Aerosol Counterflow Two-Jets Unit for Continuous

AerosolCounterflowTwo-JetsUnitforContinuousMeasurementoftheSolubleFractionofAtmosphericAerosols

PavelMikusˇka*andZbyneˇkVecˇerˇa

InstituteofAnalyticalChemistry,AcademyofSciencesoftheCzechRepublic,Veverˇ´ı97,CZ-61142Brno,CzechRepublicAnewtypeofaerosolcollectoremployingaliquidatlaboratorytemperatureforcontinuoussamplingofatmo-sphericparticlesisdescribed.ThecollectoroperatesontheprincipleofaVenturiscrubber.SampledairflowsathighlinearvelocitythroughtwoVenturinozzles“atom-izing”theliquidtoformtwojetsofapolydisperseaerosoloffinedropletssituatedagainsteachother.Counterflowjetsofdropletscollide,andwithinthisprocess,theaerosolparticlesarecapturedintodispersedliquid.Underopti-mumconditions(airflowrateof5L/minandwaterflowrateof2mL/min),aerosolparticlesdownto0.3µmindiameterarequantitativelycollectedinthecollectorintodeionizedwaterwhilethecollectionefficiencyofsmallerparticlesdecreases.Thereisverylittlelossoffineaerosolwithintheaerosolcounterflowtwo-jetsunit(ACTJU).Couplingoftheaerosolcollectorwithanannulardiffusiondenuderlocatedupstreamofthecollectorensuresanartifact-freesamplingofatmosphericaerosols.OperationoftheACTJUincombinationwithon-linedetectiondevicesallowsinsituautomatedanalysisofwater-solubleaerosolspecies(e.g.,NO2-,NO3-)withhightimeresolu-tion(ashighas1s).Undertheoptimumconditions,thelimitofdetectionforparticulatenitriteandnitrateis28and77ng/m3,respectively.Theinstrumentissufficientlyruggedforitsapplicationatroutinemonitoringofaerosolcompositionintherealtime.Atmosphericaerosolsplayanimportantroleinvariousenvironmentalissues.1,2TheaerosolparticlesaffecttheEarth’sclimatebychangingtheradiationbudgetoftheatmosphere.ByscatteringUVradiationbacktospace,theyreducesolarirradiance(so-calleddirectaerosoleffect).Inclouds,theaerosolsmayserveascloudcondensationnucleiaffectingthustheformation,albedo,andoccurrenceofclouds(indirectaerosoleffect).Theaerosolsalsocontributetoacidificationandeutrophicationoflandandwaterresourcesviawetanddrydeposition,takepartinsmogproductionandvisibilityreductionoverlargeportionsoftheglobe,orserveasthesitesonwhichheterogeneousreactionsofgaseoustraceconstituentsoccur.Inaddition,epidemiologicalstudiesreporta*Correspondingauthor.E-mail:mikuska@iach.cz.Phone:+420532290167.Fax:+4201212113.

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5534AnalyticalChemistry,Vol.77,No.17,September1,2005correlationbetweenincreasedadversehealtheffectsandmortality,respectively,andhighconcentrationofambientparticulatematter.2-4Theexactrolethatatmosphericaerosolsplaydependsprima-rilyonaerosolnumberconcentration,onsizedistribution,andespeciallyonchemicalcompositionofaerosols.Themeasurementofaerosolcompositionisverydifficultbecauseofhighcomplexityofatmosphericaerosolcomponentsandtheirconsiderablevaria-tionswithtimeandplace.Althoughvariousconstituentsofatmosphericaerosolsarefrequentlymeasuredwithinspecializedstudies,inalong-termperiod,thechemicalcompositionofatmosphericaerosolsisnotroutinelymeasuredbecause,atpresent,noreliableandlow-costautomatedinstrumentationwithareasonabletimeresolutioniscommerciallyavailable.Measurementofthechemicalcompositionofaerosolsisusuallycarriedoutbycollectionofaerosolsatfiltersorimpactorswithsubsequentoff-lineanalysisofcollectedparticlesprovidingtime-integratedresults.However,theseproceduresareknowntobesubjecttosignificantsamplingartifacts.Particlescollectedonfilterscanbelostbyvolatilizationduringandaftercollection(negativeartifacts)whilepositiveartifactsmayarisefromgasabsorptiononfilterorcollectedparticles.5Anotherapproach,anelectrostaticcollectionofaerosols,6,7allowshighefficiencyofparticlecollectionbutthenecessityofregularwashingofcollectedparticlesmakesthemeasurementdiscontinuous.Variousliquid-basedcollectorsworkingondifferentprinciplessuchasabsorp-tion,8impaction,9,10orcollectiononfrit11orfilter12havebeendescribedforaerosolsampling,butefficiencyofabsorptiondecreaseswhenparticlediametergoesbelow1µm.Applicationoffilterssuffersfromproblemswithdisintegrationoffilterandgradualaccumulationofwater-insolublesubstancesonthefilter(3)Dockery,D.W.;Pope,C.A.,III;Xu,X.;Spengler,J.D.;Ware,J.H.;Fay,

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10.1021/ac050343lCCC:$30.25

©2005AmericanChemicalSociety

PublishedonWeb07/28/2005

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presentdaycommercialscrubbersisdecreasedeffectivenessofcollectionmechanismsforsubmicrometeraerosolparticles.Inthispaper,thenewtypeofaerosolcollector,anaerosolcounterflowtwo-jetsunit(ACTJU),employingaliquidatroomtemperatureforcontinuoussamplingofatmosphericaerosolparticlesisdescribed.TheACTJUisdestinedforautomatedmeasurement(i.e.,samplingandsubsequenton-lineanalyses)ofsolubleaerosolconstituentsofbothinorganicandorganicnature.EXPERIMENTALSECTION

Apparatus.Theemployedapparatusconsistsofanimpactor(2.5-µmaerodynamiccutoffdiameter),anannulardiffusiondenuder,aparticlecollector(i.e.,ACTJU),andvariousdetectionsystems.Theanalyzedairaspiratedintothecollectorbymem-branepumpfirstpassesthroughthestainlesssteelsingle-stageimpactorwhereparticleswithaerodynamicdiameterslargerthan2.5µmareremovedandthengoesthrougha“dry”annulardiffusiondenudertoremoveinterferinggases.ParticleCollector.TheACTJU,schematicallyshowninFigure1,consistsoftwoVenturinozzles,acollisionchamber,andacyclone.Thecollisionchamberhasacylindricaldesignwithaninnerdiameterof4mmandalengthof10mm.TheanalyzedairisaspiratedintothecollisionchamberthroughtwoidenticalVenturinozzlesscrewedagainsteachotherinoppositesidesofthecollisionchamberalongalongitudinalaxisatadistanceof0.75mmbetweennozzlethroats.TheinnerdiameteroftheVenturinozzlethroatis1mmandthethroatlengthis2mm;thedistancebetweenthethroatoftheVenturinozzleandthethroatofthecapillaryforliquidfeedis2.5mm.AtaperangleattheconvergingzoneoftheVenturinozzleis40°.Liquid(deionizedwater)isdeliveredintothenozzlesthroughstainlesssteelcapillaries(i.d.0.6mm,o.d.0.9mm)bymeansofaperistalticpump(Ismatec,typeISM852)or,alternatively,isaspiratedduetoanejectioneffectofstreamingair.High-velocityairstreamspraysintroducedliquidintofinedroplets.Formedcounterflowjetsoffinedropletscollidewitheachotherinsidethecollisionchamber,andwithinthisprocess,togetherwithothermecha-nisms,theaerosolparticlesarecapturedintodispersedliquid.Theinhomogeneousmixtureinwaterwithcollectedparticlesisaspiratedoutfromthecollisionchamberthroughahole(1.5-mmdiameter)drilledinthemiddleofthechamberperpendicularlyversusthelongitudinalaxisandenterstangentiallyintothecyclone(i.d.7mm,15mmlong)whereairandliquidareseparated.Collectoreffluent,i.e.,aqueousconcentrateofcom-poundsofinteresttogetherwithinsolubleaerosolspices,iscontinuouslyaspiratedfromthebottomofthecyclonebyaperistalticpumpforsubsequentanalysis,whileairisaspiratedoutbythemembranepumpthroughastainlesssteelcapillary(i.d.1.0mm,o.d.1.5mm).Thebodyoftheaerosolcounterflowtwo-jetsunitconsistingofthecollisionchamberandthecycloneismadefromKetronPEEK-1000;Venturinozzlesaremadeofstainlesssteel.ThedimensionsoftheACTJUbodyare40×40×50mm.AnalyticalSystem.Nitriteandnitrateasrepresentativesofwater-solubleconstituentsofcollectedaerosolsareon-lineana-lyzedintheACTJUeffluentusingacontinuousdetectionsystem.Thecollectoreffluentiscontinuouslypumpedoutfrombottompartofthecycloneintoaglassdebubbler.Afterthat,thedebubbledsolutionisaspiratedbymeansoftheperistalticpumpAnalyticalChemistry,Vol.77,No.17,September1,20055535

Figure1.Aerosolcounterflowtwo-jetsunit.(A)frontview;(B)sideview.Alldimensionsaregiveninmillimeters.

Figure2.Schemeofnitriteandnitratecontinuousdetectionsystem.ACTJU,particlecollector;E,effluent;DB,debubbler;CEC,cationexchangecolumn;NC,photolyticconverter;D,detector;PP,peristalticpump;PC,computer;CL,chemiluminescentsolution;RE,reagentsolution.

throughthecationexchangecolumntoremoveinterferingcations,andthenitsplitsintotwostreamsfornitrateandnitriteanalysis.Bothnitriteandnitratearedetectedatthesametimeviaacontinuousmethod(Figure2)with1-stimeresolutionbasedonFIAmethodsreportedpreviously.37-39ContentofparticulatenitriteisdetecteddirectlyinthefirstACTJUstreamthatiscontinuouslymergedwithareagentsolutionconsistingof4mMH2O2and3mMEDTAin0.3MH2SO4.Duringtheflowthroughareactioncoil(PTFE,0.5mmi.d.×30cmlength),H2O2inacidmediumoxidizesnitrousacidtoformperoxynitrousacidthatdirectlyinsideachemiluminescent(CL)detectorismergedwiththeCLsolutioncontaining2mMluminoland3mMEDTAin0.6MKOH.LuminolisoxidizedbyperoxynitriteandemittedCLlightisdetectedbyaphotomultipliertube(model65PK518,Tesla-VacuumTechnique,Prague,CzechRepublic).37(37)Mikusˇka,P.;Zdra´hal,Z.;Vecˇerˇa,Z.Anal.Chim.Acta1995,316,261-268.(38)Mikusˇka,P.;Vecˇerˇa,Z.Anal.Chim.Acta2002,474,99-105.(39)Mikusˇka,P.;Vecˇerˇa,Z.Anal.Chim.Acta2003,495,225-232.

ParticulatenitrateinthesecondACTJUstreamisfirstpho-tolyticallyon-linereducedtonitritebytheabsorptionofUVlight,38andthesumoftheoriginalandreducednitriteisdeterminedintheparalleldetectionsystemviathewayasdescribedabovefornitritealone.Nitratecontentiscalculatedfromthedifferenceofsignalsfornitriteandthesumofnitriteandnitrate.39Peristalticpumps(Ismatec,typeISM852andISM597A)areusedfortransportationofthesampleandthereagentsolutions.AllliquidsflowthroughTeflontubingswithi.d.0.5mm(o.d.1/16in.)ori.d.0.25mm(o.d.1/16in.)tominimizetransportdelaybetweenthecycloneandthedebubbler.StandardAerosol.PerformanceoftheACTJUwastestedunderlaboratoryconditionswithbothpolydisperseandmono-disperseaerosols.AfinepolydispersesodiumnitrateaerosolwasgeneratedbypneumaticatomizationofaNaNO3solutionbyahigh-velocityairstreaminaconcentricnebulizer.40Theaerosolsprayleavingtheatomizerpassedthroughawatertrap(TSI),wherelargedropletswerecollected.Thespraywasthenmixedinaglasstubewithdryfilteredairtoevaporatewaterfromthedropletstoformdryaerosolparticles.Bychangingtheconcentra-tionofsodiumnitrateinthesprayedsolution,theNaNO3particleindifferentsizerangescanbeproduced.FortheNaNO3concentrationinthesprayedsolution,20g/L,NaNO3particlesinthesizerangeof10-750nmareobtained.Thegeometricmeandiameter(GMD)ofproducedpolydisperseaerosolis56.9nm,andageometricstandarddeviationis2.30.FortheNaNO3concentrationinthe0.2g/Lsprayedsolution,theaerosolparticlesinthesizerangeof10-133nmareproduced,geometricmeandiameteris34.2nm,andageometricstandarddeviationofproducedpolydisperseaerosolis1.61.Monodisperseaerosolwasproducedatacondensationgenera-tor(MAG-2010,Palas)operatingontheprincipleofheterogeneouscondensationofvaporizedparticlematerial(i.e.,bis(2-ethylhexyl)sebacate,DEHS)atthenucleiproducedbynebulizationofNaCl(0.02g/L)anduranine(0.3g/L)solutionbyanitrogenstream.Thetemperatureofthesaturatorandnitrogenflowratethrough(40)Mikusˇka,P.Collect.Czech.Chem.Commun.2004,69,1453-1463.

5536AnalyticalChemistry,Vol.77,No.17,September1,2005thesaturatorwerechangedinordertoobtainthemonodisperseaerosolwithparticlesofGMDvaluesof0.12,0.19,0.28,0.43,0.56,0.74,1.03,1.34,and2.26µm,respectively.Theparametersofstandardaerosolsweremeasuredwithanelectricalaerosolsizeanalyzer(EASA;model3030,TSI)andwithanopticalaerosolspectrometerWelas2000(Palas).TheWelasoperatedontheprinciplethatthesingleparticle’sscatteringofwhitelightmeasuresnumberconcentrationofparticlesinthesizerange0.3-17µm.Theparticlesareclassifiedinto60sizechannelsonthebasisofpolystyrenelatexequivalentdiameters.Maximumconcentrationis104(sensortype2200)or105particles/cm3(sensortype2100),respectively.CollectionEfficiencyandLossofParticles.Thecollectionefficiency(CE)ofaerosolintheACTJUasafunctionofparticlesizewasdeterminedusingthestandardmonodisperseaerosol.TheDEHSaerosoldopedwithuraninewassampledonaback-upfilterplaceddownstreamoftheACTJUandsimultaneouslyonareferencefilterplacedinparallelwiththecollector.Aftersampling,thefilters(quartzmicrofiberfilter,WhatmanQM-A,4.7cm)wereextractedintheultrasonicbathwith8mLofdeionizedwaterfor10minatambienttemperature.Theuranineconcentra-tioninthefilterextractwasanalyzedusingafluorometerFluoromat(modelFS950,KratosAnalyticalInstruments).TheCEwascalculatedasaratioofuraninemasscollectedonbothfilters.TheCEasafunctionofwaterflowrateintotheACTJUwasdeterminedbysamplingthestandardNaNO3aerosolonaback-upfilterasaratioofmassofnitratecollectedinthecollectortothenitratemasscollectedbothinthecollectorandontheback-upfilter.Aftersampling,thefilter(quartzmicrofiberfilter,WhatmanQM-A,4.7cm)wasextractedintheultrasonicbathwith8mLofdeionizedwaterfor15minatambienttemperature.ThenitrateconcentrationinthefilterextractwasdeterminedbymeansoftheFIAtechniquewithchemiluminescentdetection.38TheefficiencyofaerosolcollectionintheACTJUasafunctionofairflowratewasmeasuredon-lineusingthespectrometerWelas.TheCEofparticlesonanumberbasisisexpressedasthefractionofenteringparticlesthatisretainedintheACTJU,CE)(Nin-Nout)/Nin,whereNinisparticlenumberconcentration(#/cm3)atairenteringintotheACTJUandNoutisparticlenumberconcentration(#/cm3)attheoutputfromtheACTJU.ThelossofaerosolsintheACTJUwasdeterminedbywashingoutthedepositedmonodisperseDEHSaerosol(dopedwithuranine)fromtheACTJUwithsubsequentfluorescentanalysisofuranineatextract.AnnularDiffusionDenuder.Theannulardiffusiondenuder41consistsofastainlesssteeltube(470mmlength×60mmi.d.×63mmo.d.)inwhichtwotubes(withdiametersof46and51mm,respectively,andlengthof450mm)fromcopperwirenetarecoaxiallyplacedtoformanannuluswidthof2.5mm.Thespacewithinoftheinnercuprousnetandthespacebetweentheoutercuprousnetandthestainlesssteeltubearefilledwithactivatedcharcoal.Particlelossesduringpassingofairthroughthe“dry”annulardenuderweremeasuredwiththespectrometerWelasandtheelectricalaerosolsizeanalyzer.(41)Mikusˇka,P.;Vecˇerˇa,Z.;Brosˇkovicˇova´,A.;Sˇteˇpa´n,M.;Chi,X.;Maenhaut,

W.J.AerosolSci.2003,S761-S762.

Reagents.Allsolutionsarepreparedwithadistilleddeionizedwater.NaNO3(Aldrich,Milwaukee,WI),uranine(disodiumsaltoffluorescein;Aldrich),andotherchemicals(Lachema,Brno,CzechRepublic)areofanalyticalgrade;luminol(p.a.,Sigma-Aldrich,Praha,CR)isusedwithoutfurtherpurification.RESULTSANDDISCUSSION

OptimizationandEfficiencyoftheCollector.ThecollectoroperatesontheprincipleofaVenturiscrubberwhenanalyzedairflowingathighlinearvelocitythroughtheVenturinozzleatomizestheliquidtoformapolydisperseaerosolofsmalldroplets.Aerosolparticlesarecollectedbyinteractionswithdroplets.Dropletswithcollectedaerosolparticlesareseparatedon-linefromairinthesmallcyclone.Afewcollectorswithdifferentgeometryhavebeendevelopedandtestedduringthework.Atthebeginning,weemployedconfigurationswithasingleVenturinozzleincombinationwithsubsequentmultistageinputsofdeionizedwater.ThefinalcollectorversionappliedtwoVenturinozzleslocatedatoppositepositions.AllversionswithasingleVenturinozzleweremuchlargerinsize,andinaddition,theydidnotreachashighanefficiencyofaerosolcollectionasdidthetwo-nozzleversion.Forseparationofdropletsfromair,thecyclonewasusedinalltestedversions.Atthetwo-nozzleversionofthecollector,so-calledaerosolcounterflowtwo-jetsunit,formedcounterflowjetsoffinedropletscollidinginsidethecollisionchamberwitheachotherandwithinthisprocess,togetherwithothermechanisms,theaerosolparticlesarecapturedintodispersedliquid.OverallefficiencyofaerosolcollectionintheACTJUdependsonthecollectorparameters(diameterandpositionofcapillaryatVenturinozzle,throatdiameterandlength,axialdistanceofnozzles,etc.)andprimaryoperatingvariables(gasflowrate,liquidflowrate,throatgasvelocity,liquid-to-gasratio).Selectionoftheliquidappliedtothecollectionofaerosolsplaysanimportantrole.WeusedeionizedwaterastheliquidforthecollectionofaerosolparticlesfromairintheACTJU.ThelongitudinaldistancebetweenthethroatsoftheVenturinozzleswasoptimizedbysamplingofNaNO3aerosolinthesizerangeof0.3-0.7µm.TheoptimumdistancebetweentheVenturinozzlesis0.5-1.0mm.Atalargerdistance,theefficiencyofaerosolcollectiondecreaseswhileatasmallerdistancethepressuredropacrostheACTJUincreases.Thecollectionefficiencyofaerosolasafunctionoftheairflowrateintherange1-5L/minwasstudiedbysamplingofNaNO3aerosol(sizerange,0.3-0.7µm)followedbymeasurementofparticleconcentrationupstreamanddownstreamofthecollectorbymeansoftheWelasspectrometer(Figure3,waterflowrateof0.5mL/min).Itisevidentthatnearlyquantitativeaerosolcapture(CE>99.7%)intheACTJUisaccomplishedforairflowratesof4(CE)99.7%)and5(CE)99.9%)L/minfortheaerosolinthesizerangeof0.3-0.7µm(thenumberconcentrationofNaNO3particlesinthissizerangeis3.2×105#/cm3),andevenattheairflowrateof3L/min,thecollectionefficiencyofnitrateparticlesisashighas98.3%.SimilarresultswereobtainedalsofortheDEHSaerosolinthesizerangeof0.3-3.4µmalthoughtheDEHSisnotmisciblewithwater,henceproducingverysmalloildropsinsidethewaterstream.Theeffectofliquidflowratewasexaminedintherange0.1-4.0mL/minforequalliquidinputsintobothnozzlesandforAnalyticalChemistry,Vol.77,No.17,September1,20055537

Figure3.Dependenceofaerosolcollectionefficiencyonairflowrate.NaNO3aerosol(thesizerangeof300-750nm);waterflowrate0.5mL/min.

Figure4.Dependenceofaerosolcollectionefficiencyonwaterflowrate.NaNO3aerosol(thesizerangeof10-133nm);airflowrate5L/min.

variousratiosofinputsintonozzlesfromratio1:1upto40:1.Forasamplingofaerosolswithdiameterslargerthan0.3µm,waterflowrateinthestatedrangehasnoeffectontheefficiencyofaerosolcollection.Theonlyproblemarisesatawaterflowratebelow0.2mL/minwhenanevaporationlossofwaterinsidetheACTJUatlowhumidityofanalyzedairresultsintoolowoutputoftheeffluentfromthecollector,whichmaysubsequentlycauseproblemswithintheanalysisoftheeffluentcomponents.Quiteadifferentsituationexistsatthecollectionofaerosolswithparticlediameterssmallerthan0.3µmwhenwaterflowrateplaysacrucialrole.Figure4showsthedependenceofCEoftheNaNO3aerosol(inthesizerangeof10-133nm)onthewaterflowrate(atairflowrateof5L/min).TheCEincreasesfrom26%at0.5mL/mintoamaximumof77%at2mL/min;athigherwaterflowrates,theCEdecreases,probablyduetoreducedefficiencyintheproductionofwaterdropletsattheVenturinozzles.Theeffluentflowrate(i.e.,flowrateofwaterwithcollectedaerosolsoutoftheACTJU;FLeff)issmallerthanthewaterflowrateintothecollectorbecauseofevaporationofwaterinsidethecollector;nevertheless,thedependenceislinear.FortheoptimumwaterflowrateintotheACTJU(2mL/min),theeffectofrelativehumidity(RH)ofsampledairwasinvestigatedbutthechangesintheeffluentflowrateasafunctionofrelativehumidityofsampledairarenegligiblewhenFLeffincreasesfrom1.75mL/minatRH)13%to1.88mL/minatRH)86%.Infurtherwork,5538AnalyticalChemistry,Vol.77,No.17,September1,2005Figure5.Size-dependentcollectionefficiencyofDEHSaerosol.Airflowrate5L/min;waterflowrate2mL/min.

deionizedwateratnormaltemperaturewasusedastheliquidattheflowrateof2.0mL/minwhilesampledairpassedthroughtheACTJUattheflowrateof5L/min.DuringstudyofaerosolCE,wefirstsampledthepolydisperseNaNO3aerosolintheACTJU(attheairflowrateof5L/minandthewaterflowrateof2mL/min).ThepenetrationofaerosolsthroughtheACTJUwason-linecheckedbymeansoftheWelasspectrometerplacedaftertheACTJU.Wefoundthatnoparticleslargerthan0.3µmpassedthroughtheACTJU.However,wedonothaveanyinformationabouttheCEforsmallerparticles.Subsequentmeasurementwasperformedwiththemonodisperseuranine-dopedDEHSaerosol.TheCEwasevaluatedusingaerosolsamplingsimultaneouslyonafilterafterthecollectorandonaparallelreferencefilter.ThedependenceofaerosolcollectionefficiencyonthesizeofDEHSparticlesisshowninFigure5.Theobtainedcurveconfirmedthequantitativecollectionofparticleswithadiameterlargerthan0.3µmanddeterminedtheCEof67,87,and98%forparticlesofGMDvalues0.12,0.19,and0.28µm,respectively.Theseresultsthusprovethequantitativecollectionofparticleslargerthan0.3µmintheACTJUwhilebelowthissizelimittheefficiencyofparticlecollectiondecreases.LossofaerosolwithintheACTJU(attheairflowrateof5L/minandthewaterflowrateof2mL/min)measuredwiththeuranine-dopedDEHSaerosolwasfoundtobe3.07,2.75,2.65,2.59,2.44,2.35,2.22,2.40,and3.15%forparticlesofGMDvalues0.12,0.19,0.28,0.43,0.56,0.74,1.03,1.34,and2.26µm,respectively.Fromtheobtainedresults,itisevidentthatthereisverylittlelossoffineparticlesinsidetheACTJU.ParticleCollectionMechanism.ThemechanismofparticlecollectionintheVenturiscrubbershasbeensubjectofmanypapers.23,25-36Thesestudiesindicatethattheinertialimpactionistheprimarycollectionmechanismresponsibleforparticlecollec-tionfromanairstreambywaterdropletsduetotherelativevelocitydifferencebetweentheparticle-ladengasstreamandtheliquiddroplets.IntheACTJU,theairstreamwithparticlesflowingthroughtheVenturinozzleisacceleratedintheconvergingentrancesection.Waterintroducedintocoaxiallyflowingairisatomizedatthethroatinletbythehigh-velocityairstreamtoproduceaspectrumoffinedroplets.WeproposethismechanismfortheparticlecollectionintheACTJU:thefastermovingparticlesarecapturedbyimpactionandinterceptionwiththeslowermovingFigure6.Recordofon-lineparticulatenitrateconcentration(1-stimeresolution),Brno,15June2004.Airflowrate5L/min;waterflowrate2mL/min.

waterdropletsincludedattwocounterflowstreamsofliquiddropletsthatareproducedattwoVenturinozzleslocatedatoppositesitesofcollisionchamber.Moreover,thecollisionofbothjetsinthemiddleofthechambersignificantlyincreasesthenumberofimpactsbetweenparticlesanddropletswithconsequentconsiderableimprovementinoverallcollectionefficiencyofparticulatematterintheACTJU.TheaerosolcollectionefficiencyintheACTJUisfurtherenhancedbyadiabaticexpansionofbothairstreamsattheexitoftheVenturithroatsresultingincooling,whichcausesahighlyhumidenvironmentinsidethecollector.Underthesewetconditions,growthofparticlesinsizemayoccurbywateruptakeduetothedeliquescenceeffectofaerosols1,42-45.Relativehumiditycorrespondingtothedeliquescencepointofindividualaerosolcomponentsseemstobehighsometimes,butthedeliquescencerelativehumidityofparticlesinamulticompo-nentaerosolmixtureislowerthanthatofitsindividualcompo-nents.1,44Moreover,recentstudies46,47indicatethatparticlesmaystarttogrowinsizeevenbelowthereporteddeliquescencerelativehumiditywithoutanydistinctstep-sizechanges.Grownparticlesaretheneasilycollectedbyimpactionwithwaterdroplets.AnalyticalPerformance.Thecollectoreffluent(i.e.,theliquidwithdissolvedaerosolspeciesaswellasthenondissolvedparts)ispermanentlyaspiratedoutfromtheACTJUforsubsequenton/off-lineanalysisofparticulatecomponents.Nitriteandnitrateasrepresentativesofwater-solubleinorganicspeciesareanalyzedon-linebymeansofacontinuousmethod37,38thatallowsfastandsensitivedetectionofbothionsdirectlyinthecollectoreffluentwithoutneedofsamplepreconcentration.Thedetectionlimits(S/N>3)ofaqueousnitriteandnitrateare1.5and3.1nM,(42)Tang,I.N.InGenerationofAerosolsandFacilitiesforExposureExperiments;

Willeke,K.,Ed.;AnnArborScience:AnnArbor,MI,1980;pp153-167.(43)Pilinis,C.;Seinfeld,J.H.;Grosjean,D.Atmos.Environ.19,23,1601-1606.

(44)Wexler,A.S.;Seinfeld,J.H.Atmos.Environ.1991,25A,2731-2748.(45)Tang,I.N.;Munkelwitz,H.R.Atmos.Environ.1993,27A,467-473.(46)Ha¨meri,K.;Va¨keva¨,M.;Hansson,H.C.;Laaksonen,A.J.Geophys.Res.

2000,105,22231-22242.

(47)Hoffman,R.C.;Laskin,A.;Finlayson-Pitts,B.J.J.AerosolSci.2004,35,

869-887.respectively,andthecorrespondingdetectionlimitsofparticulatenitriteandnitrateare28and77ng/m3,respectively(calculatedforthewaterflowrateof2mL/minandtheairflowrateof5L/min).NegligibleinnervolumeoftheACTJU(0.8cm3)enablesashortresidencetime(12ms),andtheresponsetime(95%ofsteady-statedetectorsignal)ofthewholesystemisthenlimitedbyanalysistimeofdetectionmethodused,whichdependsonthesample(i.e.,theACTJUeffluentstream)flowrate(seeFigure2).WhentheNaNO3aerosolconcentrationinairenteringtheACTJUwassteeplyswitchedbetween0and45µg/m3,theresponsetimeofthesystemwas145,100,and82sforthesampleflowratesof100,200,and300µL/min,respectively.Thecorrespondinglimitsofdetectionofnitrateinsolutionatthecontinuousdetectionsystem(CDS)are3.1,4.4,and7.5nManddetectionlimitsofparticulatenitrateare77,109,and186ng/m3.Thetimesobtainedwhenaerosolconcentrationwaschangedtozeroareabout5-10slongerthancorrespondingrisetimes,whichstillprovidesrapidresponsetime.Itisevidentthatwithincreasingsampleflowratetheresponsetimeofthesystemdecreases;however,simulta-neouslysensitivityofnitriteandnitratedeterminationgoesdowntoo.Infurtherexperiments,thesampleflowrateof100µL/minattheCDSwasmostlyemployedbecausetheresponsetimeintheorderof2minisstillquitesatisfactoryformostfieldapplicationswherehighsensitivityratherthanhighresponsetimeispreferred.Theseresultsofferapplicationofthecollectorforthemeasurementofaerosolchemicalcompositionintherealtime.Timeresolutionforon-lineanalysisofnitriteandnitrateis1s.Itisobviousthatthisveryhighresolutionhasnopracticaluseformostfieldapplications,butthereareafewspecialutilizationssuchasspecializedkineticstudieswithinatmosphericchemistry,forexample,wheresuchtimeresolutionmaybeuseful.Theexampleofparticulatenitratemeasurement(withtimeresolutionof1s)ispresentedinFigure6,wheretheabilityoftheACTJUandconnectedanalyticalsystemtodetectquickchangesinparticulatenitrateconcentrationisclearlydemonstrated.Inthecourseof15min,weobservedfrequentvariationsinnitrateAnalyticalChemistry,Vol.77,No.17,September1,20055539

Figure7.Parallelmeasurementofparticulatenitrateandparticlenumberconcentration.Brno,14-18June2004.Airflowrate5L/min;waterflowrate2mL/min.(A)On-linemeasurementofparticulatenitrateconcentration,3-minaverages,PM2.5.(B)Totalparticlenumberconcentration,5-minmeasuringinterval,sizerange0.3-2.5mm.

concentrationswithafewpeaksofparticulatenitrate.Theothersamplingsystems15,16,18,19,21combinedwithionchromatographyduringthesametimeintervalcouldgiveonlyasingleintegratedaverageofnitrateconcentrationoverthistimeinterval.Theparticleimpactionsystemdesignedspecificallyfornitrateanaly-sis10,48providesatimeresolutionof10min,whichisalsomuchslowerthanthatofthepresentedsystem.Interferences.Atmosphericwater-solublegases(e.g.,HNO3,HONO,SO2,HCl,andNH3)coexistingwithaerosolsinanalyzedairhavetoberemovedfromthesampledairprioritsentranceintotheACTJUotherwisetheyareefficientlycollectedinthepresentedcollectorintodeionizedwater.Toeliminatethesepositiveartifacts,theaerosolcollectorduringitsoperationiscoupledwiththeannulardiffusiondenuder41placedbetweentheimpactorandtheACTJU.Duringpassingofairthroughthedenuder,thegaseousinterferencesareremovedbytheircollectionontoasolidabsorbinglayer(i.e.,activatedcharcoal)whileaerosols(48)Stolzenburg,M.R.;Dutcher,D.D.;Kirby,B.W.;Hering,S.V.AerosolSci.

Technol.2003,37,537-6.passthroughthedenuderwithoutanychange.Outgoingaircontainingaerosolparticlesthenenterstheaerosolcollectorwhereparticlesarecollected.ThecombinationoftheannulardenuderandtheACTJUthusprovidesartifact-freesamplingofatmosphericaerosols.Theused“dry”annulardiffusiondenuder41alsoremovesNO2frompassedairveryefficiently,whichisthesourceofpositiveartifactsatsteam-basedcollectors15-17,19evenifwetdenuders(althoughparallelplate15,17,19orannular16designareemployed)areusedforthesamplingofgaseousinterferences.Moreover,thisdrydenuderversion,contrarytowetdenuders,19doesnotcausehygroscopicgrowthofpassedparticles.Lossesofparticlesduetogravitationalsedimentationareminimizedbyverticalorientationofthedenuderduringthemeasurement.Diffusionlossesofparticlesinthesizerange0.1-2.5µmwerenotobserved;lossesofparticleswithdiameterssmallerthan0.1µmare2-3%.AmbientAirMeasurement.TheperformanceoftheACTJUwasverifiedduringmeasurementofatmosphericaerosolsinairinthecityofBrno.ThesamplingplacewastheroofoftheInstitute50AnalyticalChemistry,Vol.77,No.17,September1,2005ofAnalyticalChemistryatthefifthfloor.Astreetwithrelativelyheavytraffic(carsandtrams)is∼30mawayfromthebuilding.Ambientairwithaerosolparticles(PM2.5)wascontinuouslysampledintheACTJU(attheairflowrateof5L/minandthewaterflowrateof2mL/min),andparticulatenitriteandnitrateatthecollectoreffluentwereanalyzedon-line.Theambientparticulatenitratemeasurementwasperformedwitha4-mintimedelaybetweenentranceofaerosolintotheACTJUandobtainingofcorrespondingdetectorsignal.Concentrationsofparticulatenitriteduringthewholemeasurementperiodwereverylow,mostly1-2ordersbelowconcentrationofnitrateswithoutanyepisodesorcorrelationwithcorrespondingnitratesordaytime;therefore,nitriteswerenotincludedinthenextdiscussion.Theconcentrationofparticulatenitrateduring14-18June2004isshowninFigure7Aas3-minaverages.Duringthemeasurements,weobservedafewepisodeswhentheconcentrationofparticulatenitratesreachedalmostuptolevelsof1.5µg/m3.InparallelwithaerosolsamplingintheACTJUandsubsequentparticulatenitriteandnitrateanalysis,theparticlenumberconcentrationwasmeasuredwiththeWelasspectrometer(Figure7B).ResultsdepictedinFigure7AandBdemonstratesimilartrendsinthecourseofconcentrationofparticulatenitrates(3-minaverages,PM2.5)andthetotalparticlenumberconcentration(5-minaverages)inthesizerange0.3-2.5µm.CONCLUSIONS

Anewtypeofwetcollectorforthecontinuousautomatedsamplingofatmosphericaerosolshasbeendeveloped.Couplingofthecollectorwithanon-linedetectiondevicewasverifiedattheanalysisofparticulateinorganicions.Consequently,wesupposethattheACTJUcanalsobeappliedtoon-lineanalysisofaerosolwater-solublecomponentsoforganicorigin.Nosteamisrequiredforaerosolsampling,whichoffersalotofadvantagesincomparisonwithwidelyusedsteam-basedaerosolcollectors.Particularly,applicationofwateratlaboratorytemperatureisassumedtoeliminatesamplingartifactsduetolossofparticulatesemivolatilecompoundsoforganicaswellasinorganicnaturethatareobservedifsteamcollectorsorclassicalfiltertechniquesareused.Highcollectionefficiency,simpledesign,smalldeadvolume,andeasymaintenanceofACTJUofferanexcellentopportunitytoemploythiskindofcollectorforthelong-termunattendedmonitoringofatmosphericaerosolsintheframeofpollutioncontrolofambientairandinthestudyoflong-termimpactofaerosolsonenvironment.ACKNOWLEDGMENT

ThisworkwassupportedbytheGrantAgencyoftheAcademyofSciencesoftheCzechRepublicunderGrantIAA4031105andbytheGrantAgencyoftheCzechRepublicunderGrantsGACR526/03/1182and525/04/0011.WethankDr.Smolı´k,InstituteofChemicalProcessFundamentals,AcademyofSciencesoftheCzechRepublic,PragueforlendingEASAtous.ReceivedforreviewFebruary25,2005.AcceptedJuly4,2005.

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