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Journal of Environmental Radioactivity 101 (2010) 106–112

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Journal of Environmental Radioactivity

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Depositional behaviors of plutonium and thorium isotopes at Tsukuba andMt. Haruna in Japan indicate the sources of atmospheric dust

K. Hirose*,1, Y. Igarashi, M. Aoyama, Y. InomataMeteorological Research Institute, Geochemical Research Department, Nagamine 1-1, Tsukuba, Ibaraki 305-0052, Japan

a r t i c l e i n f o

Article history:Received 1 April 2009Received in revised form2 September 2009Accepted 2 September 2009Available online 4 October 2009

Keywords:Soil contaminantsPlutoniumThorium isotopesDepositionRe-suspensionDustMassic activity

* Corresponding author.E-mail address: hirose45037@mail2.accsnet.ne.jp

1 Present address: Sofia University, Faculty of Scienicho, Chiyoda-ku, Tokyo 102-8554, Japan

0265-931X/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.jenvrad.2009.09.003

a b s t r a c t

Monthly plutonium and thorium depositions at Tsukuba (28 m asl) and Mt. Haruna (1370 m asl) weremeasured during 2006 and 2007 (Jan 2006–Dec 2007 at Tsukuba, Nov 2006–Dec 2007 at Mt. Haruna).The monthly 239,240Pu depositions ranged from 0.044 to 2.67 mBq m�2 at Tsukuba and from 0.05 to0.9 mBq m�2 at Mt. Haruna during the measurement periods. Monthly 239,240Pu deposition did not differmarkedly between the two sites except in April 2007. Seasonal pattern of monthly 239,240Pu depositionsat both sites showed high in spring and low in summer, and typical of seasonal variations in northeasternAsia. Thorium deposition at Tsukuba was higher than that at Mt. Haruna except in May and June 2007.230Th/232Th activity ratios were used to partition deposition samples into locally and remotely derivedfractions. The results revealed that a major proportion of total 239,240Pu and Th deposits are derived fromremote sources, especially in spring.

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1. Introduction

Dust storms occur every spring in the arid and desert regions ofEast Asia. The long-range transport of dust, which is referred tolocally as ‘Kosa’, is of concern because of its possible effects onhuman health (Kwon et al., 2002). The Kosa contains minoramounts of toxic contaminants, including anthropogenic radionu-clides such as plutonium, 137Cs and 90Sr (Hirose et al., 2003; Igarashiet al.,1996, 2001, 2003, 2005), toxic trace metals (Feng et al., 2008),and microorganisms (Ichinose et al., 2008). The chemicals in thedust particles and the increasing load of mineral dust together forma major environmental concern in East Asia.

The Meteorological Research Institute (MRI), Japan Meteoro-logical Agency, has measured monthly atmospheric deposition ofplutonium at Tokyo (1957–1980) and Tsukuba (1980–2007) inJapan since 1957 (Hirose et al., 2001, 2003; Katsuragi et al., 1982;Miyake et al., 1968). Plutonium deposited on the Earth’s surface hasoriginated mainly from atmospheric nuclear explosions (Harley,1980; UNSCEAR, 2000). The annual deposition of 239,240Pu duringthe period 1957 to 1984 was determined by stratospheric fallout

(K. Hirose).ce and Technology, 7-1 Kio-

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from atmospheric nuclear tests (Hirose et al., 1987). The maximumannual deposition of 239,240Pu (a typical value: 7.41 Bq m�2 y�1 atTokyo) was observed in 1963 after the 1961–1962 peak in atmo-spheric nuclear testing conducted by the USA and former USSR(Harley, 1980; Hirose et al., 1987). However, the fallout rate declinedafter 1963 and, in the period 1985 to 2005, the annual 239,240Pudeposition at Tsukuba ranged from 1.7 to 7.8 mBq m�2 y�1 and didnot display systematic inter-annual variation (Hirose et al., 2001,2003, 2004a, 2007). The 239,240Pu deposition observed recently inJapan appears to be derived from the re-suspension of anthropo-genic plutonium previously deposited on the land surface.Re-suspended plutonium and other long-lived fission products inatmospheric dust have sources in remote regions, including desertand arid areas of continental East Asia as well as local areas (Hiroseet al., 2003; Igarashi et al., 2001). In the decade since 2000, higher239,240Pu depositions in spring as did a change of 137Cs/90Sr ratio indeposition (Igarashi et al., 2005) suggest a change in dust-sourceregions, which may cause a change of trace constituents in dustparticles. Those findings suggest that plutonium can be used asa proxy for changes in the chemical constituents of atmosphericdust caused by environmental changes. Although tracing pluto-nium deposition can provide a better understanding of environ-mental change in the East Asian region, we know little about thesources of recent fallout of 239,240Pu in other seasons.

K. Hirose et al. / Journal of Environmental Radioactivity 101 (2010) 106–112 107

To elucidate sources of deposited 239,240Pu, including thecontribution of suspended dust from local sources, we constructeda new sampling station at Mt. Haruna in central Honshu, Japan, andmeasured deposition of 239,240Pu and thorium isotopes. The newstation is 150 km from the long-term station at Tsukuba (Fig. 1) and,at 1370 m above sea level (asl), is within the free troposphere (>1000 m) and is not affected by contaminated surface air of theKanto Plain (Kimura, 1985), whereas the station at Tsukuba, at 28 masl, is affected by surface processes. On the other hand, aerosoldeposition measurements over mountain terrain are complicatedby the meteorological conditions characteristic of high elevations(Begeron, 1965). High rainfall at high elevation sites causes depo-sition fluxes to be higher than those at lowland sites (Likuku, 2006).Therefore, the comparison of radioactive deposits between bothstations provides new information on surface processes on theKanto Plain.

Thorium isotopes, which are typical lithogenic radionuclides(Hirose, 2000), were determined in the deposition samples at bothsites to provide better insights into the sources of 239,240Pu (Hiroseet al., 2004b). Although most of the thorium isotopes in atmo-spheric dust are derived from soil particles, significant variation inthorium isotope ratios (230Th/232Th) can exist in the atmosphericdust (Hirose, 2000), which may reflect different sources. Therefore,the thorium isotope ratio is a useful indicator of the source ofmineral dusts.

In this paper, we analyze monthly 239,240Pu and thorium depo-sition at Tsukuba and Mt. Haruna in the period of 2006–2007, anddiscuss the depositional behavior of 239,240Pu and thorium isotopesat both sites with respect to local and remote sources of soil-derived dust.

2. Methods

Monthly deposition samples were collected at the Tsukuba campus of the MRI(36�030N, 140�080E; 28 m asl) from January 2006 to December 2007 and at Mt.Haruna (36�280N, 138�520E; about 1370 m asl) from November 2006 to December2007 using plastic open surface collectors with surface areas of 4 m2 and 1 m2,respectively. The locations of the two sampling stations are shown in Fig. 1. At theMRI, the collector (4 m2) was installed on the roof of a small building at a height ofabout 5 m above the ground. At Mt. Haruna, four collectors (1 m2) were affixed to

Fig. 1. Locations of Tsukuba and Mt. Haruna sampling stations.

a steel frame at a height of about 1 m above the ground near the summit of themountain. The sampling site at Mt. Haruna is considered at higher elevation than theplanetary boundary layer. The effect of local dust blown from nearby agriculturalfields would therefore be negligible.

Monthly total (wet and dry) depositions, which corresponded to rainwatervolumes of (100 to 1800 L), were dried in an evaporation dish or in a glass flask bymeans of a rotary vacuum evaporator (Eyela NE-12). The resultant residues wereweighed after drying in an oven at 110 �C and then transferred to a plastic container.Concentrations of 137Cs in the residue samples were determined by g-ray spec-trometry (a photo-peak of 661 kev) with a Ge detector. After g-ray measurements,the residue samples were digested with nitric acid on a hot plate to eliminate mostof the organic matter. The solution obtained was subjected to radiochemical sepa-ration to determine 239,240Pu and thorium isotopes.

The activity of 239,240Pu in deposition samples was determined by a-ray spec-trometry after radiochemical separation and purification by means of anionexchange resin. Chemical recoveries of plutonium were corrected using a 242Pu yieldtracer. The precision, accuracy, and uncertainty of plutonium analysis in depositionsamples have been described in detail by Otsuji-Hatori et al. (1996). Quality assur-ance of plutonium isotope determination was achieved by analyzing a referencematerial composed of fallout samples, which was also described by Otsuji-Hatoriet al. (1996). Thorium isotopes in the monthly deposition samples were determinedby a-spectrometry after chemical separation and purification with anion exchangeresin (Hirose, 2000). The Thorium yield obtained from the separation procedureused was more than 90%, as determined from a reference sample. Counting uncer-tainties of thorium in deposition samples were less than 5%. Repeated measure-ments (8 samples) of the standard fallout material provided a standard error of 4%.Therefore, the total uncertainties for Thorium analysis were estimated to be lessthan 7%.

3. Results and discussion

3.1. Plutonium deposition

The monthly 239,240Pu deposition at Tsukuba ranged from0.044� 0.009 to 2.46� 0.09 mBq m�2 in 2006 and from0.044� 0.008 to 2.67� 0.10 mBq m�2 in 2007 (Table 1). Themonthly 239,240Pu deposition at Mt. Haruna during Nov 2006–Dec2007 ranged from 0.05� 0.01to 0.90� 0.05 mBq m�2 (Table 1). Theannual 239,240Pu depositions in 2006 and 2007 at Tsukuba were6.8� 0.2 and 5.3� 0.1 mBq m�2, respectively. The annual 239,240Pudeposition in 2007 at Mt. Haruna was 3.4� 0.1 mBq m�2. Theannual 239,240Pu depositions in 2006 and 2007 at Tsukuba and Mt.Haruna were within the range measured at Tsukuba during 1985–2005 (Hirose et al., 2003, 2004a, 2007) and at Neuherberg(Munich), Germany (2.1 to 6.4 mBq m�2) during 1987–1998 (Rosnerand Winkler, 2001). Although the annual 239,240Pu deposition at Mt.Haruna in 2007 was lower than that at Tsukuba, the difference wasattributed to the difference between the two sites in April 2007;there was no significant difference in 239,240Pu deposition betweenTsukuba and Haruna in any month other than April 2007.

Aerosol deposition is generally higher on mountain summitsthan at lowland sites because precipitation is greater at higherelevations (Likuku, 2006). Annual precipitation at Mt. Haruna(2222 mm in 2006 and 1936 mm in 2007) exceeded that at Tsukuba(1616.5 mm in 2006 and 1137.5 in 2007). However, precipitationduring the dust season (January–May) at Mt. Haruna was the sameor less than at Tsukuba (JMA, 2009), and, consequently, no markedtopographic effect on 239,240Pu deposition was observed.

Temporal variation in the monthly 239,240Pu depositions atTsukuba and Mt. Haruna during 2006–2007 is shown in Fig. 2. Themonthly 239,240Pu depositions at Tsukuba and Mt. Haruna displayedmarked seasonal differences, peaking in spring and being low insummer and fall, which is consistent with previous measurementsin Japan, at Tsukuba and Nagasaki (Hirose et al., 2003), and in Korea(Hirose et al., 2004a). The maximum monthly 239,240Pu depositionat Tsukuba during the period of 2006–2007 occurred in April 2007.The second highest 239,240Pu deposition occurred in April 2006,corresponding with an extraordinary dust event that occurred innorthern China and extended to Japan (Papayannis et al., 2007). Theseasonal variation in 239,240Pu deposition at Tsukuba was similar to

Table 1Monthly depositions of 239,240Pu and 232Th and 230Th/232Th activity ratios in deposition samples observed in Tsukuba and Mt. Haruna.

Date Tsukuba Mt. Haruna

239,240Pu mBq m�2 232Th mBq m�2 230Th/232Th 239,240Pu mBq m�2 232Th mBq m�2 230Th/232Th

Jan. 2006 0.51� 0.04 33.2� 1.0 1.01� 0.03Feb. 0.61� 0.05 33.6� 0.8 1.28� 0.03Mar. 1.76� 0.07 63.1� 1.4 1.36� 0.03Apr. 2.46� 0.09 68.8� 1.4 1.13� 0.02May 0.76� 0.04 32.5� 1.0 0.85� 0.03June 0.24� 0.02 14.7� 0.5 0.83� 0.03July 0.19� 0.02 24.8� 0.6 0.96� 0.03Aug. 0.044� 0.009 12.1� 0.3 0.96� 0.04Sept. 0.048� 0.009 13.7� 0.4 0.89� 0.04Oct. 0.044� 0.010 15.5� 0.5 0.75� 0.03Nov. 0.12� 0.01 19.0� 0.4 0.94� 0.03 0.089� 0.016 5.49� 0.2 0.75� 0.03Dec. 0.053� 0.010 10.5� 0.3 0.72� 0.03 0.097� 0.016 2.67� 0.14 0.97� 0.05Jan. 2007 0.30� 0.01 15.4� 0.5 0.71� 0.04 0.22� 0.03 14.0� 0.4 0.79� 0.03Feb. 0.25� 0.03 22.0� 0.5 0.99� 0.04 0.20� 0.02 7.74� 0.4 1.01� 0.07Mar. 0.82� 0.06 36.2� 0.6 1.34� 0.03 0.74� 0.06 24.9� 0.9 0.89� 0.03Apr. 2.67� 0.10 49.3� 0.7 0.98� 0.02 0.71� 0.05 20.7� 0.4 0.99� 0.03May 0.52� 0.04 28.7� 0.4 0.93� 0.02 0.90� 0.05 32.4� 0.6 0.96� 0.02June 0.19� 0.02 14.5� 0.3 1.04� 0.03 0.15� 0.02 19.1� 0.4 1.03� 0.03July 0.12� 0.01 17.2� 0.7 0.93� 0.05 0.12� 0.01 5.4� 0.1 1.03� 0.05Aug. 0.05� 0.01 14.4� 0.5 1.04� 0.05 0.07� 0.01 8.9� 0.2 0.97� 0.03Sept. 0.07� 0.01 14.7� 0.2 1.16� 0.05 0.05� 0.01 4.7� 0.1 0.97� 0.04Oct. 0.044� 0.008 9.4� 0.1 0.89� 0.02 0.09� 0.01 7.1� 0.1 0.89� 0.03Nov. 0.09� 0.01 15.4� 0.3 0.78� 0.00 0.07� 0.01 3.7� 0.08 0.85� 0.03Dec. 0.19� 0.02 17.7� 0.2 0.77� 0.01 0.08� 0.01 3.7� 0.1 0.83� 0.03

Uncertainty is a counting error of 1 sigma.

K. Hirose et al. / Journal of Environmental Radioactivity 101 (2010) 106–112108

that at Nagasaki, Japan, and Daejeon, Korea (Hirose et al., 2004a),although the increases in 239,240Pu depositions at Nagasaki andDaejeon in spring were more pronounced. In June, which corre-sponds to the start of the rainy season in East Asia, monthly239,240Pu depositions at Tsukuba and Mt. Haruna declined toapproximately the same levels as those observed in fall and winter(Hirose et al., 2003).

Monthly 239,240Pu depositions at Mt. Haruna were similar tothose at Tsukuba, except in April 2007, when pronounced Kosaevents at Tsukuba resulted in higher 239,240Pu deposition (Igarashiet al., 2009). A corresponding peak in 239,240Pu deposition was notobserved at Mt. Haruna. Rainfall events that coincide with dustevents usually cause major depositions of anthropogenic radioac-tivity (Igarashi et al., 2009). The rainfall in April 2007 at Mt. Haruna(71 mm) was considerably lower than that at Tsukuba (111 mm).Most notably, during a large Kosa event on the Kanto Plain during

Fig. 2. Temporal variations in the 239,240Pu deposition observed in Tsukuba (opencircles) and Mt. Haruna (solid circles).

2–3 April 2007 (Igarashi et al., 2009), only 3 mm of precipitationwas received at Mt. Haruna, whereas 19.5 mm was recorded atTsukuba (JMA, 2009). Differences in rainfall represent one expla-nation for the lower 239,240Pu deposition at Mt. Haruna in April2007. Another explanation may be that air mass including Kosapreferentially moves on lowland because air stream is affected bylocal topography near Mt. Haruna (Fig. 1).

Kosa events typically occur in Japan in the period from March toMay. The Kosa particles are important carriers of artificial radio-nuclides. The Kosa events are a recognized meteorologicalphenomenon whereby large-scale dust storms over the East Asiancontinent develop within intense low-pressure systems trailingcold fronts. Modeling and analyses of event trajectories (Hiroseet al., 2004a, b; Lee et al., 2006) suggest that a major air masscontaining soil-derived dust forms in the East Asian desert and aridareas, and then flows through northeastern China and the KoreanPeninsula and finally to Japan. The frequency of Kosa eventsdisplays inter-annual variability; there were many events in 2000–2002 and 2006 and few in 1997–1999 and 2003. Inter-annualvariation in 239,240Pu deposition at Tsukuba has followed a similartrend, high in 2000–2002 and low in 1999 and 2003 (Hirose et al.,2003, 2007). In the current study, the relatively high annual239,240Pu deposition at Tsukuba in 2006 was similar to that in 2000–2002. It appears that the inter-annual variation in 239,240Pu depo-sition at Tsukuba is closely related to the frequency of Kosa events.

Massic activities of 239,240Pu in deposition samples, which aredefined as 239,240Pu contents in residue of monthly deposition,should be a function of the resuspension (Hirose et al., 2003).Residual matter consists of surface soil, fly ash, sea salt, and otheraerosols. When new 239,240Pu is introduced into the atmosphere byaccidents at nuclear power plants and atmospheric nuclear explo-sions, the 239,240Pu massic activity in deposition should be largerthan the 239,240Pu concentrations in surface soils. The reverseapplies when the main source of 239,240Pu is sea salt, which isestimated to contain less than 0.0003 mBq g�1 of 239,240Pu (Hiroseet al., 2003), much less than that of soils. Temporal variations in the239,240Pu massic activities at Tsukuba and Mt. Haruna are shown inFig. 3. The 239,240Pu massic activity during 2006–2007 at Tsukuba

Fig. 3. Temporal variation in 239,240Pu massic activity at Tsukuba (open circles) andMt. Haruna (solid circles).

Fig. 4. Temporal variation in 232Th deposition at Tsukuba (open circles) and Mt. Har-una (solid circles).

K. Hirose et al. / Journal of Environmental Radioactivity 101 (2010) 106–112 109

and during Nov 2006–Dec 2007 at Mt. Haruna ranged from 0.036 to0.59 mBq g�1 and from 0.027 to 0.56 mBq g–1, respectively, whichare the same order of magnitude as observed and estimatedconcentrations in Japanese surface soils (0.02 to 0.4 mBq g–1; Hir-ose et al., 2003). The 239,240Pu massic activities exhibited markedseasonal changes, from high in the dust season (March–May) to lowin the non-dust season, which has been the typical pattern over thepast decade (Hirose et al., 2003). The peak 239,240Pu massic activityoccurred one month earlier at Mt. Haruna than at Tsukuba.Although the precise causes of the difference in timing areunknown, complicated local meteorological conditions such asrainfall and air streams carrying the dust are the main determi-nants of 239,240Pu massic activity.

Massic activities of other anthropogenic radionuclides, espe-cially 137Cs and 90Sr, declined during the period from 1990 to 2007(Hirose et al., 2003, 2007, 2008). However, 239,240Pu massic activitiesshowed no such decline. To the contrary, 239,240Pu massic activitiesexceeded those of local soils in the dust seasons of 2000, 2001, 2002,2006 (Hirose et al., 2007), and 2007 (this work). Zheng et al. (2009)measured 239,240Pu concentration in the surface soils of north-western China (Kumtag Desert) at 0.021–0.23 mBq g–1. The KumtagDesert is therefore not the major contributor to 239,240Pu massicactivities in the deposits at Tsukuba. We recently measured 239,240Puconcentrations in surface soil in Mongolia; the higher 239,240Puconcentrations (0.7–3.5 mBq g–1; unpublished data) are moreconsistent with the higher 239,240Pu massic activities in Tsukuba.Recent studies (Lim and Chun, 2006; Kurosaki and Mikami, 2003)have revealed that during the 1990s the dominant sources of Asiandust changed from the Chinese deserts, such as the Taklimakandesert, to more eastern regions of the North China Plain, north-eastern China, and the Korean Peninsula. This suggests that thehigher 239,240Pu massic activities in the 2000s are from new sourcesof Kosa (Igarashi et al., 2006, 2009) reflecting recent desertificationin Mongolia, the North China Plain, northeastern China, and theKorean Peninsula as a result of over-cropping or increased droughtassociated with climate change. Such new types of Kosa could bearpollutants that had been deposited in the surface soil includingagricultural chemicals (Cai et al., 2008; Wang et al., 2001).

3.2. Thorium deposition

The monthly 232Th deposition at Tsukuba in 2006 and 2007ranged from 10.5� 0.3 to 68.8� 1.4 mBq m–2 and from 9.4� 0.1 to

49.3� 0.7 mBq m�2, respectively, whereas at Mt. Haruna, it rangedfrom 2.7� 0.1 to 32.4� 0.6 mBq m�2 (Table 1). The annual 232Thdepositions in 2006 and 2007 at Tsukuba were 341�3 and255� 2 mBq m�2, respectively, which are in the same order ofmagnitude as in 2001–2003 (Hirose et al., 2007). The annual 232Thdeposition in 2007 at Mt. Haruna was 152�1 mBq m�2. Thedeposition of 232Th was markedly lower at Mt. Haruna than atTsukuba, in contrast to the mostly similar rates of deposition of239,240Pu at the two sites.

Monthly 232Th depositions at Tsukuba showed marked seasonaldifferences, with high values in spring (Fig. 4). This was the same asfor 239,240Pu, although it is a typical feature of anthropogenicradionuclide deposition in Japan (Hirose et al., 2004b, 2007). Thehighest monthly 232Th depositions at Tsukuba in this periodoccurred in March and April 2006, which were similar to the valuesin 2000–2002, which corresponded with high dust events period.The higher 232Th deposition in spring implies a greater soil dustload in the air because 232Th is essentially of terrestrial origin.Hirose et al. (2007) showed that the inter-annual differences in232Th deposition in spring appear to correspond to the frequency ofKosa events, although the annual sum of the 232Th depositionshowed no marked inter-annual differences; in fact, the annual232Th deposition in 2001 (more Kosa events) was the same as thatin 2003 (fewer Kosa events). Hirose et al. (2007) also reasoned thatthe source of 239,240Pu in the deposition samples differed from thatof 232Th because the peak in monthly 232Th deposition does notalways coincide with that of 239,240Pu deposition, even though bothare supported by atmospheric dust particles derived from soil.Monthly 232Th deposition at Mt. Haruna was generally lower thanthat at Tsukuba, except in May and June 2007, and was less than halfof that at Tsukuba in Nov 2006, Dec 2006, Jan 2007 and Mar 2007.This suggests that a considerable proportion of the atmosphericdust at Tsukuba consists of suspended soil particles derived fromlocal farmland, a source that would be less pronounced at Mt.Haruna.

The massic activity of 232Th in the monthly deposition sampleswas calculated from the weight of the corresponding monthlyresidue (Fig. 5). The 232Th massic activity during 2006–2007 atTsukuba ranged from 7.6 to 21.7 mBq g�1 and exhibited no clearseasonal variation, whereas at Mt. Haruna, larger variation (1.7 to19 mBq g�1) was observed, possibly suggesting lower values insummer and fall; however, only one year of data is not conclusive of

Fig. 5. Temporal variation in 232Th massic activity at Tsukuba (open circles) andMt. Haruna (solid circles).

Fig. 6. Fractions of 232Th deposition from remote and local sources observed atTsukuba and Mt. Haruna. Remote fraction at Tsukuba, open circles; local fraction atTsukuba, open diamonds; remote fraction at Mt. Haruna, solid circles; local fractionMt. Haruna, solid squares.

K. Hirose et al. / Journal of Environmental Radioactivity 101 (2010) 106–112110

a clear seasonal pattern. The 232Th concentrations in cultivated soilsat Tsukuba were in the range 4.4 to 28.6 mBq g�1 with an average of13.9 mBq g�1 (9 samples; authors’ unpublished data). The 232Thmassic activities in deposition samples were in the same order ofmagnitude as the concentrations in local soil. This suggests thatmineral dust originating from soil represents a significant propor-tion of the deposition residue at Tsukuba, although other possibleconstituents of deposition include fly ash, sulfate, sea salt, and otheraerosols. At Mt. Haruna, the peak 232Th massic activity occurred inMarch 2007 and was considerably higher than that at Tsukuba. Thisvalue may reflect the thorium content of mineral dusts originatingfrom Kosa in the free troposphere. The lower massic activities of232Th in summer and fall at Mt. Haruna comparing with that atTsukuba suggest that materials other than mineral dust maycontribute to the deposited residue.

The 230Th/232Th activity ratios in deposition samples were 0.72to 1.36 in 2006 and 0.71 to 1.34 in 2007 at Tsukuba and 0.75 to 1.03at Mt. Haruna. High 230Th/232Th activity ratios occurred at Tsukubain early spring (Feb and Mar 2006 and Mar 2007), whereas therewere no marked peaks in the 230Th/232Th activity ratios at Mt.Haruna. Although there is no information on the 230Th/232Thactivity ratios in Kosa, we can estimate them from the uranium/thorium activity ratios because 230Th is a daughter isotope in thedecomposition of 238U. Feng et al. (2008) measured the chemicalcomposition of heavy dust falls observed in Beijing in April 2006;the U/Th ratio in the dust was 0.45, very different from the230Th/232Th ratios in the deposition in Japan. The 230Th/232Th ratiosin surface soils (<53 mm particle size) around Tsukuba are relativelyhigh (2.1–2.6 from 9 samples in cultivated fields near the MRI;Hirose et al., 2007), which may be attributed to traces of uranium infertilizer. In contrast, the 230Th/232Th ratios of Chinese soils(<53 mm particle size) have lower values (0.47–0.65; 7 samplesfrom the Taklimakan desert; Hirose et al., 2007), which reflect theabsence of fertilizers. The 230Th/232Th activity ratios in depositionsamples from Tsukuba, which were systematically higher thanthose from Nagasaki (Hirose et al., 2007), were approximatelymidway between the 230Th/232Th activity ratios of soils from Tsu-kuba and the Taklamakan desert. This reveals that the dust withhigh 230Th/232Th activity ratios at Tsukuba could not be derivedentirely from Asian dust such as Kosa; there must be a source witha higher 230Th/232Th activity ratio involved. A high 232Th depositionand high 230Th/232Th activity ratio observed in February 2002 didnot coincide with a Pu peak (Hirose et al., 2007) strongly suggesting

that a substantial amount of the dust load in early spring at Tsukubaoriginated from local sources of suspended soil particles withhigher 230Th/232Th and lower Pu/Th ratios than those of Kosa. Thedeposition samples at Mt. Haruna showed no peak in the230Th/232Th activity ratios in spring, suggesting that fewer sus-pended soil particles derived from local cultivated fields occur atthe Mt. Haruna site because it is located in the boundary layer.

3.3. Estimation of local and remote source fractions in soil-deriveddust

Because the 230Th/232Th activity ratios in the local soil differfrom those in remote soil, we propose that the 230Th/232Th activityratio in the deposition residues can be used to estimate the frac-tions of locally and remotely derived thorium in the depositionresidues at Tsukuba and Mt. Haruna. Assuming that the 232Thdeposition observed at Tsukuba and Mt. Haruna arises from a singlemixing of the remote and local fractions, the remote fraction in the232Th deposition, D(Remote Th), is calculated from the observed232Th deposition, DTh,obs, and observed 230Th/232Th activity ratio indeposition, Robs, by the following equation:

DðRemote ThÞ ¼ DTh;obsðRL � RobsÞ=ðRL � RRÞ;

where RL and RR are the 230Th/232Th activity ratios in the local andremote fractions of dust, respectively. We assumed a 230Th/232Thactivity ratio of 2.3 for the local fraction and 0.65 for the remotefraction from mean values of local soils and East Asian continentalsoils. The results of our partitioning calculations are shown in Fig. 6.The local 232Th fraction in deposition at Tsukuba varied seasonally,with a maximum in early spring, whereas the local 232Th fraction atMt. Haruna did not display pronounced seasonal variation. Thepeaks in the local 232Th fraction at Tsukuba, which coincided withthe occurrence of local dust storms on the Kanto plain, were onemonth earlier than those in the remote 232Th fraction. Similarly, thepeaks in the remote 232Th fraction in April 2006 coincided withmonthly maximum number of Kosa events in Japan. The pertinentmonthly frequencies of Kosa events in Japan were as follows: Feb2006, 0; Mar 2006, 15; Apr 2006, 21; May 2006, 4; Feb 2007, 2; Mar2007, 7; Apr 2007, 11; and May 2007, 14 (JMA, 2009). The local 232Th

Fig. 7. Remote and local fractions of 239,240Pu deposition observed at Tsukuba andMt. Haruna. Remote fraction at Tsukuba, open circles; local fraction at Tsukuba, opendiamonds; remote fraction at Mt. Haruna, solid circles; local fraction at Mt. Haruna,solid squares.

K. Hirose et al. / Journal of Environmental Radioactivity 101 (2010) 106–112 111

fraction ranged from 4 to 43% of total deposition at Tsukuba and 6to 22% at Mt. Haruna. This suggests that the remote fractiondominates 232Th deposition at both sites through all seasons.

If the local fraction in the 239,240Pu deposition can be evaluated,the information from this study on the remote fraction in the239,240Pu deposition, which is closely related to Kosa events, isimportant because it could lead to a better understanding of othercontaminants borne by dust. We calculated the local fraction in239,240Pu deposition from the 239,240Pu/232Th ratio in local soil andthe local fraction in Th deposition, assuming that the local fractionof 239,240Pu deposition is derived from the suspension of local soilparticles that have a constant 239,240Pu/232Th ratio of 0.025 (Fig. 7).The remote fraction in 239,240Pu deposition at Tsukuba peaked inApril in both 2006 and 2007, corresponding to the monthlymaximum number of Kosa events in Japan, whereas the localfraction peaked in March in both years. At Mt. Haruna, the localfraction in the 239,240Pu deposition, which showed no marked peak,was lower than that at Tsukuba in early spring. This suggests thereis less local presence of re-suspended 239,240Pu in the deposition atMt. Haruna than in that at Tsukuba. In spring, most of the 239,240Pudeposition was sourced from remote sites, i.e., originating fromKosa, whereas in summer and fall the major fraction in the 239,240Pudeposition was attributable to the local suspension of soil particles,the reverse of the situation for 232Th deposition. This suggests thatthe remote fraction of dust particles contains a greater proportionof 239,240Pu-enriched particles in winter and spring and smallerproportion of 239,240Pu particles in summer and fall, which mightreflect different source regions of dust particles in the differentseasons.

4. Conclusions

The monthly plutonium deposition at Tsukuba and Mt. Harunaduring 2006–2007 showed the spring maximum typically seen inJapan. Monthly depositions of 239,240Pu at Mt. Haruna were similarto those recorded at Tsukuba, except in April 2007, probablydirectly related to atypically low rainfall at Mt. Haruna. Themajority of the plutonium deposition in Japan reflects thesuspension of 239,240Pu-bearing soil particles on the East Asiancontinent. Monthly 232Th deposition in Tsukuba, which peaked in

spring and reflects soil-derived particles, was greater than that atMt. Haruna, except in May and June 2007. The 230Th/232Th activityratios in deposition samples suggested the presence of suspendedparticles from local sources in deposition residue, but less than theremote sources, especially at Mt. Haruna. The remote sources of239,240Pu and 232Th depositions, including Kosa, play an importantrole in the deposition of atmospheric dust in Japan. Increases indeposition of 239,240Pu and Th isotopes, especially in spring, duringthe 2000s appear to reflect environmental changes on the Asiancontinent.

Recently, Kosa dust has begun to display enhancement in239,240Pu and appears to be originating from new arid regionsformed because of drought and over-cropping. If the new aridregion, contaminated by anthropogenic radionuclides due to rela-tively high rainfall in the past, has been also polluted by anthro-pogenic toxic materials, increases in the new type of Kosa eventsmay exacerbate the potential for human health problems related topollutants borne in the dust.

Acknowledgements

We thank M. Tomita, T. Mizo, K. Inukai, and Y. Togashi of KANSOCo. Ltd. for deposition sampling and radiochemical analysis. Thisresearch was supported by the Research Fund for the JapaneseRadioactivity Survey from the Ministry of Education, Culture, Sport,Science and Technology of Japan.

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