Observed Chemical Characteristics of Long-Range Transported Particles at a Marine Background Site in...

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Observed Chemical Characteristics of Long-RangeTransported Particles at a Marine Background Site inKoreaMylene G. Cayetano a , Young J. Kim a , Jin Sang Jung a , Tsatsral Batmunkh a , Kwang YulLee a , Sung Yong Kim a , Kwan Chul Kim a , Dong Gyu Kim a , Suk Jo Lee b , Jeong Soo Kimb & Lim Seok Chang ba Advanced Environmental Monitoring and Research Center, Gwangju Institute of Scienceand Technology (GIST), Gwangju, Koreab Air Quality Research Division, Climate and Air Quality Research Department, NationalInstitute of Environmental Research, Incheon, Korea

Available online: 31 Oct 2011

To cite this article: Mylene G. Cayetano, Young J. Kim, Jin Sang Jung, Tsatsral Batmunkh, Kwang Yul Lee, Sung Yong Kim,Kwan Chul Kim, Dong Gyu Kim, Suk Jo Lee, Jeong Soo Kim & Lim Seok Chang (2011): Observed Chemical Characteristicsof Long-Range Transported Particles at a Marine Background Site in Korea, Journal of the Air & Waste ManagementAssociation, 61:11, 1192-1203

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ISSN: 1047-3289 J. Air & Waste Manage. Assoc. 61:1192–1203DOI: 10.1080/10473289.2011.604001Copyright 2011 Air & Waste Management Association

TECHNICAL PAPER

Observed Chemical Characteristics of Long-Range TransportedParticles at a Marine Background Site in Korea

Mylene G. Cayetano, Young J. Kim, Jin Sang Jung, Tsatsral Batmunkh, Kwang Yul Lee,Sung Yong Kim, Kwan Chul Kim, and Dong Gyu KimAdvanced Environmental Monitoring and Research Center, Gwangju Institute of Science and Technology(GIST), Gwangju, Korea

Suk Jo Lee, Jeong Soo Kim, and Lim Seok ChangAir Quality Research Division, Climate and Air Quality Research Department, National Institute ofEnvironmental Research, Incheon, Korea

ABSTRACTDeokjeok Island is located off the west coast of the KoreanPeninsula and is a suitable place to monitor the long-rangetransport of air pollutants from the Asian continent. Inaddition to pollutants, Asian dust particles are also trans-ported to the island during long-range transport events.Episodic transport of dust and secondary particles wasobserved during intensive measurements in the spring(March 31–April 11) and fall (October 13–26) of 2009. Inthis study, the chemical characteristics of long-range-transported particles were investigated based on highlytime-resolved ionic measurements with a particle-into-liquid system coupled with an online ion chromatograph(PILS-IC) that simultaneously measures concentrations ofcations (Liþ, Naþ, NH4

þ, Kþ, Ca2þ, Mg2þ) and anions (F�,Cl�, NO3

�, SO42�). The aerosol optical thickness (AOT)

distribution retrieved by the modified Bremen AerosolRetrieval (M-BAER) algorithm from moderate resolutionimaging spectroradiometer (MODIS) satellite data con-firmed the presence of a thick aerosol plume coming fromthe Asian continent towards the Korean peninsula. Sevendistinctive events involving the long-range transport (LRT)of aerosols were identified and studied, the chemical com-ponents of which were strongly related to sector sources.Enrichment of acidic secondary aerosols on mineral dustparticles, and even of sea-salt components, during trans-port was observed in this study. Backward trajectory, che-mical analyses, and satellite aerosol retrievals identified twodistinct events: a distinctively high [Ca2þþMg2þ]/[Naþ]ratio (>2.0), which was indicative of a preprocessedmineraldust transport event, and a low [Ca2þþMg2þ]/[Naþ] ratio(<2.0), which was indicative of severe aging of sea-saltcomponents on the processed dust particles. Particulate

Cl� was depleted by up to 85% in spring and 50% in thefall. A consistent fraction of carbonate replacement (FCR)averaged 0.53 in spring and 0.55 in the fall. Supportingevidences of Cl� enrichment on the marine boundarylayer prior to a dust front were also found.

Supplementalmaterials are available for this article. Goto the publisher’s online edition of the Journal of the Air &Waste Management Association for sector and air mass clas-sifications of clean and LRT cases.

INTRODUCTIONAsian continental outflow transports large quantities of pri-mary and secondary aerosols over the Korean peninsuladuring the spring and fall seasons, mostly influenced bythe passage of cold fronts.1 Observations during the springhave been extensively studied during the Asian PacificRegional Aerosol Characterization Experiment (ACE-Asia)2–5 and other field campaigns (Joint Research on Long-Range Transboundary Air Pollutions in Northeast Asia[LTP],6 Transport and Chemical Evolution over the Pacific(TRACE-P)7,8) These aerosols are either emitted directly frommultiple sources or produced through various atmosphericprocesses. Furthermore, atmospheric aerosols engage in var-ious processes during transport that modify their chemicalcomposition, mixing state, and reactivity.9–12

Particularly in the northeast region (China, Mongolia,Siberia, Korea, Japan) of the Asian continent, source regionsof mineral dusts include loess soils from the arid Gobi desertof northernChina andMongolia, and the Taklimakandesertof westernChina.13Mineral dust particles are considered thelargest single contributor to transported particles in theatmosphere14 and are susceptible to processing and chemi-cal transformation during transport. Mineral dust particlescan also be coatedwith sulfate and nitrate,12,15,16 as well as acarbonaceous10 or sea-salt17 component via heterogeneousprocesses during transport. These in turn may be removedfrom their surface, making them more available for proces-sing and the production of secondary aerosols.9 Such pro-cesses would determine the chemical characteristics of theresulting aerosol. The long-range transport of mineral dustassociatedwith urbanpollution has therefore become one ofthe major challenges in Northeast Asia.

1192 Journal of the Air & Waste Management Association Volume 61 November 2011

IMPLICATIONSThe chemical characteristics of aerosol particles evolve asthey undergo long-range transport (LRT) in the atmospherefrom the source region. Aside from meteorological condi-tions, the signature of the source region influences the load-ing and chemical characteristics of LRT aerosols.

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Long-range-transported aerosol particles can impact anenvironment a considerable distance from its source.However, limitations concerning the availability of long--termmonitoring data havemade it difficult to achieve scien-tific and political consensus regarding the relative impacts oflong-range-transported aerosol particles. The NortheastAsian countries have undertaken a wide range of collabora-tive activities in order to build a scientific consensus on thebasis of coordinatedmultilateral research activities. The JointResearch on Long-Range Transboundary Air Pollutions inNortheast Asia (LTP),6,18 the so-called “LTP Project”, wasestablished to characterize the transport and transformationof atmospheric aerosols over Northeast Asia, and to identifytheir sources based on continuous, intensive, and remotesensingmeasurements and receptormodels. Long-term (con-tinuous) and intensive (seasonal) measurement campaignswere implemented atmultiple surface sites inNortheast Asia.The 2009 LTP Project initialized the use of highly time-resolved aerosol chemistry measurements to study the trans-port of pollutants to a background site in Korea.

This study reports the results from the 2009 LTP inten-sive measurement campaigns at a backgroundmarine site inKorea. The aim of the intensive measurement was to inves-tigate the chemical characteristics of local and long-range-transported Asian aerosols under different synoptic condi-tions from the unique Northeast Asian outflow. In addition,satellite-retrieved aerosol optical thickness and airmass backtrajectory analyses were used to identify the spatial distribu-tion and transport of aerosols. Furthermore, detailed analysiswas performed to characterize the degree of processing oflong-range-transported mineral dust and secondary species.

EXPERIMENTAL METHODSThe LTP ground measurement site (Deokjeok Island,37�1303300N, 126� 805100E, 50 m above sea level [ASL]) islocated off the west coast of the Korean peninsula in theeffective downwind area of the Asian continent (Figure 1).Incheon Metropolitan City is located just 40 km east,

whereas the capital city, Seoul, is located 75 km east of themeasurement site.

Measurements were taken during the spring (March 31to April 11) and fall (October 14 to October 26) of 2009. Aparticle-into-liquid system (PILS; BMI, Hayward, CA, USA)19

coupled with an online ion chromatograph (IC) that simul-taneously measures cations (Liþ, Naþ, NH4

þ, Kþ, Ca2þ,Mg2þ) and anions (F�, Cl�, NO3

�, SO42�) was installed

inside a mobile laboratory at the Deokjeok Island monitor-ing site. Ambient aerosols pass through a PM2.5 (particulatematter with an aerodynamic diameter �2.5 mm) size cutimpactor, and then through an annular and carbon denuderfor removal of reactive gaseous components. The overallPILS sampling flow rate was maintained at 12.8 L/min by acritical orifice. The remaining particles were mixed withhigh-purity steam (18 M�,) injected coaxially, and main-tained at a flow rate of 1.5 mL/min and a steam temperatureof 98 � 5 �C. The grown particles, now droplets, were col-lected by an impactor. An internal standard (LiF) of knownflow rate and concentration was used to collect and dissolvethe droplets. These droplets are then subsequently injectedinto the IC (Metrohm,model 850, Herisau) for ionic analysisin a 30-min interval. Recovery of LiF was equal to 75% �17% for Liþ and 85% � 10% for F�. Particulate matter(PM2.5) mass concentration was also measured in a 30-mininterval using an optical particle counter (OPC; Grimm,model 1.107, Ainring, Germany) 120-Hr back trajectoriesending at 200 m ASL at the Deokjeok Island site were ana-lyzed using Hybrid Single-Particle Lagrangian IntegratedTrajectory (HYSPLIT) model 4.20 Input data for HYSPLITutilized the meteorological data from the FNL (Final Run)framework that are archived from the GDAS (Global DataAssimilation System) of NCEP (National Centers forEnvironmental Protection).20 The computation periodfrom March 31 to April 11, 2009, included all possiblein situ monitoring every 6 hrs, ending at 12:00 a.m.Coordinated Universal Time (UTC) (09:00 a.m. KoreanStandard Time [KST]), 06:00 a.m. UTC (3:00 p.m. KST),12:00 p.m. UTC (9:00 p.m. KST), and 6:00 p.m. UTC (03:00

Figure 1. Sampling locations (clockwise from left): (a) Northeast Asian region (including the Asian Continent, mainly China and Mongolia, Korea,and Japan), divided by dashed lines into sector sources (Sectors I–IV, KP). (b) Deokjeok Island off the west coast of the Korean peninsula. The islandis a background marine site. (c) GIST mobile laboratory, situated on Deokjeok Island, equipped with ambient air quality measurement instruments.

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a.m. KST). The aerosol optical thickness (AOT) over themeasurement region was retrieved using the MODIS Terraand Aqua hierarchical data format (hdf) satellite data utiliz-ing the modified Bremen Aerosol Retrieval (M-BAER) algo-rithm.21–23 The study domain covered approximately theNortheast Asian region from 100�E to 145�E and 20�N to45�N, which includes all of Korea, Japan, most of easternChina, and parts of Mongolia and Russia. Aerosol opticalthickness images by M-BAER are integrated over the entireatmospheric column and at a fixed time frame.Meteorological parameters, including wind speed, winddirection, precipitation, temperature, and humidity, werealso measured by an automated weather station (AWS).Highly time-resolved ionic chemical composition, PMmassconcentration from OPC, and meteorological parameterswere averaged to hourly data.

RESULTS AND DISCUSSIONAir Mass History and Temporal Evolution of

Secondary SpeciesThe synoptic events and source regions are summarized inTable 1. Descriptive statistics and the contribution of eachchemical component to the PM2.5 mass are presented inTable 2. Aerosol optical thickness (AOT) retrieved fromsatellite data using the M-BAER19 algorithm combinedwith HYSPLIT18 back trajectory analyses were used toexamine how the aerosol was transported spatially. Theair mass pathways arriving in Deokjeok Island are categor-ized into five sectors. Those originating from Siberia, fol-lowed by Northeast China and the Yellow Sea, are

categorized as Sector I. Those coming from the arid regionsof Mongolia and Central China, through the Shandongpeninsula, and passing over the Yellow Sea are categorizedas Sector II. Those coming from the east coast of China(through the Yellow Sea) in a southern direction are cate-gorized as Sector III. Those originating from Russia, north-eastern China, and entering the Korean peninsula throughthe East Sea towards Deokjeok Island are categorized asSector IV. Lastly, stagnant air mass pathways circulatingover the Korean peninsula are categorized as KP. The sectorregions are illustrated together in Figure 1a.

Long-range transport events of secondary aerosol spe-cies andmineral dust can thus be verified over the NortheastAsian grid from satellite-based AOT distribution and path-way directions (see Supplementary Materials; available inthe publisher’s online edition). In addition, the differencesin chemical composition can be associated with air masssources (Figure 2a and b). Generally, changes in ionic com-position observed on the ground are strongly associatedwith changes in air mass origins. The chemical characteris-tics of each event are summarized in Table 1.

On the basis of local wind patterns (Figure 3), most LTPcases influenced by westerly wind experienced an increase inthe distributionof ionic components (Figure 3b, c, e) and dustcomponents (Figure 3f, g).WhenKimet al. conducted a 24-hrsampling and measurements of aerosol components inDeokjeok andGosan from about 2005 to 2007, they observeda 40% increase in secondary aerosols, and most wind casesappeared to be westerly in origin, indicating the influence oflong-range-transported (LRT) pollutants from easternChina.6

Table 2. Descriptive statistics of PM2.5 and its chemical components.a

PM2.5 Na+ NH4+ K+ Ca2+ Mg2+ Cl� NO3� SO42�

DLb 0.1c 0.001 0.0003 0.00002 0.001 0.0004 0.001 0.001 0.001

Synoptic eventsClean 14.9 � 2.7 (100) 0.6 � 0.0 (4.0) 1.1 � 0.2 (7.2) 0.08� 0.01 (0.6) 0.2� 0.2 (1.2) 0.10� 0.03 (0.7) BDLd (0.0) 0.5� 0.2 (3.5) 2.2 � 0.5 (15.3)LRT-1 43.5� 22.9 (100) 0.7 � 0.1 (2.0) 3.1 � 1.6 (6.4) 0.3 � 0.2 (0.7) 0.5� 0.3 (1.3) 0.3 � 0.1 (0.7) 0.2� 0.2 (0.5) 3.3� 2.1 (6.8) 5.6 � 3.4 (11.1)LRT-2 69.8� 20.3 (100) 0.7 � 0.1 (1.3) 6.3 � 3.2 (9.5) 0.5 � 0.5 (0.7) 0.6� 0.2 (1.2) 0.2 � 0.1 (0.4) 0.2� 0.1 (0.2) 6.0� 3.8 (8.6) 14.6� 8.1 (22.4)LRT-3 74.1� 15.1 (100) 0.3 � 0.5 (0.5) 10.3� 3.0 (14.7) 0.8 � 0.4 (1.1) 0.5� 0.4 (0.8) 0.1 � 0.1 (0.1) 0.1� 0.2 (0.2) 5.6� 3.0 (7.8) 25.4� 9.9 (36.4)LRT-4 13.2 � 7.0 (100) 0.8 � 0.1 (8.1) 0.9 � 0.8 (7.7) 0.2 � 0.1 (1.4) 0.4� 0.3 (3.3) 0.1 � 0.1 (1.2) 0.7� 0.1 (8.0) 0.9� 0.4 (8.3) 3.3 � 2.2 (28.8)LRT-5 23.0� 15.0 (100) 1.2 � 0.4 (7.0) 0.1 � 1.4 (0.4) 0.1 � 0.2 (0.4) 0.6� 0.8 (2.8) 0.2 � 0.2 (1.0) 1.3� 1.0 (8.0) 1.0� 2.5 (4.9) 1.1 � 3.9 (5.5)LRT-6 35.2� 14.7 (100) 1.0 � 0.2 (4.3) 1.2 � 1.3 (2.6) 0.4 � 0.3 (1.0) 1.1� 0.8 (3.2) 0.3 � 0.2 (1.0) 0.9� 0.2 (3.8) 3.2� 2.5 (7.8) 4.1 � 3.5 (11.5)LRT-7 37.1 � 9.6 (100) 0.79� 0.05 (2.2) 2.9 � 1.6 (6.5) 0.4 � 0.2 (0.9) 0.9� 0.4 (2.4) 0.3 � 0.1 (0.7) 0.9� 0.1 (2.5) 3.2� 1.3 (7.6) 8.4 � 4.3 (19.8)

Notes: aAverage concentration� standard deviation, mg/m3. Contribution to PM2.5, in percent, %.bDL¼ detection limit, mg/m3; calculated using the data from periodic injectionof method blank sample during sampling flow rate check and transport liquid flow rate check.cManufacturer specifications.dBDL ¼ below detection limit.

Table 1. Summary of event classifications and source regions.

Synoptic Event Source Regions Timeline (KST, 2009)

Clean Sectors II and III April 2 11:00 p.m. to April 4 09:00 a.m.LRT-1 Sector I March 31 09:00 a.m. to April 2 10:00 p.m.LRT-2 Sector II April 4 10:00 a.m. to April 8 09:00 a.m.LRT-3 Sector III-KP April 8 10:00 a.m. to April 11 6:00 p.m.LRT-4 Sectors I and II October 13 8:00 p.m. to October 15 11:00 p.m.LRT-5 Sector II October 16 12:00 a.m. to October 22 12:00 a.m.LRT-6 Sector II, extending from West Russia October 22 01:00 a.m. to October 24 08:00 a.m.LRT-7 Sector II-KP October 24 09:00 a.m. to October 26 12:00 p.m.

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Figure 2. The temporal evolution of ground-based chemical composition during clean and long-range transport events. The top arrow rangedescribes the time periods corresponding to different air mass source regions. The x-axes are scaled to 1 day, with grid lines corresponding to 0000KST. (a) Spring and (b) fall intensive periods.

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It is apparent from Figure 2b that ground-based concen-trations of chemical components of atmospheric aerosolswere minimal for the period April 2 11:00 p.m. to April 409:00 a.m. KST. Although the local wind direction indicateditwas predominantly northwest of the island (Figure 3a), themeasurement site was mostly impacted by clean marine airfrom the Yellow Sea. Moreover, the average PM2.5 mass con-centration was also found to beminimal (14.9� 2.7 µg/m3).Thus, this period has been classified as “Clean”.

The spring intensive measurement period is character-ized by air mass coming from the northeast and easternregions of China. The fall intensive measurement period ischaracterized by a 120-hr air mass originating from the aridregions of Mongolia and China, as well as LRT dust transportevents. The dust front that passed over the Korean peninsula(KP; Table 2) also brought in elevated levels of secondaryaerosol particles. The LRT-3 event during spring and theLRT-7 event during fall resulted in the highest averagePM2.5 masses of 74.1 � 15.1 and 37.1 � 9.6 µg/m3, respec-tively. A common factor between the two events is the pos-sible accumulation of aerosol particles while the air masslingers slowly around the KP (Figure 3d and h). In addition,the relativelywarm (17 to 22 �C) and humid (relative humid-ity [RH] >80%) air favored the formation of secondary aerosolsuch as (NH4)2SO4. Both events also accumulated the highestconcentration of NH4

þ (10.3 � 3.0 and 2.9 � 1.6 µg/m3

during LRT-3 and -7, respectively) and SO42� (25.4 � 9.9

and 8.4 � 4.3 µg/m3 during LRT-3 and -7, respectively).The air mass originating from Sector I during the LRT-1

event in spring exhibited the lowest concentration of sec-ondary acids: 3.3� 2.1 µg/m3 for NO3

� and 5.6� 3.4 µg/m3

for SO42�. Almost the same sector source during the LRT-4

event in the fall marked the lowest NO3� concentration of

0.9 � 0.4 µg/m3, but not of SO42�. Sector II air mass can be

seen to contribute significantly to the concentration of sec-ondary species in PM2.5, the highest being 22.4% for SO4

2�

during the LRT-2 event in spring and19.8% for SO42� during

the LRT-7 event in fall. Figure 2b also suggests the arrival of amarine anddust front between the periods LRT-5 and -6. Theincrease in sea-salt components was first observed in LRT-5,with contributions reaching as high as 7% for Naþ and 8%for Cl�. The event was succeeded by elevated levels of dustcomponents during LRT-6, having a Ca2þ concentration of1.1� 0.8 µg/m3 andMg2þ concentration of 0.3� 0.2 µg/m3.Mineral dust sampled in its preprocessing form was com-monly detected during the dust transport event fromOctober 16 12:00 a.m. to October 22 12:00 a.m. Dust incombination with secondary aerosols coming from the KPwas also sampled between October 24 09:00 a.m. andOctober 26 12:00 p.m. In general, spring LRT events con-tained aerosols thatwere already processed during transport,and thus representedmore aged aerosols. LRT events duringfallmeasurements containmineral dust and sea-salt particlesin their preprocessing form. The chemical composition canbe influenced by the enrichment of acidic secondary aero-sols on mineral dust particles, and even of sea-salt compo-nents. These observations and their processes will bediscussed in the succeeding sections.

Chemical Characteristics of Sea-Salt Componentsin Long-Range-Transported Particles

On a single-particle basis, Sullivan et al. established the leastreported occurrence of the presence of chloride on mineraldust.17 This could happen when the NaCl in the marineboundary layer has been converted into HCl(g) via the het-erogeneous reaction with SO4

2�, CH3SO3�, NO3

�, and their

Figure3. Wind rose plots of episodic events during spring and fall intensive periods. Top row, from left to right: (a) Clean, northwest wind; (b) LRT-1,west and northwest; (c) LRT-2, west; and (d) LRT-3, southeast. Bottom row, from left to right: (e) LRT-4, northwest; (f) LRT-5, west; (g) LRT-6, northand southwest; and (h) LRT-7, southeast.

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precursors,24 prior to the dust front. The released HCl(g)would then enrich the mineral dust by exchanging withmore volatile CO3

2� (ibid.). Sea-salt particles are impor-tant tracers for understanding the heterogeneous reactionwith reactive gases during transport. If the sea-salt parti-cles were freshly injected into the measured aerosols, itwould be expected that the measured particles would fol-low the seawater ratio (SWR) proposed by Wilson25 basedon the equivalent concentrations of Na and Cl ([Naþ]:[Cl�] ¼ 1:1.18). However, it can be seen from Figure 4that there were distinct characteristics of transported sea-

salt components for each seasonal measurement period,aside from the noticeable deviation from the SWR line. Inspring, there is almost no correlation (r2 ¼ 0.06) betweenthe major sea-salt components [Naþ] and [Cl�]. On theother hand, sea-salt aerosol components during the fallintensive period are moderately correlated (r2 ¼ 0.74). Thedeviations from the SWR line indicate that the sea-saltaerosols have already been processed and have undergonedepletion. Furthermore, the correlations show that thespring sea-salt aerosol components have undergone moreextensive processing compared with the fall intensive sea-salt aerosol components. Thus, a formula for the calcula-tion of Cl� depletion (% Cl� depleted) in the marine back-ground site is proposed as follows:

% Cldepleted ¼ SWR� Cl�=Naþ

SWR� 100 (1)

based on the assumption that freshly released sea-salt aero-sols have an SWR of 1.18. More processed sea-salt aerosolcomponent in spring was depleted (% Cl depleted¼ 85%�12%) compared to the fall sea-salt aerosol component (%Cldepleted ¼ 50% � 9%).

Specifically, we observed remarkable concentrationsof Naþ (LRT-5: 1.2 � 0.4 µg/m3, LRT-6: 1.0 � 0.2 µg/m3)and Cl� (LRT-5: 1.3 � 1.0 µg/m3, LRT-6: 0.9 � 0.2 µg/m3) during the arrival of marine and dust fronts in thefall, as seen in Table 2. This could have occurred afterCl� liberation24 prior to the transport of mineral dust,followed by Cl� enrichment in the mineral dust. Thus,HCl(g) was most likely elevated in the boundary layerbefore the arrival of the dust at Deokjeok Island. TheHCl(g) produced by an acidic gas displacement reactionwith NaCl was then mixed with incoming dusts, andthe liberated Cl� could then enrich the mineral dustmaking the particles susceptible to a CO3

2� and Cl�

heterogeneous reaction. On the other hand, prior toreaching the measurement site, the LRT-7 air masspassed over dust-laden Sector II of Northeast Asia, fol-lowed by the reactive-species-rich (NO3

� and SO42�)

Korean peninsula (see Supplementary Figure 7;Figure 2b). Note that this event resulted in NO3

� andSO4

2� enrichment, which could have displaced existingCl� in the transported mineral dust, thus producingNO3

�- and SO42�-enriched aerosols in the LRT-7 case. The

sea-salt component has been actively modified in the mar-ine boundary layer, prior to its association with dust com-ponents and later associations with secondary acids.

Figure 4. Scatter plot of themajor sea-salt components Naþ andCl�,expressed in µEq/m3. Dotted line corresponds to the sea water ratio.(a) Spring and (b) fall.

Table 3. Degree of dust processing in Northeast Asia using the FCR method.

Location Periods Results Methoda References

West Sea 11 April 2001 Average FCR 0.13 ACE Asia Flight 6 Song et al.4

12 April 2001 Average FCR 0.32 ACE Asia Flight 7Kyushu Island 13 April 2001 Average FCR 0.61 ACE Asia Flight 8b

Deokjeok Island (Spring) 31 March�8 April 2009 Ave. of all qualified FCR 0.53 Background marine This studyDeokjeok Island (Fall) 13�26 October 2009 Ave. of all qualified FCR 0.55 Background marine This study

Notes: aMeasurements were made using PILS-IC instrument.bThe ACE-Asia Flight 8 passes over the Marine Boundary Layer (MBL) over the South of Kyushu Island.

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Chemical Characteristics of Long-Range-Transported Mineral Dust and Secondary AerosolsThe atmospheric chemistry, processing, uptake and forma-tion of NO3

� and SO42� on mineral dust particles during

long-range transport has been studied at field sites inNortheast Asia,2,15,16,26,27 as well as in laboratory studies15

and with aerosol interface modeling.1 Previously, it wasshown that the sea-salt particles sampled with the trans-ported mineral dust had already undergone depletion andprocessing. The mineral dust components associated withthese sea salts had undergone aging to a similar extent. It isknown that the reactive constituents of dust particles inEast Asia are CaCO3 (calcite), MgCO3 (magnetite), andCaMg(CO3)2 (dolomite), amongwhich CaCO3 is dominantand accounts for �90% of the total dust component.28

Other major components of mineral dust such as Al2O3,SiO2, and Fe2O3 can be considered unreactive,29 whereasthe volatile CO3

2� is expected to be reactive. Ourmethod isunable to measure CO3

2� directly due to its interference inthe measured anionic conductivity. Assuming the particleis neutrally charged, it was assumed that CO3

2� is the onlyanionic species not measured by the PILS-IC instrument,and thus the difference between cations and anions mea-sured is designated as [CO3

2�]estimated4:

½CO2�3 �estimated ¼

Xn

i

½Cation�measured;i

�Pm

j½Anion�measured; j

; ð2Þ

where n and m are the total number of cations and anionsmeasured by the IC, respectively.

Considering the major cationic and anionic species,and the assumption that the only unmeasured anionicspecies is CO3

2�, we maintain that the association of Ca2þ

andMg2þwith carbonate represents preprocessed dust. Thefraction of carbonate replacement (FCR; eq 3) method pro-posed by Song et al.29 was used to investigate the degree ofaging of mineral dust aerosols.

FCR ¼ ½CO2�3 �replaced

½CO2�3 �original

; (3)

where [CO32�]replaced is the amount of CO3

2� that remainedin the aerosol after deducting the estimated CO3

2� (inequivalent concentrations), that is, [CO3

2�]original �[CO3

2�]estimated. [CO32�

original], on the other hand, is esti-mated from the sum of Ca2þ and Mg2þ, assuming that allCO3

2� is the product of the dissociation of CaCO3 (calcite),MgCO3 (magnetite), and CaMg(CO3)2 (dolomite) along itstransport into the atmosphere.

Results from FCR estimations are presented in Table 3,as well as comparable results from previous studies in theNortheast Asia region. FCR data were collected from aircraftmeasurements of aerosols in the marine boundary layer,whereas this study dealt with ground-based measurements.As such, the differences in the levels of reactive acidic gasesand the interaction time with gas-phase precursors couldhave caused the differences between the independent FCRmeasurements.29

Despite the direct observation that Ca2þ and Mg2þ donot always show a good correlation with CO3

2�, whichcan be interpreted as processed or aged mineral dust, cor-relation analysis (Figure 5) between the same charged ionsstill suggests that (SO4

2� þNO3�) and (NH4

þ þ Kþ) show aslope, s, of >1 (1.1 < s < 1.3), which means that the acidiccomponents are in excess. Moreover, a strong correlationcoefficient of r2 ¼ 0.95 suggests that these componentshave been emitted from the same source or have resultedfrom the same aerosol processes. The range in concentra-tion of SO4

2� (spring: 1.4�48.6 µg/m3; fall: 0.25�15.9 µg/m3) and NO3

� (spring: 0.1�14.3 µg/m3; fall: 0.3�23.3 µg/m3) during the spring intensive campaign is higher thanthat of the fall campaign. These major components areindicators of dust aerosol processing because they are sec-ondary in origin.12 Results of atmospheric chemical pro-cessing show dust processing on pre-emitted mineral dusttransported to Deokjeok Island. Secondary acids that haveaccumulated in mineral dust can be a sink for NH3,through heterogeneous nucleation30 or direct uptakefrom gas-phase coagulation with nitrate or sulfate richammonium particles.31 Looking at the magnitude of thecomponents, in which the spring transported aerosolevent is higher than the fall event, it can be said that thespring event producedmore secondary aerosols during theprocessing. High RH during the spring intensive periodmay have contributed more to the fine-particle formationinduced by a damp atmosphere and abundant acidicsources, as explained previously.

In this study, the presence of Ca2þ andMg2þ in varyingamounts is thought to characterize the long-range-transportedmineral dust. When themineral dust is prepro-cessed, we would expect it to be in a carbonate form, andwith a strong association with depleted sea-salt particles.However, whenmineral dusts are chemically modified dur-ing transport, the volatile CO3

2� is replaced by less volatile

Figure 5. Equivalent concentrations of major anion components(SO4

2� and NO3�) and major cation components (NH4

þ þ Kþ) forthe intensive measurements during the spring and fall seasons. Themagnitude of the spring intensive measurement data is higher thanthat of the fall intensive measurement period. Their slopes lie between1.1 < s < 1.3, and for both periods r2 ¼ 0.95.

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and non-volatile NO3� and SO4

2�, respectively.29

Meanwhile, after subtracting the contribution of theequivalent [NH4

þ] from the combined equivalent [SO42�]

and [NO3�], the difference is believed to represent excess

acids, which would process the transported dust. Thus,strong associations of Ca2þ and Mg2þ with excess acidsare regarded as indicators of processed mineral dust.

To complement the FCR of Song et al.,29 we propose asimple indicator of mineral dust aging and suggest a rangefor the degree of associations. The triplot in Figure 6 showsthe associations in normalized equivalent concentrationsof excess acids with mineral dust components and

[CO32�]estimated. In the triplot, a sample containing primar-

ily Ca2þ and Mg2þ would appear at the top vortex, primar-ily [CO3

2�]estimated at the right vortex, and primarily excessacids at the left vortex. Using this tool, we can see theextent to which the transported mineral dust has beenprocessed. From both measurements, it can be seen thatCa2þ þMg2þ are present in a smaller percentage comparedwith the excess acids. However, the range of the fraction ofmineral dust components and excess acids measured dur-ing the spring intensive period are less varied, mostlybetween about 0.25 and 0.4 for Ca2þ þ Mg2þ, and about0.6 and 0.75 for the excess acids. In contrast, data for the

Figure 6. Ternary plots of the associated chemical components. Plotted data are normalized such that equivalent concentrations of excess acids,Ca2þ þ Mg2þ and CO3

2� are summed to unity. Most of the mineral dust has already undergone complete aging, although not all because levels of[CO3

2�] were estimated in 10% of the samples in spring and in 80% of the aerosol measured in the fall. Symbols correspond to the episodic eventsdescribed in each intensive measurement period. (a) Spring and (b) fall intensive periods.

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fall intensive measurement period show that the range ofthe fraction ofmineral dust components and excess acids iswider, between about 0.1 and 0.5 for Ca2þ þ Mg2þ andabout 0.5 and 0.8 for the excess acids. In addition, thereare more fractions of mineral dust components that areassociated with excess acids.

The spring intensive data (Figure 6a) show the equiva-lent ratio of Ca2þ þ Mg2þ in the range of 25�50%. Whenthe proportion of CO3

2� is 40�70%, the mineral dust com-ponent is present in the preprocessed form, as in the case ofLRT-1. Mineral dust during local events does not showevidence of associations with secondary acids. However,mineral dust components during the LRT-2 period are asso-ciated at a constant amount with CO3

2� and excess acids,which are in inverse proportions. This indicates theexchange in the anionic component of the transportedmineral dust processed by reactive acids such as NO3

� andSO4

2�. The fall intensive data show unique results. Around65�90% of excess acids are in strong association with10�25% of mineral dust components in the LRT-4 event.The poor association of mineral dust with the secondaryacid component in the case of LRT-7 only confirmed thatmost of the aerosols sampled are in the processed form, andmostly comprise secondary acids.

One remarkable feature of the fall intensive data is thatthe degree of acidity of associated secondary acids inmineral dust can be seen by just looking at the percentageof associations with excess acids (Figure 6b). For example,by looking at the left-most panel of Figure 6b (excess acids),the LRT events are shown to be grouped in increasing orderof acidity: LRT-4 aerosols have an association of 75�87%,LRT-6 aerosols possess an association of 62�75%, and LRT-7aerosols display an association of 50�60%. Most of thesampled aerosols during LRT-5 were associated withCO3

2� (ranging from about 15% to 70%), owing to thearrival of a marine and dust front during this period.Thus, the order of acidity of the LRT events is representedby the relationship LRT-4 > LRT-6 > LRT-7.

It is therefore apparent that transported mineral dustparticles can process and accumulate reactive acids. Alaboratory-scale experiment has shown that nitrate productsafter calcite mineral processing are still intact even aftersignificant water uptake,32 whereas reaction with SO2 is

relatively slow.33 Competitive reactions between SO2 andHNO3 show no apparent effect on the reactivity of bothgases (ibid.). On the basis of single-particle measurements,Zhang et al.34 observed that the degree of SO4

2� formationmay have been dependent on the oxidation of SO2, butwould later be neutralized by surface-adsorbed or gas-phaseNH3 present in the troposphere at the time of the measure-ments. Observations of Fairlie et al.35 have shown that par-ticulate nitrate is primarily associated with dust, sulfate isprimarily associated with ammonium, and mineral dustremained alkaline across their observational grid over thePacific. Their results were even reproduced using models.

There are several studies in the Northeast Asian regionthat also utilized the ratio method for indicators of dustepisodes, some of which are tabulated in Table 4. Most ofthe values were obtained by analyzing filter-based samplesfor chemical composition, followed by the ratio method ofmineral dust components usingCa as the commonchemicalindicator.36–40 However, most of the studies were conductedat continental sites (Beijing,Mongolia, China), such that thepartner species in their ratio method was either Al or Si.Because our study site was conducted at a background mar-ine site, we employed the ratio method using Ca and Mg inpartnership with the sea-salt component Na. As shown inFigure 7, the degree of associations of mineral dust withreactive acids was explored. The extent of processing wasbased on themagnitude of reactive acids, utilizing the scatterplot of NO3

� and SO42� in relation to Ca2þ. The sea-salt and

dust components were reconciled using the ratio methodbetween mineral dust and sea-salt indicators, namely,[Ca2þ þ Mg2þ] and [Naþ], respectively. To reconcile theassociations of reactive acids with transported mineral dust,the ratio of NO3

� and SO42� individually to Ca2þ was used.

The color bar indicates the degree of association of mineraldust to sea-salt components. The range of the [Ca2þ þMg2þ]/[Naþ] ratio itself reflects the extent of the differencein aerosol events. A higher [Ca2þ þ Mg2þ]/[Naþ] (ratio >2)indicates that major dust events dominate during that per-iod, which decreases the possibility of dust processing. Alower [Ca2þ þ Mg2þ]/[Naþ] (ratio <2) ratio suggests thatsea-salt aging has already occurred on the dust particles.The equivalent ratio of SO4

2�/Ca2þ and NO3�/Ca2þ shows

the degree of association of the acidic component on dust

Table 4. Ratio method for mineral dust and soil indicators at different sites in Northeast Asia.

Ratio Method Indicative Site Period Method Reference

Ca/Si<0.2, Al/Ca>1.7 Dust storm Urban Beijing Jan, Apr, Jul, Oct 2000 24 hrs filter based, XRF Song et al38

Ca/Si 0.37 Al/Ca 1.00 Mineral dust Chinese Loess Area Not applicable Single particle-EDX Krueger et al39

Al/Ca 1.6�1.7 Surface Soils Mongolia and Inner

Mongolia

Not available Emission inventory Xuan37

Al/Ca 2.05 Mineral Dust Hunshan Dake

Sandland

Spring, 2001 PM2.5 Cheng et al40

Al/Ca2þ �2.17 Dust Urban 2001–2004 PM2.5 Wang et al36

Al/Ca2þ �0.74 Haze Beijing

Al/Ca2þ �1.04 Clear

[Caþ2þMgþ2]/[Naþ] ratio > 2.0* [Caþ2þMgþ2]/

[Naþ] (ratio < 2)**

Transported mineral dust seasalt

ageing on the processed dust

particles

Deokjeok island Spring and Fall, 2009 PILS-IC This study

*[Caþ2 þMgþ2]/[Naþ] ratio ranged from2.0 to 23.4 **[Caþ2 þMgþ2]/[Naþ] ratio ranged from 0.02 to 2.0

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particles. Observedhighermagnitudes for the SO42�/Ca2þ or

NO3�/Ca2þ ratio (>10) suggest that the ground aerosols have

alreadybeenprocessedby the reactive gaseous acids,whereasa lower ratio (<10) suggests that the dust has been processedto a lesser extent with estimated amounts of CO3

2�.

SUMMARY AND CONCLUSIONThe chemical characteristics of long-range-transportedaerosols downwind of the Asian continent were investi-gated to identify the levels of mineral dust transport andprocessing using ground-based measurements. Two

Figure 7. Ratio method of major reactive acids to Ca2þ to describe the extent of LRT aerosol processing. For both intensive periods, aerosolparticles with aged sea salt (i.e., [Ca2þþMg2þ]/[Naþ] ratio <2) are shown to have higher magnitudes of acidic components. (a) The Ca2þ-normalizedequivalent ratio of NO3

� and SO42� during the spring intensive measurement period occurs mostly between the 1:2�1:6 line, and (b) between the

1:1�1:6 line for the fall intensive measurement data. (Inset) Larger x-y range to show the acid/[Ca2þ] ratio >10.

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mineral dust transport events and details of the degrees ofassociations of their components have been identified.Evidence was found for the preprocessing of mineral dustby Cl� and reprocessing by reactive acids (NO3

� andSO4

2�). Sea-salt components were aged to the same extentas the mineral dust. The prior reaction of HCl(g) withmineral dust caused the displacements of various amountsof CO3

2�, and activated the mineral dust for further reac-tion with nitrates and sulfates. The spring observationsindicate that CO3

2� was almost fully depleted by excessNO3

� and SO42�, whereas the fall observations show that

fine-particle mode dust particles were not completelydepleted by these excess acids. Additionally, the estimated[CO3

2�] was strongly associated with [Ca2þ] and [Mg2þ] inthe fall. The analysis of measurement data suggests that thefraction of carbonate replacement (FCR), which is a mea-sure of the degree of chemical interaction between mineraldust and reactive gases, was comparable with that obtainedfrom independent ground-based measurements. In addi-tion, our triplot analysis and ratio approach supported thedegree of association between CO3

2�, excess acids, andmineral dust components. This study is important for anunderstanding of the chemical evolution of dust particlesin Northeast Asia, and clarifies the chemical processes ofaerosols from anthropogenic and natural sources. It waspossible to estimate CO3

2� from the difference of thesummed anions and excess cations, and the degree of repla-cement as shown by the FCR method, utilizing time-resolved ion data. Our observations on the processing oflong-range-transportedmineral dust and associated sea-saltcomponents suggest important evidence on the temporalevolution of naturally occurring atmospheric particles andthe subsequent anthropogenic reactive components.Generally, source regions influence the loading and chemi-cal characteristics of LRT aerosols.

ACKNOWLEDGMENTSThis workwas supported by aNational Research Foundationof Korea (NRF) grant funded by the Korean government(MEST) (Project No. 2008-0060618) and by the 2009 LTPProject supported by the National Institute ofEnvironmental Research (NIER). It was also funded by theKorea Meteorological Administration Research andDevelopment Program under grant RACS_2010-1002through the Advanced Environmental MonitoringResearch Center at Gwangju Institute of Science andTechnology. The authors also gratefully acknowledge theNOAA Air Resources Laboratory (ARL) for the provision ofthe HYSPLIT transport and dispersion model and/or READYWeb site (http://www.arl.noaa.gov/ready.php) used in thispublication. Lastly, the authors would like to acknowledgethe assistance of local residents from Deokjeok Island.

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About the AuthorsMylene G. Cayetano is currently a Ph.D. candidate at theGwangju Institute of Science and Technology (GIST). YoungJ. Kim is the Director of the Advanced EnvironmentalMonitoring and Research Center (ADEMRC) and a professorat the School of Environmental Science and Engineering inGIST. Jin Sang Jung obtained his Ph.D. at GIST, and iscurrently a postdoctoral associate at the Institute of LowTemperature Science, Hokkaido University, in Sapporo,Japan. Tsatsral Batmunkh, Kwang Yul Lee, Sung YongKim, and Kwan Chul Kim are fellow graduate students atGIST. Dong Gyu Kim was formerly a researcher inADEMRC. Suk Jo Lee, Jeong Soo Kim, and Lim SeokChang are officials of the Air Quality Research Division,Climate and Air Quality Research Department of theNational Institute of Environmental Research. Pleaseaddress correspondence to: Young J. Kim, ADEMRC,GIST, 261 Cheomdan Gwagiro (1 Oryongdong) Bukgu,Gwangju, 500-712 Korea; phone: +8262 715 3401; fax:+8262 715 3404; e-mail: [email protected].

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