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Atmospheric Environment 130 (2016) 84e94

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Springtime variations of organic and inorganic constituentsin submicron aerosols (PM1.0) from Cape Hedo, Okinawa

Bhagawati Kunwar a, K. Torii a, b, Chunmao Zhu a, Pingqing Fu a, c, Kimitaka Kawamura a, *

a Institute of Low Temperature Science, Hokkaido University, N19 W8, Kita-ku, Sapporo, Japanb Graduate School of Environmental Science, Hokkaido University, N11 W5, Kita-ku, Sapporo, Japanc State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences,Beijing, 100029, China

h i g h l i g h t s

� Springtime aerosols (PM1.0) were collected and analyzed for diacids, oxoaikcids, and a-dicarbonyls.� Anthropogenic activities are more significant during long-range atmospheric transport.� Study site is more influenced from East Asian aerosols.

a r t i c l e i n f o

Article history:Received 27 May 2015Received in revised form30 August 2015Accepted 1 September 2015Available online 5 September 2015

Keywords:Submicron aerosolsDiacids and related compoundsOrganic nitrogenOkinawaAtmospheric aging

* Corresponding author.E-mail address: kawamura@lowtem.hokudai.ac.jp

http://dx.doi.org/10.1016/j.atmosenv.2015.09.0021352-2310/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

During the spring season with enhanced Asian outflow, we collected submicron aerosol (PM1.0) samplesat Cape Hedo, Okinawa Island in the western North Pacific Rim. We analyzed the filter samples for di-acids, oxoacids, pyruvic acid, a-dicarbonyls and fatty acids to better understand the sources and atmo-spheric processes in the outflow regions of Asian pollutants. Molecular distributions of diacids show apredominance of oxalic acid (C2) followed by malonic (C3) and succinic (C4) acids. Total diacids stronglycorrelated with secondary source tracers such as SO2�

4 (r ¼ 0.87), NHþ4 (0.90) and methanesulfonate

(MSA�) (0.84), suggesting that diacids are secondarily formed from their precursor compounds. We alsofound good correlations among C2, organic carbon (OC) and elemental carbon (EC) in the Okinawaaerosols, suggesting that diacids are mainly derived from anthropogenic sources. However, a weakcorrelation of diacids with levoglucosan, a biomass burning tracer, suggests that biomass buring is notthe main source of diacids, rather diacids are secondarily formed by photochemical oxidation of organicprecursors derived from fossil fuel combustion. We found a strong correlation (r ¼ 0.98) betweeninorganic nitrogen (NO3eN þ NH4eN) and total nitrogen (TN), to which organic nitrogen (ON)contributed 23%. Fatty acids were characterized by even carbon number predominance, suggesting thatthey are derived from biogenic sources. The higher abundances of short chain fatty acids (<C20) than longchain fatty acids (>C20) further suggest that fatty acids are largely derived from marine phytoplanktonduring spring bloom.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Atmospheric aerosols are generally enriched with water-soluble oraganic (dominated by oxalic acid) and inorganic (sul-fate) components (e.g., Jimenez et al., 2009; Kawamura et al.,2010). Those atmospheric particles have an impact on theEarth's radiative balance and hydrological cycles (Ramanathan

(K. Kawamura).

et al., 2001) as well as adverse health effect to the human pul-monary tract (Mitchell et al., 1987). The extent of such impactslargely depends upon the chemical composition of atmosphericaerosols. Aerosols are emitted from the various primarily sourcessuch as soil dust, fossil fuel combustion, vehicular and biomassburning emission, and sea salt via the sea spray. They are alsosecondarily formed via the atmospheric oxidation of inorganicand organic precursors in the presence of sunlight and oxidants.Primary and secondary aerosols together with their atmosphericcirculations have an influence on local to regional air quality,

B. Kunwar et al. / Atmospheric Environment 130 (2016) 84e94 85

atmospheric loadings and chemical compositions (Pavuluri et al.,2015). Based upon the model study, organic aerosols are known tobe largely subjected to chemical transformation (Kanakidou et al.,2005).

In East Asia, emissions of pollutants (both organic and inor-ganic) are increasing due to the growing activities of Chineseindustries and biomass burning in Southeast and East Asia (Kunwarand Kawamura, 2014a,b; Zhu et al., 2015). The anthropogenicemissions in East Asia are muchmore severe than any other regionsin the world and this situation will continue to increase for thecoming decades (Ohara et al., 2007). For example, the pollutantsemitted from China are recognized to have a significant impact onthe air quality in the outflow regions, such as Okinawa Island in thewestern North Pacific Rim, by long range atmospheric transport(Kunwar and Kawamura, 2014a). Similarly, major sources of organicnitrogen in the atmosphere include vehicle exhaust, landfill, ma-rine biological algal blooms and bacteria, dust and biomass burning(Westerholm et al., 1993; Kallinger and Niessner,1999;Wedyan andPreston, 2008; Timperley et al., 1985). Wang et al. (2013) reportedthe abundant presence of water soluble organic nitrogen (WSON)in Chinese aerosols.

In a previous study, we reported the seasonal variations oforganic and inorganic aerosols in the TSP samples from Cape Hedo,Okinawa (Kunwar and Kawamura, 2014a,b). However, there are nostudies of diacids together with organic nitrogen and relatedcompounds in PM1.0 filter samples in marine aerosols in the Asianoutflow region, although there aremany studies on total suspendedparticles (Fu et al., 2013 and references therein; Kundu et al., 2010;Wang et al., 2006; Kawamura and Sakaguchi, 1999). Cape Hedo issituated on the northwestern edge of Okinawa Island, Japan, anoutflow region of East Asian pollutants. Around the samplinglocation, there is no major anthropogenic activity (Yamamoto andKawamura, 2010) and thus Cape Hedo has been used as a super-site of Atmospheric Brown Cloud (ABC) project to study the EastAsian aerosols (Takami et al., 2007).

In this study, we report the day-by-day variations of diacids,oxoacids, a-dicarbonyls and benzoic acid, and fatty acids, togetherwith organic carbon, elemental carbon, major ions and organicnitrogen in PM1.0 samples from Cape Hedo. To better understandthe sources and atmospheric processing of organic aerosols at CapeHedo, we compare the data of diacids and related compoundstogether with inorganic ions and a biomass buring tracer, i.e.,levoglucosan, a specific pyrolysis product of cellulose and hemi-cellulose (Simoneit et al, 1999).

2. Samples and analytical procedure

2.1. Site description and aerosol sampling

Aerosol sample collection was performed at the rooftop of thefacility of Cape Hedo Atmosphere and Aerosol MeasurementStation (CHAAMS, 26� 90 N, 128� 20 E) during 17 March to 13April, 2008. Fig. 1 shows a map of Okinawa along with East Asiaand the Pacific Ocean. PM1.0 samples (n ¼ 28) were collected atCHAAMS station using low volume air sampler and pre-combusted (450 �C, 4 h) quartz fiber filters (Pallflex 2500QAT,47 mm in diameter). The flow rate was 16.7 l/min. Blank filters(n ¼ 4) were collected every week. Each sample was collected for24 h. Before and after sampling, the filters were stored in apreheated glass vial (50 mL) with a Teflon-lined screw cap. Thesamples were stored in darkness at �20 �C until the analysis.During the sampling period, average temperature and relativehumidity were 19.7 �C and 71%, respectively. The weather con-ditions were cloudy and fine during the campaign, but therewere rainfall events in March 18 and 30 and April 13. The

ambient temperature, rainfall, and relative humidity are shownin Fig. S2.

2.2. Chemical analysis

Filter samples were analyzed for water-soluble diacids, oxoacids,and a-dicarbonyls by the method reported previously (Kawamuraand Ikushima, 1993; Kawamura, 1993). Known area of filter wasextractedwith organic-free purewater and then carboxylic acids anda-dicarbonyls in the extracts were derivatized with 14% BF3/n-butanol to butyl esters and/or dibutoxy acetals. The derivatives weredetermined using a capillary gas chromatography (GC; HP 6890). TheGC peaks were identified by comparing the GC retention times withthose of authentic standards and the peak identifications wereconfirmed by mass spectral examination using a GC/mass spec-trometry (GC/MS) system. Both field and laboratory blanks wereanalyzed. Oxalic acid and other organic species were detected in theblanks. However, their concentrations were less than 5% of the realsamples. The concentrations of all species reported here are cor-rected for blanks. We also performed the recovery test by spikingauthentic dicarboxylic acids to the quartz filter. The recoveries ofspiked diacidswere 94% for oxalic acid (C2) andmore than 97% for C3,C4, C5 and C6 diacids. The reproducibility in the measurements ofmajor diacids (C2, C3, and C4) was ca. 10%.

For the determination of levoglucosan, a small punch of eachfilter was extracted with dichloromethane/methanol (2:1, v/v)under ultrasonication. The extracts were concentrated using a ro-tary evaporator, and then reacted with N,O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA) to derive TMS ethers of levoglucosan.Levoglucosan was measured by gas chromatography/mass spec-trometry (GC/MS). Detailed analytical procedures can be foundelsewhere (Fu et al., 2012).

To measure total nitrogen (TN), we combusted a small filter discplaced in a tin cup at 1400 �C using elemental analyzer (EA)(Thermo Scientific, Trace GC Ultra, Delta V Advantage Isotope RatioMS). All the nitrogen species are converted to NO and furtherreduced to N2 in a reduction column. N2 is separated on a packedcolumn of gas chromatograph installed in EA and measured with athermal conductivity detector. Organic nitrogen (ON) is calculatedusing following equation.

ON ¼ TNeIN

where IN (inorganic nitrogen) means the summation of nitrateðNO�

3 Þ and ammonium ðNHþ4 Þ nitrogen. NO�

3 , NHþ4 and other major

ions were determined using an ion chromatograph (761 CompactIC, Metrohm, Switzerland). Details of analytical procedures for ionsare reported in Kunwar and Kawamura (2014a).

Organic carbon (OC) and elemental carbon (EC) were measuredusing a Sunset Laboratory carbon (OC/EC) analyzer followingInteragency Monitoring Protected Visual Environments (IMPROVE)thermal/optical evolution protocol. Detailed procedure is describedin Kunwar and Kawamura (2014a).

2.3. Air mass backward trajectory analysis and fire counts

Fig. 1a shows the 5-day back trajectories above the 500 m forevery day using the Hybrid Single-Particle Lagrangian IntegratedTrajectory (HYSPLIT4) model (Draxler and Hess, 1998). Except forfew days, air masses in the study area were transported fromEast Asia. During March 17 and 18 and April 6 and 9, air masseswere delivered from the Pacific Ocean over the sampling site. Fig.1bshows the fire counts in East Asia obtained by satellite (https://earthdata.nasa.gov/data/near-real-time-data/firms/active-fire-data) with 5-day back trajectories.

Fig. 1. (a) 5-day backward trajectories during study period. Back trajectory analysis was performed at 500 m above ground level with the NOAA HYSPLIT model. (b) 5-day backwardtrajectories with fire counts for specific days.

B. Kunwar et al. / Atmospheric Environment 130 (2016) 84e9486

3. Results and discussion

3.1. Molecular distributions of dicarboxylic acids, oxoacids and a-dicarbonyls

We detected homologous series of a,u-dicarboxylic acids(C2eC12), unsaturated diacids (phthalic, isophthalic, terephthalic,maleic, fumaric andmethylmaleic), multi-functional diacids (malic,

ketomalonic, and 4-ketopimelic), oxocarboxylic acids (uC2�uC9and pyruvic acid), a-dicarbonyls (glyoxal and methylglyoxal) andbenzoic acid in the PM1.0 samples. In addition, we also detectedhomologous series of fatty acids. Fig. 2 shows the averagemoleculardistribution of diacids, oxoacids and a-dicarbonyls.

Concentrations of total diacids, oxoacids and a-dicarbonylsranged from 114 to 537 ng m�3 (av., 273 ± 121 ng m�3),5.5e53 ng m�3 (27 ± 15 ng m�3), and 2.9e19 ng m�3

Fig. 2. Averaged molecular distributions of straight chain diacids (C2eC12), branched chain diacids (iC4�iC6), unsaturated diacids (M, F, mM, Ph, iPh, and tPh), multifunctionaldiacids (hC4, kC3 and kC7), oxoacids (uC2�uC9, and pyruvic), and a-dicarbonyls (Gly and MeGly) in PM1.0 aerosols collected at Cape Hedo. The error bars represent the standarddeviation.

B. Kunwar et al. / Atmospheric Environment 130 (2016) 84e94 87

(7.8 ± 4.2 ng m�3), respectively (Table 1). Oxalic acid (C2) was foundas the most abundant species followed by malonic (C3), succinic(C4), glyoxylic (uC2), ketomalonic (kC3) and phthalic (Ph) acid. Themolecular distributions of diacids in the Cape Hedo aerosols aresimilar to those reported from Gosan site, Jeju Island, South Korea(C2 > C3 > C4) (Kundu et al., 2010). The predominance of uC2 and

Table 1Concentrations of diacids and related compounds in atmospheric aerosols fromCape Hedo, Okinawa (2007 spring).

Concentrations (ng m�3)

Species AV ± SD Range

DiacidsOxalic, C2 172 ± 73 78e329Malonic, C3 26 ± 13 7.5e58Succinic, C4 20 ± 11 5.2e53Glutaric, C5 4.5 ± 2.5 1.0e9.9Adipic, C6 2.9 ± 2.0 0.31e9.3Pimeric, C7 0.60 ± 0.71 BDL-3.1Suberic, C8 0.93 ± 2.2 BDL-11Azelaic,C9 3.3 ± 0.75 0.37e4.5Sebacic, C10 0.39 ± 0.37 BDL-1.2Dodecanedioic, C12 0.91 ± 0.43 0.32e1.9Methylmalonic, iC4 4.3 ± 2.5 1.3e9.8Methylsuccinic, iC5 1.2 ± 0.91 0.17e3.1Methylglutaric, iC6 0.36 ± 0.26 BDL-0.95Maleic, M 1.6 ± 1.0 0.33e4.9Fumaric, F 0.68 ± 0.47 0.20e2.2Methylmaleic, mM 2.0 ± 3.9 BDL-11Phthalic, Ph 12 ± 8.4 2.39e35Isophthalic, iPh 0.74 ± 0.63 0.04e2.2Terephthalic, tPh 0.22 ± 2.2 BDL-7.2Hydroxysuccinic, hC4 0.21 ± 0.15 0.07e0.87Ketomalonic, kC3 15 ± 14 0.45e69Ketopimelic, kC7 3.4 ± 2.0 0.61e7.8Total diacids 273 ± 121 114e537OxoacidsGlyoxylic, uC2 22 ± 12 4.7e443-Oxopropanoic, uC3 0.02 ± 0.04 BDL-0.144-Oxobutanoic uC4 2.9 ± 2.0 0.15e7.329-Oxononoic, uC9 2.2 ± 1.3 0.46e6.2Total oxoacids 27 ± 15 5.5e53Pyruvic acid 4.9 ± 2.5 1.7e10Benzoic acid 17 ± 9.2 4.3e36a-DicarbonylsGlyoxal, Gly 2.9 ± 2.3 0.44e9.0Methylglyoxal, MeGly 4.9 ± 2.0 2.0e9.6Total a-dicabonyls 7.8 ± 4.2 2.9e18

Note: BDL means below detection limit (<0.001 ng m�3).

kC3 after C2 to C4 indicates a significant photochemical processingduring long range transport. Both uC2 and kC3 are the precursors ofC2 (Kawamura et al., 1996). The highest relative abundance of C2 intotal C2eC12 (78%) was observed in April 13, whereas the lowestvalue (55%) was observed in March 20.

3.2. Temporal variations of diacids, oxoacids and a-dicarbonyls

Fig. 3 shows temporal variations of diacids, oxoacids and a-dicarbonyls. Except for azelaic acid (C9), short chain diacids (C2 toC6) present similar temporal variations. The temporal variationssuggest that the source and/or formation process of diacids aresimilar except for C9. Oxalic acid shows the highest concentration inApril 3, whereas the lowest concentration was observed in March18 (Fig. 3a). Back trajectory analysis revealed that air masses of April3 originated fromMongolia passing over the northern part of Chinawhereas the air masses of April 18 were delivered from the PacificOcean. The temporal variations of C3 and C4 are similar with that ofC2. Interestingly, C3 and C4 peaked on April 8 (Fig. 3b and c). On thesame date, 9-oxononanoic acid (uC9), an oxidation product of un-saturated fatty acids (Kawamura and Gagosian, 1987), maximized(Fig. 3i). The different temporal variation betweenuC9 and C9 is dueto the presence of active eCHO group in uC9 that makes uC9 morereactive and semi-volatile. The air masses of April 8 originated fromSouth Asia. These results suggest that unsaturated fatty acids areoxidized to result in C3 and C4 diacids as well asuC9 during the longrange transport from South Asia.

Adipid acid (C6), a tracer of anthropogenic sources (Kawamuraand Ikushima, 1993), showed the highest concentration in April10 followed by April 3 whereas the lowest concentrationwas foundin April 13. Although air masses originated from the Pacific Oceanpassing over the Japanese Islands, the air parcels were encounteredwith rainfall around Cape Hedo in April 13 and thus the concen-tration was declined. During April 10 and April 5, air masses fromEast Asia were delivered to Cape Hedo with air pollutants. Phthalicacid (Ph), a tracer of anthropogenic source (Kawamura andIkushima, 1993), also showed the highest concentration in April 5(Fig. 3f), whereas the lowest concentration was found in April 6,one day before a small rain event was recorded in Okinawa.

Terephthalic acid (tPh) has been proposed as an organic tracer ofplastic burning (Kawamura and Pavuluri, 2010; Pavuluri et al., 2010;Simoneit et al., 2005). This tracer maximized in April 3 (not shownas a figure). Azelaic acid (C9), a tracer for the oxidation of biogenicunsaturated fatty acids such as C18:1 (Kawamura and Gagosian,

Fig. 3. Temporal variations of (a) oxalic acid (C2), (b) malonic acid (C3), (c) succinic acid (C4), (d) adipic acid (C6), (e) azelaic acid (C9), and (f) phthalic acid (Ph), (g) glyoxylic acid(uC2), (h) 4-oxobutanoic acid (uC4), (i) 9-oxononanoic acid (uC9), (j) glyoxal (Gly), (k) methylglyoxal (MeGly), and (l) benzoic acid in PM1.0 aerosols collected at Cape Hedo, Okinawa.

B. Kunwar et al. / Atmospheric Environment 130 (2016) 84e9488

1987), peaked in April 3 (Fig. 3e), whereas lower concentrationwasfound in April 7 although the air mass source region is East Asia. C9is the first oxidation product of unsaturated fatty acids emittedfrom biogenic sources, biomass burning of plants, meat cookingoperation and plant leaf abrasion (e.g., Ho et al., 2010). Glyoxylicacid (uC2), which is a precursor of C2 and is formed by the oxidationof aromatic hydrocarbans such as benzene and toluene, showed thehighest concentration in April 3. The peaks of C2 and uC2 in April 3suggest a significant oxidation of precursor compounds. Bothglyoxal (Gly) and methylglyoxal (MeGly) show higher concentra-tions in April 1. The temporal variation of uC2 is similar to C2whereas that of uC4 is similar to C4. These results suggest that C2and C4 are formed by the oxidation of corresponding oxoacids (uC2and uC4). The temporal variations of Gly and MeGly are similar tothat of C2, suggesting that C2 may be the oxidation product of a-dicarbonyls.

Benzoic acid has been proposed as a primary pollutant emittedfrom vehicular exhaust (Kawamura et al., 1985; Ho et al., 2006) andsecondary photochemical oxidation of aromatic hydrocarbons suchas toluene from automobiles (Suh et al., 2003). Very high ambientlevels of toluene (11.4 mg m�3) were detected in Beijing (Duan et al.,

2008). Guo et al. (2006) reported daily concentration of toluene upto 50 mg m�3 in Hong Kong. Higher concentrations of benzoic acidwere recorded in March 20, 28 and 29, and April 3 and 8 in CapeHedo. The high abundance of benzoic acid in this study suggests asignificant contribution of toluene and subsequent oxidation dur-ing the long range atmospheric transport from the Asian Continent.

During the campaign, thereweremany butminor rainfall eventsin the study site (March 17, 18, 23 and 30, and April 3, 10 and 13),suggesting that these organic species were washed out from theatmosphere near the sampling site. In fact, during these rainy days,we found lower concentrations of diacids, oxoacids and a-dicar-bonyls, except for April 3.

Malonic (C3) to succinic (C4) acid ratio has been used as an in-dicator of photochemical aging of organic aerosol (OA) (e.g.,Kawamura and Ikushima, 1993). The C3/C4 ratios are higher inMarch 30 and April 6, whereas it was lowest in April 5. Except forthe samples of March 30 and April 5 and 6, C3/C4 ratios do not showa significant variation, suggesting that Cape Hedo aerosols arephotochemically more aged probably during long-range atmo-spheric transport. In fact, the air mass source regions associatedwith the high C3/C4 ratios are located in Northwest and Northeast

Fig. 4. Temporal variations in the concentration ratios of (a) malonic/succinic (C3/C4), (b) adipic/azelaic (C6/C9), (c) phthalic/azelaic (Ph/C9), and (d) fumaric/maleic (F/M) acids inPM1.0 aerosols collected at Cape Hedo, Okinawa.

B. Kunwar et al. / Atmospheric Environment 130 (2016) 84e94 89

China, from which polluted aerosols were transported long dis-tances to the sampling site in Okinawa. The average C3/C4 ratio inthis study is 1.3. Similar but higher ratio (1.7) was observed for TSPsamples from the same site (Kunwar and Kawamura, 2014b), theSouthern Ocean (1.7) (Fu et al., 2013) and Western North Pacific(0.91e1.5) (Fu et al., 2013). Average C3/C4 ratio in this study is lowerthan those from the Indian Ocean (2.1) (Fu et al., 2013), South ChinaSea (2.7) (Fu et al., 2013) and Pacific Ocean (4) (Kawamura andSakaguchi, 1999), suggesting that photochemical aging of Oki-nawa aerosols are less significant than the open ocean aerosols.

C6 and Ph acids are produced by the oxidation of anthropogeniccyclohexane and aromatic hydrocarbons, respectively (Kawamuraand Ikushima, 1993), whereas C9 is produced by the oxidation ofbiogenic unsaturated fatty acids having double bond at C9 positionas stated above. Hence, C6 to C9 and Ph to C9 ratios can be usefultracers to evaluate the relative contributions of anthropogenic andbiogenic sources (Ho et al., 2006; Wang et al., 2009). Both C6/C9 andPh/C9 ratios peaked in April 7 (Fig. 4). On this date, the air masssource regions are the coasts of China (Guangzhou, Shantou andXiamen, not shown as a figure). The lowest concentration of C9 atthis day is due to the difference in source region. Lower C6/C9 ratioswere observed inMarch 18,19, 23, 30, and April 5 and 13. Lower Ph/C9 ratios were also observed in March 23 and 30. These resultssuggest that there are more biogenic contributions on those days.Spring is a growing season with more biogenic emissions. Maleicacid (M) can be isomerized to fumaric acid (F) under strong solarradiation during long range atmospheric transport. A significantphoto-isomerization of M to F was observed in April 2 and 3(Fig. 4d) when air masses were delivered from North China andlocal temperature in Okinawa was not high (Fig. S2). This resultsuggest that photo-isomerization is enhanced during long rangeatmospheric transport.

The average F/M ratio in this study is 0.43. This value is severaltimes lower than that of the springtime TSP aerosols (1.9) from thesame sampling site (Kunwar and Kawamura, 2014b), the SouthChina Sea (1.9) (Fu et al., 2003), North Atlantic (2) (Fu et al., 2013),Indian Ocean (3.8) (Fu et al., 2013), and Western North Pacific (1.1)(Fu et al., 2013). These comparison suggests that larger particlesmay be photochemically more aged in the marine atmosphereduring the accumulation and coagulation process of fine particules.

3.3. Carbonaceous components and organic nitrogen

Concentrations of organic carbon (OC) and elemental carbon

(EC) ranged from 0.41 to 2.49 mg m�3 (av. 1.2 mg m�3) and 0 to 0.85(av. 0.36 mg m�3), respectively. Higher concentrations of OC wereobserved in March 28 and April 3, whereas the lowest concentra-tions were observed in March 23 and April 6 (Fig. 5a). Similarly, thehighest concentration of EC was found in March 28, whereas thelowest value was observed in March 18. A strong correlation(r ¼ 0.82) between OC and EC (Fig. 6a) during the whole campaignsuggests that they have similar sources. OC to EC ratio is a usefultool to discuss the sources of aerosols (Kunwar and Kawamura,2014a). The OC/EC ratios >2.0 indicate a significant contributionof secondary organic aerosol (SOA) (Cao et al., 2003). OC/EC ratios inthe study area are >2, suggesting a significant fraction of OC in CapeHedo aerosols is of SOA origin. The highest OC/EC ratio (12.7) wasfound in March 18 (not shown as a figure), which may be due tomore biogenic emission of organic aerosols and precursors frommarine sources and/or lower contributions of EC. Back trajectoryanalysis shows that air masses of March 18 were delivered fromoceanic regions (Fig. 1). We found similar temporal variations oftotal diacids with OC and EC. A strong correlation between OC withtotal diacids (r ¼ 0.84) suggests that diacids are mostly derivedfrom anthropogenic sources from East Asia via long-rangetransport.

Concentrations of organic nitrogen (ON) ranged from 0.0 to1.5 mg m�3 (av. 0.54 mg m�3) (Fig. 5b). The temporal variation of ONis similar to those of OC, NHþ

4 and nss� SO2�4 , suggesting that

source of ON is East Asia. There are various sources of organic ni-trogen in the atmosphere; e.g., emission from vehicular exhaust(Westerholm et al., 1993) marine algal blooms and bacteria(Sorooshian et al., 2008), biomass burning, cooking (Simoneit et al.,2002), etc. Higher concentration of ON was observed in April 3,when air mass came from northern part of China. Contribution ofON to total nitrogen is 23%. We found higher percentage of ON forone year observation of TSP samples (37%) (unpublished data),suggesting that organic nitrogen is present in larger size fraction ofaerosols. In fact, very high concentration of WSON have been re-ported in China during the dust storm event (Wang et al., 2013). Wefound higher contribution of NH4eN to TN in PM1.0 samples thanTSP samples (unpublished data). The higher concentration ofNO3eN in coarse fraction suggests that NO�

3 is associated withcoarse particles during long range atmospheric transport.We foundstrong correlation between IN and ON (r¼ 0.98) (Fig. 7), suggestingthe similar sources of these nitrogenous compounds. We did notfind statistically significant correlation between levoglucosan andON, suggesting that ON is not emitted from the biomass burning in

Fig. 5. Temporal variations of (a) organic carbon (OC) and elemental carbon (EC), (b)organic nitrogen (ON), (c) NHþ

4 , (d) nss� SO2�4 , (e) MSA�, (f) nss-Kþ, and (g) levo-

glucosan in PM1.0 aerosols collected at Cape Hedo, Okinawa.

Fig. 6. Scatterplot between (a) OC and EC, and (b) SO2�4 and NHþ

4 in aerosols collectedfrom Cape Hedo, Okinawa.

B. Kunwar et al. / Atmospheric Environment 130 (2016) 84e9490

this region. However, Mace et al. (2003) reported that organic ni-trogen is emitted from biomass burning.

Fig. 7. Scatterplot between total nitrogen (TN) and NH4eN þ NO3eN in aerosolscollected from Cape Hedo, Okinawa.

3.4. Temporal variation of anthropogenic tracers and evidence forsecondary formation of diacids

nss� SO2�4 can be used as an anthropogenic tracer of industrial

origin and NHþ4 is a tracer of biomass burning, agricultural wastes

and animal excreta (Pavuluri et al., 2011). The temporal variationsof NHþ

4 and SO2�4 are similar (Fig. 5c, d). Both nss� SO2�

4 and NHþ4

show higher concentrations in March 29. A strong correlation(r ¼ 0.98) between nss� SO2�

4 and NHþ4 supports a similar oxida-

tion pathway to form NH4HSO4 or (NH4)2SO4 (Fig. 6b). nss� SO2�4

and MSA� are formed by the photochemical oxidation in theatmosphere.

Total diacids show strong correlations with NHþ4 (r ¼ 0.90),

nss� SO2�4 (0.87) and MSA� (0.84) (Fig. 8aec). MSA� is formed by

the oxidation of dimethyl sulfide (DMS) that is emitted from in-dustrial emission (Yuan et al., 2004) although DMS is also emittedfrom the ocean by microbial activity (Saltzman et al., 1983). A

Fig. 8. Scatterplots between (a) NHþ4 and total diacids, (b) SO2�

4 and total diacids, (c) MSA� and total diacids, and (d) Naþ and total diacids in aerosols collected from Cape Hedo,Okinawa.

B. Kunwar et al. / Atmospheric Environment 130 (2016) 84e94 91

strong correlation between MSA� and total diacids suggests thatdiacids are mainly formed from the secondary formation from in-dustrial emission. Yuan et al. (2004) reported significant concen-trations of MSA� in Beijing. Although Naþ is emitted primarily fromthe ocean, we found a negative correlation (r ¼ 0.31) between Naþ

and diacids (Fig. 8d), suggesting that diacids are not of marineorigin. nss-Ca2þ is a major component of dust. In spring, our sam-pling site is influenced from dust emitted from the arid regions inEast Asia (Kunwar and Kawamura, 2014a). Although nss-Ca2þ wasnot detect in the PM1.0 samples, we found abundant nss-Ca2þ in TSPsamples collected at the same site in spring (Kunwar andKawamura, 2014a).

Levoglucosan is a specific sugar compound formed by the py-rolysis of cellulose at high temperature (>300 �C) (Simoneit et al.,1999). Concentrations of levoglucosan ranged from 0.43 to8.1 ng m�3 (3.4 ± 2.5 ng m�3). Higher concentrations of levoglu-cosan were observed in March 20, 22, 28, and April 1, 2, 3, 4 andApril 10. nss-Kþ is another tracer for biomass burning, whichshowed co-varied peaks with levoglucosan (Fig. 5g). However, weobserved relatively weak correlation between the two tracers(r ¼ 0.44). We checked the back trajectory along with fire countsduring March 20, 22 and 28, and April1 2, 3, 4, and 10. Except forMarch 28 and April 2, air masses were influenced from the areaswith high fire counts, whereas during March 28 and April 2, airmasses did not passed over the areas with high fire counts. Thisresults may suggest that, in addition to open biomass burning,closed or domestic biomass burning is also important in East Asia(Sang et al., 2011 and references therein).

To better understand the sources and transformation process,

we performed correlation analyses for selected diacids, oxoacids,inorganic ions, and levoglucosan (Table 2). We found that C2 isstrongly correlated with its precursor compounds such as C3(r ¼ 0.94), C4 (0.90), uC2 (0.83), and pyr (0.73). These strong cor-relations suggest that the predominance of C2 is involved withchain reactions of its precursors. The fair correlation (0.64) betweenC2 and benzoic acid suggest that C2 come in part from vehicularemission or atmospheric oxidation of aromatic hydrocarbonsbecause benzoic acid is a tracer of vehicular emission (Ho et al.,2010; Kawamura and Kaplan, 1987) and is produced by the oxida-tion of toluene (Suh et al., 2003). It is interesting to note that thereis no correlation between C9 and short chain (C2eC4) diacids,suggesting that small diacids are not derived from biologicalsources. We observed a weak correlation (r ¼ 0.46) between oxalicacid and levoglucosan, instead, correlations are strong amongoxalic acid, NHþ

4 and nss� SO2�4 , suggesting that the Okinawa

aerosols are largely involved with the secondary formation withvigorous photochemical processing.

3.5. Possible sources of diacids and ionic species

To understand the possible sources of diacids andmajor ions, weperformed principal component analysis using varimax rotationwith Kaiser Normalization (SPSS software). As shown in Table 3,four components were found. C2 to C5, uC2, NHþ

4 , MSA� andnssSO2�

4 show high loadings in Component 1. Component 1 isassociated with anthropogenic sources followed by photochemicaloxidation because nss� SO2�

4 , NHþ4 , MSA� are emitted from

anthropogenic sources including industrial emissions, biofuel

Table 2Correlation (r) analyses among diacids species and selected major ions in aerosols from Cape Hedo, Okinawa, Japan.

C2 C3 C4 C6 C7 C9 Ph uC2 uC9 Pyr Naþ nssKþ NO�3 nssSO2�

4 MSAe Levo

C2 1.00C3 0.94 1.00C4 0.90 0.95 1.00C6 0.58 0.63 0.62 1.00C7 0.44 0.49 0.50 0.86 1.00C9 0.19 0.11 0.14 0.32 0.32 1.00Ph 0.42 0.46 0.65 0.61 0.57 0.31 1.00uC2 0.83 0.89 0.87 0.70 0.55 0.35 0.62 1.00uC9 0.81 0.90 0.86 0.47 0.33 0.10 0.36 0.78 1.00Pyr 0.73 0.86 0.81 0.69 0.61 0.27 0.62 0.93 0.78 1.00Naþ �0.02 0.10 0.17 �0.07 �0.02 �0.17 0.00 �0.02 0.20 0.08 1.00nssKþ 0.57 0.61 0.54 0.33 0.36 0.26 0.28 0.58 0.45 0.43 �0.17 1.00NO�

3 �0.10 �0.25 �0.26 �0.19 �0.13 �0.04 �0.16 �0.27 �0.27 �0.27 �0.09 �0.22 1.00nssSO2�

4 0.81 0.85 0.84 0.55 0.45 0.42 0.45 0.85 0.81 0.75 0.10 0.69 �0.29 1.00MSA- 0.82 0.80 0.86 0.56 0.43 0.21 0.52 0.74 0.72 0.64 0.16 0.47 �0.14 0.84 1.00Levo 0.46 0.52 0.46 0.64 0.60 0.33 0.50 0.76 0.32 0.71 �0.27 0.44 �0.20 0.49 0.38 1.00

Table 3Principal component analysis of selected diacids, oxoacid and major ions in theaerosols from Cape Hedo, Okinawa.

Component

Species 1 2 3 4

C2 0.94 0.14 0.09 �0.03C3 0.94 0.22 0.08 �0.12C4 0.91 0.30 0.05 �0.12C5 0.83 0.30 0.28 �0.04C6 0.44 0.79 0.31 �0.01C7 0.30 0.79 0.35 �0.03C8 0.03 0.92 �0.12 �0.07C9 0.04 0.10 0.79 �0.04Ph 0.38 0.58 0.31 0.02uC2 0.82 0.24 0.43 �0.09Naþ �0.27 0.09 �0.01 0.88NHþ

4 0.90 0.12 0.18 �0.24nssKþ 0.61 �0.02 0.34 �0.27NO�

3 �0.08 �0.15 �0.06 0.91nssSO2�

4 0.90 0.15 0.17 �0.22MSA- 0.85 0.28 0.00 �0.04levo 0.41 0.28 0.69 �0.00Variance 54% 13% 9% 7%

Fig. 9. Averaged molecular distributions of fatty acids in PM1.0 aerosols collected fromCape Hedo, Okinawa. The first number of fatty acid indicates the carbon numberswhereas the second number indicates the number of double bond.

B. Kunwar et al. / Atmospheric Environment 130 (2016) 84e9492

burning, and animal excreta. uC2 is formed by the oxidation ofaromatic hydrocarbons. Component 2 is associated with biogenicsources because C7 and C8 diacids are produced by photochemicaloxidation of biogenic unsaturated fatty acids emitted from terres-trial higher plants andmarine phytoplankton (Pavuluri et al., 2010).Higher loadings of C9 and levoglucosan in Component 3 suggestthat C9 diacid is associated with biomass burning. High loadings ofNaþ and NO�

3 in Component 4 suggest the adsorption of HNO3 onthe surface of sea-salts (Deshmukh et al., 2013). These resultssuggest that springtime aerosols are largely influenced from in-dustrial emission, terrestrial and marine biological emission, agri-cultural activity and biomass burning.

3.6. Molecular distributions of fatty acids

Fig. 9 shows average molecular distribution of fatty acids. Shortchain fatty acids (�C19) are emitted from vascular plants, microbes,and marine phytoplankton, while long chain fatty acids (�C20) arelimited to terrestrial higher plant waxes (Kawamura et al., 2004;Simoneit, 1978; Kolattukudy, 1976). Average molecular distribu-tion showed the peak of C14:0 fatty acid followed by C16:0 and C18:0.The predominance of even carbon number fatty acids suggests animportant contribution of biological sources to the Okinawa

aerosols. C18:1 is highly reactive compared to C18:0 in the atmo-sphere due to the presence of double bond. Thus, the ratios of C18:1/C18:0 can be used to evaluate the aging process of organic aerosols(Ho et al., 2011). Very high ratios were observed in April 7 followedby April 10 and 9 whereas lower ratios were observed in March 31,April 1 and 2. The lower ratios are associated with the photo-chemical oxidation process. The fresh organic aerosols were ob-tained in April 7 whereas the aged aerosols were obtained duringMarch 31 and April 1. In April 7, air masses were delivered from thecoastal region of China, whereas air masses of March 31 weredelivered from Mongolia via North China and air masses of April 1were originated from North China.

4. Summary and conclusions

We measured dicarboxylic acids, oxoacids, a-dicarbonyls andbenzoic acid as well as fatty acids in the springtime PM1.0 aerosolsfrom Cape Hedo, Okinawa to better understand the source andformation processes. Among diacids and related compounds, C2 ismost abundant species (172 ± 73 ng m�3) followed by C3(26 ± 13 ng m�3) and C4 (21 ± 11 ng m�3). Molecular distribution ofdiacids suggests that they are formed by the photochemicaloxidation of anthropogenic and biogenic precursors during longrange atmospheric transport. Higher concentrations of C2, C6 andbenzoic acid in the aerosols (April 3) suggest significant influencesfrom anthropogenic activities including vehicular emission. Strongcorrelations between total diacids and EC suggest that diacids aremostly transported from the polluted regions of East Asia. Wefound a strong correlation between MSA� and total diacids, sug-gesting that some fractions of diacids were derived with industrial

B. Kunwar et al. / Atmospheric Environment 130 (2016) 84e94 93

emissions from Chinese megacities. Temporal variations of OC, EC,nss� SO2�

4 , NHþ4 and ON are similar, suggesting that organic ni-

trogen is emitted from the anthropogenic sources in East Asia. Nocorrelation between ON and levoglucosan suggests that ON is notemitted from biomass burning. The even carbon number predom-inance of short chain fatty acids (<C20) further suggests that marineemission is also important source during the campaign.

Acknowledgment

This study was in part supported by the Japan Society for thePromotion of Science (grant-in-aid No. 24221001) and the Envi-ronment Research and Technology Development Fund (B-0903)from the Ministry of the Environment, Japan. We thank to Y. Kita-mori and S. Aggarwal for their helps during sample collection.Wealso thank the NOAA Air Resource Laboratory (ARL) for the provi-sion of the HYSPILT transport model and READY website (http://www.arl.noaa.gov/ready.php) used in this paper.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.atmosenv.2015.09.002.

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