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Atmospheric Environment 46 (2012) 299e308

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Atmospheric Environment

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Long-range transport of spring dust storms in Inner Mongolia and impacton the China seas

Sai-Chun Tan a,b,*, Guang-Yu Shi a, Hong Wang c,d

a State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences,Beijing 100029, ChinabKey Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, Nanjing 210044, ChinacCentre for Atmosphere Watch and Services, Chinese Academy of Meteorological Sciences, Beijing 100081, Chinad State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081, China

a r t i c l e i n f o

Article history:Received 29 March 2011Received in revised form18 August 2011Accepted 27 September 2011

Keywords:Dust stormTransportDepositionProbabilityThe China seas

* Corresponding author. State Key Laboratory of Nuspheric Sciences and Geophysical Fluid Dynamics, InstChinese Academy of Sciences, Beijing 100029, Chifax: þ86 10 82995097.

E-mail address: sctan@mail.iap.ac.cn (S.-C. Tan).

1352-2310/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.atmosenv.2011.09.058

a b s t r a c t

Analysis of daily observations from 43 meteorological stations in the Inner Mongolia Autonomous Region,China, showed the distribution of spring dust storm events during 2000e2007. Guaizihu and Sunitezuoqistations had the highest frequencies of dust storms. The interannual and seasonal variations of duststorms were closely related to weather conditions, especially the wind speed. A forward trajectory modeland satellite observations were used to investigate the transport paths and dust layers of dust storms fromGuaizihu and Sunitezuoqi stations to the China seas and their probability of influencing the seas duringspring 2000e2007. Forward trajectories showed that dust storms at Guaizihu and Sunitezuoqi stationshad the highest probability of affecting the Yellow Sea, followed by the Bohai Sea, the East China Sea, andthe northern South China Sea. The dust particles from Sunitezuoqi station affected these four seas directlythrough coastal areas, while those from Guaizihu station were transported via the Inner Mongoliandeserts and/or the Loess Plateau. The dust storms from Sunitezuoqi station impacting the four seas werecharacterized by a single dust source and a short transport distance, while those from Guaizihu stationwere characterized by multiple sources and relatively long transport distances. The dust particles fromthese two stations were mostly transported in a <4 km layer from the source regions to the seas. Thesatellite vertical profile also indicated that dust particles were mainly contained in a 0e4 km layer overthe source regions and the four seas. An aerosol index retrieved from satellite observations and theestimated dust deposition also supported the influence derived from the forward trajectory model, withlarge aerosol index and dust deposition values occurring on the dust days affecting the four seas. Theaverage deposition over the four seas was 18.7 g m�2 during spring 2000e2007.

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1. Introduction

The dust aerosols carried by dust storm events in East Asia havemajor effects on local ecosystems, the environment, and humansociety, as well as large impacts in downwind areas such as Japan,Korea, the North Pacific, and North America through long-rangetransport (Mikami et al., 2006). Eolian dust particles in the atmo-sphere could affect climate via radiative forcing (Sokolik et al.,2001) and could play important roles in marine biological

merical Modeling for Atmo-itute of Atmospheric Physics,na. Tel.: þ86 10 82995215;

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activities, which are closely related to the global carbon cycle andmarine aerosol production (Bishop et al., 2002; Meskhidze et al.,2005; Levasseur et al., 2006; Yuan and Zhang, 2006; Jo et al.,2007; Tobo et al., 2010). This, in turn, could cause feedbackeffects on climate and dust production (Jickells et al., 2005).Consequently, understanding the characteristics of dust stormevents and their influence on seas is important.

Asian dust events primarily occur in springtime (MarcheMay)(Qian et al., 2006). Simulated springtime dust emissions for theperiod 1960e2002 indicated that Asian dust mainly originatedfrom 10 source areas in the deserts of Mongolia, northern China,and Kazakhstan (Zhang et al., 2003b). Both observations andsimulations showed that the Inner Mongolia Autonomous Region(Inner Mongolia) in China is one of the most important sourceregions for Asian dust (Zhang et al., 2003b; Wang et al., 2005, 2010;

S.-C. Tan et al. / Atmospheric Environment 46 (2012) 299e308300

Qian et al., 2006) and is responsible for approximately 31% of thetotal dust production (Zhang et al., 2003b).

Transport of dust particles is closely related to synoptic condi-tions and the characteristics of the source regions (Tsai et al., 2008).Asian dust is usually associated with frontal systems and/orcyclones (Sun et al., 2001; Uno et al., 2004; Tsai et al., 2008). Dustparticles can be transported by surface-level northwesterly windsassociatedwith the Asianwintermonsoon over the Asian continentand by westerly winds in the free troposphere from the easternAsian continent to the Pacific Ocean (Uematsu et al., 1983; Unoet al., 2004; Zhao et al., 2006). Lidar observations and modelsimulations indicated that dust particles from the Gobi Desert areusually entrained in a lower layer (�3 km) and can be deposited inproximal regions. Dust from the Taklimakan Desert uplifted to�5 km has a better chance of being transported over long distances,because the surrounding high mountains exceed 5 km in heightand the stronger westerly jet dominates in this altitude range(Iwasaka et al., 1983, 2003; Sun et al., 2001; Matsuki et al., 2003;Uno et al., 2004).

Many studies of the transport of Asian dust to downwind areas,including the China seas, have been reported. Zhang and Gao(2007) analyzed the source and movement route of dust stormsto downwind seas during 2000e2002 based on meteorology datafrom the Meteorological Information Comprehensive Analysis andProcess System (MICAPS). Gao et al. (2009) reviewed previousstudies of the transport pathways of Asian dust and its influenceareas. However, most studies concentrated on the transport of duststorms across the continent (Zhang et al., 2003a; Chen et al., 2006)or performed a case study of the influence of Asian dust storm ondownwind areas or seas (Iwasaka et al., 1983; Liu et al., 1998; Fanget al., 1999; Husar et al., 2001; Kim and Park, 2001; Lin et al., 2007).Studies of the long-term impacts of dust storms on the seas are stillrare. In addition, lidar and balloon observations and model simu-lations were used to monitor and understand the vertical distri-bution and transport processes of dust storms generally. Withadvances in remote sensing technology, satellites will increasinglybe able to monitor dust transport, including even the verticalstructure of transported dust (Winker et al., 2009). In this study, wetried to clarify the influence of dust storms on the China seas duringa long-term period.

We investigated the impacts of spring (MarcheMay) duststorms on the China seas (the Bohai Sea, Yellow Sea, East China Sea,and South China Sea) through combinations of surface observa-tions, satellite observations, trajectories, and dust model simula-tions during the period 2000e2007. First, the transport processesof dust storms from the source regions to the seas were analyzedusing a forward trajectorymodel and satellite vertical observations.Second, the probability of spring dust storms affecting the Chinaseas was examined through the forward trajectory model. Finally,satellite-derived aerosol index and dust deposition from a numer-ical model were used to test the influence of dust storms on theChina seas derived from the forward trajectory model.

Fig. 1. The distribution of averaged annual occurrence frequency of spring dust storms(days year�1) for the period 2000e2007 in Inner Mongolia. Plus sign shows thelocations of 43 meteorological stations.

2. Data and methods

The daily occurrence time records of dust storms at 43 meteo-rological observation stations in Inner Mongolia, China during2000e2007 were obtained from the National MeteorologicalInformation Center of the China Meteorological Administration(NMIC/CMA). The records contained the starting and ending timesof dust storms each day at each station, allowing for calculation ofthe occurrence frequency of dust storms (days month�1 ordays year�1) at each station. At each station, dust storm weatherphenomenon were defined as storms with minimum visibility of

�1 km and instantaneous maximum wind speed of �10 m s�1

(Qian et al., 1997).Daily mean wind speed, pressure, temperature and relative

humidity and daily rainfall (mm) datasets at the same 43 meteo-rological stations were used to examine the relationships of thesefactors to dust storm occurrence frequency. These data were alsoprovided by the NMIC/CMA.

The aerosol index is an effective measure of the dust loading inthe atmosphere (Herman et al., 1997). Many studies found that theaerosol index correlated closely with modeled dust loading (Unoet al., 2004; Zhao et al., 2006). Here, we used the aerosol index tomeasure the dust loading over the China seas. From 2000 to 2004,the aerosol index was retrieved from the Total Ozone MappingSpectrometer (TOMS, resolution 1.25� �1�), and from 2005 to 2007it was retrieved from the Ozone Monitoring Instrument (OMI,resolution 1� �1�). The TOMS data were interpolated into the samespatial resolution as the OMI data.

Dust deposition (including dry and wet depositions) from theGlobal/Regional Assimilation and Prediction System/the ChineseUnified Atmospheric Chemistry Environment for Dust AtmosphericChemistry Module (GRAPES/CUACE-Dust) numerical model wasalso used to detect the impacts of spring dust storms in InnerMongolia on the China seas. The GRAPES/CUACE-Dust model wasdeveloped by the Numerical Prediction Research Center, CMA, andCenter for AtmosphereWatch and Services of the Chinese Academyof Meteorological Sciences (Wang et al., 2010). The model domaincovers 70�e140�E and 15�e60�N, including the source regions ofdust storms in East Asia. Comparisons of real-time predictionoutputs with surface observations and the TOMS aerosol indexhave indicated that the model can predict the outbreak, develop-ment, transport and depletion processes of dust storms accuratelyover China and the East Asian region (Wang et al., 2010). Details ofthe model have been described by Wang et al. (2010). Dust depo-sition was used in this study at 0.5� � 0.5� spatial resolution.

The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)is a two-wavelength polarization lidar. It is the primary instrumenton the Cloud-Aerosol Lidar and Infrared Pathfinder SatelliteObservations (CALIPSO) satellite. CALIOP has provided a directmeasure of the vertical structure of aerosols in the troposphere andlower stratosphere since June 2006 (Winker et al., 2009). Theaerosol type data from CALIOP Level 2 products were used to detectthe vertical structure of dust aerosols in this paper. Six distinctaerosol types were labeled desert dust, smoke, clean continental,polluted continental, clean marine and polluted dust (Dubovik andKing, 2000; Omar et al., 2005; Winker et al., 2009). The verticalresolution was 30 m for altitudes from �0.5 to 8.2 km, 60 m foraltitudes from 8.2 to 20.2 km and 180 m for altitudes from 20.2 to30.1 km.

To understand the impacts of dust storms from different sourceareas on the China seas, a forward trajectory analysis was

1 2 3 4 5 6 7 8 9 10 11 12

0

1

2

3

4

-202468

1012

Dust storm Wind speed

Month

)raey/syad(mrotstsud

gnirpS)s/

m(deeps

dniw

naem

yliaD

b

)htnom/syad(

ecnerruccomrotstsu

D)s/

m(deeps

dniw

naem

yliaD

Wind speed Dust storm a

Year00 01 02 03 04 05 06 07

Fig. 2. a. The interannual variation in spring dust storm occurrence frequency (days year�1) and daily mean wind speed (m s�1). b. Monthly climatological dust storm occurrencefrequency (days month�1) and daily mean wind speed (m s�1). Error bar of dust storms is the standard deviation.

S.-C. Tan et al. / Atmospheric Environment 46 (2012) 299e308 301

performed using the Hybrid Single Particle Lagrangian IntegratedTrajectory (HYSPLIT) model (Draxler and Rolph, 2010) with inputsfrom the National Centers for Environmental Prediction/theNational Center for Atmospheric Research (NCEP/NCAR) globalreanalysis meteorological data. The data were on a latitude-longitude grid with a spatial resolution of 2.5�. The internalscaling height of HYSPLIT was 25 km. An air parcel appearing in themiddle of a dust storm event at the source stations was tracedforward.

3. Results and analysis

3.1. Characteristics of the occurrence frequency of spring duststorms in Inner Mongolia during 2000e2007

Fig. 1 shows the distribution of the annual mean occurrencefrequency of spring dust storm events (days year�1) averaged from2000 to 2007, as derived from the 43 meteorological observationstations. The standard deviation (SD) at the 43 stations is from 0.5

0 3 6 9 12 150

2

4

6

8

10

c

a WindSpeedWind speed = 0.21x + 2.93R = 0.68, P<0.0001

)s/m(

deepsdni

wnae

mylia

D

Spring dust storm occurrence (days/year)

)%(

HR

naem

yliaD

0 3 6 9 12 150

5

10

15

20 Temperature

Temperature = 0.19x + 6.38R = 0.19, P=0.21

(erutarep

metnae

mylia

Do

)C

Spring dust storm occurrence (days/year)

)aPh(erusserp

naem

yliaD

Fig. 3. The relationship between spring dust storm occurrence frequency (days year�1) ancoefficient; P is the significance level of fitting.

to 9.5 days year�1. There were two regions with high dust stormfrequency: the deserts in the west of Inner Mongolia (including theBadain Jaran Desert, Ulan Buh Desert, and Tengger Desert) andHunshandake Desert. The two stations with the highest frequenciesof dust storms were Guaizihu (960 m above sea level) and Sunite-zuoqi (1036.7 m above sea level) stations. The annual meanoccurrence frequencies of spring dust storms at these two stationswere 11 days year�1 (SD ¼ 3) at Guaizihu and 14 days year�1

(SD ¼ 9.5) at Sunitezuoqi (Fig. 1). Guaizihu and Sunitezuoqi arelocated in the Gobi Desert, which is bounded by the Hexi Corridorand the Tibetan Plateau to the southwest and by the North ChinaPlain to the southeast. The simulated springtime dust emissions for1960e2002 also indicated that deserts in western Inner Mongoliaand the Hunshandake Desert were the two source areas of Asiandust. The deserts in western Inner Mongolia, in particular, havebeen identified as one of three major sources (the others beingMongolia and the Taklimakan Desert) (Zhang et al., 2003b).

There was apparent interannual variation in spring dust stormsin Inner Mongolia during 2000e2007. The maximum occurrence

d

b

0 3 6 9 12 150

20

40

60

80

100

RainfallRainfall = -2.18x + 44.89R = -0.39, P<0.01

Relative humidity (RH)RH= -1.16x + 53.42, R= -0.46, P<0.01

)m

m(llafniardetalu

muccA

Spring dust storm occurrence (days/year)

0 3 6 9 12 15800

850

900

950

1000

1050 Pressure

Pressure = -6.99x + 928.51R = -0.48, P<0.01

Spring dust storm occurrence (days/year)

d environmental factors. The formulas are the results of linear fitting. R is correlation

S.-C. Tan et al. / Atmospheric Environment 46 (2012) 299e308302

frequency (days year�1) appeared in 2001, followed by 2006 withthe second highest frequency (Fig. 2a). Dust storms in Inner Mon-golia occurred most frequently in spring (Fig. 2b). During2000e2007, spring dust storms accounted for 77.2% of the annualtotal in Inner Mongolia.

The interannual and seasonal variations in dust storms wereclosely related to weather conditions. Significant correlation exis-ted between the dust storms and environmental factors, includingdaily mean wind speed (m s�1), relative humidity (%), pressure(hPa), and rainfall (mm) (Fig. 3). Dust storms were positivelycorrelated to wind speed and negatively correlated to rainfall,relative humidity, and pressure. However, there was no significantcorrelation between dust storms and temperature. The interannualvariation in spring dust storms correlated to wind speed witha correlation coefficient of 0.75 (significance level ¼ 0.05), whereasthe monthly climatological variation correlated to wind speed witha correlation coefficient of 0.94 (significance level ¼ 0.01) and torelative humidity with a correlation coefficient of �0.82 (signifi-cance level ¼ 0.01).

3.2. Characteristics of spring dust storms during transport

Statistically, the stronger the intensity of the dust storm, thelonger its duration timewill be. This may induce a greater chance of

Fig. 4. The transport paths of dust storms at Guaizihu station (pentacle in the figure) to theand South China Sea (d) using 96-h trajectory. The top and bottom panels show horizonShandong, Jiangsu, Zhejiang, Fujian and Guangdong Province. AGL means above ground lev

long-range transport of dust particles. Therefore, dust storms withshort duration time were excluded from the analysis, and duststorms with duration times over 1 h at representative stations wereused for the forward trajectory analysis. Because Guaizihu andSunitezuoqi had the highest dust storm occurrence frequencies, thestorms at these two stations were used to analyze the influence ofdust on the China seas. During spring 2000e2007, 77 and 79 duststorms with durations over 1 h were recorded at Guaizihu andSunitezuoqi, respectively.

3.2.1. Trajectory analysis of transport paths from dust sourceregions to the seas

The transport paths of dust storms from Guaizihu station to theChina seas passed by some Inner Mongolia deserts and/or the LoessPlateau and the North China Plain (sometimes over Beijing/Tianjincity) and then (1) entered the Bohai Sea from Tianjin city, Hebei,Liaoning, or Shandong Province (Fig. 4a); (2) entered the Yellow Seathrough Tianjin city and Bohai Sea, Hebei, Liaoning, Shandong, orJiangsu Province (Fig. 4b); (3) entered the East China Sea throughShandong, Jiangsu, Zhejiang, or Fujian Province, Shanghai city and/or Yellow Sea (Fig. 4c); or (4) entered the South China Sea throughShandong, Jiangsu, Zhejiang, Fujian, or Guangdong Province and/orBohai Sea, Yellow Sea, and East China Sea (Fig. 4d). These pathswere characterized by long distances and multiple sources.

Bohai Sea (a), Yellow Sea (b), East China Sea (c) using 72-h forward trajectory analysistal and vertical motion, respectively. LP: Loess Plateau area. A-G are Liaoning, Hebei,el.

S.-C. Tan et al. / Atmospheric Environment 46 (2012) 299e308 303

The transport paths of dust storms from Sunitezuoqi station tothe China seas were as follows. The dust particles were depositedinto the China seas through Beijing/Tianjin city and/or Hebei,Liaoning, or Shandong Province (Fig. 5). As with the paths fromGuaizihu, sometimes the dust entered East China Sea via Bohai Seaand/or Yellow Sea and entered South China Sea via Bohai Sea,Yellow Sea and/or East China Sea. Rarely, the dust arrived over theBohai Sea and Yellow Sea by way of the Horqin Desert (Fig. 5a andb). These paths were characterized by relatively short movementdistances and a single dust source.

The movement routes to the Bohai Sea, Yellow Sea and EastChina Sea were similar to the routes observed by Zhang and Gao(2007) based on MICAPS meteorology data for 2000e2002. There,an Asian dust storm from Mongolia/eastern Inner Mongolia wasdeposited into the Bohai Sea and Yellow Sea by way of Hun-shandake and Horqin deserts; Asian dust from Mongolia/westernInner Mongolia branched into three paths and then entered theBohai Sea, Yellow Sea and East China Sea. The movement routes tothe Yellow Sea, East China Sea, and South China Sea were alsoconsistent with previous backward trajectories and model simu-lations. Dust in Korea in April 1998 was traced to Mongolia by wayof Inner Mongolia and Yellow Sea (Chun et al., 2001). Combinedsimulations and backward trajectories of the Asian PacificRegional Characterization Experiment (ACE-Asia) perfect duststorm (Julian days 94e104 in 2001) indicated that the dust layer

Fig. 5. Same as Fig. 4, but for Sunitezuoqi station

during one of the dust storms from the Gobi region during thetransport was below 2 km in height. This dust stormwas split intotwo parts: one was transported to the north (affecting Bohai Seaand Yellow Sea), while the other was transported to lower lati-tudes around Taiwan (affecting East China Sea) (Uno et al., 2004).The path to South China Sea sometimes passing through EastChina Sea was similar to the result of Fang et al. (1999). Theyfound that the Asian dust aerosols reached Hong Kong by way ofEast China Sea.

Asian dust is usually associated with frontal systems and/orcyclones (Sun et al., 2001; Uno et al., 2004; Tsai et al., 2008). Dustparticles can be transported southeastward or eastward bynorthwesterly winds associated with frontal systems and/orcyclones and westerly winds in the free troposphere (Uematsuet al., 1983; Sun et al., 2001; Uno et al., 2004; Zhao et al., 2006;Tsai et al., 2008). The transport type of a dust event stronglydepends on the source areas relative to the synoptic conditions. Thedust particles from Guaizihu usually passed deserts in the OrdosPlateau (including the Kubuqi and Maowusu deserts) and/or LoessPlateau during transport, and thus the dust could mix with localdust storms. In contrast, most dust storms at Sunitezuoqi stationcould be transported southeastward or southward to the China seasdirectly, without passing the Horqin Desert. This was why theformer station observed dust from multiple sources, while thelatter station observed dust from a single source.

and all figures are 72-h forward trajectories.

Fig. 6. The altitude-orbit cross-section of CALIPSO aerosol types during transport process of dust storm at Guaizihu and Sunitezuoqi on 30 March 2007. The altitude refers to abovelocal mean sea level. The purple line displays the local surface elevation. (For interpretation of the references to color in this figure legend, the reader is referred to the web versionof this article.)

S.-C. Tan et al. / Atmospheric Environment 46 (2012) 299e308304

3.2.2. The transport of two dust stormsThe transport pathways of two dust storms observed by satellite

were consistent with those derived from the forward trajectories.Figs. 6 and 7 show the vertical structures of aerosol types fromCALIPSO satellite along the altitude-orbit cross-section. The brightyellow color in the figures represents dust aerosol.

The first dust storm event occurred on 30 March 2007 atGuaizihu and Sunitezuoqi. Meteorological station observationsindicated that the dust storm occurred at Guaizihu at UTC01:26e04:15 and Sunitezuoqi at UTC 07:35e12:00 on 30 March2007. The trajectories of the dust storm (Supplementary Fig. S1)suggest that the dust storm at Guaizihu arrived over the Bohai Seaaround half a day later (30 March) and Yellow Sea after about1 day (31 March). The dust storm at Guaizihu and Sunitezuoqiarrived over the East/Japan Sea about a day and a half later (31March)and over the northwestern Pacific after around 2 days (1April). This result was consistent with the CALIPSO satelliteobservations (Fig. 6). The CALIPSO data showed that the dust fromGuaizihu was transported to the east of Guaizihu after around 4 h(Fig. 6a), to the Bohai Sea around 15 h later (Fig. 6b) and to theYellow Sea around 27 h later (Fig. 6c). The dust over the East/JapanSea (Fig. 6d) and the Pacific (Fig. 6e) was attributed to Guaizihuand Sunitezuoqi.

Fig. 7 shows the transport of another dust storm event thatoccurred at Guaizihu on 29 April 2007. The dust occurred atGuaizihu (Fig. 7a) and was transported to the southeast on 30 April

2007 (Fig. 7b), to the Yellow Sea and East China Sea on 1e2May (Fig. 7c and d), and finally to South China Sea on 3 May(Fig. 7e). This was consistent with the results of forward trajectories(Supplementary Fig. S2).

3.2.3. Dust layers over dust source regions and the China seasFigs. 6 and 7 also show vertical information on the dust layers.

The results showed that dust layers over the source regions,including thewest andmiddle of Inner Mongolia and Loess Plateauareas, were mainly located up to 4 km above ground level (Figs. 6aand 7a). The dust particles weremainly contained in the layer fromthe surface to 6 km above the mean sea level over Bohai Sea(Fig. 6b and c), from 0 to 4 km over Yellow Sea (Fig. 6b and c, andFig. 7c and d), from 0 to 6 km over East China Sea (Fig. 7c and d),and from 0 to 4 km over South China Sea (Fig. 7e). Fig. 7c showsthat sometimes a higher dust layer (>7 km) appeared over theYellow Sea and East China Sea. The dust layers (<4 km) over sourceregions and the China seas were consistent with the trajectoriesshown in Figs. 4 and 5.

3.3. Probability of impact of different dust sources over the Chinaseas

The probability of spring dust storms impacting the China seaswas defined by the ratio of dust storms arriving over the seas to thetotal dust storms, using a trajectory analysis. Forward trajectory

Fig. 7. Same as Fig. 6, but for dust storm at Guaizihu station on 29 April 2007.

S.-C. Tan et al. / Atmospheric Environment 46 (2012) 299e308 305

analysis showed that dust storms at Guaizihu and Sunitezuoqistations had the largest probability of affecting Yellow Sea. Nextmost probable was Bohai Sea, followed by East China Sea, with thesmallest probability in South China Sea (Table 1). Dust stormsfromGuaizihu had a larger probability of affecting the seas than didthose from Sunitezuoqi.

Dust particles from Guaizihu to Yellow Sea were mostly trans-ported at altitudes below 4 km (Table 1 and Fig. 4), as with duststorms from Sunitezuoqi (Table 1 and Fig. 5). For example, for thedust storm events occurring at Guaizihu station that affectedYellow Sea, there were 39% events transported through the >4 kmlayer, but 61% transported through the <4 km layer. At Sunitezuoqistation, the percentage of dust storms transported in the <4 kmlayer (90%) was nine times larger than that in the >4 km layer(10%). This was the same for dust storms transported from thesetwo stations to the other three seas (i.e., the Bohai Sea, East ChinaSea and South China Sea).

Table 1The percentage of spring dust storms at Guaizihu and Sunitezuoqi stations affecting the

Seas Percentage

Guaizihu

Affecting the sea Affecting the sethrough <4 km

Bohai Sea 50.6 26Yellow Sea 59.7 36.4East China Sea 27.3 23.4South China Sea 10.4 10.4

3.4. Aerosol index and dust deposition on the days of spring duststorms in Inner Mongolia affecting the seas during 2000e2007

3.4.1. Aerosol indexThe satellite-retrieved aerosol index was used to further verify

the influence of dust storms on the China seas derived from theforward trajectory model. According to the forward trajectories,after 1e4 days of dust storms occurring at Guaizihu and Sunite-zuoqi stations, the dust arrived over the Bohai Sea, Yellow Sea, EastChina Sea, or northern South China Sea (north of 19�N). The averageaerosol index values for the days of dust storms affecting thedifferent seas are shown in Fig. 8a. The total numbers of dust daysaffecting the Bohai Sea, Yellow Sea, East China Sea and South ChinaSeawere 54, 64, 28 and 11, respectively. The aerosol indexwas largewhen dust particles arrived over the seas, with average indexvalues of 2.4, 1.9, 1.4, and 1.7 for the Bohai Sea, Yellow Sea, EastChina Sea, and northern South China Sea, respectively. Previous

China seas during 2000e2007 through HYSPLIT model.

Sunitezuoqi

alayer

Affecting the sea Affecting the seathrough <4 km layer

24.1 1925.3 22.810.1 10.12.5 2.5

Fig. 8. Average aerosol index (a) and dust deposition (b) for the days of spring dust storms affecting the Bohai Sea (BH), Yellow Sea (YS), East China Sea (ECS), the northern SouthChina Sea (NSCS) and southern South China Sea (SSCS) during 2000e2007.

S.-C. Tan et al. / Atmospheric Environment 46 (2012) 299e308306

research indicated that an average aerosol index of 1.5e2 mayreflect dust source regions well. Large aerosol index can be used toidentify the outbreaks of dust storms over source regions anddownwind areas (Darmenova et al., 2005). Hence, the high aerosolindex supported that the forward trajectory analysis.

The aerosol index over most areas of the southern South ChinaSea was <1.3 and the average value was small (0.9). This indicatedthat Asian dust may not affect the southern South China Sea. This isidentical to the results of a previous study (Lin et al., 2007).

3.4.2. Dust deposition amountDust deposition in spring 2000e2007 over the China seas was

estimated by the deposition in spring 2006 as simulated by theGRAPES/CUACE-Dust model. First, dust deposition over the Chinaseas simulated from GRAPES/CUACE-Dust model was compared toOMI aerosol index (AI) in spring 2006. Results showed a closecorrelation between them, with a relationship of deposition(g m�2) ¼ 22.292e50.370 � AI þ 28.127 � AI2 (R2 ¼ 0.55, signifi-cance level < 0.0001). This relationship was used to estimate dustdeposition for spring 2000e2007 over the China seas, according tothe aerosol index shown in Fig. 8a. The spatial distribution of theestimated dust deposition is shown in Fig. 8b.

The average deposition values for the days of dust storms inInner Mongolia affecting the different seas during spring2000e2007 were 58, 30, 7 and 21 g m�2 for the Bohai Sea, YellowSea, East China Sea, and northern South China Sea respectively,with an average of 18.7 gm�2. The total deposition amount over theabove four seas (total area of w1.95 � 106 km2) was about 36million tons. Uematsu et al. (2003) calculated a value of 21 g m�2

over the coastal sea for the whole year from March 1994 toFebruary 1995. The spring deposition should be at least 12.6 g m�2

because the ratio of spring dust storms to the whole year is >60%,which is comparable to our result.

The total atmospheric input of mineral dust was 225 milliontons for the seven major events in April 2006, based on GRAPES/CUACE-Dust model simulation (Wang et al., 2010). The total emis-sion of dust storms in Inner Mongolia in April 2006 was about 70million tons on the basis of spring dust emission flux accounting for31% of the total Asian dust (Zhang et al., 2003b). From the ratio ofdust storm occurrence days in April 2006 (2.26 days) to spring2006 (5 days, see Fig. 2a), the estimated total emission in spring2006 was about 155 [70/(2.26/5)] million tons. The estimatedtotal emission of spring dust storms in Inner Mongolia during

2000e2007 was about 785 million tons (the emission amount ineach year was estimated from the dust occurrence days in each yearshown in Fig. 2a/5 days in spring 2006 � 155 million tons). Hencethe deposition amount over the China seas was about 4.6% of thetotal emission.

4. Conclusions

In this study, the transport of spring dust storms in InnerMongolia during 2000e2007 and its influence to the China seaswere examined based on daily observations at meteorologicalstations, satellite observations, forward trajectory model, andmodeled dust deposition. The main conclusions were as follows:

(1) During spring 2000e2007, dust storms occurred mostly in thewest of Inner Mongolia and Hunshandake Desert, and Guaizihuand Sunitezuoqi were the two stations with the highest duststorm frequencies. The interannual variation in spring duststorms correlated well with wind speed. The monthly clima-tological variation correlatedwell with wind speed and relativehumidity.

(2) Forward trajectories showed that the dust from Sunitezuoqistation arrived over the China seas directly through coastalareas. The dust coming from Guaizihu station affected theChina seas through coastal areas by way of the Inner Mongoliadeserts and/or the Loess Plateau. The transport paths of duststorms at Sunitezuoqi to the China seas were characterized byshort distance and a single dust source, while those at Guaizihuwere characterized by relatively long distance with multiplesources. In addition, two case studies of dust storms occurringat Guaizihu and Sunitezuoqi showed that the paths of thestorms estimated from forward trajectories were consistentwith CALIPSO satellite observations.

(3) Dust storms from Guaizihu and Sunitezuoqi stations weremostly transported in a <4 km layer from source regions to theChina seas. CALIPSO satellite observations indicated that dustparticles were mainly contained in the 0e4 km layer over thedust source regions and the China seas, and sometimes dust ata high level (>7 km) could occur over the Yellow Sea and EastChina Sea. This lower dust layer (<4 km) over the sourceregions and the China seas was consistent with the forwardtrajectories.

S.-C. Tan et al. / Atmospheric Environment 46 (2012) 299e308 307

(4) Forward trajectory analysis indicated that dust storms atGuaizihu and Sunitezuoqi stations had the highest probabilityof affecting the Yellow Sea, followed by the Bohai Sea and theEast China Sea, with the smallest probability in the South ChinaSea. The probability of affecting the seas was larger for duststorms from Guaizihu than for storms from Sunitezuoqi.

(5) The satellite-retrieved aerosol index and modeled dust depo-sition also supported the influence derived from the forwardtrajectory model. The average index values for the days of duststorms affecting the different seas were 2.4, 1.9, 1.4, and 1.7 forthe Bohai Sea, Yellow Sea, East China Sea, and northern SouthChina Sea, respectively. The average deposition over these fourseas was 18.7 g m�2. The total deposition amount for the daysof dust storms affecting the China seas was about 36 milliontons, which amounts to 4.6% of the total emission of spring duststorms in Inner Mongolia during 2000e2007.

Acknowledgments

The authors would like to thank the National Natural ScienceFoundation of China (Grant No. 41005080), Open Research Programof Key Laboratory of Meteorological Disaster of Ministry of Educa-tion, Nanjing University of Information Science and Technology(Grant No. KLME1110) and the Ministry of Science and Technologyof China (Grant No. 2010DFA22770) for funding this study.

Appendix. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.atmosenv.2011.09.058

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