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Chemosphere 215 (2019) 413e421

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Chemosphere

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

Evolution of aerosol chemistry in Xi'an during the spring dust stormperiods: Implications for heterogeneous formation of secondaryorganic aerosols on the dust surface

Yan Qin Ren a, c, d, Ge Hui Wang b, c, d, *, Jian Jun Li c, Can Wu c, d, Cong Cao c, d, Jin Li c, d,Jia YuanWang c, d, Shuang Shuang Ge b, Yu Ning Xie b, Xing Ru Li b, e, Fan Meng a, Hong Li a

a State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing, 100012, Chinab Key Lab of Geographic Information Science of Ministry of Education of China, School of Geographic Sciences, East China Normal University, Shanghai,200142, Chinac State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, 710061, Chinad University of Chinese Academy of Sciences, Beijing, 100049, Chinae Department of Chemistry, Analytical and Testing Center, Capital Normal University, Beijing, 100048, China

h i g h l i g h t s

� n-Alkanes were the dominant organic compound class during the whole campaign.� Low-volatile anthropogenic compounds dominated in the fine mode particles.� SOA were predominantly enriched on the coarse particles during the dust storm.� Multiphase reaction has an important role in evolution of aerosol chemistry.

a r t i c l e i n f o

Article history:Received 3 August 2018Received in revised form7 October 2018Accepted 10 October 2018Available online 11 October 2018

Handling Editor: R Ebinghaus

Keywords:Primary organic aerosolsSecondary organic aerosolsHygroscopicitySize distributionsDust storm

* Corresponding author. Key Lab of Geographic InforEducation of China, School of Geographic Sciences, EShanghai, 200142, China.

E-mail addresses: ghwang@geo.ecnu.edu.cn, wang

https://doi.org/10.1016/j.chemosphere.2018.10.0640045-6535/© 2018 Elsevier Ltd. All rights reserved.

a b s t r a c t

TSP and 9-stage size-segregated samples were simultaneously collected in Xi'an during the spring of2013 and analyzed for organic aerosols (OA) on a molecular level. n-Alkanes were the dominant com-pound class during the whole campaign, followed by fatty acids. High molecular weight (HMW) n-al-kanes and fatty acids dominated in the coarse mode particles (>1.1 mm) during the dust event, indicatingthey were mostly originated from surface soil and plants in the upwind regions. Low-volatile anthro-pogenic compounds such as benzo(e)pyrene (BeP) and bisphenol A (BPA) dominated in the fine modeparticles during the whole campaign. In contrast, semi-volatile anthropogenic compounds such asphenanthrene (Phe) and di-n-butyl phthalates (DBP) showed a bimodal size distribution with a signif-icant increase in the coarse mode during the dust event due to their vaporization from the fine modeparticles and the subsequent adsorption on the dust surface. Secondary organic aerosols (SOA) in Xi'anduring the dust storm period were predominantly enriched on the coarse particles, which can beascribed to the adsorption and subsequent oxidation of gas-phase hydrophilic organics on the aqueous-phase of hygroscopic dust surface (e.g., mirabilite). Our work suggested an important role of multiphasereaction in evolution of aerosol chemistry during the dust long-range transport process.

© 2018 Elsevier Ltd. All rights reserved.

mation Science of Ministry ofast China Normal University,

gh@ieecas.cn (G.H. Wang).

1. Introduction

Taklimakan desert in western China and Gobi deserts insouthern Mongolia and northern China are the major source re-gions of East Asia dust (Wang et al., 2018b; Wu et al., 2012), fromwhich more than 800 Tg of dust was annually emitted toward thedownwind regions spaning from the inland China to the westernUS. Atmospheric dust particles affect climate directly by scattering

Y.Q. Ren et al. / Chemosphere 215 (2019) 413e421414

and absorbing the solar radiation and indirectly by acting as cloudcondensation nuclei (CCN) or ice nuclei (IN) (Zhang et al., 2015).Moreover, during the transport process, the dust particles couldinternally mix with pollutants by absorption and reaction andbecome more toxic. Thus, they have adverse health effects afterbeing deposited into human respiratory tract (Khaniabadi et al.,2017; Wang et al., 2012b, 2017a, b). Major fraction of dust parti-cles was inorganic and organic species in dust were relatively mi-nor (Li et al., 2016; Wang et al., 2014). Therefore, studies on dustparticles have mostly focused on inorganic components such aselements and inorganic ions, whereas study on organic compoundsduring the dust storm periods was sparsely (Li et al., 2016b; Wanget al., 2012b, 2015). Although they were relatively less abundantthan mineral components, absolute concentrations of organicaerosols were still comparable and even higher than those in non-dust storm period (Wang et al., 2012b, 2015). Our previous obser-vation on aerosols fromMt. Hua and Mt. Tai in East Asia found thatdust storm originating from Gobi desert in north China could resultin a sharp increase of organic aerosols over the mountaintoptroposphere; those organics largely derived from plants in thedesert region (Wang et al., 2012b). In addition, Stone et al. (2011)found that during the dust transport process primary and sec-ondary carbonaceous particles become internally mixed with dust,which could significantly modify the dust's chemical compositionand surface properties. Park and Cho (2013) also observed hydro-philic water-soluble organic carbon was produced by atmospherictransformation processes, which were believed to be similar tothose seen with SO4

2� and oxalate.In the March of 2013 we conducted an intensive sampling in

Xi'an city, inland China to identify the changes in aerosol chemistrycaused by dust storm (Wang et al., 2014). The previous workidentified the sources and formation mechanisms of ambient par-ticulate nitrate and sulfate in Xi'an during the presence of the duststorm. We found that sulfate in the event was almost entirelytransported from the surface soil in Gobi deserts and existedmostlyas mirabilite (Na2SO4$10H2O) and gypsum (CaSO4$2H2O) whilenitrate during the episode was largely derived from heterogeneousreaction of gaseous HNO3 and NH3 on the dust surface and existedas NH4NO3 (Wang et al., 2014). To further investigate the evolutionof aerosol chemistry, i.e., their sources and formation mechanisms,during the dust storm period in the city, herewe firstly explored thehourly variations in concentrations and compositions of primaryand secondary organic aerosols during the campaign on a molec-ular level, and then discussed their size distributions with a focuson secondary organic aerosols (SOA).

Fig. 1. Percentages of organic compounds in the TSP samples collected during the duststorm, transition and non-dust storm periods.

2. Experimental section

2.1. Collection of TSP and size-segregated particles

TSP and size-segregated samples were collected from 9 to 12March 2013 on the rooftop of a three-story building on the campusof Institute of Earth Environment in Xi'an (33º290�34º440N,107º400�109º490E), which was situated in Guanzhong Basin, centralChina. The TSP samples were collected using a high-volumesampler (TCH-1000, Tianhong Company, China) with an airflowrate of 1.0m3min�1 and 1-hr interval. The size-segregated sampleswere collected for 5e24 h for each set depending on the dustloading by using a 9-stage sampler (Andersen, USA) at an airflowrate of 28 Lmin�1 with 9 size bins as <0.4, 0.4e0.7, 0.7e1.1, 1.1e2.1,2.1e3.3, 3.3e4.7, 4.7e5.8, 5.8e9.0 and> 9.0 mm. All samples werecollected onto quartz fiber filters, which were combusted in our labat 450�Cfor 8 h before the sampling to remove organic contami-nants. Field blank samples were collected before and after the

sampling by mounting a blank filter onto the sampler about 15minwithout sucking any air. All the samples were sealed in analuminum foil bag individually and stored in freezer under �20 �Cbefore analysis.

2.2. Organic aerosol analysis

Detailed analytical procedure has been published elsewhere (Liet al., 2018; Ren et al., 2017a). Briefly, aliquot of the filter sampleswere cut in pieces, followed by extracted dichloromethane/meth-anol (2:1, v/v). The extracts were concentrated into dryness andreacted with 60 mL N, O-bis-(trimethylsilyl) trifluoroacetamide(BSTFA) at 70 �C for 3-hr. Afterwards, the derivatives were dilutedby the addition of 40 mL of n-hexane with 3.0 ng mL�1 of C13 n-alkane (as the internal standard) to 100 mL. The derivatives werefinally quantified using gas chromatography coupled with massspectroscopy detector (HP7890A, HP5975, Agilent Co., USA) (GC/MS). All the target compounds in the field blanks were less than 5%of those in the real samples, suggesting no significant contamina-tion. Recoveries of all the target compounds were 80e120%.

3. Results and discussions

3.1. General description

During the sampling period, the annual heaviest dust stormevent arrived in Xi'an, which originated from the Mongolian Gobidesert, with the highest TSP level of 7527 mgm�3 at 9th 18:00 (localtime, LT). The whole campaign was classified into three periods byour previous work (dust storm period (DS), transition time (tran-sition), and non-dust storm period (NDS)), based on the changes inTSP levels and the 48-hr air mass backward trajectories (Wanget al., 2014). Here we follow this classification to further discussthe organic aerosol chemistry.

A total of 11 classes of organic compounds in the TSP sampleswere determined and their relative abundances were plotted inFig. 1. All the determined organics were 2875± 1086 (1224e4996)ng m�3, 2455± 1096 (874e4596) ng m�3 and 4031± 1928(1218e7252) ng m�3 during the DS, transition and NDS period,

Y.Q. Ren et al. / Chemosphere 215 (2019) 413e421 415

accounting for 0.17± 0.09%, 0.4± 0.19%, 0.61± 0.34% of the TSPmasses, respectively.

n-Alkanes (C18eC36) were the most abundant organics with C31and C29 being the dominant congeners in the DS and NDS periods,respectively, which was similar to those on the mountaintoptroposphere of Mt. Hua and Mt. Tai, China (Wang et al., 2012b). n-Alkanes in the TSP samples were 820± 308, 682± 299 and1111± 582 ngm�3 and account for 29%, 29% and 27% of the totaldetermined organic compounds in the DS, transition and NDS pe-riods, respectively (Table 1, Fig. 1). CPI (concentration ratio of odd toeven) of n-alkanes was indicative of the relative contributions frombiogenic and anthropogenic sources, because n-alkanes derivedfrom terrestrial higher plant were typically of a CPI value largerthan 5 while those from fossil fuel combustion was commonly of aCPI value closed to unity (Li et al., 2016; Wang and Kawamura,2005; Wang et al., 2006). In the current work, CPI values of n-al-kanes in the TSP samples were 2.0± 0.5 (1.2e2.9) and 1.4± 0.1(1.2e1.6) during the DS period and NDS period, respectively. Theywere 2e3 times lower than those on the mountaintop of Mt. Hua,which was situated nearby Xi'an, during the spring of 2009(5.0± 3.5 for the DS and 3.3± 1.1 for the NDS, respectively) (Wanget al., 2012b), indicating that n-alkanes in the urban ground surfacewere largely derived from fossil fuel combustion even in the dust

Table 1Hourly concentrations (ng m�3) of organic compounds, element carbon (EC) and organicnon-dust periods.

Time Dust storm (n¼ 27) (9 March, 18:00e10March, 21:00LT)

Transition (March, 12:0

I. Major species in TSP samples(a) n-alkanesSn-alkanes(C18�36) 820± 308(246e1290)B 682± 299(1LMWa n-alkanes(C18�27) 233± 80(91e370)B 253± 145(8HMWa n-alkanes(C28�36) 587± 245(135e1000)AC 430± 224(9plant wax n-alkanesb 269± 135(88e652)A 157± 57(44fossil fuel n-alkanesb 507± 213(113e873)B 479± 243(1CPIc 2.0± 0.5(1.2e2.9)A 1.6± 0.3(1.1(b) fatty acidsSfatty acids(C10�32) 769± 295(287e1264)A 499± 217(1LMWa fatty acids(C10�20) 500± 189(185e1050)A 352± 160(1HMWa fatty acids(C21�32) 269± 148(88e645)A 147± 61(31CPIc 8.7± 2(5.4e14)A 7.4± 0.9(6e(c) fatty alcoholsSfatty alcohols (C16�32) 391± 204(96e982)A 235± 96(58LMWa fatty alcohols(C16�23) 45± 19(11e90)B 45± 23(15eHMWa fatty alcohols(C24�32) 346± 189(85e906)A 189± 77(33CPIc 8.7± 2.4(4.4e15)A 4.2± 1.8(1.6(d) sugars 393± 252(123e961)BC 427± 371(1(e) polyols and polyacids 255± 137(82e588)BC 308± 158(1(f) phthalates 184± 108(67e669)B 186± 84(86II. Minor species in the TSP samples(g) PAHsSPAHs 22± 13(6.1e59)B 34± 23(10eLMWa PAHs(3&4�ring) 13± 6.7(3.6e31)B 19± 12(5.9HMWa PAHs(5&6�ring) 9.6± 6.2(2.4e28)B 15± 11(3.6(h) phthalic acids 24± 14(2.8e60)C 61± 37(26e(i) lignin and resin products 11± 15(1.6e61)A 6.1± 4.2(1.7(j) hopanes 4.2± 2.4(1.0e11)B 4.1± 2.8(0.8(k) Biphenol A (BPA) 2.9± 2.7(0.6e13)B 12± 13(1.3Total 2875± 1086(1224e4996)B 2455± 109OC, mg m�3 68± 50(8.7e254)A 33± 8.9(20EC, mg m�3 6.7± 8.6(0e32)A 7.1± 2.3(3.4TSP, mg m�3 2116± 1385(774e7527)A 634± 150(4Total/OC, % 6.8± 8.1(1.4e38)A 7.8± 3.2(2.1Total/TSP, % 0.17± 0.09(0.05e0.37)C 0.4± 0.19(0

A, B, CDifferent capital letters (A, B, and C) within the same species denote significant difa LMW: low molecular weight; HMW: high molecular weight.b Plant wax n-alkanes: calculated as the excess odd homologues-adjacent even homo

amount.c CPI, carbon preference index: (C19þC21þ...þC35)/(C18þC20þ…þC36) for n-alkanes, (C

(C17þC19þ…þC31) for fatty alcohols.

storm period.Fatty acids (C10:0�C32:0) were the second most abundant com-

pounds, accounting for 27%, 21% and 23% of the total determinedorganics during the DS, transition and NDS periods, respectively(Fig. 1). Fatty acids presented a similar composition during thewhole campaign, which were dominated by C16:0, C18:0 and C30:0with concentrations being 769± 295, 499± 217 and913± 447 ngm�3, respectively. Carbon preference indexes (CPI,even to odd) of the fatty acids during the three periods were similar,which were 8.7± 2 (5.4e14), 7.4± 0.9 (6e9) and 6.8± 1.1 (4.5e10),respectively (Table 1 and S1). Low molecular weight (LMW) fattyacids (C10:0-C20:0) were derived from vascular plants, microbes, andcooking emissions (Simoneit et al., 2004). While high molecularweight (HMW) fatty acids (C21:0� C32:0) were characteristic ofterrestrial higher plant wax origins, such as C24:0, C26:0, C28:0, andC30:0 (Fu et al., 2008, 2010). The abundant saturated HMW fattyacids during the three periods suggested an important contributionof terrestrial plant emissions to the ambient organic aerosols.

The third highest organic class in NDS was sugars, followed bypolyols and polyacids. In contrast, during DS period fatty alcoholswas the third highest class, followed by sugars. As shown in Fig. 1,sugars accounted for 13%,16% and 15% in the DS, transition and NDSevents, while fatty alcohols accounted for 14%, 10% and 10% of the

carbon (OC) in total suspended particles (TSP) during the dust storm, transition and

n¼ 15) (10 March, 21:00e110LT)

Non-dust storm (n¼ 22) (11 March, 12:00e12March, 10:00LT)

80e1236)B 1111± 582(310e2237)A

7e555)B 451± 228(127e872)A

4e1041)BC 660± 393(177e1454)A

e228)BC 224± 94(74e370)AC

29e977)B 822± 465(214e1798)A

e2.2)B 1.4± 0.1(1.2e1.6)B

90e975)B 913± 447(256e1706)A

42e724)B 605± 296(179e1240)A

e251)B 307± 172(71e653)A

9)B 6.8± 1.1(4.5e10)B

e395)B 438± 233(99e777)A

105)B 84± 46(18e175)A

e290)B 353± 189(75e634)A

e6.8)B 3.5± 1.1(2.0e6.3)B

00e1485)AC 626± 342(175e1496)A

13e634)AC 399± 160(172e855)A

e349)B 350± 219(102e1079)A

99)B 61± 37(14e145)A

e55)B 32± 19(7.6e72)A

e45)B 30± 18(6.6e73)A

145)B 102± 55(18e181)A

e14)A 13± 9.2(2e34)A

e11)B 7.5± 4.9(1.9e24)A

e48)A 12± 10(1.2e38)A

6(874e4596)B 4031± 1928(1218e7252)A

e49)B 56± 13(37e84)A

e12)A 8.9± 6.3(0e28)A

12e1037)B 687± 198(476e1399)B

e12)A 7.1± 3.2(3e15)A

.14e0.71)B 0.61± 0.34(0.21e1.3)A

ferences at p< 0.05, with the same capital letters suggesting no statistic difference.

logues average and the difference from the total n-alkanes is the fossil fuel-derived

10:0þC12:0þ...þC32:0)/(C11:0þC13:0þ…þC31:0) for fatty acids and (C16þC18þ...þC32)/

Y.Q. Ren et al. / Chemosphere 215 (2019) 413e421416

total detected organic compounds in the three phases, respectively.As a key tracer of biomass burning, levoglucosanwas the dominantsugar with concentrations being 293± 141 ngm�3 during the NDSperiod, which was two times more than that in the DS time (Table 1and S1). Galactosan was another important anhydrosugar found inbiomass smoke and formed during pyrolysis of hemicelluloses.Galactosan well correlated with levoglucosan during the wholecampaign, but the slope was lower for the samples collected duringMarch 9 at 23:00 �March10 at 3:00 LT (Fig. 2a and S1). Levoglu-cosan/galactosan ratio was usually different in different types ofbiomass smokes, which was higher for wood smoke types (Schmidlet al., 2008b) but lower for leaf smoke types (Schmidl et al., 2008a).Thus, we surmised that there was a pollution event influenced byleaf burning emissions in the period of March 9 at23:00 �March10 at 3:00 LT.

Phthalates, which were softener in polyvinyl chloride (PVC)products, could be released into the air from the matrix by evap-oration, because they were not chemically bonded to the matrix (Liand Wang, 2015). Phthalates in the TSP samples were 184± 108,186± 84 and 350± 219 ngm�3, accounting for 6.6%, 7.8% and 8.6%of the total organic compounds in the DS, transition and NDSevents, respectively (Table 1, Fig. 1). Six phthalates were detected inall the samples, including dimethyl (DMP), diethyl (DEP), diisobutyl(DIBP), di-n-butyl (DBP), benzylbutyl and bis (2-ethylhexyl) (DEHP)phthalates. DEHP is the dominant species in this class and accountsfor more than 50% of the total, followed by DIBP and DBP (Table S1).DIBP and DBP show a strong linear correlation during the wholecampaign (r¼ 0.94, P< 0.01) (Fig. S2), because they werecommonly used plasticizers in China (Wang et al., 2007b).

In this study, three phthalic acids were detected, among whicho-phthalic acid (o-Ph) was secondarily derived from anthropogenicprecursor oxidation, while m-phthalic acid (m-Ph) and p-phthalicacid (p-Ph) were primarily derived from source emissions(Alnaiema and Stone, 2017; Kautzman et al., 2010). Phthalic acidswere 18e181 ngm�3 (average 102± 55 ngm�3) in the NDS time,about 4 times higher than that during the DS period (Table 1). Thehigh concentrations of phthalic acids indicated local emissionswere the main sources of anthropogenic pollutants in the urbanatmosphere.

Polycyclic aromatic hydrocarbons (PAHs) originated fromincomplete combustion of carbon-containing compounds. Theyubiquitously occurred in the environment and were of high

Fig. 2. Linear fit regressions for primary (aef) and secondary (gej) organic aerosols in the Tthe dust storm/transition time, and (gej) the dust storm (Red dots in Fig. 2a were the samplwere indicative of all other samples collected during the whole campaign). (For interpretatversion of this article.)

carcinogenicity and mutagenicity (Ren et al., 2017b, 2017c). Con-centrations of the total PAHs during the NDS period was61± 37 ngm�3, nearly 3 times higher than in the DS period (Table 1,Fig. 1). As shown in Table S2, mass ratios of indeno(1,2,3-cd)pyrene/benzo(ghi)perylene (IP/BghiP) and benzo(ghi)perylene/benzo(e)pyrene (BghiP/BeP) were very close to the ratios of PAHs in coalburning especially in the NDS hours. Moreover, there was a robustlinear correlation between PAHs and fossil fuel derived n-alkanes(r¼ 0.80, P < 0.01) (Fig. 2b), further suggesting that coal combus-tion could be an important source of the pollutants in the city,although emissions from vehicle exhausts cannot be neglected dueto sharp increase of vehicle number (Zhao et al., 2018).

Hopanes were abundant in coal and crude oils and enriched inlubricant oil fraction. In the TSP samples, only 17a(H), 21b(H)-30-norhopane (C29ab) and 17a(H), 21b(H)-hopane (C30ab) weredetermined, and other hopanes in the TSP aerosols were unde-tectable. The total concentrations of hopanes were 4.2± 2.4,4.1± 2.8 and 7.5± 4.9 ngm�3 during the DS, transition and NDSperiods, respectively. Linear fit regression showed that in the NDSperiods hopanes well correlated with fossil fuel n-alkanes (r¼ 0.81,P< 0.01) and PAHs (r¼ 0.80, P< 0.01) (Fig. 2c and d). Diagnosticratios of C29ab/C30ab were 0.6e0.7, 0.4 and 0.6e2.0 for gasoline,diesel (Rogge et al., 1993), and coal burning emissions (Oros andSimoneit, 2000), respectively. As shown in Table S2, the C29ab/C30ab ratioswere 0.78± 0.22, 0.86± 0.28 and 0.91± 0.37 during theDS, transition and NDS periods, respectively. The above correlationand the ratios indicated that hopanes in the city were largelyemitted from the residential coal burning and vehicle exhausts.

3.2. Hourly variations

3.2.1. Primary organic aerosolsAirborne particulate fossil fuel n-alkanes, benzo(b)fluo-

ranthene(BbF), hopanes, p-phthalic acid and levoglucosan weretaken as primary organic aerosols (POA), because they wereemitted directly from combustion processes of coal, petroleum oil,plastic waste and biomass due to incomplete combustion. Asshown in Fig. 3, those POA display a similar variation pattern, whichwere lower in the DS and higher in the NDS. Particularly, temporalvariation pattern of p-phthalic acid was similar to that of levoglu-cosan, as shown in Fig. 3d and e, which sharply increased to morethan 147 and 628 ngm�3 at 11th 21:00 during the NDS time,

SP samples collected during (a) the whole campaign, (bed) the non-dust period, (eef)es collected from March 9 at 23:00 to March 10 at 3:00, while the open circles in Fig. 2aion of the references to colour in this figure legend, the reader is referred to the Web

Fig. 3. Temporal variations of primary organic aerosols during the campaign.

Y.Q. Ren et al. / Chemosphere 215 (2019) 413e421 417

respectively. And at 11th 11:00, there was a moderate peak maybedue to the cooking or burning garbage activities. But for levoglu-cosan, there was another pronounced peak at 10th 00:00 duringthe event time, which could be explained by the pollution of leafburning emissions as mentioned section 3.1. In contrast, during thewhole sampling period, trehalose was very low in the NDS time butmuch higher in the DS period (Fig. 3f and Table S1). Furthermore,trehalose showed a robust linear correlation with elemental cal-cium (Ca) (Fig. 2e, r¼ 0.94, P< 0.01) and Na2SO4 (Fig. 2f, r¼ 0.94,P< 0.01) during the dust storm and transition periods, respectively.Trehalose is a metabolism product of highly desiccation-tolerantspecies (Garg et al., 2002), which was believed to be abundantlyexists in the surface dust of Gobi desert rather than in the soil ofdownwind regions (Wang et al., 2012b). Therefore, the strong cor-relations further confirmed our previous findings that in the Asiandust storm period airborne particulate Na2SO4$10H2O was directlytransported from Gobi desert surface soil (Wang et al., 2014).

As seen in Fig. S3, CPI values of n-alkanes, fatty acids and fattyalcohols during the whole observation period displayed a contin-uously decreasing trend. During the DS period CPI of n-alkanes wasvery high in the morning of March 10th but sharply decreased to1.5 at 10th 09:00 (Fig. S3a), while CPI for fatty acids was muchhigher than in other times (Fig. S3b). Such a phenomenon can beexplained by the transport pathway of the air mass moving to Xi'anat that moment, which passed through the nearby upwind citiessuch as Weinan and Huayin and thus brought more fossil fuelderived n-alkanes and residential cooking derived fatty acids (i.e.,C16:0 and C18:0), being consistent with the simultaneous increase ofblack carbon observed by our previous work at the same site (Wanget al., 2014).

LMW n-alkanes (C18eC27) were attributable to fossil fuel com-bustion with no odd/even carbon number preference, whereasHMW n-alkanes (C28eC36) were from higher plant emissions with asignificant odd/even carbon number predominance (Wang et al.,2007a). As shown in Fig. S4a, the ratios of HMW/LMW n-alkaneswere higher in the DS period (average 2.5± 0.8, 2.0± 1.3, 1.5± 0.6during the three periods, respectively). However, the value sud-denly increased between 11th 3:00 and 11th 5:00, of which the realreason was unknown and was possibly related to the road dustemissions around the sampling site. High concentrations ratios(18± 2.0%, Fig. S4a) of C31 to total n-alkanes were observed duringthe DS period, which decreased to 14± 2.4% in the transition timeand 12± 1.8% in the nonevent, respectively. These changes sug-gested that HMW organic aerosols during the dust were derivedfrom plants in Gobi desert regions, being consistent with theobservation on dust storm at the mountaintops of Mt. Hua and Mt.Tai by Wang et al. (2012b). Unsaturated fatty acid (C18:1) wasdetected in all the samples and relative abundance concentrationsof C18:1 to total fatty acids (C18:1%) displayed an increasing trendfrom 4.8± 1.3% during the DS event to 6.7± 1.9% in the NDS stormperiod. Meanwhile, the concentration ratio of fatty acid C18:0 toC18:1 was decreasing gradually from DS to NDS periods (Fig. S4b).Fatty acid C18:1 was liable to be oxidized by O3 due to the unsatu-rated C]C bond. Thus, the above changes in the relative abun-dances indicated that aerosols were more aged during the DSperiod. Fig. S4d showed the temporal trend of the benzo(e)pyrene/benzo(a)pyrene (BeP/BaP) ratios, which also decreased from the DSperiod to the NDS period, further demonstrating that aerosols inthe dust event were more aged. During the whole sampling period,HMW/LMW fatty alcohols ratios displayed a variation pattern

Y.Q. Ren et al. / Chemosphere 215 (2019) 413e421418

similar to that of the HMW/LMW n-alkanes ratios, decreasing from7.8± 2.4 in the DS hours to 4.2± 0.9 in the NDS hours. Abundance ofC28 to the total fatty alcohols (C28%) showed a similar temporalvariation pattern to that of HMW/LMW fatty alcohols ratio, whichwas 25± 10% in the event and decreased to 17± 1.5% in the non-event, because HMW organic aerosols in the DS period mostlyoriginated from plants in Gobi regions (Fig. S4c) (Wang et al.,2012b).

3.2.2. Secondary organic aerosolsDuring the whole sampling period, o-phthalic acid (o-Ph)

continuously increased from the DS period to the NDS period, andsharply increased to more than 64 ngm�3 at 12th 06:00 with asecond peak of about 53 ngm�3 at 11th 11:00 (Fig. 4a), indicatingthat o-Ph was related to local anthropogenic pollutants. Temporalvariation pattern of succinic acid was similar to that of fossil fuel n-alkanes (Fig. 4b), indicating that they were of similar localanthropogenic sources, e.g., coal burning and vehicle exhaust(Wang and Kawamura, 2005, 2006; 2012a). As seen in Fig. 4c,relative abundance of cis-pinonic acids (PA) to SOA (PA/SOA, SOA:

Fig. 4. Temporal variations of secondary organic aerosols during the whole campaign. (POAfatty acids, sugars, hopanes, BPA, dehydroabietic acid, vanillic acid and glycerol. SOA: includinmalic acid, suberic acid, 2-methyl glyceric acid, glyceric acid and erytritol).

including o-phthalic acid, cis-pinonic acid, maleic acid, fumaricacid, succinic acid, glutaric acid, malic acid, suberic acid, 2-methylglyceric acid, glyceric acid and erytritol) was higher with an averagevalue of 5.6% during the DS period; it was nearly constant duringthe transition and non-dust storm periods. Thus, we surmise thatduring the dust storm period this BSOA largely originated fromlong-range transport rather than from local sources. The mass ratioof SOA/POA (%) (POA: including n-alkanes, phthalates, m-phthalicacid, p-phthalic acid, PAHs, fatty alcohols, fatty acids, sugars,hopanes, BPA, dehydroabietic acid, vanillic acid and glycerol.) dur-ing the whole sampling period increased from 5.9± 2.8% in theevent, 7.9± 1.4% in the transition to 8.2± 2.7% in the non-duststorm event (Fig. 4d), again indicating that dust storm broughtmore POA in the whole campaign.

3.3. Size distribution

To further recognize the impact of the dust storm on the urbanaerosol chemistry, size distributions of some primary and second-ary organic compounds were investigated (Fig. 5). Cumulative

: including n-alkanes, phthalates, m-phthalic acid, p-phthalic acid, PAHs, fatty alcohols,g o-phthalic acid, cis-pinonic acid, maleic acid, fumaric acid, succinic acid, glutaric acid,

Fig. 5. Differences in size distributions of primary (aef) and secondary (gej) organic aerosols during the non-dust storm and the dust storm periods.

Y.Q. Ren et al. / Chemosphere 215 (2019) 413e421 419

percentages of these compounds in the fine (<1.1 mm) and coarse(>1.1 mm) modes were presented in Table S3.

Fig. 6. Relative concentration of LMW PAHs to that of HMW PAHs (a) and relativeconcentration of fluoranthene (Flu) and pyrene (Pyr) to that of benzo(ghi)perylene(BghiP) (b) in each stage during the nonevent and event periods.

3.3.1. Primary organic aerosolsHMW n-alkanes (e.g., C31) and fatty acids (e.g., C30:0) (Fig. 5a and

b) showed a bimodal size distribution during the NDS period,peaking at the fine mode (<1.1 mm) and the coarse mode (>1.1 mm),respectively. However, during the DS period both showed a mon-omodal distribution with a peak at the coarse mode. Our previouswork found that those two compounds in Asian dust storm periodswere largely emitted from biota in Gobi desert surface soil, thusboth dominated in the coarse particles (Wang et al., 2012b).

LMW PAHs were of high vapor pressure and thus may evaporatefrom the fine particles into the atmosphere and re-adsorb/-condense onto the coarse particles (e.g. phenanthrene (Phe),pPhe¼ 2.0� 10�2 Pa at 298 K) (Finlayson-Pitts and Pitts, 2000).Thus, Phe showed a bimodal pattern in the non-event (45% and 55%in the fine and coarse modes, respectively) with a slight increase inthe coarse mode during the dust event (about 28% and 72% in thefine and coarse modes, respectively) (Fig. 5c and Table S3). Oncontrast, BeP (HMW PAH) was characterized by an accumulationmode in the whole sampling period due to its low volatility(pBeP¼ 7.4� 10�7 Pa at 298 K) (Finlayson-Pitts and Pitts, 2000)(Fig. 5d and Table S3). Such a repartitioning effect resulted in therelative abundances of LMW PAHs/HMW PAHs higher in largeparticles (Fig. 6a), which could further be demonstrated by thedifference between the concentration ratios of fluoranthene (Flu)and pyrene (Pyr) to that of BghiP in each stage (Fig. 6b). In com-parison with Pyr, Flu had a higher vapor pressure(pFlu¼ 1.2� 10�3 Pa at 298 K, pPyr¼ 6.0� 10�4 Pa at 298 K)(Finlayson-Pitts and Pitts, 2000) and therefore transferred to largerparticles faster than does Pyr (Wang et al., 2009). Such a discrep-ancy in repartitioning effect led to the difference between massratios of Flu/BghiP and Pyr/BghiP getting more significant with anincrease in the size of particles especially in the dust storm period(Fig. 6b).

Both DBP and BPA were plasticizers and were usually emittedinto the atmosphere as fine particles by open burning of plasticwastes (Li and Wang, 2015; Teil et al., 2006). However, due to itssemi-volatile nature (0.3e9 kPa) (Katayama, 1988; Chuang andDonahue, 2015), DBP might evaporate from fine particles and re-adsorb onto coarse particles, resulting in a bimodal size

distribution with 70% of the total enriched in the coarse modeduring the NDS period and 76% of the total enriched in the coarse

Y.Q. Ren et al. / Chemosphere 215 (2019) 413e421420

mode during the DS period (Fig. 5e and Table S3). In contrast, BPAwas much less volatile (1.2� 10�10�5.3� 10�8 kPa) (Staples et al.,1998). Therefore, BPA showed no changes in size distributionwith a fine mode pattern during the whole campaign (Fig. 5f andTable S3).

3.3.2. Secondary organic aerosolso-Ph aerosol was formed via a gaseous oxidation of naphthalene

and followed by a subsequent adsorption/condensation onto pre-existing particles (Li et al., 2013). So it presented a bimodalpattern during the none-event, peaking at 0.4e1.1 mm (54% of theorganic in the fine mode). But the coarse fraction sharply increasedduring the DS period (about 64% in the coarse mode) (Fig. 5g andTable S3). Succinic acids also exhibited a bimodal pattern in NDSperiod with a slight increase in the coarse mode during the event(Fig. 5h and Table S3), consisting with that reported previously byWang et al. (2015). o-Ph and succinic acid weremostly derived fromanthropogenic volatile organic compounds (VOC), which werefirstly oxidized as gaseous dicarboxylic acid and then partitioninginto the particle phase (Wang et al., 2011). Similarly, cis-Pinonicacid (PA) was also formed by a gas-phase oxidation of pinene withozone and subsequently adsorbed onto pre-existing particles (Liet al., 2013). Thus PA exhibited a bimodal pattern during the NDSperiod with a significant increase in the coarse mode during thedust event (Fig. 5i and Table S3). Concentrations of these three SOAmarkers well correlated one another during the dust storm period(r� 0.83, P< 0.01) (Fig. S5), because they were formed via a similarpathway, i.e., gas-phase oxidation and subsequent gas-aerosolaqueous phase partition.

In addition, we found that succinic acid and cis-pinonic acidshowed a linear correlation with mirabilite (Na2SO4$10H2O) duringthe dust storm episode (Fig. 2h and i). Both acids and other organics(e.g., o-Ph) could be further oxidized in the aqueous phase of thedust surface and finally become as oxalic acid, resulting in a strongcorrelation of oxalic acid with mirabilite in the dust storm period(r¼ 0.93, p< 0.01) (Fig. 2j). On the contrary, there was no correla-tion observed for mirabilite with POAmarkers such as Phe and DBPduring the dust storm event (Fig. S6). Previous field observation(Wang et al., 2014, 2018a) and the laboratory chamber simulation(Wu et al., 2018) revealed that hygroscopic salts such as mirabilite(Na2SO4$10H2O) and gypsum (CaSO4$2H2O), which were abundantin the Gobi desert surface soil, could take upwater vapor during thedust transport process and form an aqueous-phase on the dustparticle surface, which further result in NH4NO3 forming on thedust surface. Such an aqueous phase favored the partitioning ofVOC oxidation products, which could futher be oxidized into di-acids, resulting in the linear corrections of oxalic acid and relatedSOA markers with mirabilite.

4. Summary and conclusion

Composition and size-distribution of primary and secondaryorganic aerosols in Xi'an during the dust storm, the transition andthe non-dust storm periods of 2013 were measured on a molecularlevel. A total of 11 classes of organics were found during thecampaign with n-alkanes being the most abundant, followed byfatty acids, sugars and fatty alcohols. HMW n-alkanes and fattyacids dominated in the coarse mode of particles (>1.1 mm) duringthe dust event, indicating theywere mostly originated from surfacesoil and plants in the upwind regions. Low-volatile anthropogeniccompounds such as BeP and BPA dominated in the fine modeparticles during the whole campaign. In contrast, semi-volatileanthropogenic compounds such as Phe and DBP showed abimodal size distribution with a significant increase in the coarsemode during the dust storm event due to their evaporation from

the fine particles and the subsequent repartitioning to the coarsedusts. We found that unlike POA, oxalic acid and related SOAshowed a mild/strong correlation with mirabilite during the duststorm period. Moreover, in contrast with those in the non-duststorm event, the SOA markers significantly shifted to the coarseparticles in the dust storm event. Such correlations with mirabiliteand changes in size distributions can be ascribed to the heteroge-neous formation of SOA on the dust surface (e.g., hygroscopic saltmirabilite).

Acknowledgements

Financial support for this work was provided by National KeyR&D Plan (Quantitative Relationship and Regulation Principle be-tween Regional Oxidation Capacity of Atmospheric and Air Quality(No. 2017YFC0210000), the China National Natural Science Fundsfor Distinguished Young Scholars (No.41325014), the program fromNational Nature Science Foundation of China (No. 41773117).

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.chemosphere.2018.10.064.

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