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Atmospheric Environment 43 (2009) 2971–2980

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Atmospheric Environment

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Possible change in Asian dust source suggested by atmospheric anthropogenicradionuclides during the 2000s

Yasuhito Igarashi a,*, Yayoi Inomata a, Michio Aoyama a, Katsumi Hirose a, Hiroshi Takahashi a,Yoshihiro Shinoda a,1, Nobuo Sugimoto b, Atsushi Shimizu b, Masaru Chiba a

a Meteorological Research Institute, Japanb National Institute of Environmental Studies, Japan

a r t i c l e i n f o

Article history:Received 29 August 2008Received in revised form19 January 2009Accepted 8 February 2009

Keywords:Asian dustAtmospheric depositionAnthropogenic radionuclidesClimate change

* Corresponding author. Fax: þ81 29 853 8728.E-mail address: yigarash@mri-jma.go.jp (Y. Igarash

1 Present affiliation: Kobe Marine Observatory, JaJapan.

1352-2310/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.atmosenv.2009.02.018

a b s t r a c t

Decades-long monitoring of anthropogenic radionuclides in the atmospheric deposition in Tsukuba,Japan suggests not only the substantial impacts of the Asian dust (Kosa) on the deposition but also thepossible change of the Kosa source region, especially during springs of the 2000s. In order to know moreabout such change, 4 single wet deposition events occurred in the spring of 2007 were scrutinized. Thelargest anthropogenic radionuclides wet deposition was supplied by the April 2–4 event. It broughtseveral tens % of the monthly depositions (April 2007) of the dust (residue) mass (4.5 g m�2) andanthropogenic radionulides (90Sr: 16, 137Cs: 97 and Pu: 3 mBq m�2). None of the events observed fulfilledboth criteria of the specific activities and 90Sr/137Cs activity ratio to the Tsukuba soil; they did not exhibitlocal soil dust signature. The Kosa events in fact have extensive impacts on the atmospheric environmentover Japan in spring season. Considering the elevated specific activities as well as greater 137Cs/90Sractivity ratio in the deposited dust, it is hypothesized that the dust source areas in Asian continent wouldbe shifting from the arid zone to the desert-steppe zone suffering from desertification during the 2000s.This type of the Kosa may be called as the ‘new-regime Kosa’. Chemical observation in the far downwindregion of the Kosa dust could allow us to know possible shift in the source regions.

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1. Introduction

We have been continued the observation of anthropogenicradionuclides having large environmental impacts since thelate1950s, at Geochemical Research Department, MeteorologicalResearch Institute (MRI), Japan, so as to clarify the dynamics of theradionuclides in the atmosphere (e.g., Katsuragi, 1983; Hirose et al.,1987, 2003; Aoyama et al., 1991; Igarashi et al., 1996, 2003). Espe-cially, from the long-term record of the monthly deposition of 90Sr,137Cs (Igarashi et al., 1996, 2001, 2005, 2006) and Pu (Hirose et al.,2003, 2007), we have pointed out the important role of the large-scale aeolian dust such as Kosa as the carrier of the anthropogenicradionuclides since the early 1990s (after the stratospheric falloutbecame negligible). Those radionuclides had been injected by theatmospheric nuclear tests, deposited over the globe during the late1950s to early 1960s in large amount and have remained over the

i).pan Meteorological Agency,

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land surface (besides marine environment) for several decades.Aoyama et al. (2006) compiled the deposition, soil and watercolumn inventory data and created the global 137Cs fallout map. Atthe northern middle latitude, the total of cumulative 137Cs depo-sition, being almost identical to the soil column inventory over theland, was in the range of several hundred to 10 thousand Bq m�2 at1970. The soil 137Cs inventory at present would be a little smallerthan the half considering its physical decay (T1/2: about 30 years).As a result, the surface soil particles have been contaminated byanthropogenic radionuclides, and being suspended when the duststorm takes place. This means that long-lived anthropogenicradionuclides such as 90Sr and 137Cs are applicable as potentialtracers in the aeolian dust research (Igarashi et al., 2001, 2005) aswell as Pu (Hirose et al., 2003, 2007, 2008). Atmospheric tracer useof anthropogenic radionuclides was also studied by the globaltransport model studies (Lee et al., 2003, 2006).

It is well-known that 137Cs in soil has been applied to the soilerosion research (e.g. Ritchie et al., 1990). Moreover, the increaseof the 137Cs deposition at many sites over Japan during springs ofthe early 2000s, which is due to the frequent occurrence of Kosaphenomenon in East Asia since 2000. This becomes therefore an

Y. Igarashi et al. / Atmospheric Environment 43 (2009) 2971–29802972

active research target in Japan (Akata et al., 2007; Fujiwara et al.,2007; Fukuyama and Fujiwara, 2008; Igarashi et al., 2006).According to the soil 137Cs data obtained by Fujiwara et al. (2007)over the Asian continent, it seems that 137Cs would be lost fromtopsoil in the region where wind erosion is severe. Furthermore,Fujiwara et al. (2007) and Akata et al. (2007) have described thatthe heavier 137Cs deposition (Max. 0.8 Bq m�2 month�1) recordedin March 2002 in Japan (especially in northern districts) wasoriginated from Mongolia and northeastern China. However,a sort of incomplete convincing may still remain (e.g. Arimotoet al., 2005) on such aeolian dust hypothesis for the anthropo-genic radionuclides’ re-suspension (e.g. Igarashi et al., 1996, 2001;Hirose et al., 2003). The dust research should be a current topicwith a big scientific concern. Because, it is related to the globalclimate change in various manners (e.g. see Fig. 1 in Mikami et al.,2006).

Here, at first, the 90Sr and 137Cs monthly deposition time seriesat the MRI in Tsukuba, where the less impact was found during theearly 2000s, are addressed to know about their decadal change.And then the results of 90Sr, 137Cs and Pu analysis for single wetdeposition events occurred in spring of 2007 in Tsukuba areanalyzed. Fukuyama and Fujiwara (2008) collected weekly depo-sition samples during the same spring at National Institute ofAgro-Environmental Sciences, Tsukuba to investigate the impactof the Kosa and local dusts on the 137Cs deposition, but theystressed the local dust. Thus the present paper offers moreprofound considerations on the Asian dust transport during thespring by using additional radionuclides data, lidar measurements,meteorological analyses, etc.

2. Materials, methods and analysis

Monthly atmospheric deposition (total deposition ¼wet þ dry)were collected using a 4-m2 plastic open surface collector installedin the observation field of the MRI, Tsukuba (36.1�N, 140.1�E). Thesingle wet precipitation event sample was collected at the samesite in a similar way by using more plastic samplers to obtainlarger sample volume to ensure the radiochemical analysis. Theevent samples were collected during following date and time in2007;

0

4

8

12

16

20

24

28

1993 1994 1995 1996 1997 1998 1999 200

137Cs

90Sr

Sp

ecific activity (m

Bq

/g

d

ry)

Year

Fitting curve for

Fig. 1. Trends of 90Sr and 137Cs in monthly atmospheric deposition observed at the Meteoractivity (activity in the unit residue mass) and the 137Cs/90Sr activity ratio, respectively.

Number 1: April 2 12:00–April 4 10:00,Number 2: April 4 17:45– April 5 9:45,Number 3: April 13 21:00–April 14 9:00 andNumber 4: May 10 10:00–May 11 10:00.

They were evaporated and the residue finally obtained wassubjected to the conventional radiochemical analysis for 90Sr,137Csand Pu (Igarashi et al., 1996, 2003; Otsuji-Hatori et al., 1996). Soilsamples were collected in Tsukuba in 2000. Most of them werefrom agricultural field including one from the MRI observationfield. They were all sieved by 54 mm mesh to obtain suspension-able fraction. The identical analytical procedure was applied to allsamples, and the quality control/assurance of the analysis was keptby using a homemade reference material (Otsuji-Hatori et al.,1996). Some data were already appeared in our previous publica-tions (Igarashi et al., 1996, 2001, 2003, 2005, 2006).

Meteorological data were taken from the homepage of JapanMeteorological Agency on the internet. Based on the data, a map oflow visibility phenomena was created for each event.

The continuous lidar observation in Tsukuba was carried out atthe National Institute of Environmental Sciences (NIES) next to theMRI, Tsukuba. It employs a probe of flash lamp-pumped, secondharmonic Nd:YAG laser and a receiver telescope (20 cm). Thetransmitted laser (532 nm) was linearly polarized, and two polar-ization components of the scattered light were detected with twophotomultiplier tubes. Five-minute measurements were takenautomatically every 15 min (Shimizu et al., 2004; Sugimoto et al.,2005). Dust extinction coefficient (extinction estimates for the non-spherical particles) was calculated according to the algorithmdeveloped by Shimizu et al. (2004), in which backscatter signal of532 nm and its total depolarization ratio are used as inputparameters. The lidar profile gives the temporal change of the dustextinction vertical profile over Tsukuba.

We used the popular HYbrid Single-Particle Lagrangian Inte-grated Trajectory (HYSPLIT) model that was developed and madepublic by National Oceanic and Atmospheric Administration AirResources Laboratory, USA (NOAA ARL) (Draxler and Rolph, 2003;Rolph, 2003) for the calculation of the backward trajectory of the airmass reaching to Tsukuba. NCAR/NCEP reanalysis data for windfield were used in the calculations. Daily trajectory was calculated

0 2001 2002 2003 2004 2005 2006 2007

137Cs/

90Sr Winter

Spring

Summer

Fall

137C

s/90S

r ratio

Fitting curve for fall

spring

0

2

4

6

ological Research Institute, Tsukuba, Japan–Right and left ordinates show the specific

Y. Igarashi et al. / Atmospheric Environment 43 (2009) 2971–2980 2973

leaving the site at 03 UTC (12 JST) at 1000, 1500 and 3000 m abovesea level, considering the geography (height) of the Kosa sourceareas.

The simulation of dust distribution over the Far East region wasdone by a regional-scale aerosol transport model, ChemicalWeather Forecasting System (CFORS) installed at NIES. The CFORS(Uno et al., 2003) is designed as a multi-tracer, online system builtwithin the Regional Atmospheric Modeling System (RAMS). TheCFORS system treats size-resolved mineral dust using 12 particlesize bins with corresponding effective radii of 0.13, 0.21, 0.33, 0.52,0.82, 1.27, 2.01, 3.19, 4.06, 8.01, 12.7, and 20.1 mm. The total dustuplift flux is calculated online using a fourth-power law function ofsurface friction velocity u*. The calculation domain contains100 � 90 boxes (Dx ¼ Dy ¼ 80 km) centered by 25�N in the polarstereo coordinate. Vertically, the CFORS calculated 23 layers (up to23 km). The dust particles are then subjected to transport and bothwet and dry depositions in the model.

3. Results and discussion

3.1. Temporal change of specific activity and activity ratioin the total deposition

Monthly 90Sr and 137Cs deposition during the 1990s and 2000sat the MRI are given in Fig. 1 in terms of two parameters; specificactivity (activity per unit residue mass) and their activity ratio(137Cs/90Sr). The specific activity would indicate the concentrationof each radionuclide in the soil at the origin of the dust suspension,also suggesting the extent of the global fallout at the dust originduring the 1950s to 1960s by the atmospheric nuclear test. Thedeposition amount will mainly be controlled by wet deposition(Aoyama et al., 2006). On one hand, if the higher average precipi-tation rate during the global fallout period was at the dust sourceregion, the greater 137Cs specific activity would be in the surfacesoil. On the other hand, the 137Cs/90Sr activity ratio suggests theextent of fractionation between two radionuclides also reflectingthe precipitation at the dust source region. These two radionu-clides, having similar decay constants, in surface soil, co-depositedby the global fallout, have been slowly but surely fractionated

Fig. 2. Schematic explanations (hypothesis) for possible shift of the Kosa dust source overpogenic radionuclides by the global fallout by nuclear weapon tests could be proportional towhile they were fractionated through the wet precipitation in temperate zone (Shrub and/oractivity and higher 137Cs/90Sr activity ratio would come out by storm.

through the leaching processes to give a comparatively rich in 137Csover 90Sr, which is due to the quicker migration of 90Sr than 137Csin the soil column (see e.g. Miller and Reittemeier, 1963). In otherwords, 137Cs (Pu, too) is very reactive with soil particles but 90Sr isnot. The major factor governing the fractionation between 90Sr and137Cs could thus be precipitation rate at the dust source region.

Fig. 2 schematically depicts what specific activities and the137Cs/90Sr ratios can provide information on the source climaticconditions (e.g., Igarashi et al., 2001, 2005, 2006). It has beenclaimed that 90Sr and 137Cs deposited by the global fallout reflectsthe original activity ratio (137Cs/90Sr) due to the fission of Pu and U(Eisenbud and Gesell, 2000). In the arid area the 137Cs/90Sr (havingsimilar halflives) ratios in soil would remain unchanged due to thecomparatively lesser precipitation, while they would be elevated inthe temperate areas, as a result of preferential migration of 90Sr dueto the higher precipitation. Similarly, the specific activities shouldbe lower in the arid area, while they would be higher in thetemperate area as shown in Japanese soil data (Igarashi et al., 2005,2006). In addition, the enrichment of eluted 90Sr by precipitation insurface soil by the salt crust formation would be likely in the aridarea. Therefore, supposed that once temperate area (possibly of thedesert-steppe zone) became ruined, soil dust having differentsignature from that of arid one should be emitted. The cultivationshould dilute the specific activity but in the most of the dust sourcearea, farming would be absence. The present work concerns suchsignatures of the anthropogenic radionuclides in the depositeddust, which is exerted mostly by the source climatic conditions.Igarashi et al. (2006) have proposed to describe the recent outbreakof the Kosa phenomena as ‘‘new-regime Kosa’’, which means thedust with different chemical signatures would come out of differentgeographical regions/newly degraded lands from those of thepreceding decades as shown in Fig. 2. The individual wet deposi-tions are interpreted based on the hypothesis in later sections.

In Fig. 1 the 137Cs/90Sr ratios are plotted with different symbolsaccording to seasons in order to see if any temporal changes havetaken place depend on seasons. Igarashi et al. (2005) suggested thatthe origins of atmospheric deposition may be different amongseasons in Japan. Because the prevailing air mass over Japanexhibits the seasonal change. It is noted that the 90Sr/137Cs activity

the Asian continent deduced from the radionuclides data: The deposition of anthro-the wet precipitation. In the arid zone the deposited radionuclides remain undisturbed,grassland). Such dust-steppe zone became ruined, dust particles with elevated specific

Y. Igarashi et al. / Atmospheric Environment 43 (2009) 2971–29802974

ratio in spring shows increasing trends, as a whole. Also the 137Csspecific activity in spring seems elevated especially in 2000–2002,2006 and 2007. Hirose et al. (2008) noted the similar elevation ofspecific activity of Pu during 2000, 2001, 2002, and 2006 springs. Itis well-known that the spring is the favorable season for the Kosaoccurrence and subsquent long-range transport (e.g. Husar et al.,2001). The Kosa was active over Japan throughout the 2000s except2003 (See e.g. http://www.data.kishou.go.jp/obs-env/kosahp/kosa_shindan.html). So, this temporal change in the deposition couldimply the change of dust (Kosa) source region due possibly to theanthropogenic and/or climatic change. In order to analyze thesepossible trends in the dust source region further, the observation ofsingle precipitation event for 90Sr, 137Cs and Pu depositions wascarried out in the spring of 2007.

3.2. Characteristics of each precipitation event

Figs. 3–6 depict the surface weather map, the low visibilityobservation over middle of the Honshu island, lidar profileobtained at the NIES, Tsukuba, meteorological data in Tsukuba,HYSPLIT backward trajectories from 1000, 1500 and 3000m overTsukuba, and the CFORS simulation map for the dust transport foreach precipitation event. Table 1 summarizes the analytical results.In general, Japanese islands were under the strong Kosa activity allthrough the 2007 spring and that situation also appears in eachevent’s information.

Fig. 3. Information on No. 1 wet deposition event (April 2, 2007 12:00–April 4 10:00); a) theHonshu island, c) lidar profile obtained at the NIES, Tsukuba, d) meteorological data in Tsukuthe CFORS simulation map for the dust transport

3.3. Number 1 event during April 2, 2007 12:00–April 4 10:00

First precipitation event evidently occurred at a stage in theheavy dust event (Kosa) over Japan (Fig. 3) in early April. In Tsukuba,the daylight was extensively fainted with brownish dust during thisevent (April 2). Also it was relatively calm conditions without strongwind, which was memorable. The dust transport phenomena andlow visibility were admitted on April 2 at many meteorologicalobservatories over wide areas including the Kansai, Chubu andKanto districts in the Honshu island. Lows were passing off thesouthern coast of the Japanese islands during April 2–3, whichshould bring the dust plume behind the cold front. This transport iswell portrayed by the CFORS simulation. The dust plume extendedfrom China to the Northwestern Pacific passing over Japaneseislands. The lidar also captured the dense dust plume within theboundary layer in Tsukuba, especially from night of April 1 to nightof April 2 prior to the rain (19.5 mm in total) which caused the dustdeposition. The backward trajectories on April 2 were a bitcomplicated, while they back-tracked towards the continent. Thoseon April 3 exhibit that the dust would be originated from middle toeast of Mongolia, eastern Inner Mongolia and Northeast China. Suchareas are not typical Kosa source regions but could be considered asthe recent active dust source as discussed later (e.g. Kurosakai andMikami, 2003; Lim and Chun, 2006). The heavy dust deposition ofabout 1 g m�2 in mass took place during early morning of April 2.Also the depositions of 90Sr, 137Cs and Pu were remarkable (Table 1).

surface weather map, b) distribution of the low visibility observation over middle of theba, e) HYSPLIT backward trajectories from 1000, 1500 and 3000 m over Tsukuba, and f)

Fig. 4. Information on No. 2 wet deposition event (April 4, 2007 17:45–April 5 9:45); the same items as for Fig. 3.

Y. Igarashi et al. / Atmospheric Environment 43 (2009) 2971–2980 2975

3.4. Number 2 event during April 4, 2007 17:45–April 5 9:45

Next precipitation event took place (11 mm in total) in the finalstage of the intense Kosa event mentioned above (Fig. 4). Its dustdeposition was smallest (0.08 g m�2) among the events observed inthe present work. However, the second largest depositions of 90Sr,137Cs and Pu were observed (Table 1). On April 4, a small lowmoving eastward was on the Sea of Japan, which might beresponsible for the dust transport. However, the CFORS simulationindicated that, on that day, the major dust plume had been leavingfrom the Japanese islands towards the Pacific. This was alsoexpressed by the visibility observations; only at a few sitesdecreased visibilities were reported during midnight between April4 and 5. The lidar profile showed that the dust plume over Tsukubaat low altitude preceded to the precipitation during night of April 4.The backward trajectories suggest that the origin of the depositeddust seems to be in Northeast China.

3.5. Number 3 event during April 13, 2007 21:00–April 14 9:00

Third one took place during very weak Kosa event in the earlymorning of April 14 (Fig. 5) with precipitation as small as 0.5 mm.The dust mass deposited was about 0.1 g m�2. This event resulted inthe smallest depositions of 90Sr, 137Cs and Pu. In particular, 90Sr wasof less than its detection limit (Table 1). The dust cloud wastransported by a low passing above the Sea of Japan and northernJapan during April 13–14, 2007. The reduced visibilities wererecorded over coastal Chubu, Kanto and southernmost Tohokudistricts in the Honshu island. The CFORS simulates the dust plumecovering slightly over Japan in the morning of April 14. Although

the lidar did not record the thick dust layer within the boundarylayer, at higher altitude some signal was seen around the time ofthe precipitation. The backward trajectories exhibit the air masswhich contained the dust could be from Mongolia and northeast ofInner Mongolia.

3.6. Number 4 event during May 10, 2007 10:00–May 11 10:00

This sample was collected in May (Fig. 6). The dust depositionwas substantial, being recorded about 0.6 g m�2 in mass, but itleaded relatively little 90Sr, 137Cs and Pu depositions. The precipi-tation took place (27 mm in total) in a relatively clear Kosa event. Inthis case, a low that played a role in the dust transport clearlyappeared in the surface weather charts of April 10 and 11. Thereduced visibilities were recorded at many sites including Chubu,Kanto and southernmost Tohoku districts and coastal area of theSea of Japan. Although the lidar profile did not show the dense dustplume around the time of precipitation, relatively intense signalswere seen around the noon of April 10 up to 3–4 km in the air. TheCFORS simulates the dust plume stretching over Japan during theperiod of concern. The backward trajectories suggest the dust couldbe from Northeast China.

It should be noted that, according to the backward trajectories,none of the case was produced by the air mass from the westernChina where the distinctive large dust source area exist.

3.7. Wet deposition of anthropogenic radionuclides

In Table 1 the analytical results for single events of 90Sr, 137Csand Pu depositions are given along with the monthly total

Fig. 5. Information on No. 3 wet deposition event (April 13, 2007 21:00–April 14 9:00); the same items as for Fig. 3.

Y. Igarashi et al. / Atmospheric Environment 43 (2009) 2971–29802976

deposition (April 2007). It is notable that those depositions did notdepend upon the precipitation amount.

The No. 1 event recorded not only the highest dust massdeposition, but also the maximum 90Sr, 137Cs and Pu depositions(about 10, 40 and 1 mBq m�2, respectively) among the eventsampled. This is simply due to that the precipitation occurredwithin the course of the serious Kosa event among all. Moreover,the contamination by local wind-blown dust should be negligiblebecause that the wind was relatively weak (less than 4 m s�1)during the rain. Dust mass, 90Sr, 137Cs and Pu depositions by thisevent occupied 30, 64, 42 and 36% of relevant monthly deposition,respectively. The data also accord with the weekly data obtained byFukuyama and Fujiwara (2008). These data evidently confirm thatthe Kosa transports anthropogenic radionuclides with dust parti-cles from the Asian continent to Japan and its contribution to themonthly depositions seems not trivial but quite substantial.

The values of lower rows for each event data in Table 1 indicatethe specific activity (activity relative to the deposited mass;mBq g�1) of the individual radionuclide, which should reflect thosein the surface soil at the dust occurrence region. Fig. 7 shows thecomparison of those for 90Sr, 137Cs and Pu with Tsukuba surface soildata. In general, 90Sr, 137Cs and Pu specific activities of the deposi-tion samples are much higher than those of the Tsukuba soil.Monthly average specific acrivities of 90Sr, 137Cs and Pu (the lowestrow in Table 1) were about 4, 22 and 0.6 mBq g�1, respectively,which are greater than the medians of the corresponding Tuskubasoil data.

Individually, No. 2 event exhibited the highest 90Sr, 137Cs and Puspecific activities of about 15, 80 and 1.5 mBq g�1, respectively,although this was not typical Kosa event. Similar high 137Cs valuewas reported for the corresponding weekly sample by Fukuyamaand Fujiwara (2008), they attributed it as the dust particle size; thefiner size dust particles should have the greater 137Cs specificactivity. Present 137Cs specific activity level was, however, previ-ously reported for Inner Mongolian surface soil of grassland(Fujiwara et al., 2007), where is one of the possible dust sourceregions. Surface soil 137Cs concentration as high as 46–86 mBq g�1

(the latter corresponding to soil column inventory of 3700 Bq m�2)was found near Xilinhot, Inner Mongolia. This inventory should beat least doubled at the early 1970s (close to 8000 Bq m�2). Grass-land over the Asian continent might have such great fallout asJapan. Among Japanese surface soil the higher 137Cs concentrationtends to accompany with the greater 137Cs/90Sr activity ratio, whichis due to the fractionation between 90Sr and 137Cs by precipitation.The No. 2 sample should have similar nature.

Although No. 3 event showed very low 90Sr specific activity (thedatum was ND), other radionuclides’ data were higher than thelevel of the Tsukuba soil. The No. 4 event exhibited the low specificactivities of 137Cs and Pu, falling in the Tsukuba soil data range.However, 90Sr specific activity of the event seems to be off theTsukuba soil data range. Signatures of anthropogenic radionuclidesspecific activity in deposition revealed that it is difficult to explainthe dust components in the individual rainfall only by washout ofthe local soil dust.

Fig. 6. Information on No. 4 wet deposition event (May 10, 2007 10:00–May 11 10:00); the same items as for Fig. 3.

Y. Igarashi et al. / Atmospheric Environment 43 (2009) 2971–2980 2977

Closer examination on the 137Cs/90Sr activity ratio also allows usto deduce the source. On one hand, No. 4 event showed quite lower137Cs/90Sr activity ratio (2.2) than those of the Tsukuba soil (medianand average were 4.2 and 7.2, respectively), suggesting arid natureof the dust source. Specific activities of this event could be close tothe Tsukuba soil level but the low 137Cs/90Sr ratio of 2.2 is hard tocreate by diluting the Kosa with the Tsukuba soil; excess 90Sr wascontained. The backward trajectories for the No. 4 event leaving at2000 m over Tsukuba can approach to northern central region ofChina (the Gobi region in Inner Mongolia), the dust source could bethe Gobi region. The dust source region for No. 4 event may have

Table 1Comparison of the individual wet depositions and monthly depositions for anthropogen

Sampling duration Residue mass g m�2 9

Start End D

S

Wet deposition event2007/April/2 12:00 2007/April/4 10:00 1.327 1

72007/April/4 17:45 2007/April/5 9:45 0.083 1

12007/April/13 21:00 2007/April/14 9:00 0.113 <

<

2007/May/10 10:00 2007/May/11 10:00 0.574 23

Monthly deposition2007/April/1 2007/May/1 4.499 1

been dry for decades (as in the past) and still served as a dust sourceregion (conventional type of the Kosa). This fact exhibits that thedust source change process would not be gradual one but more likesporadic.

On the other hand, those ratios for other events (3.9 to largerthan 8.1) were in the range of the Tsukuba soil data. Consequently,the elevated specific activity along with greater 137Cs/90Sr activityratio of Nos. 1–3 events suggests the dust source region of themwere desert-steppe zone (DSZ) as depicted in Fig. 2.

None of the present wet deposition events, accordingly, fulfillthe criteria of the specific activity and 137Cs/90Sr activity ratio for

ic radioactivities observed at the MRI, Tsukuba, Japan.

0Sr 137Cs Plutonium 137Cs/90Sr

epo./mBq m�3 Depo./mBq m�2 Depo./mBq m�2 activity ratio

pc. actv./mBq g�1 Spc. actv./mBq g�1 Spc. actv./mBq g�1

0.5�0.5 40.5�2.6 0.97�0.04 3.9�0.3.88�0.34 30.6�1.9 0.73�0.03.24�0.21 6.63�1.23 0.13�0.02 5.4�1.35.0�2.5 80.1�14.9 1.56�0.210.3, ND 2.76�0.44 0.12�0.02 >8.13.0, – 24.3�3.9 1.03�0.14.24�0.10 4.92�0.76 0.18�0.02 2.2�0.4.90�0.17 8.58�1.32 0.32�0.036.3�0.28 96.5�6.33 2.67�0.10 5.9�0.43.6�0.06 21.5�1.41 0.59�0.02

Fig. 7. Comparison of atmospheric wet depositions and Tsukuba soils for anthropogenic radionuclides’ signatures of the specific activity and the 137Cs/90Sr activity ratio. Aprl.:Monthly deposition of April 2007 sample; Soil: Tsukuba surface soil samples collected in 2000 (decay corrected to 2000).

Y. Igarashi et al. / Atmospheric Environment 43 (2009) 2971–29802978

the Tsukuba soil samples, without contradiction. The major dustplayer of the individual wet deposition events in 2007 spring was,as a consequence, the Kosa being transported long-range.

Fig. 8. The apparent correlation between nationwide Kosa observation over Japan(visibility observation at the ground; total day�site) and the 137Cs/90Sr activity ratiofound in the monthly atmospheric deposition at the MRI, Tsukuba during the 1990sand 2000s

3.8. Possible shift of the Kosa source region suggested by the presentobservation and related factors

It is probable that the vigorous aeolian dust source influencingJapan would be shifting from the conventional arid area to the DSZduring the 2000s. Fig. 8 depicts the relations between the nation-wide Kosa observation in Japan (visibility observation at theground) and the 137Cs/90Sr ratio found in the MRI deposition in thespring month. Igarashi et al. (2005) reported that the Kosa obser-vation and the 90Sr and 137Cs depositional peaks at the MRI accordin spring during the 1990s, though the relationship is apparent. InFig. 8, for the 1990s the correlation was unclear, while for the 2000sthere is a weak but apparent positive correlation (significant at 95%confidence level). The enhanced Kosa observation during the 2000sover Japan could be connected with the change of the 137Cs/90Srratio; hence the source change.

As mentioned earlier, according to the field survey by Fujiwaraet al. (2007) on the vertical profile of 137Cs in uncultivated soil inInner Mongolia, 137Cs was enriched in the surface layer. This patternwas remarkable for grassland. It seems natural that the grasslandhas been receiving higher precipitation than the so-called arid area;the 137Cs/90Sr ratio should be elevated due to the higher precipi-tation and resulted fractionation in the soil column. Without thevegetation cover, surface clay and silt minerals are easily blown off

Y. Igarashi et al. / Atmospheric Environment 43 (2009) 2971–2980 2979

by the strong wind. Fujiwara et al. (2007) and Akata et al. (2007)have suggested that the heavier 137Cs deposition in March 2002 innorthern Japan was from Mongolia and northeastern China. Butthey did not analyze 90Sr and Pu, of which data profound thehypothesis further; we are going to fill such data blank regardingthe source in the near future.

The DSZ is spreading over the north to east China and Mongoliacovered by shrub and grass (e.g. see Fig. 6 in Fujiwara et al., 2007; ofthe web version). Certainly, the degradation of such area over theAsian continent would be continuing (Normile, 2007). Kurosakaiand Mikami (2003) have pointed out, during 2000–2002, theoutbreak of dust storms on the Asian continent dramaticallyincreased in different areas from the conventional dust sourceregions such as Taklimakan. Their analysis on the dust outbreakfrequency apparently depicts the geographical shift in the dustoccurrence region over the Asian continent. Lim and Chun (2006)also pointed out that source regions of Asian dust have beenextending eastward.

We could thus assume that the elevated specific activity alongwith higher 90Sr/137Cs activity ratio found in the present wetdeposition samples should be attributed to such character ofuncultivated continental grassland soil (in the DSZ). In thisconnection, Igarashi et al. (1995) have analyzed one Saharan dustdeposited in Europe. The dust gave elevated 137Cs and lesser 90Srconcentrations (34.0 � 1.9 and2.69 � 0.1, respectively); the137Cs/90Sr ratio was 12.6. Such signatures could be of the dust fromthe DSZ, the newly ruined area may be expanding in Saharanregion, too.

In the present work, the source shift temporal trends wereexpressed in Figs. 1 and 8. The shift has occurred not steadily butsporadically, because it should relate, on one hand, to the large-scale atmospheric circulation (e.g. Ding et al., 2005; Han et al.,2008; Hara et al., 2006; Wang et al., 2008). Hara et al. (2006)concluded that the simulated Asian dust transport could have theconnection with El Nino/Southern Oscillation. Han et al. (2008)mentioned the possible relation between the dust outbreak in Asiaand Arctic Oscillation. On the other hand, Zou and Zhai (2004)investigated the vegetation coverage and spring dust outbreak overnorthern China by using normalized difference vegetation index(NDVI); the poor vegetation coverage could be an important factorfor the frequent dust storm. Kurosaki and Mikami (2005) addressedalso the effect of the snow cover on the threshold of the dustemission. Although Zhang et al. (2003) looked at the role of climatechange in the dust emission and transport by using a modelsimulation, as many factors are involved, it is still difficult to givethe straightforward conclusion on the whole mechanism of shift ofthe Kosa dust source.

Nevertheless the possible shift of the dust source seems incoincidence with the change in the present anthropogenic radio-nuclides deposition time series as depicted in Figs. 1 and 8. Toimprove further the present hypothesis, the actual dust sourceinformation including not only 137Cs but also 90Sr data, etc. isindispensable; we have conducted such survey in Mongolia as aninternational joint collaboration. In the near future, we will reportthe results.

4. Summary

1. Monthly depositions of 90Sr and 137Cs at the MRI, Tsukuba,Japan are presented during 1993 up to the spring of 2007. Thetime series exhibited deposition peaks in spring. The 137Cs/90Sractivity ratio had an increasing trend only in spring.

2. Four wet deposition events occurred in spring of 2007 in Tsu-kuba, Japan were analyzed. Each event was characterized bythe surface weather map, distribution of the low visibility

observation over Japan, lidar profile in Tsukuba, meteorologicaldata, HYSPLIT backward trajectory and the CFORS simulationfor the dust transport.

3. One typical wet deposition event, which brought the anthro-pogenic radionuclides from the Asian continent to Japan, wasfound. The event exhibited 30–60% of the monthly depositions(April 2007) of the dust mass and anthropogenic radionulides.

4. Signatures of the anthropogenic radionuclides by all depositionevents were distinguishable from those of the Tsukuba surfacesoil. During the spring of 2007 the influence of the Kosa wassignificant.

5. The radionuclides observation in the far down wind confirmsthat the Asian dust source expanse has been continuing since2000.

Acknowledgement

Mr. Masatoshi Tomita, KANSO, is acknowledged for his workinvolving the sample analysis. Part of the expenses of this work iscovered by the Radioactivity Survey in Japan by the Ministry ofEducation, Culture, Sports, Science and Technology. Ms. KazueInukai and Ms. Yoshiko Togashi, KANSO, helped in the tedioussample preparation for which we are truly grateful. We gratefullyacknowledge the NOAA ARL for the provision of the HYSPLITtransport and dispersion model and/or READY website used in thisstudy.

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