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Atmosphere 2015, 6, 164-182; doi:10.3390/atmos6020164

atmosphere

ISSN 2073-4433

www.mdpi.com/journal/atmosphere

Article

PM2.5 Chemical Compositions and Aerosol Optical Properties in

Beijing during the Late Fall

Huanbo Wang 1, Xinghua Li 2, Guangming Shi 1, Junji Cao 3, Chengcai Li 4, Fumo Yang 1,*,

Yongliang Ma 5 and Kebin He 5

1 Key Laboratory of Reservoir Aquatic Environment of CAS, Chongqing Institute of Green and

Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China;

E-Mails: hbwang@cigit.ac.cn (H.W.); shigm@cigit.ac.cn (G.S.)

2 School of Chemistry and Environment, Beihang University, Beijing 100191, China;

E-Mail: lixinghua@buaa.edu.cn

3 Key Lab of Aerosol Chemistry & Physics, Institute of Earth Environment, Chinese Academy of

Sciences, Xi’an 710075, China; E-Mail: cao@loess.llqg.ac.cn

4 School of Physics, Peking University, Beijing 100871, China; E-Mail: ccli@pku.edu.cn

5 State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex,

School of Environment, Tsinghua University, Beijing 100084, China;

E-Mails: liang@tsinghua.edu.cn (Y.M.); hekb@tsinghua.edu.cn (K.H.)

* Author to whom correspondence should be addressed; E-Mail: fmyang@cigit.ac.cn;

Tel.: +86-23-6593-5921.

Academic Editors: Ru-Jin Huang and Robert W. Talbot

Received: 21 November 2014 / Accepted: 7 January 2015 / Published: 26 January 2015

Abstract: Daily PM2.5 mass concentrations and chemical compositions together with the

aerosol optical properties were measured from 8–28 November 2011 in Beijing. PM2.5

mass concentration varied from 15.6–237.5 μg·m−3 and showed a mean value of

111.2 ± 73.4 μg·m−3. Organic matter, NH4NO3 and (NH4)2SO4 were the major constituents

of PM2.5, accounting for 39.4%, 15.4%, and 14.9% of the total mass, respectively, while

fine soil, chloride salt, and elemental carbon together accounted for 27.7%. Daily

scattering and absorption coefficients (σsc and σap) were in the range of 31.1–667 Mm−1

and 8.24–158.0 Mm−1, with mean values of 270 ± 200 Mm−1 and 74.3 ± 43.4 Mm−1.

Significant increases in σsc and σap were observed during the pollution accumulation

episodes. The revised IMPROVE algorithm was applied to estimate the extinction

coefficient (bext). On average, organic matter was the largest contributor, accounting for

OPEN ACCESS

Atmosphere 2015, 6 165

44.6% of bext, while (NH4)2SO4, NH4NO3, elemental carbon, and fine soil accounted for

16.3% 18.0%, 18.6%, and 2.34% of bext, respectively. Nevertheless, the contributions of

(NH4)2SO4 and NH4NO3 were significantly higher during the heavy pollution periods than

those on clean days. Typical pollution episodes were also explored, and it has been

characterized that secondary formation of inorganic compounds is more important than

carbonaceous pollution for visibility impairment in Beijing.

Keywords: PM2.5; visibility; aerosol optical properties; chemical composition

1. Introduction

The atmospheric visibility in China has been deteriorating with economic growth during the past

40 years [1,2]. Visibility impairment is resulted from light scattering and absorption by atmospheric

particles and gases, especially from the scattering by the particles of similar size range as the

wavelength range of visible light. Numerous studies have indicated that the fine particles caused most

of the visibility impairment, while the influence of gas and coarse particles on visibility degradation

was commonly weak [3,4]. Moreover, meteorological parameters, such as wind, rain, and temperature,

especially the relative humidity, have their contributions as well [5]. Additionally, atmospheric particles

have significant impacts on climate change, which is one of the greatest sources of uncertainty in

estimating the direct radiative forcing [6,7]. Generally, inorganic and organic aerosols have a cooling effect

on climate by scattering light, while black carbon (BC) has a warming effect by absorbing light.

Previous studies usually determined the chemical compositions and optical properties of atmospheric

aerosols separately [8–10]. Yang et al. [11] compared the characteristics of PM2.5 in representative

megacities of China. Results showed that five major species including organic carbon (OC),

elemental carbon (EC), SO42−, NO3−, and NH4+ amounted to 54%–59% of PM2.5 mass in Beijing,

Chongqing and Guangzhou, and the percentages of total carbon and secondary inorganic ions were

very close, implying that both primary and secondary particles had a significant contribution to the

PM2.5 mass. Recently, Zhang et al. [12] indicated that secondary inorganic aerosols, mineral dust and

organic matter (OM) each accounted for about 20% of PM2.5 in Beijing, respectively, suggesting both

primary and secondary components of PM2.5 in Beijing were equally important.

The parameters of light extinction (bext) can be measured directly using optical instruments such as

an integrating nephelometer for the light scattering coefficient (σsc), or an aethalometer for the

absorption coefficients (σap). Optical properties of PM2.5 have been conducted in Beijing, Shanghai,

and Guangzhou [13–17]. In recent years, a few studies have focused on the relationship between the

chemical compositions and optical properties of aerosols [18,19]. Results showed that NH4+, SO42−,

NO3−, and OC are the main contributors to aerosol scattering, and the light absorption coefficients had

strong linear correlations with EC in Shanghai. Furthermore, as an alternative method, the Interagency

Monitoring of Protected Visual Environments (IMPROVE) formula could be used to estimate bext

based on the chemical compositions of particulate matter (PM). The original and revised IMPROVE

algorithms for estimating bext were developed from the particle data at 21 rural/remote sites with low

scattering coefficients. Furthermore, the IMPROVE formula usually assumes externally mixed status

Atmosphere 2015, 6 166

of PM and fixed mass extinction efficiency for each species [20]. However, the actual mass absorption

efficiency (MAE) and mass scattering efficiency (MSE) were not constant due to large temporal and

spatial variations of chemical compositions of PM. Thus, it is necessary to evaluate the applicability of

the IMPROVE formula to the calculation of bext in more polluted urban Beijing in China.

In the present study, aerosol optical properties including σsc and σap, as well as the chemical

compositions of PM2.5 were measured in Beijing during the late fall of 2011. The applicability of the

IMPROVE formula to the calculation of bext was then evaluated, and the contribution of PM2.5

chemical compositions to visibility impairment was discussed. In addition, the formation mechanisms

of typical pollution episodes during the late fall were also explored.

2. Experimental Section

2.1. Sampling

Samples were collected from 8–28 November 2011 at the campus of Tsinghua University (39°98ʹN,

116°32ʹE) in urban Beijing, about 600 m north of the Fourth Ring Road. The campus is mainly

surrounded by residential areas without significant factory emissions. Beijing is connected to the

industrialized cities of the Great North China Plain in the South, and surrounded by the Yanshan

Mountains in the west, north, and northeast.

Daily 23 h integrated PM2.5 samples were collected using a five-channel Spiral Ambient Speciation

Sampler (SASS, MetOne Inc., Grants Pass, OR, USA) with a flow rate of 6.7 L·min−1. The first

channel was used for PM2.5 mass and elemental analysis with a 47 mm Teflon filter. The second channel

collected the particles for the analysis of water-soluble inorganic ions with a 47 mm Teflon filter. The

third channel was used to collect PM2.5 on quartz filters for organic and elemental carbon analysis.

2.2. Gravimetric and Chemical Analysis

The PM2.5 mass concentrations were determined using an electronic balance with a detection limit

of 1 µg (Sartorius, Göttingen, Germany) after stabilizing at a constant temperature (22 ± 5 °C) and

relative humidity (40% ± 5%) for 24 h.

Four anions (SO42−, NO3−, Cl−, and F−) and five cations (Na+, NH4+, K+, Mg2+, and Ca2+) were

determined in aqueous extracts of the filters by Ion chromatography (ICS-1000 and ICS-2000 for

anion and cation, respectively, Dionex, Sunnyvale, CA, USA). To extract the water-soluble ions from

the Teflon filters, each sample was extracted twice with 7.5 ml Milli-Q water via an ultrasonic bath for

20 min, and then filtered through a 0.45 μm PTFE syringe filter and stored in a refrigerator at 4 °C

until analysis.

A 0.5 cm2 punch from each quartz filter was analyzed for OC and EC using a DRI Model 2001

Thermal/Optical Carbon Analyzer (Atmoslytic Inc., Calabasas, CA, USA), following the IMPROVE

thermal optical reflectance (TOR) protocol [21].

Crustal elements including Al, Si, Ca, Fe, and Ti were analyzed by Energy Dispersive X-ray

fluorescence spectrometry (Epsilon 5 ED-XRF, PANalytical Company, Almelo, The Netherlands) on

Teflon filters. Quality assurance/Quality Control (QA/QC) procedures of the XRF analysis procedure

were described by Xu et al. [22].

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Atmosphere2015,6,164-182;doi:10.3390/atmos6020164atmosphereISSN2073-4433www.mdpi.com/journal/atmosphereArticlePM2.5ChemicalCompositionsandAerosolOpticalPropertiesinBeijingduringtheLateFallHuanboWang1,XinghuaLi2,GuangmingShi1,JunjiCao3,ChengcaiLi4,FumoYang1,,YongliangMa5andKebinHe51KeyLaboratoryofReservoirAquaticEnvironmentofCAS,ChongqingInstituteofGreenandIntelligentTechnology,ChineseAcademyofSciences,Chongqing400714,China;E-Mails:hbwang@cigit.ac.cn(H.W.);shigm@cigit.ac.cn(G.S.)2SchoolofChemistryandEnvironment,BeihangUniversity,Beijing100191,China;E-Mail:lixinghua@buaa.edu.cn3KeyLabofAerosolChemistry&Physics,InstituteofEarthEnvironment,ChineseAcademyofSciences,Xi’an710075,China;E-Mail:cao@loess.llqg.ac.cn4SchoolofPhysics,PekingUniversity,Beijing100871,China;E-Mail:ccli@pku.edu.cn5StateEnvironmentalProtectionKeyLaboratoryofSourcesandControlofAirPollutionComplex,SchoolofEnvironment,TsinghuaUniversity,Beijing100084,China;E-Mails:liang@tsinghua.edu.cn(Y.M.);hekb@tsinghua.edu.cn(K.H.)Authortowhomcorrespondenceshouldbeaddressed;E-Mail:fmyang@cigit.ac.cn;Tel.:+86-23-6593-5921.AcademicEditors:Ru-JinHuangandRobertW.TalbotReceived:21November2014/Accepted:7January2015/Published:26January2015Abstract:DailyPM2.5massconcentrationsandchemicalcompositionstogetherwiththeaerosolopticalpropertiesweremeasuredfrom8–28November2011inBeijing.PM2.5massconcentrationvariedfrom15.6–237.5μg·m−3andshowedameanvalueof111.2±73.4μg·m−3.Organicmatter,NH4NO3and(NH4)2SO4werethemajorconstituentsofPM2.5,accountingfor39.4%,15.4%,and14.9%ofthetotalmass,respectively,whilefinesoil,chloridesalt,andelementalcarbontogetheraccountedfor27.7%.Dailyscatteringandabsorptioncoefficients(σscandσap)wereintherangeof31.1–667Mm−1and8.24–158.0Mm−1,withmeanvaluesof270±200Mm−1and74.3±43.4Mm−1.Significantincreasesinσscandσapwereobservedduringthepollutionaccumulationepisodes.TherevisedIMPROVEalgorithmwasappliedtoestimatetheextinctioncoefficient(bext).Onaverage,organicmatterwasthelargestcontributor,accountingforOPENACCESSAtmosphere2015,616544.6%ofbext,while(NH4)2SO4,NH4NO3,elementalcarbon,andfinesoilaccountedfor16.3%18.0%,18.6%,and2.34%ofbext,respectively.Nevertheless,thecontributionsof(NH4)2SO4andNH4NO3weresignificantlyhigherduringtheheavypollutionperiodsthanthoseoncleandays.Typicalpollutionepisodeswerealsoexplored,andithasbeencharacterizedthatsecondaryformationofinorganiccompoundsismoreimportantthancarbonaceouspollutionforvisibilityimpairmentinBeijing.Keywords:PM2.5;visibility;aerosolopticalproperties;chemicalcomposition1.IntroductionTheatmosphericvisibilityinChinahasbeendeterioratingwitheconomicgrowthduringthepast40years[1,2].Visibilityimpairmentisresultedfromlightscatteringandabsorptionbyatmosphericparticlesandgases,especiallyfromthescatteringbytheparticlesofsimilarsizerangeasthewavelengthrangeofvisiblelight.Numerousstudieshaveindicatedthatthefineparticlescausedmostofthevisibilityimpairment,whiletheinfluenceofgasandcoarseparticlesonvisibilitydegradationwascommonlyweak[3,4].Moreover,meteorologicalparameters,suchaswind,rain,andtemperature,especiallytherelativehumidity,havetheircontributionsaswell[5].Additionally,atmosphericparticleshavesignificantimpactsonclimatechange,whichisoneofthegreatestsourcesofuncertaintyinestimatingthedirectradiativeforcing[6,7].Generally,inorganicandorganicaerosolshaveacoolingeffectonclimatebyscatteringlight,whileblackcarbon(BC)hasawarmingeffectbyabsorbinglight.Previousstudiesusuallydeterminedthechemicalcompositionsandopticalpropertiesofatmosphericaerosolsseparately[8–10].Yangetal.[11]comparedthecharacteristicsofPM2.5inrepresentativemegacitiesofChina.Resultsshowedthatfivemajorspeciesincludingorganiccarbon(OC),elementalcarbon(EC),SO42−,NO3−,andNH4+amountedto54%–59%ofPM2.5massinBeijing,ChongqingandGuangzhou,andthepercentagesoftotalcarbonandsecondaryinorganicionswereveryclose,implyingthatbothprimaryandsecondaryparticleshadasignificantcontributiontothePM2.5mass.Recently,Zhangetal.[12]indicatedthatsecondaryinorganicaerosols,mineraldustandorganicmatter(OM)eachaccountedforabout20%ofPM2.5inBeijing,respectively,suggestingbothprimaryandsecondarycomponentsofPM2.5inBeijingwereequallyimportant.Theparametersoflightextinction(bext)canbemeasureddirectlyusingopticalinstrumentssuchasanintegratingnephelometerforthelightscatteringcoefficient(σsc),oranaethalometerfortheabsorptioncoefficients(σap).OpticalpropertiesofPM2.5havebeenconductedinBeijing,Shanghai,andGuangzhou[13–17].Inrecentyears,afewstudieshavefocusedontherelationshipbetweenthechemicalcompositionsandopticalpropertiesofaerosols[18,19].ResultsshowedthatNH4+,SO42−,NO3−,andOCarethemaincontributorstoaerosolscattering,andthelightabsorptioncoefficientshadstronglinearcorrelationswithECinShanghai.Furthermore,asanalternativemethod,theInteragencyMonitoringofProtectedVisualEnvironments(IMPROVE)formulacouldbeusedtoestimatebextbasedonthechemicalcompositionsofparticulatematter(PM).TheoriginalandrevisedIMPROVEalgorithmsforestimatingbextweredevelopedfromtheparticledataat21rural/remotesiteswithlowscatteringcoefficients.Furthermore,theIMPROVEformulausuallyassumesexternallymixedstatusAtmosphere2015,6166ofPMandfixedmassextinctionefficiencyforeachspecies[20].However,theactualmassabsorptionefficiency(MAE)andmassscatteringefficiency(MSE)werenotconstantduetolargetemporalandspatialvariationsofchemicalcompositionsofPM.Thus,itisnecessarytoevaluatetheapplicabilityoftheIMPROVEformulatothecalculationofbextinmorepollutedurbanBeijinginChina.Inthepresentstudy,aerosolopticalpropertiesincludingσscandσap,aswellasthechemicalcompositionsofPM2.5weremeasuredinBeijingduringthelatefallof2011.TheapplicabilityoftheIMPROVEformulatothecalculationofbextwasthenevaluated,andthecontributionofPM2.5chemicalcompositionstovisibilityimpairmentwasdiscussed.Inaddition,theformationmechanismsoftypicalpollutionepisodesduringthelatefallwerealsoexplored.2.ExperimentalSection2.1.SamplingSampleswerecollectedfrom8–28November2011atthecampusofTsinghuaUniversity(39°98ʹN,116°32ʹE)inurbanBeijing,about600mnorthoftheFourthRingRoad.Thecampusismainlysurroundedbyresidentialareaswithoutsignificantfactoryemissions.BeijingisconnectedtotheindustrializedcitiesoftheGreatNorthChinaPlainintheSouth,andsurroundedbytheYanshanMountainsinthewest,north,andnortheast.Daily23hintegratedPM2.5sampleswerecollectedusingafive-channelSpiralAmbientSpeciationSampler(SASS,MetOneInc.,GrantsPass,OR,USA)withaflowrateof6.7L·min−1.ThefirstchannelwasusedforPM2.5massandelementalanalysiswitha47mmTeflonfilter.Thesecondchannelcollectedtheparticlesfortheanalysisofwater-solubleinorganicionswitha47mmTeflonfilter.ThethirdchannelwasusedtocollectPM2.5onquartzfiltersfororganicandelementalcarbonanalysis.2.2.GravimetricandChemicalAnalysisThePM2.5massconcentrationsweredeterminedusinganelectronicbalancewithadetectionlimitof1µg(Sartorius,Göttingen,Germany)afterstabilizingataconstanttemperature(22±5°C)andrelativehumidity(40%±5%)for24h.Fouranions(SO42−,NO3−,Cl−,andF−)andfivecations(Na+,NH4+,K+,Mg2+,andCa2+)weredeterminedinaqueousextractsofthefiltersbyIonchromatography(ICS-1000andICS-2000foranionandcation,respectively,Dionex,Sunnyvale,CA,USA).Toextractthewater-solubleionsfromtheTeflonfilters,eachsamplewasextractedtwicewith7.5mlMilli-Qwaterviaanultrasonicbathfor20min,andthenfilteredthrougha0.45μmPTFEsyringefilterandstoredinarefrigeratorat4°Cuntilanalysis.A0.5cm2punchfromeachquartzfilterwasanalyzedforOCandECusingaDRIModel2001Thermal/OpticalCarbonAnalyzer(AtmoslyticInc.,Calabasas,CA,USA),followingtheIMPROVEthermalopticalreflectance(TOR)protocol[21].CrustalelementsincludingAl,Si,Ca,Fe,andTiwereanalyzedbyEnergyDispersiveX-rayfluorescencespectrometry(Epsilon5ED-XRF,PANalyticalCompany,Almelo,TheNetherlands)onTeflonfilters.Qualityassurance/QualityControl(QA/QC)proceduresoftheXRFanalysisprocedureweredescribedbyXuetal.[22].Atmosphere2015,61672.3.QualityControlThefreshquartzfilterswerepre-heatedat450°Cinamufflefurnacefor6htoremoveanyvolatilecomponentsbeforesampling.Furthermore,aftercollection,thesamplesweresealedincleanplasticbags,andwerestoredinafreezerat−18°Cbeforechemicalanalysistominimizetheevaporationofvolatilecomponents.Beforeandaftersampling,theTeflonfiltersinthefirstchannelwereweighedafterbeingequilibratedfor24h.Theartifactsduringthesamplingandanalysiswereestimatedbyafieldblankfilter.2.4.MeasurementsofAerosolOpticalandMeteorologicalParametersBCmassconcentration,σscandmeteorologicaldataweremeasuredontheroofofthePhysicsBuildingabout30mabovethegroundinPekingUniversity,whichisabout1kmawayfromthesamplingsitesofTsinghuaUniversity.Anautomaticweatherstation(VaisalaLtd.,Helsinki,Finland)wasusedtorecordwindspeed(WS),winddirection,relativehumidity(RH),temperature(Temp),andvisibility(VR).σscwasmonitoredusingasinglewavelength(525nm)integratingNephelometer(M9003,Ecotech,Melbourne,VIC,Australia).ThisinstrumentdrewambientairthroughaheatedinlettubetomaintainRHintheNephelometerchamberbelow60%.Thescatteringintensityoveranglesfrom7°to170°wasmeasuredandintegratedtoyieldthescatteringcoefficient.Zerocalibrationwasperformedeverytwodayswithparticle-freeairtosubtracttheRayleighscattering,whilespancalibrationwascarriedouteverymonthusingR-134gas.BCmassconcentrationwasmeasuredwithanAethalometer(AE-16,MageeScientific,Berkeley,CA,USA).TheprincipleofthisinstrumenttocalculatetheBCconcentrationisbasedontheattenuationofanincidentbeamatawavelengthof800nmcausedbytheparticlesloadedinthequartzfilter.Nosize-selectiveinletwasusedforboththenephelometerandaethalometer.Consideringthenegligiblecontributionofcoarseparticlestolightextinction,themeasuredσsccanbeapproximatelyattributedtothePM2.5.2.5.DataAnalysis2.5.1.ReconstructionofPM2.5MassPM2.5componentscanbegroupedasfollows:secondaryinorganicaerosols(SNA),OM,EC,finesoil(FS),andchloridesalt(CS).SNAisthesumofSO42−,NO3−,andNH4+,andOMisderivedfrommultiplyingOCconcentrationsbyafactorof1.6toaccountforunmeasuredatomsaccordingtoXingetal.[23],whichdemonstratedthatthecalculatedOM/OCmassratioinsummerwasrelativelyhigh(1.75±0.13)andinwinterwaslower(1.59±0.18)inPM2.5collectedfrom14Chinesecities.Beijingisfarfromthecoastaloceans,andseasaltisnottransportedtoBeijing,thusithasaminorcontributiontoPM2.5inBeijing.TheCSwasconsideredinsteadofseasalt,andestimatedbysummingconcentrationsofCl−,K+,andNa+accordingtoZhangetal.[12].TheconcentrationsofFSareoftenestimatedbyassumingtheoxidesoftheelementsmainlyassociatedwithsoil(Al2O3,SiO2,K2O,CaO,FeO,Fe2O3,andTiO2),whichiscalculatedasfollows[24]:Atmosphere2015,6168[FS]=2.20[Al]+2.49[Si]+1.63[Ca]+2.42[Fe]+1.94[Ti](1)2.5.2.ReconstructionoftheLightExtinctionCoefficientAccordingtotherevisedIMPROVEalgorithm,thereconstructedbextisshownfromthefollowingequationassuminganexternallymixedaerosol[20]:ScatteringRayleighppbNOPMSaltSeaRHfSoilFineECeOMLSmallOMNOeNHLRHfNOSmallNHRHfSONHeLRHfSONHSmallRHfbbbbbssLsLssgagapspext+×+×+××+×+×+×+×+××+××+××+××≈+++=)]([33.0][6.0][)(7.1][1][10]arg[1.6][8.2]arg[)(1.5][)(4.2])(arg[)(8.4])([)(2.2210~5.23434424424(2)ThealgorithmdividestheconcentrationsofSO42−,NO3−,andOMintosmallandlarge-sizedfractions.Thesizemodesaredescribedbylog-normalmasssizedistributionswithgeometricmeandiameterandgeometricstandarddeviations.Thefractionofacomponentinthelarge-orsmall-sizedmodewasestimatedbyanempiricalapproach[20].Theapportionmentofthetotalconcentrationsof(NH4)2SO4intotheconcentrationsofthesmallandlargesizefractioninPM2.5isaccomplishedusingthefollowingequations:34244244244243[()][arg()][()],[()]20.20.TotalNHSOLeNHSOTotalNHSOforTotalNHSOgmgmμμ−−=×<(3)3424424424.20])([],)([])(arg[−>=mgSONHTotalforSONHTotalSONHeLμ(4)])(arg[])([])([424424424SONHeLSONHTotalSONHSmall−=(5)SimilarequationsareusedtoapportiontotalNH4NO3andtotalOMconcentrationsintosmallandlargesizefractions.Thewatergrowthadjustmenttermfs(RH),fL(RH)forsmallandlargesizedistributionof(NH4)2SO4andNH4NO3,andfss(RH)forseasaltareusedaccordingtothewatergrowthcurvesprovidedbyPitchfordetal.[20].AccordingtotherevisedIMPROVEmethod,SO42−andNO3−areassumedtobefullyneutralizedbyNH4+intheformsof(NH4)2SO4andNH4NO3,respectively.Therefore,(NH4)2SO4massisestimatedbytheSO42−massmultipliedbyafactorof1.38,andtheNH4NO3massisestimatedbytheNO3−massmultipliedbyafactorof1.29.InordertocomparewiththereconstructedbextcalculatedusingMietheoryat550nm,σscmeasuredat525nmwiththeintegratingnephelometershouldbeconvertedtothatat550nmaccordingtothemethodbyJungetal.[16].αλλσσ−=)525()550()525()550(nmnmnmnmscsc(6)whereαisthescatteringAngströmexponent,anaverageαvalueof1.18determinedinBeijingduringthesummerof2012byTianetal.[18]wasusedinthepresentstudy.σabat550nmwascalculatedbasedonBCconcentrationfollowingtheequation:Atmosphere2015,6169σab=K×[BC](7)whereKistheconversionfactor,whichwassetto8.1m2·g−1inthisstudyaccordingtoapreviousstudy[25].Thesinglescatteringalbedo(SSA)isdefinedastheratiooftheaerosolscatteringcoefficienttotheextinctioncoefficientataknownwavelength,asderivedfromtheformula:abscscSSAσσσ+=(8)3.ResultsandDiscussion3.1.PM2.5ChemicalCompositionsThetimeseriesofdailyPM2.5massconcentrationsandthemeteorologicalparameters,includingRH,temperature,andWSareshowninFigure1.ThemassconcentrationsofPM2.5rangedfrom15.6–237.5μg·m−3andaveraged111.2±73.4μg·m−3.ComparedwithotherstudiesconductedinurbanBeijing,theaveragePM2.5concentrationinthisstudywaslowerthanthatmeasuredduringautumnof2006(194.2μg·m−3)[26]and2009(135μg·m−3)[12],whilecomparablewiththatobservedduringthesameseasonin2005(115.0μg·m−3)and2012(106.9μg·m−3)[27].ThereweresixtypercentofdayswithdailyPM2.5massconcentrationexceedingtheChinaAmbientAirQualityStandards(75μg·m−3).ThehighestPM2.5massconcentrationoccurredon26November,whichwasassociatedwiththehighrelativehumidityandlowwindspeed.01530456075900123456RH(%),Temp(oC)TempRHWS(ms-1)WS2011-11-92011-11-112011-11-132011-11-152011-11-172011-11-192011-11-212011-11-232011-11-252011-11-2703060901201501802102400510152025303540PM2.5PM2.5concentration(μgm-3)VRVR(km-1)01530456075900123456RH(%),Temp(oC)TempRHWS(ms-1)WS2011-11-92011-11-112011-11-132011-11-152011-11-172011-11-192011-11-212011-11-232011-11-252011-11-2703060901201501802102400510152025303540PM2.5PM2.5concentration(μgm-3)VRVR(km-1)Figure1.DailyvariationsofPM2.5massconcentrationandmeteorologicalparameters.Atmosphere2015,6170Thetemporalvariationsofninewater-solubleinorganicions(WSIIs)arepresentedinFigure2.Theaverageconcentrationoftotalnineionswas46.9±33.8μg·m−3,accountingfor41.5%ofPM2.5massconcentration.NO3−wasthemostabundantspeciesinWSIIswithanaverageconcentrationof14.7±11.2μg·m−3,followedbySO42−(12.2±9.63μg·m−3),NH4+(9.13±7.26μg·m−3),andCl−(6.62±4.62μg·m−3),accountingfor28.9%,25.6%,17.7%,and14.6%ofWSIIs,respectively.TherestofK+(1.66±1.41μg·m−3),Na+(0.95±0.52μg·m−3),Ca2+(0.81±0.32μg·m−3),F−(0.61±0.34μg·m−3),andMg2+(0.25±0.18μg·m−3)eachhadaminorcontributiontotheWSIIs,totallyaccountingfor13.2%ofWSIIs.SNAtypicallyconstituted33.5%–87.1%ofthetotalWSIIsand15.3%–46.0%ofPM2.5,respectively.0.00.30.60.91.21.51.82.12.42011-11-92011-11-112011-11-132011-11-152011-11-172011-11-192011-11-212011-11-232011-11-252011-11-27020406080100120Concentration(μgm-3)NH4+Ca2+Mg2+k+Na+Cl-F-NO3-SO42-[NO3-]/[SO42-]Figure2.Dailyvariationsofwater-solubleionsand[NO3−]/[SO42−]ratios.NO3−andSO42−aremainlyformedbyatmosphericreactionsofprecursorgasessuchasNOxandSO2.Generally,SO2emitsfromcoalcombustion,whileNOxistheresultofanytypeofcombustionsuchascoal-firedpowerplantsandautomobiles.Themassratio[NO3−]/[SO42−]hasbeenusedtoidentifytheinfluenceofthestationaryandmobilesourcesofsulfurandnitrogen[28].The[NO3−]/[SO42−]ratiorangedfrom0.41–2.42,withanaveragevalueof1.19.Itwashigherthanthatofvalues(around0.68)measuredinBeijingfrom2001–2006[26,29–31],butrathermorecomparabletothoseobservedinrecentyears[12].AsillustratedinFigure2,theratiowasusuallylowerduringweekenddays(12,18–19November)thanonworkdays,indicatingthatthehigher[NO3−]/[SO42−]ratiointhepresentstudywasprobablyassociatedwiththerapidincreaseofmotorvehiclesinrecentyears.AccordingtothestatisticsfromtheChinaVehicleEmissionControlAnnualReportin2013,theamountofvehiclesreached5millioninBeijingby2012,whichwasanincreaseofaboutthreetimescomparedwiththeamountofvehiclesin2001[32].PreviousstudyshowedthatCl−mightbederivedfromcoalcombustionwhentheCl−/Na+equivalentconcentrationratioswerelargerthanthemeanratio(1.17)forseawater.TheratiosofCl−/Na+wereintherangeof1.6–11.6withameanvalueof6.63duringthestudyperiod,implyingthatCl−maybeoriginatedfromcoalcombustionratherthanseaspray[12].Theequivalentmolarratiooftotalcationstototalanions(CE/AE)rangedfrom0.71–1.40,withanaveragevalueof0.95±0.14duringthestudyperiod.Figure3illustratesthescatterplotsofthesumofcationsversusanions.Resultsshowedthattheslopewasslightlylowerthan1,implyingthatthefineparticlescollectedinthestudyperiodwereweaklyacidic.Moreover,theratiosof[NH4+]/[SO42−+NO3−]Atmosphere2015,6171werecloseto1,demonstratingthatSO42−andNO3−werefullyneutralizedbyNH3.Therefore,thedominantchemicalformofSO42−was(NH4)2SO4ratherthanNH4HSO4,whichcanbeestimatedbytheSO42−massconcentrationmultipliedbyafactorof1.38,whileNO3−existedasNH4NO3,andcanbeestimatedbytheNO3−massconcentrationmultipliedbyafactorof1.29.0.00.20.40.60.81.01.21.41.61.80.00.20.40.60.81.01.21.41.61.8y=0.91x;R=0.991CE(μeqm-3)AE(μeqm-3)(a)0.00.20.40.60.81.01.21.40.00.20.40.60.81.01.21.4(b)NH4+(μeqm-3)SO42-+NO3-(μeqm-3)y=1.04x;R=0.981Figure3.Relationshipsofequivalentconcentrationsofcationsversusanions(a)and[NH4+]versus[SO42−+NO3−](b).AsillustratedinFigure4,OCvariedfrom2.1to64.3μg·m−3,averaging27.5±19.9μg·m−3,whileECrangedfrom0.86to19.6μg·m−3,averaging9.62±6.24μg·m−3.ThecontributionofOCandECtoPM2.5were24.5%and8.96%,respectively.SuchlevelsofOCandECwereclosetothoseobservedinthesameseasoninrecentyears[10,26,30,33],whereaslowerthanthosemeasuredtenyearsago[29,34].2011-11-92011-11-112011-11-132011-11-152011-11-172011-11-192011-11-212011-11-232011-11-252011-11-270102030405060700.00.51.01.52.02.53.03.54.0OC/ECConcentration(μgm-3)OCECOC/EC(a)2011-11-92011-11-112011-11-132011-11-152011-11-172011-11-192011-11-212011-11-232011-11-252011-11-2701020304050607001020304050SOC/OC(%)Concentration(μgm-3)POCSOCSOC/OC(b)Figure4.VariationsofOC,EC,andOC/EC(a)aswellasprimaryorganiccarbon(POC),secondaryorganiccarbon(SOC),andSOC/OC(b).TherelationshipsbetweenOCandECcanbeusedtoidentifytheoriginsofcarbonaceousparticles[35,36].AsshowninFigure5,strongcorrelationsbetweenOCandECwereobservedwithacorrelationcoefficientof0.97,indicatingthatOCandECwerelikelyderivedfromthesamemajorprimarysourcesduringthecampaigns.Ontheotherhand,theOC/ECratiosdidnotvarydistinctlyduringthestudyperiod,especiallyduringthespaceheatingdays.Theratiosrangedfrom1.96–3.52,averaging2.79,andwereveryclosetothevalueof2.7fromcoalcombustionsuggestedbyAtmosphere2015,6172Watsonetal.[37].ThispointedtothefactthatOCandEClikelyoriginatedmainlyfromcoalcombustions.Furthermore,themeanOC/ECratiowashigherthan2,indicatingthatSOCmightbepresentduringthestudyperiod[38].03691215182101020304050607080OCconcentrations(μgm-3)ECconcentrations(μgm-3)y=2.88xR=0.97Figure5.RelationshipsofOCandECconcentrations.ThemethodofEC-tracerhasbeenwidelyusedtoestimatetheSOCconcentrationsinceitwasfirstintroducedbyCastroetal.[9,39].ThisapproachsuggestedthatsampleshavingthelowestOC/ECratiocontainedalmostexclusivelyPOC.Then,theconcentrationofSOCcanbeestimatedbythefollowingformula:POC=EC×(OC/EC)min(9)SOC=OC–POC(10)where(OC/EC)minwasthevalueofthelowestOC/ECratio.Basedonthe(OC/EC)minof1.63,theSOCconcentrationsvariedfrom0.02–25.9μg·m−3withanaveragevalueof9.03μg·m−3.AsillustratedinFigure4,itisinterestingtonotethattheconcentrationsofSOCwerestillhighwithlowtemperatureduringthestudyperiodexcepton10,12and22November.Thismaybecausedbythecombinationofthehighprecursoremissionduetothelargelyincreasedcoalcombustionforresidentialheatingandlowwindspeed(averaging0.79m·s−1),whichwasfavorableforthepollutantsaccumulationandformationofsecondaryorganicaerosol.Thelowtemperaturewasnotfavorableforthegastoparticleconversion,whereasthefrequentinversionconditionswerelikelyfavorablefortheformationofSOC[40].DailyvariationsofcrustalelementsareshowninFigure6.FivecrustalelementshaveasimilarvariationasthePM2.5massconcentrations.Theirconcentrationsvariedsignificantlyfromdaytoday.TheaverageconcentrationforAl,Si,Ca,Fe,andTiwas0.51±0.26,1.01±0.58,0.74±0.36,1.10±0.64,and0.06±0.03μg·m−3,respectively.Increasingwindspeedcouldbeexpectedtoincreasetheconcentrationsofcrustalelements,buttheconcentrationofthefiveelementshadaweakcorrelationwiththewindspeed(R<0.4)inthepresentstudy.However,whenthewindspeedexceeded1.5m·s−1(11and18November),theconcentrationsofcrustalelementswerehigherthanthoseonanyotherday.TheFSmassconcentrationwasestimatedbysummingtheabovefivecrustalelementsplusoxygenforAtmosphere2015,6173thenormaloxidesasEquation(1).TheaveragemassconcentrationsofFSwere7.66±3.62μg·m−3,rangingfrom1.99–15.1μg·m−3,andaccountingfor9.42%ofPM2.5.2011-11-92011-11-112011-11-132011-11-152011-11-172011-11-192011-11-212011-11-232011-11-252011-11-270.00.30.60.91.21.51.82.12.42.73.0AlSiCaFeTiElementconcentration(μgm-3)Figure6.Timeseriesofcrustalelementsconcentration.3.2.PM2.5MassBalanceThereconstructedPM2.5massconcentrationswereclosetothemeasuredoneswithstrongcorrelation(Figure7),indicatingthatthereconstructionofPM2.5couldbereasonable.Nevertheless,afewbiaseswereobservedinthereconstructedPM2.5mass.Waterabsorptionofthewater-solublecomponentsmayleadtopositivebiasesandoverestimatethePM2.5mass,whilethevolatilizationofNH4NO3andvolatileorganicmattermayresultinnegativebiases.Moreover,theconversionusedtoestimateOMfromOCalsocausedanuncertaintyincalculatingthePM2.5mass.050100150200250300050100150200250300ReconstructedPM2.5(μgm-3)MeasuredPM2.5(μgm-3)y=0.97xR=0.9971:1lineFigure7.ScatterplotsofmeasuredandreconstructedPM2.5massconcentrations.Figure8presentsthereconstructedchemicalcompositionsinPM2.5.Onaverage,thefractionsofmajorchemicalcompositionsfollowedtheorderofOM>NH4NO3>(NH4)2SO4>FS,CS,andEC.OMwasthemostabundantcomponentinPM2.5(averaging45.8±31.7μg·m−3),accountingfor39.4%Atmosphere2015,6174ofPM2.5.ThecontributionofFS(averaging7.66±3.62μg·m−3),CS(averaging9.47±6.24μg·m−3),andEC(averaging9.97±6.28μg·m−3)toPM2.5wassimilar,eachapproximatedto9%.ThepercentageofSNA(30.2%)wasmuchhigherthanthethreespeciesofFS,CS,andEC,butslightlylowerthanthatofOM.Thepercentagesof(NH4)2SO4andNH4NO3(averaging16.7±13.2and19.0±14.4μg·m−3,respectively)were14.9%and15.4%,respectively.Comparedwiththeresultsdeterminedoverthesameperiodinearlieryears[11],itisnotedthatthepercentageofSNAinourstudydecreasedby3%–10%comparedwiththatmeasuredduring2003–2007,whiletheOMfractionroseabout5%.2011-11-92011-11-112011-11-132011-11-152011-11-172011-11-192011-11-212011-11-232011-11-252011-11-270306090120150180210240ECCSFSOMNH4NO3(NH4)2SO4Concentration(μgm-3)PM2.5measuredFigure8.DailyvariationsofthereconstructedchemicalcompositioninPM2.5.3.3.AnalysisofAerosolOpticalPropertiesThetimeseriesofdailyaveragedopticalpropertiesincludingσsc,σap,andSSAareshowninFigure9.Dailyσscrangedfrom31.1–667Mm−1,withameanvalueof270±200Mm−1,whileσapwasintherangeof8.24–158.0Mm−1,withameanvalueof74.3±43.4Mm−1.ThemeanσscvaluewasconsiderablelowerthanthatmeasuredinurbanBeijingin2009andduring2005–2006[41,42],buthigherthanthatobtainedatasuburbansite(Changping)andruralsite(Shangdianzi)[15].Comparedwiththeresultsmentionedabove,themeanσapvaluewaslowerthanthatmeasuredin2009aswell,buthigherthanthatduring2005–2006.Theincreasedσapinrecentyearsislikelyattributabletotherapidincreaseofvehiclepollution,sincevehicularexhaustwasoneoftheprimaryfactorsaffectingaerosolabsorption.ThemeanvalueofSSAwas0.76,whichwascomparablewiththeresultsdeterminedinBeijingduring2005–2006[42]andin2009[41].MSEisanimportantparameterforestimatingradiativeforcingofaerosolsandchemicalextinctionbudgetsforvisibilityimpairment.Generally,therearetwomethodstoestimateMSE,i.e.,measurementmethodandmultilinearregressionmethod[43].MSEwasdefinedastheratioofmeasuredσsctoaerosolmassconcentrationaccordingtothemeasurementmethod.Onealternativemethodcanalsobeusedbyregressionofthemeasuredσscagainstaerosolmassconcentration.SincetheRHinthenephelometerwasmaintainedbelow60%andthePM2.5massconcentrationsweremeasuredatRHof40%,thosedayswithRHbelow50%wereselectedforregressingtominimizetheimpactofparticlehygroscopicgrowthonσsc.Accordingtothemeasurementmethod,dailyMSEvariedfrom1.70–3.02m2·g−1,withameanvalueof2.32±0.44m2·g−1.ItisnotedthatastrongAtmosphere2015,6175correlationbetweenthemeasuredσscandPM2.5massconcentrationwasobservedwithahighcorrelationcoefficientof0.988(Figure9).Theslopewas2.67fromalinearregression,whichwasslightlyhigherthanthevalueobtainedbythemeasurementmethod,alsofoundbyTitosetal.[43].ComparedtothatmeasuredinurbanBeijingbyZhaoetal.,andJingetal.[41],therelativelowerMSEinourstudywaslikelyrelatedtotheheavilypollutedevents.2011-11-92011-11-112011-11-132011-11-152011-11-172011-11-192011-11-212011-11-232011-11-252011-11-2701002003004005006007008000.00.20.40.60.81.0σscandσap(Mm-1)σscσapSSASSA(a)0501001502002500100200300400500600700Measuredσsc(Μm-1)PM2.5concentration(μgm-3)y=2.67xR=0.988(b)Figure9.Thetimeseriesofdailyaveragedaerosolopticalproperties(a)andrelationshipbetweenthemeasuredσscandPM2.5concentration(b).3.4.ChemicalApportionmentoftheAerosolOpticalParametersInordertoappointthecontributiontothevisibilityimpairment,bextwasreconstructedbasedonthechemicalcompositionsofaerosol.Inthepresentstudy,theextinctioneffectbyfineparticleswasstudied,whilethecontributionsofgaseswereexcludedbecausetheyonlyaccountedforasmallfraction(about2%–4%)ofbext[44].TheimpactofseasaltonbextwasignoredsinceBeijingisabout150kmawayfromtheEastChina’scoastaloceans.Moreover,thecontributionofcoarsemasstobextwasnotincludedbecauseoflackoftheconcentrationsofcoarsematter.Then,therevisedIMPROVEformulaofEquation(2)wasmodifiedasfollows:][1][10]arg[1.6][8.2]arg[)(1.5][)(4.2])(arg[)(8.4])([)(2.23434424424SoilFineECeOMLSmallOMNOeNHLRHfNOSmallNHRHfSONHeLRHfSONHSmallRHfbLsLsext×+×+×+×+××+××+××+××=(11)ThemeasuredandreconstructedbextareillustratedinFigure10.Itisfoundthatthemeasuredbextwereconsiderablylowerthanthereconstructedvalue,especiallyduringtheheavilypollutionlevels.Thedeviationvariedfrom18.1%–140%,withanaveragevalueofabout70%.Jungetal.[45]alsofoundthatthebextwasoverestimatedby36.7%basedontherevisedIMPROVEalgorithm.However,afewotherstudiesusingtheIMPROVEformulafoundthatthereexistedagoodcorrelationbetweenthemeasuredandreconstructedbext,andtheslopeswerecloseto1.0inShanghai[19]andGuangzhou[4].ComparedtotheresultsconductedinShanghai,alowerMAE(7.7)wasusedtocalculatebap.Ifthesamevalueof7.7wasusedinthepresentstudy,thebiaseswoulddecreaseby8%.Additionally,theMSEusedtocalculatebextinGuangzhouwasmuchhigherthanthevalueinthepresentstudy.Thus,itcanbededucedthatalowerMSEthanthevalueadoptedaccordingtotherevisedIMPROVEAtmosphere2015,6176algorithminthepresentstudyshouldbeusedtoreconstructbext,whichmayresultinareconstructedbextapproximatelyequaltothemeasuredbext.Althoughmorelocally-derivedMSEandMAEwerenecessaryforeffectivelyreconstructingthebext,wedidnotobtainthesevaluesinthepresentstudyduetolackoftheamountofinsituandsamplingdata.2011-11-92011-11-112011-11-132011-11-152011-11-172011-11-192011-11-212011-11-232011-11-252011-11-270300600900120015001800ReconstructedbextMeasuredbextbext(Mm-1)(a)0100200300400500600700020040060080010001200140016001800ScatteringcoefficientAbsorptioncoefficientExtintioncoefficienty=1.829x+0.708R=0.989y=1.289x+6.906R=0.967y=1.759x-10.33R=0.989Reconstructedbsp,bap(Mm-1)Measuredσsp,σap(Mm-1)(b)Figure10.Thetemporalvariationsofmeasuredandreconstructedbext(a)andthecorrelationbetweenmeasuredandreconstructedopticalparameters(b).Althoughthereconstructedbextwashigherthanthatmeasuredone,bothwerecorrelatedwell(R=0.989),sodidbspandbap(Figure11).Therefore,therelativecontributionofeachchemicalcompositiontobextcanalsobeanalyzedbythemodifiedIMPROVEalgorithm.AsshowninFigure11,OM,(NH4)2SO4,NH4NO3andECwerethedominantcontributors,accountingfor97.7%ofbexttogether,whilethecontributionofFSwassmall,accountingforonly2.3%ofbext.Onaverage,OMwasthelargestcontributortothebext,accountingfor44.7%ofbext,whileSNAaccountedfor34.4%ofbext.(NH4)2SO4andNH4NO3contributed16.4%and18.0%ofbext,respectively.OurresultsweredifferentfromthosedeterminedinBeijinginpreviousstudiesconductedinsummer[16,18,46],whichshowedthat(NH4)2SO4andNH4NO3werethelargestcontributortothebext.2011-11-92011-11-112011-11-132011-11-152011-11-172011-11-192011-11-212011-11-232011-11-252011-11-27020406080100ECFSOMNH4NO3(NH4)2SO4Percentage(%)meanFigure11.RelativecontributionsofeachchemicalcompositioninPM2.5tobext.Atmosphere2015,61773.5.TypicalPollutionEpisodesAsshowninFigure1,fourobviouspollutionepisodeswereobservedduringthecampaign,withthevisibilitydeterioratingtolessthan10km.Theywereobservedon9–11November,14–17November,20and21November,and23–27November.Obviously,inthefirstpollutionperiod,pollutantsaccumulatedgraduallyfrom8–10November,withdailyPM2.5massconcentrationincreasingfrom36.3–98.8μg·m−3,andthendecreasingdramaticallyto17.1μg·m−3.AllthechemicalcomponentsincreasedwithPM2.5mass,especiallyforNO3−,whichincreasedbysixtimescomparedwiththevalueon8November.Furthermore,itcanbefoundthatduringthepollutionaccumulationperiod,thewindspeedwaslessthan0.5m·s−1.On11November,strongwindwasfavorablefordispersionofthepollutantsandaccumulationofthecrustalmaterial.Moreover,12Novemberwasweekend,andthereductionofvehiclesmayalsocontributetothelowerconcentrationofPM2.5.Fromanextinctionperspective,inthefirstpollutionstage,σscincreasedfrom71.6–177.2Mm−1,whileσapvariedlittle,implyingthatthevisibilitydegradationwasmainlycausedbytherapidincreaseofSNA.Inthesecondpollutionperiodfrom12–19November,dailyPM2.5massconcentrationincreasedfrom17.1–215.5μg·m−3,withthemaximumvalueoccurringon15November,andthenplungingto32.0μg·m−3withinthreedays.AsillustratedinFigure1,itcanbefoundthatthewindspeedswereverylowfrom12–15November,andinfavoroftheaccumulationofthepollutants.Ontheotherhand,theRHincreasedfrom30%to82%,whichwasfavorablefortheformationofSNA.ResidentialheatingstartinginthemiddleofNovembermightbetheprimaryreasonfortheheavierpollutioninthesecondstage.AsshowninFigure2,theconcentrationofCl−on13Novemberhadadramaticincreasecomparedwiththepreviousdays,approximatelyto10timeshigherthanthaton12November.ChloridemaybeessentiallycontributedbycoalcombustioninBeijingduringtheheatingseason.Thus,thehighPM2.5massconcentrationwasmainlyassociatedwithcoalcombustion.Infact,theconcentrationsofOCandECduring13–27Novemberweremuchhigherthanthosebefore12November.ThesharpincreaseofOCandECalsoverifiedtheinfluenceofcoalcombustionontheincreaseofPM2.5massconcentration.Unlikethefirstpollutionstage,σaponthemostheavypollutionday(15November)wasabout15timeshigherthanthatonthecleandays.Meanwhile,σscrosefrom31.0–439.2Mm−1,withasimilargrowthrateasσap.AsshowninFigure11,duringthesecondpollutionepisode,thecontributionofOMandECtobextdecreasedfrom54.2%and25.6%to32.7%and11.5%asthepollutantsaccumulated,respectively,whilethecontributionofSNAtobextincreasedfrom18.3%–54.8%ontheaccumulationperiod.On16November,althoughthemassconcentrationofPM2.5reducedby40%,thevisibilitywasstilllessthan3km,whichwasascribedtothelargestcontributionofSNAtobext.AspresentedinFigure11,thecontributionofSNAreachedupto73.7%whereasthatofOMandECdecreasedto26.7%tobext.ThePM2.5massconcentrationhadasignificantdecreaseon18Novemberduetorainandstrongwind(Figure1).Basedontheanalysisofatypicalpollutionepisode,itcanbeconcludedthatthesecondaryformationofaerosolwasmoreimportantthanthecarbonaceouspollutionforthehazeformationinBeijing.Intheothertwopollutionperiods,asimilartrendofthechemicalcompositiontothatduringthesecondpollutionstagewasobserved.Ingeneral,thepollutionaccumulationwasinaccordancewiththeincreaseoftheSNA,OCandECconcentrationsunderstableweatherconditionsuntilarrivalofstrongwind.Atmosphere2015,61784.ConclusionsDuringtheheatingperiodfrom8–28November2011,aerosolopticalpropertiesaswellaschemicalcompositionswereinvestigatedsimultaneouslyinBeijing.DailyPM2.5massconcentrationvariedfrom15.6–237.5μg·m−3andpresentedameanvalueof111.2±73.4μg·m−3.AmongthechemicalcomponentsinPM2.5,NO3−wasthemostabundantspeciesinWSIIswithanaverageconcentrationof14.7±11.2μg·m−3,followedbySO42−,NH4+,andCl−,accountingfor28.9%,25.6%,17.7%,and14.6%ofWSIIs,respectively.TherestofK+,Na+,Ca2+,F−,andMg2+haveaminorcontributiontotheWSIIs,accountingfor13.2%ofWSIIstogether.Themeanσsc,σapandSSAvaluesat550nmwere270±200Mm−1,74.3±43.4Mm−1and0.76duringtheentireobservationperiod,respectively.Bothoftheσscandσapincreasedsignificantlyduringthepollutionaccumulationepisode.ThebextwereestimatedbytherevisedIMPROVEformulabasedonthechemicalcompositionsofPM2.5.Comparedwiththemeasuredσscandσap,thereconstructedbextwasoverestimated,buthadastrongcorrelationwithahighcorrelationcoefficientof0.989.OMwasthelargestcontributor,accountingfor44.7%ofbext,followedbyNH4NO3,(NH4)2SO4,withminorcontributionfromsoildust(2.3%).PollutionepisodesinBeijingwerestronglyinfluencedbybothemissionsandmeteorologicalconditions.Pollutantwasaccumulatedincalmorweakwindswhilediffusedunderstrongwindconditions.Additionally,thecoalcombustionforresidentialheatingwasanothermajorreasonfortheheavypollutionduringthesamplingperiod.Fourtypicalpollutionepisodesduringthestudyperiodwereobserved,itwasfoundthatNH4NO3and(NH4)2SO4werethelargestcontributortothebextratherthancarbonaceouscomponentsduringthepollutionaccumulationepisodes,implyingthatthesecondaryinorganicpollutantsweremoreimportantthanthecarbonaceouspollutionforheavypollutionformation.Therefore,thereductionoftheirprecursorssuchasSO2,NOxandNH3couldeffectivelyimprovethevisibilityinBeijing.AcknowledgmentsThisstudywassupportedbytheNationalNaturalScienceFoundationofChinaprojects(41075093,41275121and41375123),the“StrategicPriorityResearchProgram”oftheChineseAcademyofSciences(KJZD-EW-TZ-G06-04),theMinistryofEnvironmentalProtectionofChina(201209007),StateEnvironmentalProtectionKeyLaboratoryofSourcesandControlofAirPollutionComplex(SCAPC201310),JiangsuKeyLaboratoryofAtmosphericEnvironmentMonitoringandPollutionControlofNanjingUniversityofInformationScienceandTechnology,andJiangsuProvinceInnovationPlatformforSuperioritySubjectofEnvironmentalScienceandEngineering(KHK1201).TheauthorsthankLian-fangWei,Jin-luDong,andRongZhangfortheircontributionstothefieldandlaboratorywork.AuthorContributionsTheworkwascompletedwithcollaborationbetweenalltheauthors.Thecorrespondingauthordesignedtheresearchtheme,organizedthePM2.5samplingwithXinghuaLi,checkedtheexperimentalresults,anddesignedthemanuscriptwithHuanboWang.HuanboWanganalyzedthedata,interpretedAtmosphere2015,6179theresultsandwrotethemanuscript.XinghuaLiwasinchargeofPM2.5samplingandcollectedallrelevantdata,andChengcaiLiwasinchargeofobservationofopticalparameters.JunjiCaowasinchargeofinorganicelementsanalysis.YongliangMaandKebinHeprovidedanalysesofwater-solubleionsandOC/EC.GuangmingShiwasinvolvedinrelevantdatainterpretationanddiscussion.ConflictsofInterestTheauthorsdeclarenoconflictofinterest.Reference1.Wang,Q.Y.;Cao,J.J.;Tao,J.;Li,N.;Su,X.O.;Chen,L.W.A.;Wang,P.;Shen,Z.X.;Liu,S.X.;Dai,W.T.Long-termtrendsinvisibilityandatChengdu,China.PLoSOne2013,doi:10.1371/journal.pone.00688942.Zhang,X.Y.;Wang,Y.Q.;Niu,T.;Zhang,X.C.;Gong,S.L.;Zhang,Y.M.;Sun,J.Y.AtmosphericaerosolcompositionsinChina:Spatial/temporalvariability,chemicalsignature,regionalhazedistributionandco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