Filter-based measurement of light absorption by brown carbon in PM 2.5 in a megacity in South China

2018·2.2 MB·English

by  Sheng Li

ScienceoftheTotalEnvironment633(2018)1360–1369
ContentslistsavailableatScienceDirect
Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv
Filter-based measurement of light absorption by brown carbon in PM
2.5
in a megacity in South China
ShengLia,c,MingZhua,c,WeiqiangYanga,c,MingjinTanga,XueliangHuanga,YuegangYua,HuaFanga,c,
XuYua,c,QingqingYua,c,XiaoxinFud,WeiSonga,b,YanliZhanga,b,XinhuiBia,XinmingWanga,b,c,⁎
aStateKeyLaboratoryofOrganicGeochemistry,GuangdongKeyLaboratoryofEnvironmentalProtectionandResourcesUtilization,GuangzhouInstituteofGeochemistry,ChineseAcademyof
Sciences,Guangzhou510640,China
bCenterforExcellenceinRegionalAtmosphericEnvironment,InstituteofUrbanEnvironment,ChineseAcademyofSciences,Xiamen361021,China
cUniversityofChineseAcademyofSciences,Beijing100049,China
dSchoolofEnvironmentandResource,SouthwestUniversityofScienceandTechnology,Mianyang621010,China
H I G H L I G H T S G R A P H I C A L A B S T R A C T
• Filter-basedmeasurementoflightab-
sorptionbyBrCandBC.
• Larger seasonal variation of light ab-
sorptionbyBrCinruralsites.
• Biomass burning was a significant
sourceofBrCinautumn.
• SecondaryformationofBrCmightbe
greaterinsummerthaninautumn.
• HigherapparentMAEforECatsitesim-
pactedbyfossilfuelcombustion.
a r t i c l e i n f o a b s t r a c t
Articlehistory: Carbonaceousaerosolsrepresentanimportantnexusbetweenairpollutionandclimatechange.Herewecol-
Received11January2018 lectedfilter-basedPM samplesduringsummerandautumnin2015atoneurbanandtworuralsitesinGuang-
2.5
Receivedinrevisedform20March2018 zhou,amegacityinsouthernChina,andgotthelightabsorptionbyblackcarbon(BC)andbrowncarbon(BrC)
Accepted20March2018
resolvedwithaDRIModel2015multi-wavelengththermal/opticalcarbonanalyzerapartfromdetermining
Availableonline3April2018 theorganiccarbon(OC)andelementalcarbon(EC)contents.OnaverageBrCcontributed12–15%ofthemea-
suredabsorptionat405nm(LA )duringsummerand15–19%duringautumnwithsignificantincreasein
Editor:JianminChen 405
theLA byBrCattheruralsites.Carbonaceousaerosols,identifiedastotalcarbon(TC),yieldedaveragemass
405
Keywords: absorptionefficiencyat405nm(MAE405)thatwereapproximately45%higherinautumnthaninsummer,an
Browncarbon 83%increasewasnotedintheaverageMAE405forOC,comparedwithanincreaseofonly14%intheaverage
Blackcarbon MAE forEC.TheLA byBrCshowedagoodcorrelation(pb0.001)withtheratiosofsecondaryOCto
405 405
Elementalcarbon PM insummer.However,thiscorrelationwaspoor(pN0.1)inautumn,implyinggreatersecondaryformation
2.5
PM2.5 ofBrCinsummer.Thecorrelationsbetweenlevoglucosan(amarkerofbiomassburning)andtheLA405byBrC
Lightabsorption weresignificantduringautumnbutinsignificantduringsummer,suggestingthattheobservedincreaseinthe
Guangzhou
LA byBrCduringautumninruralareaswaslargelyrelatedtobiomassburning.Themeasurementsoflightab-
405
sorptionat550nmpresentedinthisstudyindicatedthattheuseoftheIMPROVEalgorithmwithanMAEvalueof
⁎ Correspondingauthor.
E-mailaddress:[email protected](X.Wang).
https://doi.org/10.1016/j.scitotenv.2018.03.235
0048-9697/©2018ElsevierB.V.Allrightsreserved.
S.Lietal./ScienceoftheTotalEnvironment633(2018)1360–1369 1361
10m2/gforECtoapproximatelightabsorptionmaybeappropriateinareasnotstronglyaffectedbyfossilfuel
combustion;however,thispracticewouldunderestimatetheabsorptionoflightbyPM inareasheavilyaf-
2.5
fectedbyvehicleexhaustsandcoalburning.
©2018ElsevierB.V.Allrightsreserved.
1.Introduction etal.,2016).Thesecondmethodinvolvesmeasuringthetransmission
oflightthroughparticle-loadedfilterstoquantifyaerosolopticalprop-
Atmosphericaerosols,whichdispersegloballybutdisplaysignifi- ertiesusinginstrumentsincludingparticlesootabsorptionphotometer
cantregionalvariations,impacttheradiativeforcingreceivedbythe (PSAP)(Bondetal.,1999),multi-angleabsorptionphotometer(MAAP)
Earthbothdirectly (by enhancing thescatteringandabsorption of (PetzoldandSchonlinner,2004),multi-wavelengthabsorbanceana-
solarradiation)andindirectly(throughtheirimportantrolesincloud lyzer(MWAA)(Massaboetal.,2013),aethalometers(Hansenetal.,
condensationandasicenuclei)(Akimoto,2003;Ramanathanetal., 1984)and,mostrecently,multi-wavelengththermal/opticalcarbonan-
2001).Carbonaceousaerosols,whichrepresentanimportantpartofat- alyzers(Chenetal.,2015).
mosphericparticulatematter,areverycomplexmixtures.Theyaretyp- TheburningofbiomassandbiofuelcontributesubstantiallytoBrC
icallydividedintoorganiccarbon(OC)andblackcarbon(BC)(Chung (Gustafssonetal.,2009;Lacketal.,2012;Mohretal.,2013;Liuetal.,
andSeinfeld,2002).Manystudieshavedemonstratedthatcarbona- 2015;Moketal.,2016),andnearly3billionpeople,mostlyindevelop-
ceousaerosolsareresponsibleforalargefractionofthelightabsorbed ingcountries,stillburnvarioustypesofbiomasstocooktheirfoodand
byairborneparticles(Trijonis,1982;Shahetal.,1984;Malmetal., heattheirhomes(Subramanian,2014).Thus,theoccurrenceandeffects
1994;Novakovetal.,1997).BCisconsideredtobethemajorcarbona- ofBrCfromopenandresidentialburningofbiomassisalsoofincreasing
ceous aerosol component that efficiently absorbs visible concern.However,dataonairborneBrCinthedenselypopulateddevel-
(400–700nm)light(Kirchstetteretal.,2004).MostOCabsorbslight opingcountries,includingChina,arefarfromsufficient.Ithasbeenes-
stronglyintheinfrared(IR)andultraviolet(UV)rangesbutisrelatively timated that the annual mean atmospheric burden of BrC exceeds
transparentinthevisibleandnear-IRranges(Laskinetal.,2015).How- 4mgm−2intheeasternandsouthernregionsofChina(Fengetal.,
ever,anumberofrecentstudieshavefoundthatsometypesofOCcan 2013).FieldworksalsohaveshownthatBrCcontributed10–30%to
absorblightinUV–visibleranges(AndreaeandGelencser,2006;Bond totalaerosollightabsorptionintheBeijing-Tianjin-Hebeiregionin
and Bergstrom, 2006; Ramanathan et al., 2007; Feng et al., 2013). northChina(Yangetal.,2009;Chengetal.,2011;Duetal.,2014a,
These types of OC are termed brown carbon (BrC) (Pöschl, 2005; 2014b). In the Pearl River Delta (PRD) region, which is one of the
AndreaeandGelencser,2006),whichincludespolycyclicaromatics,at- world's largest megacities and lies in southern China, Yuan et al.
mospherichumic-likesubstances(HULIS)andbiopolymers.Fieldmea- (2016)recentlymeasuredtheabsorptionoflightbyaerosolparticles
surementssuggestthatBrCmaycontributeapproximately10–30%of usinga3-wavelengthphotoacousticsootspectrometer;theresultssug-
thetotalabsorptionbycarbonaceousaerosolsatnear-UVwavelength gestthatBrCcontributesN10%ofthetotalaerosollightabsorptionat
andapproximately10%atawavelengthof550nm(Bahaduretal., shorterwavelengthsinthisregion.Consideringthespatialandtemporal
2012;Lacketal.,2012;Washenfelderetal.,2015).BrCmayevenbere- imbalancesinthesourcesandpropertiesofBrCintheambientair,field
sponsibleforN50%ofthetotallightabsorptionat370nmandapproxi- measurementsintendedtocharacterizeBrCareurgentlyneededin
mately15%ofthetotallightabsorptionatthemiddlewavelengths China.
duringbiomassburningevents(Favezetal.,2009).Aglobalmodeling Inthisstudy,aDRIModel2015Multi-wavelengthCarbonAnalyzer
studysuggeststhatBrCcouldcontributeupto+0.25Wm−2or19% wasusedtomeasurethelightabsorptionbyBrCinPM collectedon
2.5
oftheabsorptionbyanthropogenicaerosols(Fengetal.,2013).Conse- quartzfiltersatthreesites(oneurbansiteandtworuralsites)inthe
quently,theopticalpropertiesofBrCneedtobeexplicitlyincludedin PRDregion.Althoughthisoff-linemethoddoesnotevaluatetheeffects
modelstoaccuratelyrepresenttheradiativeforcingproducedbyaero- ofparticlesizeandmorphologyontheaerosollightabsorption,itpro-
sols(Alexanderetal.,2008). videsasimpleandconvenientmeansofmeasuringthelightabsorption
BrC is relatively inefficient in absorbing light in the near-UV ofBrCbasedontheopticalnatureofthebulkparticlesonfilters.Theef-
(300–400nm)andvisiblerangeswhencomparedtoBC,anditseffi- fectsofthequartzfiltersonthemeasurementscanbecalculatedandex-
ciencydecreasesasthewavelengthincreases(Formentietal.,2003; cludedusingempiricalformulasdevelopedforthermal/opticalanalysis.
Kirchstetteret al.,2004;AndreaeandGelencser, 2006;Yanget al., Thisstudyfocuson:1)estimatingthecontributionsbyBrCtolightab-
2009).BrCcanoriginatefromprimaryemissions,suchasbiomassand sorptioninamegacityinsouthernChinausingfilter-basedsamples
biofuel burning (Bond, 2001; Bergstrom et al., 2007; Laskin et al., and2)investigatingthespatialandseasonalvariationsintheabsorp-
2015),anditcanalsobesecondarilyformedbymultiphasereactions tionoflightbyBrCaswellasthefactorsthatinfluencetheamountof
(Noziereetal.,2007;NoziereandEsteve,2007;DeHaanetal.,2009; lightitabsorbs.
DeHaanetal.,2011;Nguyenetal.,2012;Leeetal.,2013;Drozdand
McNeill,2014;Powelsonetal.,2014;Tangetal.,2016).Detailedcharac- 2.Samplingandanalysis
terizationofBrCisdifficultbecauseofitsinherentchemicalcomplexity
andthemixingstatewithothersubstances(Laskinetal.,2015). 2.1.Descriptionofthesamplingsites
LightabsorptionbyBCandBrCcanbedifferentiatedbytheabsorp-
tionAngstörmexponent(AAE)values,assumingthattheAAEvaluesof ThePRDregionhasbecomethelargesturbanareaintheworldin
BC (sometimes alsotermedelemental carbon orEC)arecloseto 1 termsofbothsizeandpopulation(WorldBank,2015).Therapidgrow-
(Bergstrom et al., 2002; Bond and Bergstrom, 2006; Drozd and ingindustryandeconomyintheregionhasbeenaccompaniedbylarge
McNeill,2014).TherearetwowaystomeasurelightabsorptionofBrC emissionsofprimarypollutants(ChanandYao,2008),includinglight-
basedonAAEvalues.Onemethodinvolvesdirectmeasurementsof absorbingBCandBrC,whichhaveclimaticeffects.Theregionexperi-
theextinctionandabsorptioncoefficientsofaerosolparticlesbyinstru- ences a typical Asian monsoon climate; hot and humid conditions
mentssuchascavityring-downspectroscopy(CRDS)(Schereretal., occurinthesummerwhentheSouthChinaSeamonsoonisdominant,
1997),cavity-enhancedabsorptionspectroscopy(CEAS)(Moosmuller andrelatively cool anddryconditionsoccurinthefallandwinter,
etal.,2005;Washenfelderetal.,2013)andphotoacousticabsorption whichfeatureprevailingnortheasterlymonsoonwindsfromnorthern
spectrometer(PAAS)(Arnottetal.,1999;Flowersetal.,2010;Yuan China(DingandChan,2005).
1362 S.Lietal./ScienceoftheTotalEnvironment633(2018)1360–1369
Thethreesamplingsitesinthisworkincludeoneurbansiteinthe usingaDRIModel2015multi-wavelengththermal/opticalcarbonana-
citycenter(SZ;23.13°N,113.27°E)andtworuralsites,oneinthe lyzer(DesertResearchInstitute,Nevada,USA)withtheIMPROVE_A
smalltownofJiulong,whichislocatedinthenortheasternportionof protocol (Chow et al., 2007; Chow et al., 2011). Four OC fractions
Guangzhou(JL;23.30°N,113.57°E)andanotherinasmalltownof (OC1,OC2,OC3,andOC4withcuttingtemperatureof140,280,480,
Wanqingsha in the central portion of the PRD (WQS; 22.71° N, and580°C,respectively,inaheliumatmosphere)andthreeECfractions
113.55°E).TheirgeographicallocationsareshowninFig.1.SZandJL (EC1,EC2,andEC3withcuttingtemperatureof580,740,and840°C,re-
aretwostationsinthegovernmentalambientairqualitymonitoring spectively,ina2%oxygen/98%heliumatmosphere).Thedetailedproce-
networkinGuangzhou,andthelocationofWQSenablesthemonitoring duresfortheOC/ECanalysisaredescribedinChenetal.(2015).
oftheregionalbackgroundairpollutioninthePRDregion.Detailedde- LightabsorptionwasalsomeasuredbytheDRIModel2015multi-
scriptionsofthesamplingsitescanbefoundelsewhere(Zhangetal., wavelengththermal/opticalcarbonanalyzer.Theanalyzerequipped
2012,2013;Yuetal.,2016). sevendiodelaserswithwavelengthsof405,455,532,635,780,808
and980nmtomonitorspectralreflectanceandtransmittanceoffilter
2.2.Samplecollection samples.SinceBrCisweakinabsorbinginfraredlight,wechosethe
light absorption at 405, 455 and 635 nm for the discussion in this
FieldsamplingwasconductedfromAugust11toAugust31(inthe study(lightabsorptionat532nmwasnotincludedduetoinsufficient
summer)andfromNovember1toNovember20(intheautumn)in detectorsignal-to-noiseratioratios)(Chenetal.,2015).Mostprevious
2015.The24-hPM sampleswerecollectedsimultaneouslyatthe studieshaveusedtransmittanceattenuation(ATN)toestimatelightab-
2.5
threesitesusinghigh-volume samplers(Tisch Environmental, Inc., sorptionbyparticlesonafilter:
OH,USA)withaconstantflowrateof1.1m3min−1.ThePM samples
2.5 (cid:1) (cid:3)
UwKe)re,wcohlilcehctwederoenp2re0-.3ba×ke2d5.a4tc4m50q°uCarftozrfi6ltherbse(foWrehuatsme.an,Mainstone, ATNλ¼− ln FFTTλλ;;if ð1Þ
2.3.Chemicalanalysisandlightabsorptionmeasurement whereFTλ,iandFTλ,farethefiltertransmittancemeasuredbeforeand
Sulfate,nitrateandammoniumwereanalyzedwithanionchro- maftiettratnhceermofaalabnlaanlyksifisl,treers.pDeucteivteolyfi;ltaenrdefFfTeλc,tfsa,ptphreoaxbimsoartpetsiothneotprtaincsa-l
matograph(883BasicICplus,Metrohm,Switzerland)(Fuetal.,2016).
depthcannotbedirectlyobtainedfromATNatagivenwavelength.
The detailed procedure for chemical analysis is given in Fu et al.
Chenetal.(2015)arguedthattherelationshipbetweentheabsorption
(2015).Detailedproceduresfortheanalysisoflevoglucosanarede- opticaldepthτ andATNisquadratic:
scribedelsewhere(Yuetal.,2016).Briefly,filtersampleswereextracted a
by sonication with dichloride methane/methanol (1:1, v/v), and
derivatized(silylation)with100μLofpyridineand200μLofN,O-bis- τa;λ¼Aλ(cid:2)ATN2λþBλ(cid:2)ATNλ ð2Þ
(trimethylsilyl)-trifluoroacetamide (BSTFA) plus 1%
trimethylchlorosilane. The silylated extracts were analyzed by an whereτa,λistheabsorptionopticaldepthatwavelengthλ,andAλand
Agilent7890/5975Cgaschromatography/massspectrometerdetector Bλ are coefficients that include wavelength-specific multiple-
(GC/MSD) equipped with an HP-5 MS capillary column (30 m × scatteringandloadingeffects.ThevaluesofAλandBλwerereported
0.25mm×0.25mm).ForthemeasurementsofOCandEC,acircular inChenetal.(2015).Therelationshipbetweenτa,λandATNλcouldbe
punch (0.8 cm in diameter) of each filter was taken and analyzed appliedtoambientsamplestocalculateτa,λ(Chenetal.,2015).
Fig.1.Geographicallocationsofthethreesamplingsites(SZ,JL,andWQS)inthePearlRiverDelta(PRD)region.ThelocationofthePRDregioninChinaisshowedinthebottomright
corner.
S.Lietal./ScienceoftheTotalEnvironment633(2018)1360–1369 1363
SinceBCandBrCarethelight-absorbingmaterialsinaerosolsam- 18.2and12.7Mm−1atWQS,respectively.Duringautumn,theaverage
ples,asimplifiedtwo-componentmodelcanbeusedtorepresentthe total lightabsorption at 405,455 and 635 nm were 56.6, 47.8and
opticalabsorption(Chenetal.,2015): 32.8Mm−1atSZ,56.3,46.7and31.3Mm−1atJLand44.9,37.4and
25.7Mm−1atWQS,respectively.Theseresultsarecomparabletoaero-
τa; λ¼KBC(cid:2)λ−AAEBCþKBrC(cid:2)λ−AAEBrC ð3Þ solopticalabsorptionvaluesmeasuredinthePRDregionin2014by
Yuanetal.(2016)usinga3-wavelengthphotoacousticsootspectrome-
whereτa,λistheabsorptionopticaldepthatwavelengthλ;KBCandKBrC ter.Theyobservedthattheaveragelightabsorptionat405nm(LA405)
arethefittingcoefficientsforBCandBrC;andAAEBCandAAEBrCarethe atHeshan(aruralsite)was32.5Mm−1fromNovember1toNovember
AAEvaluesofBCandBrC,respectively.Theabsorptionopticaldepthsat 22,whereasthatatShenzhen(anurbansite)was21.6Mm−1fromSep-
awavelengthλforBCandBrC(τa,λ,BCandτa,λ,BrC,respectively)canbe tember12toOctober9and25.6Mm−1fromJanuary15toFebruary19
expressedas (Yuanetal.,2016).
ConsideringthatBCplaysamajorroleintheabsorptionoflightby
τa; λ;BC¼KBC(cid:2)λ−AAEBC ð4Þ aerosols(Cappaetal.,2012;Chungetal.,2012;Bondetal.,2013),the
totalaerosollightabsorptionisexpectedtobelargelycontrolledbythe
and
BCconcentrationsatthethreesites.SZislocatedinthecitycenter,andan-
thropogenicemissionsfromvehicleexhaustandindustrialactivitiesthere
τa; λ;BrC¼KBrC(cid:2)λ−AAEBrC ð5Þ arethehighestamongthethreesitesexaminedinthisstudy.Morespecif-
ically,industrialactivitiesandtransportationhavebeenestimatedtoac-
whereτa,λ,BCandτa,λ,BrCaretheabsorptionopticaldepthsofBCandBrC, count for 70% of the total BC emission in Guangzhou (Verma et al.,
respectively,atawavelengthλ.Inthisstudy,weassumethatAAEvalue 2010).Therefore,theaveragetotallightabsorptionatSZwassignificantly
ofBCisequalto1(Bergstrometal.,2002;BondandBergstrom,2006;
higherthanthatatJLandWQSinsummer.Fromsummertoautumn,the
DrozdandMcNeill,2014).ThefittingcoefficientsinEq.(3)wereob-
aerosolLA byPM increasedby38%,92%and115%atSZ,JLandWQS,
tainedforAAE valuesbetween2and8byleast-squarelinearregres- 405 2.5
BrC respectively,whichispartlyattributedtothechangesinatmospheric
sion,andtheAAE thatledtothebestfitwasselectedasthevalidAAE
BrC boundarylayerheight.However,theseasonalvariationsinthelightab-
ofBrC.Asanexample,theτa,λdecompositionasafunctionofwave- sorptionbyPM aremorepronouncedattheruralsites,suggesting
lengthfromasamplecollectedatJLinsummerwasshowninFig.2. 2.5
thatlocalemissionsalsocontributedsubstantiallyinruralareasduring
Theambientlightabsorptioncoefficient,b ,canbecalculatedusing
abs theautumn.AspointedoutbyHeetal.(2011),biomassburningemis-
Eq.(6):
sionsoccurprimarilyinruralareaswithinthePRDregionsandaregreater
inNovemberthaninAugust(Dingetal.,2012;Yuetal.,2016).
A
babs¼τa;λ(cid:2)V ð6Þ AsshowninFig.3,higherPM2.5concentrationswerenotassociated
withhighertotallightabsorption;forexample,duringperiodofAugust
whereAisthefilterareaandVisthesamplingvolume.Moredetailed 20–28,whenthePM2.5concentrationreacheditshighestlevel,theab-
descriptionsoftheproceduresandempiricalformulasusedarepro- sorptionoflightbyPM2.5remainedunchangedcomparedtothaton
videdinChenetal.(2015). normaldays(Fig.3aandb).Thisobservationmaybelargelyexplained
byincreasesintheproportionofscatteringmaterialsinthePM during
2.5
3.Resultsanddiscussion theperiod.Sulfate,nitrateandammonium(SNA)arekeycomponents
ofPMthatefficientlyscattervisibleradiation(SeinfeldandPandis,
3.1.LightabsorptionofPM 2006).AsdepictedinFig.3dande,duringthisperiod,themassratios
2.5
ofSNAtoPM increased,whereasthemassratiosofsecondaryOC
2.5
Timeseriesofthetotallightabsorptionatthreewavelengths(405, (SOC)toPM2.5decreasedfurther.Thisresultdiffersfromtheobserva-
455and635nm)indifferentsitesareshowninFig.3.Duringsummer, tionsmadebyYuanetal.(2016);intheirresults,thetotalabsorption
theaveragetotallightabsorptionat405,455and635nmwere44.0, oflightbyPM2.5andthemassconcentrationsoforganicaerosolsdisplay
37.5and27.1Mm−1atSZ;29.3,25.4and17.9Mm−1atJL;and20.9, similartrends.
BrCfraction
BC fraction
Measured
Fig.2.Antypicalexampleshowingthedecompositionofmeasuredabsorptionopticaldepth(τa,λ)foraPM2.5filtersamplefromtheJLsiteintotheBCandBrCcontributionsbasedontheir
distinctspectraldependenceoflightabsorption.
1364 S.Lietal./ScienceoftheTotalEnvironment633(2018)1360–1369
Fig.3.ThetimeseriesofPM2.5lightabsorption(a:WQS;b:SZ;c:JL)incomparisonofthatformassratiosofSOCtoPM2.5(d)andmassratiosofSNA(thesumofsulfate,nitrateand
ammonium)toPM2.5(e).
3.2.LightabsorptionbyBrC 12%ofthetotalaerosollightabsorption,respectively.Duringautumn,
the average LA by BrC was 8.8, 9.9 and 8.8 Mm−1 at SZ, JL and
405
SeasonalvariationsoftheaverageLA byBrCatthethreesitesare WQS,andBrCaccountedfor15%,19% and 19% of the total aerosol
405
showninFig.4.Duringsummer,theaverageLA byBrCwas6.4,4.3 lightabsorption,respectively.BCisthemainlight-absorbingmaterial
405
and2.3Mm−1atSZ,JLandWQS,andBrCaccountedfor14%,15%and inboththeruralandurbanareas;however,thecontributionofBrCto
Fig.4.ResolvedaverageLA405byBrCinthesummerandautumnatthethreesites.Errorbarrepresentstandarddeviation(1σ).Thepercentagesoverthebarsarecontributionpercentages
oftotalabsorptionbyBrC.
S.Lietal./ScienceoftheTotalEnvironment633(2018)1360–1369 1365
totalaerosollightabsorptionwasespeciallyimportantatshorterwave- 3.3.Seasonalvariationinthemassabsorptionefficiencyofcarbonaceous
lengthsandduringautumn.Inautumn,BrCcontributesnearly20%of aerosols
thetotalaerosollightabsorptionattheruralsites.
TheabsorptionvaluesofBrCwerelowandstableatsiteWQSin Ascarbonaceousaerosolsarethemajorlight-absorbingcomponent
summer,likelybecausetheairmassesarrivingatthesitefromthe ofPM (Trijonis,1982;Shahetal.,1984;Malmetal.,1994;Novakov
2.5
oceandrivenbythesummermonsoonswerecleaner.Attheurban etal.,1997),thetotalLA showedahighlysignificant(pb0.001)cor-
405
siteSZ,theaveragelevelofBrCwasthehighestofthethreesitesduring relationwithtotalcarbon(TC=OC+EC)inbothsummerandautumn
thesummer,whichisattributabletothehighercontributionoflocal (Fig.5).However,theslopeinsummer(3.42m2/g)waslowerthanthat
emissionsinthisurbanarea.Fromsummertoautumn,theaverageab- inautumn(4.95m2/g),indicatingthatcarbonaceousaerosolsinautumn
sorptionvaluesofBrCincreased,whichmaybeattributabletothelower absorbedgreateramountsoflightandhadhighermassabsorptioneffi-
boundarylayerinautumnthaninsummer.However,astheincreasesat ciency(MAE)value.TheMAEofOCcanbeobtainedbydividingtheOC
theruralsiteswerehigherthanthatattheurbansite,theemission concentrationsintothelightabsorptionbyBrC.TheMAEofECcanbe
sourcesattheruralsitesarelikelystronger,andbiomassburningisa represented by the ratios of the light absorptions by BC to the EC
likelyprimarysource(Heetal.,2011).Previousstudieshavedemon- concentrations.
strated thatin rural areasof thePRDregion,theconcentrationsof TheaverageMAE ofECwas11.8m2/ginsummer,andthisquan-
405
levoglucosan,atracerofbiomassburning,weresignificantlyhigherin tityincreasedby13.6%to13.4m2/ginautumn.However,theaverage
autumn and winter when compared to those in the summer (Gao MAE ofOCincreasedby83.1%fromsummertoautumn,whenthe
405
etal.,2011;Dingetal.,2012;Yuetal.,2016).Biomassburningisanim- MAE valuewere0.7m2/gand1.3m2/g,respectively.Therefore,the
405
portantsourceofBrC(Gustafssonetal.,2009;Lacketal.,2012;Mohr significantincreaseintheaverageMAEofOCwasthemajorreason
etal.,2013;Liuetal.,2015;Moketal.,2016),anditbecamethemajor forthechangeintheaverageMAEofTC.TheaverageSOC/PM in-
2.5
sourceofpolycyclicaromatichydrocarbons,whichareimportantcom- creasedbyonly0.03%,andtheaverageSNA/PM decreasedby1.8%.
2.5
ponentsofBrCduringthewinterinGuangzhou(Yuetal.,2016).At Theseresultsindicatedthatprimaryemissions,insteadofsecondary
siteSZ,althoughtheabsorptionbyBrCincreasedfromsummertoau- products,wereprobablythemajorcontributorstotheincreaseinthe
tumn,itscontributionstothetotalaerosollightabsorptiondidnot MAEofOC.TheLA byBrCshowedafairlygoodcorrelation(R=
405
showasignificantchange.Thissiteismainlyaffectedbylocalemissions 0.71,pb0.001)withlevoglucosanconcentrationsinautumn,butthis
inurbanareas,andmajorsourcesforBCandBrC,includingvehicleex- correlationwasinsignificantinsummer(pN0.1)(Fig.6),suggesting
haust,industrialemissionsandresidentialcooking,donotdisplaysig- thatbiomassburningcontributedmoretoBrCinautumnthaninsum-
nificantseasonalvariations. mer. A previous study has demonstrated that biomass burning-
ApreviousstudyconductedinthePRDregionbyYuanetal.(2016) emittedBrCdisplaysstrongabsorptionatnear-UVwavelengths(Lack
suggestedthatinautumnBrCcontributed6.3%oftheLA atShenzhen etal.,2012);therefore,biomassburningmaybeamajorsourceofBrC
405
(anurbansite)and11.7%atHeshan(aruralsite).Thecontributionsby duringtheharvestseasonsandhasasignificanteffectontheenhance-
BrCtoaerosollightabsorptioninotherregionsaresimilartothatinthe mentofthelightabsorptionbycarbonaceousaerosolsinthePRDre-
PRDregion.Forexample,BrCcontributed~30%ofthetotalaerosolLA gion.AsshowedinFig.7,thecorrelationbetweentheLA byBrC
307 405
and~10%ofthetotalaerosolLA inHebei,China(Yangetal.,2009), andtheSOC/PM ratioswassignificantinsummer(pb0.001)butin-
550 2.5
andBrChasbeenobservedtoaccountfor~10%oftheLA inCalifornia, significantinautumn(pN0.1),indicatingthatsecondaryproductshada
405
USA(Cappaetal.,2012).Generally,thecontributionsofBrCtolightab- greater effect on BrC in summer than in autumn although photo-
sorptionarelargerinbackgroundareasthatareimpactedtoagreater bleaching effect would also be very strong in summer (Zhong and
degreebybiomassburning.Forexample,thecontributionsofBrCto Jang,2014).
aerosolLA areashighas~50%atJeju,SouthKorea(Flowersetal.,
405
2010) and 27 ± 15% (at 404 nm) in Boulder, Colorado, USA (Lack 3.4.RelationshipbetweenthelightabsorptionbyPM andEC
2.5
etal.,2012).Lightabsorptionbydustwasexpectedtobenegligible
andthusnotconsideredinthisstudy,becausethecontributionofcarbo- TheInteragencyMonitoringofProtectedVisualEnvironments(IM-
naceousmaterialstoaerosolmass(N30%)ismuchlargerthanthatof PROVE)projectintheUnitedStatesprovidesasimplemethodtoesti-
dust(b5%)(Huangetal.,2014). mate the light extinction coefficient of PM (IMPROVE Report V,
2.5
Fig.5.RelationshipbetweenLA405andTCconcentrationsmeasuredwithfilter-basedPM2.5samplesduringthesummerandautumninGuangzhou.
1366 S.Lietal./ScienceoftheTotalEnvironment633(2018)1360–1369
Fig.6.RelationshipbetweenLA405byBrCandlevoglucosanconcentrationsduringthesummerandautumninGuangzhou.
2011).TheIMPROVEalgorithmwasusedtocalculatethelightabsorp- orderofWQSNSZNJL.TheslopeatJL(10.1m2/g)wasalsothesame
tionbyPM withanMAEvalueof10m2/gforEC(IMPROVEReport asthe10m2/gfortheMAEofECusedintheIMPROVEalgorithmtoes-
2.5
V, 2011). This MAE of EC corresponds to a wavelength of 550 nm timatethelightabsorptionbyPM ,andtheslopesatWQSandSZare
2.5
(HandandMalm,2007).However,theMAEintheIMPROVEalgorithm approximately30%higher.Thisresultimpliesthat,inareaslessstrongly
isonlyanapproximation,anditsapplicationinsomeheavilypolluted affectedbyfossilfuelcombustion,anMAEvalueof10m2/gforECcanbe
areasisunderquestion(Fuetal.,2015). usedtoestimatethelightabsorptionbyPM ;however,theuseofthis
2.5
TherelationshipbetweentheLA byPM andtheECconcentra- valuewouldunderestimatethelightabsorptionbyPM inurbanand
550 2.5 2.5
tionsatthethreesitesareshowninFig.8.ThetotalabsorptionandEC industrialareas.
concentrationsaresignificantlycorrelated(R=0.94,0.91and0.98at
WQS,SZandJL,respectively;pb0.001)atallthreesites(bothofthe 4.Conclusion
ruralsitesandtheurbansite),suggestingthatECmay,infact,bea
goodindicatorofthetotallightabsorptionbyPM inGuangzhou. Amulti-wavelengththermal/opticalcarbonanalyzerwasusedto
2.5
However,theslopesatWQS,SZandJLwere13.4,12.8and10.1,respec- determineOC/ECcontents;further,thelightabsorptionbythecarbona-
tively.Asdiscussedinsection3.2,theaveragelightabsorptionbyBrC ceousaerosolsinfilter-basedPM samplescollectedatoneurbansite
2.5
rangedfrom12to19%ofthetotallightabsorption,andthecontribution andtworuralsitesinGuangzhou,amegacityinsouthernChina,was
percentageswerethereforequitesimilaratthethreesites.Therefore, characterized.ThelightabsorptionsbyBCandBrCweredifferentiated.
thedifferentslopesmayresultlargelyfromthedifferentMAEvalues ItisfoundthatBrCcouldcontribute12–15%ofthetotalaerosollightab-
forECatthethreesites.Accordingtopreviousstudies,theMAEvalues sorptionat405nmduringthesummer,and15–19%ofthatduringthe
oftheECfromdifferentsourcesvaryandtypicallyfollowtheorderof autumn.TheaverageoverallMAEat405nmforcarbonaceousaerosols
woodburningbcropstrawburningbvehicleemissionsbcoalbriquette in terms of TC (OC + EC) increased by approximately 45% (from
burning(Chengetal.,2011;Shenetal.,2013;Xingetal.,2014).Atsite 3.42m2/ginsummerto4.95m2/ginautumn);theaverageMAEofEC
SZinthecitycenter,vehicleemissionsmaybethemajorsourceofEC.At at405nmincreasedbyonlyapproximately14%(from11.8m2/gin
siteJL,whichliesinaruralareasurroundedbyforestsandfarmlands, summerto13.4m2/ginautumn)andtheMAE ofOCincreasedap-
405
biomassburningtheremaymakelargercontributionstoEC.Finally, proximately83%(from0.7m2/ginsummerto1.3m2/ginautumn).
siteWQSislocatedinasmalltowninthecentralPRDregion,anditis SOC/PM ratiosandtheLA byBrCshowedgoodcorrelations(R
2.5 405
mainlyaffectedbypollutantstransportedfromsurroundingindustrial =0.46,pb0.001)insummerandpoorcorrelationsinautumn(pN
cities;theECisderivedlargelyfromvehicleemissionsandcoalburning. 0.1),indicatingthatsecondaryproductsmayhaveagreaterimpacton
ThesedifferencesmayexplainwhytheslopesinFig.8displayinthe BrCinsummerthaninautumn.Instead,theLA byBrCshoweda
405
Fig.7.RelationshipbetweenLA405byBrCandSOC/PM2.5duringthesummerandautumninGuangzhou.
S.Lietal./ScienceoftheTotalEnvironment633(2018)1360–1369 1367
Fig.8.RelationshipbetweenLA550byPM2.5(calculatedfromtherelationshipbetweenabsorptionandwavelength)andECconcentrationsatthethreesitesinGuangzhou.
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