Here we report the first measurementsof 35S in sulfate aerosols and rain water collected at six Japanese sampling sites, Hokkaido,Tsukuba, Kashiwa, Fuchu, Yokohama, and Fukushima, during March-September 2011.The measured 35SO4

2- concentrations in aerosols vary significantly. The Kashiwa (AORI)site shows the highest 35SO4

2- concentration (6.1� 104� 200 atoms/m3) on 1 April 2011,which is nearly 100 times higher than the natural background activity. Considering thepercentage loss of 35SO4

2- resulting from dry and wet deposition and dilution of theradiation plume in the boundary layer during transport, it was determined that the surfaceair concentration of 35SO4

2- at the Fukushima would have been 2.8� 105 atoms/m3 duringthe week after the earthquake, which is in agreement with the model prediction[Priyadarshi et al., 2011a]. 35SO4

2- activity in rain water collected during March-May 2011at Tokyo Tech Yokohama varies from 1.1� 105 to 9.8� 105 atoms/liter, whereas streamwater collected near Fukushima was found to have 1.2� 105 atoms/liter during April. Evenafter 6months, 35SO4

2- activity remains very high (9.9� 104� 770 atoms/m3) in the marineboundary layer in the Fukushima region, which implies that the reactor core was producingradioactive sulfur.

Citation: Priyadarshi, A., et al. (2013), Detection of radioactive 35S at Fukushima and other Japanese sites, J. Geophys.Res., Atmos., 118, 1020–1027, doi:10.1029/2012JD018485.

1. Introduction

[2] On 11 March 2011, a magnitude 9.0 earthquakeoccurred in the western Pacific Ocean, with its epicenterapproximately 72 km east of the Oshika peninsula ofTohoku Japan (see http://earthquake.usgs.gov/earth-quakes/eqinthenews/2011/usc0001xgp/). The earthquaketriggered a catastrophic tsunami with severe collateral

damage. In addition to a significant loss of life and infra-structure destruction, the tsunami caused damage to theFukushima Di-ichi nuclear power plant (http://www.iaea.org). Immediately after the earthquake, the reactors at theFukushima nuclear power plant were automatically shutdown and boron–carbon control rods inserted betweenthe fuel columns to absorb neutrons and halt the nuclearchain reaction. Even after the shutdown, the reactor corerequired a prolonged cooling, because the uranium fuelcontinues to decay into radioactive byproducts and torelease heat. A preliminary computer model by the U.K.National Nuclear Laboratory showed that, even after shutdown of the Fukushima nuclear power plant, the radioac-tive byproducts of the fission reaction still generated 7megawatts of heat [Brumfiel, 2011a]. Because both theregular and the emergency backup power supply (dieselgenerators) that powered the cooling system of the powerplant were severely damaged by the tsunami waves, thereactor core became sufficiently hot that it initiated a meltdown and released hydrogen gas that eventually ignited andcaused explosions. The core damage was estimated to be55%, 35%, and 30% for Units 1, 2, and 3, respectively(http://www.iaea.org). Even after 6months of continuouscooling, the reactor core was too hot to allow access [Brumfiel,2011b]. As the result of these events, a significant amount of ra-dioactivity was released into the atmosphere [Chino et al., 2011;

1Department of Chemistry and Biochemistry, University of CaliforniaSan Diego, La Jolla, California, USA.

2Department of Environmental Chemistry and Engineering, TokyoInstitute of Technology, Yokohama, Japan.

3Department of Environmental Science and Technology, TokyoInstitute of Technology, Yokohama, Japan.

4Atmosphere and Ocean Research Institute, University of Tokyo, Chiba,Japan.

5Institute of Agriculture, Graduate School of Agriculture, TokyoUniversity of Agriculture and Technology, Tokyo, Japan.

6Institute of Environmental Sciences, Hokkaido Research Organization,Sapporo, Japan.

7National Institute for Environmental Studies, Tsukuba, Japan.

Corresponding author: A. Priyadarshi, Department of Chemistry andBiochemistry, University of California San Diego, 9500 Gilman Drive,La Jolla, CA 92093, USA. (fantra@ucsd.edu)

©2012. American Geophysical Union. All Rights Reserved.2169-897X/13/2012JD018485

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JOURNAL OF GEOPHYSICAL RESEARCH: ATMOSPHERES, VOL. 118, 1020–1027, doi:10.1029/2012JD018485, 2013

Morino et al., 2011]. Potentially dangerous levels of radiationwere detected nearly 30-40km away from the nuclear plant.Approximately 4� 1011 neutrons/m2 were emitted during thefirst week following the earthquake, which led to the formationof radioactive 35S [Priyadarshi et al., 2011a]. During the firstfew weeks (13 March to 26 March 2011) after the earthquakeand tsunami, several hundred tons of sea water were pumped intothe partially melted reactor core as a coolant. Neutrons emittedfrom the reactor core were absorbed or captured by the constitu-ents of sea water to produce a variety of radioactive isotopes,mainly 24Na by interaction of neutrons with stable sodium viaan (n,g) reaction and 35S produced by interaction of neutronswithstable chlorine via an (n,p) reaction [Dryssen and Nyman, 1955;Love and Sam, 1962]. Since 24Na has a very short half-life (15h),the total radioactivity after a week was due nearly exclusively to35S [Dryssen and Nyman, 1955].[3] Once produced, 35S was oxidized to 35SO2 gas, which

was eventually further oxidized to 35SO42- aerosol in the

atmosphere, similar to the natural atmospheric process[Brothers et al., 2010; Priyadarshi et al., 2011b; Tanakaand Turekian, 1991]. It was subsequently transported inthe atmosphere depending on air mass trajectories andremoved from the atmosphere by dry/wet deposition andradioactive decay. 35S is a unique tracer in that it providesinformation on the number of neutrons emitted from thereactor core and can be used to probe the condition of thereactor core as well as the containment vessel. Because ofthe unavailability of samples from near the Fukushimapower plant, there have been no measurements of 35S untilnow. Based on 35S measurements of atmospheric sulfate and

SO2 samples collected at La Jolla, California, and concomitantmodel calculations, the concentration of 35SO4

2- in surface airwas estimated to be 2� 105 atoms/m3 at Fukushima duringMarch 2011 [Priyadarshi et al., 2011a]. Here we report the firstmeasurement of radioactive 35S in sulfate aerosols and rainwater collected at different sites in Japan (Figure 1) and comparethe data with the model calculation of Priyadarshi et al.[2011a].

2. Materials and Methods

[4] Aerosol samples were collected at six different places inclose proximity to the Fukushima nuclear power plant(37.25�N, 141.02�E; Figure 1): Tokyo Institute of Technology(Tokyo Tech) Yokohama (35.52�N, 139.48�E); AtmosphereandOcean Research and Institute (AORI), University of Tokyo,Kashiwa, Chiba Prefecture (35.90�N, 139.94�E); TokyoUniversity of Agriculture and Technology (TUAT), Fuchu,Tokyo (35.68�N, 139.48�E); National Institute of Environmen-tal Studies (NIES), Tsukuba, Ibaraki Prefecture (36.05�N,140.12�E); Hokkaido Research Organization, Sapporo(43.08�N, 141.34�E); and Kawamata town, Fukushima prefec-ture (37.40�N, 140.36�E). These sampling sites, except forSapporo, are located within a 250km radius and south of theFukushima nuclear plant. The aerosol samples were collectedfrom March through September 2011 using a high-volume airsampler. Rain water samples were collected only at Tokyo TechduringMarch-May 2011. In addition, a water sample from a lo-cal water stream, situated in Fukushima prefecture, nearly50 km away from the Fukushima nuclear power plant, was

Figure 1. Details of the sampling sites. Aerosol samples were collected at six different places in closeproximity to the Fukushima nuclear power plant (37.25�N, 141.02�E): Tokyo Institute of Technology(Tokyo Tech), Yokohama (35.52�N, 139.48�E); Atmosphere and Ocean Research and Institute (AORI),University of Tokyo, Kashiwa, Chiba Prefecture (35.90�N, 139.94�E); Tokyo University of Agricultureand Technology (TUAT), Fuchu, Tokyo (35.68�N, 139.48�E); National Institute of Environmental Stud-ies (NIES), Tsukuba, Ibaraki Prefecture (36.05�N, 140.12�E); Hokkaido Research Organization, Sapporo(43.08�N, 141.34�E); and Kawamata town, Fukushima prefecture (37.40�N, 140.36�E). These samplingsites except for Sapporo are located within a 250 km radius and south of the Fukushima nuclear plant.

PRIYADARSHI ET AL.: 35S MEASUREMENT IN FUKUSHIMA DISASTER

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collected in April 2011. The samples were analyzed at theUniversity of California, San Diego. The details of the sampleprocessing for analyzing 35S activity in sulfate aerosols havebeen described by Brothers et al. [2010]. Three or four litersof rainwater was passed through a pre-prepared anion resincolumn to trap sulfate on the resin surface, and 15ml of HBr(1M) was passed through the resin column to elute sulfate ionsfrom the resin. The solution was then neutralized by addingAg2O and oven dried. Sulfate was cleaned of organics asdiscussed by Brothers et al. [2010]. The 35S activity wascounted in an ultralow-level liquid scintillation spectrometer(Wallac 1220 Quantulus) and was corrected for the backgroundactivity (which is 1.07 DPM and probably is due to the radioac-tivity contributed from vial material and the scintillation gelused for counting) and the decay time from the samplecollection date. The natural variation in 35SO4

2- concentration,as reported by Priyadarshi et al. [2011b, 2012], is from 130to 900 atoms/m3 and from 200 to 1600 atoms/m3 at Scripps Pierin southern California and in Antarctica, respectively.

3. Results and Discussion

[5] 35S activities measured in sulfate aerosols collected atTokyo Tech, AORI, TUAT, NIES, Hokkaido, and Fukushima(different aerosol size fraction) are shown in Figures 2 and 3.Even though the sampling sites are relatively close to eachother (southward direction from Fukushima, except forHokkaido), a large variation in 35SO4

2- activities was observedduring March-April 2011. 35SO4

2- concentrations were 0.26�103� 11.78� 103, 0.69� 103� 61.4� 103, 0.51� 103� 1.42� 103, 1.41� 103� 18.05� 103, 0.56� 103 � 2.63� 103

atoms/m3 at Tokyo Tech, AORI, TUAT, NIES, and Hok-kaido, respectively (Table 1). The 35S activities observed atTokyo Tech, AORI and NIES are significantly higher thanthose observed at TUAT, NIES, and Hokkaido sites. AtFukushima, 35SO4

2- varies from 8.1� 103 to 9.9� 104 atoms/m3 and from 46 to 62 atoms/m3 in fine and coarse fractions, re-spectively (Table 2). 35SO4

2- concentrations at AORI (6.1� 104

atoms/m3) and Fukushima (1.2� 105 atoms/m3) are the highest35S activities ever measured in any atmospheric sample and arenearly 100 times higher than the natural background.

Radioactive 35S (half-life 87 days) is also produced by the inter-action of cosmic rays with 40Ar in the Earth’s atmosphere [Laland Peters, 1967]. The natural background 35SO4

2- concentra-tion in the atmosphere varies from 300 to 900 atoms/m3

[Brothers et al., 2010; Priyadarshi et al., 2012]. Even at Ant-arctica, where the production rate of 35S is maximal [Lal andPeters, 1967], 35SO4

2- varies from 120 to 1600 atoms/m3

[Priyadarshi et al., 2011b]. Such a high concentration of35SO4

2- at AORI indicates the presence of another source of35S in the marine boundary layer. Priyadarshi et al. [2011b]demonstrated that the partially damaged reactor core of theFukushima nuclear power plant produced radioactive 35S via35Cl(n,p)35S reaction during the first few weeks following theearthquake. The Hybrid Single Particle Lagrangian IntegratedTrajectory (HYSPLIT) model developed by NOAA’s AirResources Laboratory (ARL) [Draxler and Rolph, 2011] wasused to calculate the air mass back-trajectories to determinethe origin and pathway of the air masses affecting the samplingsites involved in this study. We considered the air mass mixingand its transport within the boundary layer; the backward airmass trajectories were calculated for 72h at three differentaltitudes (10, 500, and 1000m) over each sampling station. Asshown in Figure 4, surface air masses at AORI arrived from thenorth near Fukushima on 1 April 2011 and are thus responsiblefor the observed spike in 35SO4

2- activity (6.1� 104 atoms/m3).The sampling site at Hokkaido lies north of Fukushimaand is thus not affected by the 35S emission at Fukushima.A possible reason for lower 35SO4

2- activity at TUAT is itsgeographical location and the prevailing air mass transportvectors. TUAT is situated in Fuchu city, which has manyterraces and hills across it. The Fuchu andKokobunji hills alignfrom west to east whereas the Sengen-Yama hill is located inthe northeastern side, which may affect the air mass trajectoriescoming from Northeast Fukushima. Fine-scale air masstrajectory analysis is not available, but future modeling effortscould be instrumental in developing this.

Figure 2. 35SO42� activity measured at five different sam-

pling sites near Fukushima. The maximum 35SO42� activity

(6.1� 104 atoms/m3) was observed at AORI on 1 April 2011.

Figure 3. 35SO42� measured in different aerosol size frac-

tions (Q5, Q6, Q7, representing aerosol size fractionbetween 0.69 and 1.3 mm, between 0.39 and 0.69 mm, andless than 0.39 mm, respectively) collected at Fukushimaprefecture. 35SO4

2� in the fine particles (Q7< 0.39 mm) isnearly 100 times higher than the natural background andshows that, even after 6months, of the nuclear disaster, thereactor core was active and producing 35S.

PRIYADARSHI ET AL.: 35S MEASUREMENT IN FUKUSHIMA DISASTER

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Table 1. Measurement of radioactive 35S in sulfate aerosol collected at different sampling sites near Fukushima

Sample ID

Collection Time (m/d/y)

Air Volume (m3) 35SO42- Atoms per m3 (

a

103)Starting Date End Date

Tokyo Institute of Technology (Tokyo Tech), Yokohama1 3/25/11, 3/26/11 991 3.06� 0.12

a

2 3/26/11, 3/26/11 735 0.56� 0.113 3/26/11, 3/27/11 724 0.72� 0.124 3/27/11, 3/28/11 2001 1.25� 0.045 3/28/11, 3/30/11 2945 11.78� 0.076 3/30/11, 4/1/11 2960 0.73� 0.037 4/1/11, 4/3/11 2931 0.48� 0.038 4/3/11, 4/5/11 2980 0.35� 0.039 4/5/11, 4/7/11 2969 0.41� 0.0310 4/7/11, 4/9/11 2973 1.03� 0.0711 4/9/11, 4/11/11 2962 1.75� 0.0812 4/11/11, 4/13/11 2959 0.94� 0.0713 4/13/11, 4/15/11 2980 1.11� 0.0714 4/15/11, 4/17/11 2984 0.88� 0.0715 4/17/11, 4/19/11 2993 1.25� 0.0816 4/19/11, 4/21/11 2961 0.46� 0.0717 4/21/11, 4/23/11 2980 0.79� 0.0718 4/23/11, 4/25/11 2985 0.77� 0.0719 4/25/11, 4/28/11 4479 0.76� 0.0520 4/28/11, 5/1/11 4413 0.70� 0.0521 5/1/11, 5/4/11 4495 0.93� 0.0722 5/4/11, 5/8/11 2884 1.03� 0.0923 5/8/11, 5/10/11 2980 0.88� 0.0924 5/10/11, 5/13/11 4511 0.62� 0.0625 5/13/11, 5/17/11 5949 1.22� 0.0526 5/17/11, 5/20/11 4467 1.32� 0.0627 5/20/11, 5/24/11 5959 0.35� 0.0428 5/24/11, 5/27/11 4493 0.49� 0.0529 5/27/11, 5/30/11 4138 0.50� 0.0530 5/30/11, 6/3/11 6200 0.74� 0.0431 6/3/11, 6/7/11 5900 0.97� 0.0432 6/7/11, 6/10/11 4415 0.34� 0.0433 6/10/11, 6/14/11 5969 0.73� 0.0434 6/14/11, 6/17/11 4469 0.26� 0.0435 6/17/11, 6/21/11 5887 0.29� 0.0336 6/21/11, 6/24/11 4492 0.55� 0.0437 6/24/11, 6/28/11 5961 0.34� 0.0338 6/28/11, 7/1/11 4451 0.62� 0.04Atmosphere and Ocean Research Institute (AORI), Kashiwa, Chiba Pref.39 3/23/11, 3/25/11 1303 17.08� 0.1040 3/25/11, 3/28/11 2353 0.70� 0.0441 3/28/11, 3/30/11 1597 13.90� 0.1142 3/30/11, 4/1/11 1568 61.40� 0.2043 4/1/11, 4/3/11 1553 0.96� 0.0544 4/3/11, 4/5/11 1586 1.14� 0.0545 4/5/11, 4/7/11 1584 1.65� 0.1346 4/7/11, 4/9/11 1541 1.62� 0.1447 4/9/11, 4/11/11 1584 3.44� 0.1548 4/11/11, 4/13/11 1574 2.58� 0.1549 4/13/11, 4/15/11 1558 1.54� 0.1450 4/15/11, 4/17/11 1539 1.78� 0.1551 4/17/11, 4/19/11 1566 4.11� 0.1752 4/19/11, 4/21/11 1579 2.35� 0.15Tokyo University of Agriculture and Technology, Fuchu, Tokyo53 3/24/11, 3/26/11 2837 0.65� 0.0354 3/26/11, 3/28/11 2885 0.52� 0.0355 3/28/11, 3/30/11 2855 1.22� 0.0456 3/30/11, 4/1/11 2851 1.16� 0.0357 4/1/11, 4/3/11 2939 0.52� 0.0358 4/3/11, 4/5/11 2779 1.00� 0.0859 4/5/11, 4/7/11 2869 0.97� 0.0860 4/7/11, 4/9/11 2868 1.05� 0.0861 4/9/11, 4/11/11 2885 1.42� 0.0862 4/11/11, 4/13/11 2882 0.81� 0.0863 4/13/11, 4/15/11 2743 0.93� 0.0864 4/15/11, 4/20/11 2899 0.87� 0.0765 4/20/11, 4/22/11 2911 0.70� 0.07

(Continues)

PRIYADARSHI ET AL.: 35S MEASUREMENT IN FUKUSHIMA DISASTER

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[6] Based on 35S activity measurements in California and amoving box model, the surface air concentration of 35SO4

2- atFukushima was predicted to be 2� 105 atoms/m3 in the weekfollowing the earthquake [Priyadarshi et al., 2011a]. Thepresent data show a maximum activity of 6.1� 104 atoms/m3 at AORI on 1 April 2011, which is about three times lowerthan the model estimated value. Because the AORI site isnearly 250 km away from the Fukushima nuclear plant, adilution of the radiation plume in the boundary layer and lossof 35SO4

2- aerosol particles by dry and wet deposition duringthe transport are expected to decrease the 35SO4

2- concentrationsignificantly before reaching the AORI site. Morino et al.[2011] used a 3D chemical transport model to simulate thedistribution of radioactive 131I and 137Cs over Japan (bothinland and the surrounding nearby oceanic surface) during10-30 March 2011. The model considered the emission ofradionuclides (131I and 137Cs) at the Fukushima nuclear powerplant, horizontal and vertical advection, diffusion affecting theradiation plume during transport, dry and wet deposition ofgas and aerosols, and radioactive decay to determine thepercentage loss of radioactive 131I and 137Cs over Fukushimaand nearby prefectures. Since 137Cs behaves as an aerosol par-ticle due to its attachment to fine aerosols [Sportisse, 2007], wecompare its distribution with 35SO4

2- aerosols. According toMorino et al. [2011], nearly 15% of 137Cs emitted from theFukushima nuclear power plant were deposited over theFukushima prefecture, whereas on average, 22% of emitted137Cs was deposited over land in Japan during March. Thepercentage loss was greater (8-41%) during a transient cyclonethat passed over Japan during 15-17 March and 19-23 March[Morino et al., 2011].[7] Based on the observed 35SO4

2- concentration peak atAORI on 1 April 2011 (Figure 2) and the model estimation

of Morino et al. [2011], the surface air concentration of35SO4

2- at Fukushima was estimated to be between 1.1� 105

and 2.8� 105 atoms/m3 during March, whereas the predictedconcentration was 2� 105 atoms/m3 [Priyadarshi et al.,2011a]. This difference may be due partially to uncertaintiesrelated to the calculation of depositional rates of radionuclides,because the collection efficiency of dry-deposited aerosols isdifficult to quantify precisely [Morino et al., 2011]. In addition,the model used by Priyadarshi et al. [2011a] to calculate theconcentration of 35SO4

2- (2� 105 atoms/m3) at Fukushimacontains unavoidable uncertainty. The model is particularlysensitive to the dilution rate of the radiation plume during thelong-range transport from Fukushima to La Jolla, California.A 10% change in dilution rate in the model would change themodel output (surface air concentration of 35SO4

2-, i.e.,2� 105 atoms/m3) by 20%.[8] At Fukushima, 35SO4

2- activity measured in fine sulfateaerosols are higher than in the coarse fraction during Augustthrough September 2011 (Figure 3 and Table 3). This isbecause 35S produced at the reactor was oxidized to 35SO2

and subsequently oxidized to 35SO42- in the fine fraction by

gas-phase oxidation of 35SO2. We note that, even after6months, 35S activity was very high in the marine boundarylayer in the Fukushima region, which implies that the reactorcore was still active and releasing neutrons. However, the pres-ence of a viable chlorine source is not known. The neutronsmight be reacting either with residual evaporated salt depositsor with sea water coming in and out across a crack developedin the containment vessel. The reason for the higher 35S activityobserved during September compared with July-August is notyet clear and warrants extended future sampling.[9] The 35SO4

2- concentration in rain water collectedduring March-May 2011 at Tokyo Tech Yokohama varies

Table 1. (continued)

Sample ID

Collection Time (m/d/y)

Air Volume (m3) 35SO42- Atoms per m3 (

a

103)Starting Date End Date

66 4/22/11, 4/24/11 2857 0.97� 0.1067 4/24/11, 4/26/11 2857 1.26� 0.1068 4/26/11, 4/28/11 2849 1.05� 0.1069 4/28/11, 5/13/11 2825 0.78� 0.09National Institute of Environmental Studies (NIES), Tsukuba, Ibaraki Pref.70 3/24/ 11, 3/29/11 7065 5.02� 0.0371 3/29/11, 4/1/11 4230 18.05� 0.1472 4/1/11, 4/4/11 4224 4.39� 0.0873 4/4/11, 4/7/11 4284 1.41� 0.0674 4/7/11, 4/11/11 5686 1.94� 0.05Hokkaido Research Organization, Sapporo75 3/25/11, 3/28/11 3953 1.26� 0.1076 3/28/11, 3/30/11 2933 1.20� 0.0877 3/30/11, 4/1/11 2933 0.96� 0.0878 4/1/11, 4/3/11 2918 0.78� 0.0779 4/3/11, 4/5/11 2919 0.76� 0.0780 4/5/11, 4/7/11 2857 0.95� 0.0881 4/7/11, 4/9/11 2919 1.08� 0.0882 4/9/11, 4/11/11 2923 1.05� 0.0883 4/11/11, 4/13/11 2922 0.96� 0.0884 4/13/11, 4/15/11 2924 1.04� 0.0785 4/15/11, 4/17/11 2929 2.63� 0.1086 4/17/11, 4/19/11 2930 0.56� 0.0787 4/19/11, 4/21/11 2932 0.83� 0.1088 4/21/11, 4/23/11 2931 0.75� 0.1089 4/23/11, 4/25/11 2937 0.62� 0.09

a

The standard error associated with individual measurement.

PRIYADARSHI ET AL.: 35S MEASUREMENT IN FUKUSHIMA DISASTER

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Table 2. 35S Measurement in Aerosol Particles of different Size Fraction Collected From Fukushima Prefecture, ~100 km From theFukushima Nuclear Power Plant

a

Sample ID Collection Time (m/d/y) Start Time End Time Air volume (m3) 35SO42- atoms/m3 (�103)

Cedar forest, Kawamata-cho, Fukushima prefecture0718-Q1 7/9/11, 7/18/11 6616 0.34� 0.040718-Q2 0.65� 0.050718-Q3 0.52� 0.040718-Q4 1.41� 0.060718-Q5 1.34� 0.060718-Q6 30.41� 0.180718-Q7 57.53� 0.240725-Q1 7/18/11, 7/25/11 5783 0.20� 0.050725-Q2 0.15� 0.050725-Q3 0.13� 0.050725-Q4 0.56� 0.060725-Q5 8.12� 0.120725-Q6 10.31� 0.140725-Q7 6.02� 0.110801-Q1 7/25/11, 8/1/11 5746 0.27� 0.050801-Q2 0.43� 0.050801-Q3 0.46� 0.050801-Q4 2.06� 0.080801-Q5 14.56� 0.150801-Q6 17.97� 0.170801-Q7 50.38� 0.260815-Q1 8/8/11, 8/15/11 5870 0.20� 0.050815-Q2 0.48� 0.0500815-Q3 0.50� 0.050815-Q4 4.55� 0.100815-Q5 28.39� 0.200815-Q6 56.0 � 0.270815-Q7 34.12� 0.220824-Q1 8/15/11, 8/24/11 7444 0.16� 0.040824-Q2 0.19� 0.040824-Q3 0.25� 0.040824-Q4 1.64� 0.060824-Q5 10.72� 0.120824-Q6 0.52� 0.050824-Q7 28.03� 0.190830-Q1 8/24/11, 8/30/11 4975 0.21� 0.050830-Q2 0.18� 0.050830-Q3 0.22� 0.050830-Q4 1.60� 0.080830-Q5 11.95� 0.150830-Q6 16.74� 0.170830-Q7 14.75� 0.160916-Q1 8/30/11, 9/16/11 12,762 0.05� 0.020916-Q2 0.04� 0.020916-Q3 0.05� 0.020916-Q4 0.43� 0.030916-Q5 4.59� 0.060916-Q6 8.58� 0.080916-Q7 8.14� 0.07School ground, Kawamata-cho, Fukushima prefecture0715-Q1 7/9/11, 7/13/11 3105 0.63� 0.090715-Q2 0.48� 0.090715-Q3 0.77� 0.100715-Q4 3.96� 0.140715-Q5 7.24� 0.170715-Q6 20.74� 0.260715-Q7 62.32� 0.41Agriculture field, Kawamata-cho, Fukushima prefecture0722-Q1 7/9/11, 7/22/11 9909 0.12� 0.030722-Q2 0.15� 0.030722-Q3 0.21� 0.030722-Q4 0.68� 0.030722-Q5 16.52� 0.130722-Q6 1.14� 0.040722-Q7 50.64� 0.200819-Q1 8/17/11, 8/19/11 1460 0.56� 0.160819-Q2 0.44� 0.160819-Q3 0.41� 0.16

(Continues)

PRIYADARSHI ET AL.: 35S MEASUREMENT IN FUKUSHIMA DISASTER

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from 1.1� 105 to 9.8� 105 atoms/liter, whereas stream wa-ter collected near Fukushima was found to have 1.2� 105

atoms/liter during April (Table 3). The concentration in rainwater is nearly 10 times higher than the 35SO4

2- concentration(6.2� 104 atoms/liter) contained in rain water collected atLa Jolla, California, during January 2010. Atmosphericprocesses such as rainout or washout cleanse the atmosphereby removing aerosol particles and gases. Model simulationdealing with long-range transport of dust and aerosols anddeposition on the surface/ocean depends sensitively on theadapted aerosol-scavenging coefficient. We utilized 35Smeasurements in aerosols and rainwater to calculate theaerosol-scavenging coefficient. The scavenging coefficient(k) is defined as [Okita et al., 1996]:

k ¼ Cw=Cað Þ�106h i� P=hð Þ

[10] where Cw and Ca are the35S concentration in rain water

(mg/liter) and air (mg/m3), respectively; P is the precipitationrate (cm/s); and h is the height of the cloud top (cm). We donot have measurements of the rate of precipitation of each indi-vidual event. The average precipitation rate (varying between1.4 and 2.6mm/h) was taken from the Japan MeteorologicalAgency and is given in Table 3. The height of the cloud topwas assumed to be 2km based on the fact that rainfall rate ismore significant below the melting height (2-3 km), wherethe main wet removal occurs in cloud [Mittermaier andIllingworth, 2003]. After this height, the precipitation ratedecreases and becomes less intense [Scott, 1982]. A recentmeasurement also shows that the mean cloud top height inthe Hawaiian region is 2.1 km [Zhang et al., 2012]. For our

Table 2. (continued)

Sample ID Collection Time (m/d/y) Start Time End Time Air volume (m3) 35SO42- atoms/m3 (�103)

0819-Q4 1.35� 0.180819-Q5 7.74� 0.270819-Q6 12.39� 0.310819-Q7 8.84� 0.280908-Q1 8/27/11, 9/7/11 1460 0.56� 0.170908-Q2 0.50� 0.170908-Q3 0.49� 0.180908-Q4 1.21� 0.200908-Q5 17.02� 0.380908-Q6 1.17� 0.190908-Q7 99.98� 0.77

a

Higher 35S activity is associated with fine sulfate particles, which are produced mainly from gas-phase oxidation of 35SO2. Q1, Q2, Q3, Q4, Q5, Q6, andQ7 represent aerosol size fraction of >10.2 mm, 4.2-10.2 mm, 2.1-4.2 mm, 1.3-2.1 mm, 0.69-1.3 mm, 0.39-0.69 mm, and <0.39 mm, respectively.

Figure 4. HYSPLIT-3days air masses back-trajectories werecalculated over the AORI sampling site at three different altitudes(10, 50, and 1000m). The surface air masses arrive from north-western regions of Japan near Fukushima and are mainly respon-sible for the spike in 35SO4

2 activity observed on 1 April 2011.

Table 3. 35S measured in rain water samples collected in tokyo and Fukushima during March-May 2011 shows that 35S/liter of rain waterwas 10 times higher than that in background rain water, as observed at La Jolla, (California) rain

a

Rain Samples Sampling date Average Rainfall Rate (mm/h) 35SO42- (Atoms/Liter) Ca

b

(Atoms/m3) K (s-1)

Tokyo Tech #1 3/24/11 1.4 9.8E+ 05 -- --#2 4/25/11 2 2.1E+ 05 770 7.6

b

10�3

#3 5/11/11 2 1.9E+ 05 616 8.7b

10�3

#4 5/13/11 2.6 1.1E+ 05 616 6.5b

10�3

#5 5/24/11 1.7 2.2E+ 05 344 1.5b

10�2

Fukushima stream waterc 4/28/11 1.2E+ 05La Jolla rain 1/26/10 6.2E+ 04

a

The rain-scavenging coefficient (k) was calculated based on the 35SO42� concentration measured in rainwater and air.

b

The concentration of 35SO42- in air collected at Tokyo Tech, Yokohama.

c

Water sample collected from a stream situated in Fukushima prefecture, 50 km away from Fukushima nuclear power plant.

PRIYADARSHI ET AL.: 35S MEASUREMENT IN FUKUSHIMA DISASTER

1026

sample, the value of k varies from 6.5� 10-3/s to 1.5� 10-2/s,which agrees with other measurements [Andronache, 2004;Chate et al., 2011; Laakso et al., 2003; Maria and Russell,2005; Okita et al., 1996; Schumann, 1989]. The calculatedvalues from Andronache [2004] vary from 2.1� 10-3/s to4.3� 10-4/s, for corresponding precipitation rates of 1.5mm/hand 4.7mm/h.[11] It has been demonstrated that 35S is a unique tracer in

understanding air mass mixing and quantifying aerosol dryand wet deposition [Cho et al., 2011; Priyadarshi et al.,2011b, 2012; Tanaka and Turekian, 1991; Turekian andTanaka, 1992]. Approximately 2� 1011 atoms of 35S wereproduced from Fukushima within the first 6months followingthe earthquake, which is lower by a factor of 1011 comparedwith the total global production of 2� 1022 atoms of 35SO4

2-

by cosmic rays, of which 5� 1020 atoms of 35S are containedin boundary layer itself. However, because Fukushima is a verylocalized point source, the released 35SO4

2- was detected severalthousand miles away from the source.[12] The presence of excess 35SO4

2- (two orders of magni-tude higher than the natural background) provides a uniqueopportunity to understand the chemical transformation, theair mass transport, and potentially the fate of 35SO4

2- in soilon short time scales. A yearlong sampling of aerosol, soil,and rain water collected at several sampling sites in Japanfor 35S analysis along with a regional transport modelwill further resolve and quantify the distribution rate ofregional radiogenic (and by proxy stable) sulfur in thenatural environment.

4. Conclusions

[13] We report the first measurement of radioactive 35S insulfate aerosol collected at six different sampling sites close tothe Fukushima nuclear power plant during March-September2011. A very high 35SO4

2- activity was observed at Fukushima,AORI, NIES, Tsukuba, and Tokyo Tech Yokohama that isnearly 100 times higher than the natural background 35S. Basedon 35SO4

2- concentrations measured at AORI in April, thesurface air concentration of 35S was estimated to be 2.8� 105

atoms/m3 at Fukushima during March. Even after 6months,35S activity was very high in the marine boundary layer in theFukushima region, which implies that the reactor core was stillactive. The reason for higher 35SO4

2- during September com-pared with July-August is probably the periodic use of seawateras a coolant. 35SO4

2- concentration in rainwater collected atTokyo Tech Yokohama varies from 1.1� 105 to 9.8� 105

atoms/liter. The 35SO42- concentration measured in air and

rainwater was employed to determine the rain-scavengingcoefficient, which varies from 6.5� 10-3 to 1.5� 10-2 s-1.

[14] Acknowledgments. We thank the reviewers of the manuscriptfor their insightful comments that improved this manuscript. We also thankK. Kita for aerosol sampling at Kawamata town. This work was supportedby KAKENHI (23224013 and 24110003) of the Ministry of Education,Culture, Sports, Science and Technology, Japan.

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