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348

Appendix D
*
Evidence for Organosulfates in Secondary Organic Aerosol

* This chapter is reproduced by permission from “Evidence for Organosulfates in Secondary Organic
Aerosol” by J. D. Surratt, J. H. Kroll, T. E. Kleindienst, E. O. Edney, M. Claeys, A. Sorooshian, N. L. Ng,
J. H. Offenberg, M. Lewandowski, M. Jaoui, R. C. Flagan, J. H. Seinfeld, Environmental Science and
Technology, 41: 517-527 2006. Copyright 2006. American Chemical Society.
349
Environ.Sci.Technol.2007,41,517-527
Evidence for Organosulfates in (1).Particle-phaseoligomerizationleadstotheformationof
high-molecular-weight(MW)species(2-4);suggestedoli-
Secondary Organic Aerosol
gomerizationreactionsincludethereactiveuptakeofvolatile
aldehydesorketonesviaperoxyhemiacetalformation(5,6),
JASON D. SURRATT,† JESSE H. KROLL,‡,X hydration,hemiacetal/acetalformation,andaldolconden-
TADEUSZ E. KLEINDIENST,§ sation (7, 8). Esterification in isoprene photooxidation (9,
EDWARD O. EDNEY,§ MAGDA CLAEYS,^ 10) has also been reported in SOA formation. The role of
ARMIN SOROOSHIAN,| NGA L. NG,| these reactions remains in some doubt as some of the
JOHN H. OFFENBERG,§ proposedreactions(e.g.,hemiacetal/acetalformationand
MICHAEL LEWANDOWSKI,§ aldolcondensation)arenotthermodynamicallyfavorableat
MOHAMMED JAOUI,# ambientconditions(11,12).
RICHARD C. FLAGAN,‡ AND Laboratorychamberstudieshavedemonstratedthatthe
JOHN H. SEINFELD*,‡ presence of acidic seed aerosol enhances the SOA yields
DepartmentofChemistry,CaliforniaInstituteofTechnology, observed from the oxidation of various volatile organic
Pasadena,California91125,DepartmentsofEnvironmental compounds(VOCs),suchasR-pinene(2,3,13,14),isoprene
ScienceandEngineeringandChemicalEngineering, (8, 14, 15), and several model cycloalkenes (2) over those
CaliforniaInstituteofTechnology,Pasadena,California withalessacidicseedaerosol.Despiterecentadvancesin
91125,NationalExposureLaboratory,OfficeofResearchand understanding particle-phase SOA chemistry, the role of
Development,EnvironmentalProtectionAgency,Research particle-phaseacidityinenhancingSOAformationremains
TrianglePark,NorthCarolina27711,Departmentof essentiallyunexplained.Recentstudieshavepresentedmass
PharmaceuticalSciences,UniversityofAntwerp(CampusDrie
spectrometricevidencethatthereactiveuptakeofglyoxal
Eiken),Universiteitsplein1,BE-2610Antwerp,Belgium,
andpinonaldehyde(principalgas-phaseoxidationproducts
DepartmentofChemicalEngineering,CaliforniaInstituteof fromaromaticsandR-pinene,respectively)onacidicaerosol
Technology,Pasadena,California91125,andAlionScience
involvesorganosulfateformation(sulfateestersorderivatives;
andTechnology,P.O.Box12313,ResearchTrianglePark,
forsimplicity,wewillusehereafterthetermsulfateesters
NorthCarolina27709
to also denote sulfate derivatives; i.e. sulfate derivatives
formed from a carbonyl compound) (16-18). In addition,
severalfieldstudieshavereportedsulfateestersignaturesin
Recent work has shown that particle-phase reactions aerosolcollectedonfiltersusingFouriertransforminfrared
spectroscopy(19,20)andelectrosprayionization(ESI)-mass
contribute to the formation of secondary organic aerosol
spectrometry(MS)(21,22).Nevertheless,theimportanceof
(SOA),withenhancementsofSOAyieldsinthepresenceof
organosulfateformationtoSOAremainsunclear.Traditional
acidicseedaerosol.Inthisstudy,thechemicalcomposition
analytical methods, such as gas chromatography/mass
of SOA from the photooxidations of R-pinene and
spectrometry(GC/MS)withpriorderivatization,maynotbe
isoprene, in the presence or absence of sulfate seed wellsuitedforidentifyingorganosulfates.Itislikelythatsingle
aerosol, is investigated through a series of controlled derivatization protocols, such as trimethylsilylation, GC
chamberexperimentsintwoseparatelaboratories.Byusing injectionandcolumntemperaturescouldcausethedegra-
electrospray ionization-mass spectrometry, sulfate dationormisinterpretationofsuchspecies(23).Ontheother
esters in SOA produced in laboratory photooxidation hand,ESI-MShasbeenshownasaneffectivemethodfor
experiments are identified for the first time. Sulfate esters the detection and quantification of organosulfate species
(24,25).
are found to account for a larger fraction of the SOA
In the present study ESI-MS is used to detect and
masswhentheacidityofseedaerosolisincreased,aresult
structurallyelucidatesulfateestersinSOAformedfromthe
consistent with aerosol acidity increasing SOA formation.
photooxidationsofisopreneandR-pineneunderdiffering
Many of the isoprene and R-pinene sulfate esters
combinationsofNO levelsandseedaerosolacidities.Asa
x
identified in these chamber experiments are also found in
result, the formation of sulfate esters may be a major
ambient aerosol collected at several locations in the contributortotheobservedenhancementinSOAyieldsin
southeastern U.S. It is likely that this pathway is important thepresenceofacidicaerosol.
for other biogenic terpenes, and may be important in the
Experimental Section
formationofhumic-likesubstances(HULIS)inambientaerosol.
IsopreneChamberExperiments.Asummaryofexperimental
Introduction conditionsforallisoprenephotooxidationexperimentscan
befoundinTable1.Isoprenephotooxidationexperiments
Particle-phase reactions are now understood to play an were conducted in Caltech’s dual indoor 28 m3 Teflon
importantroleinsecondaryorganicaerosol(SOA)formation chambers(26,27)andinEPA’sfixedvolume14.5m3indoor
chamber(15).Thetemperatures,aerosolsizedistributions,
*Correspondingauthorphone: [phone];fax: (626)796- andrelativehumidities,aswellastheO,nitricoxide(NO),
2591;e-mail: [email protected]. 3
NO concentrations were continuously measured in both
†DepartmentofChemistry,CaliforniaInstituteofTechnology. x
‡DepartmentsofEnvironmentalScienceandEngineeringand facilities.Theisopreneconcentrationsinbothfacilitieswere
ChemicalEngineering,CaliforniaInstituteofTechnology. monitoredbyGCwithflameionizationdetection.TheCaltech
§EnvironmentalProtectionAgency. experimentswereconductedinthestaticmode(i.e.,batch
^ UniversityofAntwerp. reactor) whereas the EPA experiments were conducted in
|Department of Chemical Engineering, California Institute of
thedynamicmode(i.e.,continuousstirredtankreactor)with
Technology.
theexceptionofEPA-326whichwasastaticmodeexperiment.
#AlionScienceandTechnology.
XCurrentaddress: AerodyneResearch,Inc.,45ManningRoad, Hydroxyl radical (OH) precursors (H2O2 or HONO) were
Billerica,MA01281. employedintheCaltechexperiments(28,29).ForallCaltech
10.1021/es062081qCCC:$37.00 ª 2007AmericanChemicalSociety VOL.41,NO.2,2007/ENVIRONMENTALSCIENCE&TECHNOLOGY9517
PublishedonWeb12/07/2006
350
TABLE 1.Summary of Experimental Conditions and Sulfate Ester Formation from Isoprene Photooxidation
initial initial
inorganicseed OH isoprene] [NOx] average
experiment aerosola precursor (ppb) (ppb) T((cid:176) C) [M-H]-detectedsulfateesterions(m/z)
Caltechlow-NO noneadded H O 500 noneadded 23.7 nonedetected
x 2 2
Caltechlow-NO AS H O 500 noneadded 23.9 215,333
x 2 2
Caltechlow-NO AAS H O 500 noneadded 23.8 153,155,169,215,333,451
x 2 2
Caltechhigh-NO noneadded H O /NO 500 891 24.3 nonedetected
x 2 2
Caltechhigh-NO AS H O /NO 500 963 24.9 199,215,244
x 2 2
Caltechhigh-NO AAS H O /NO 500 904 24.6 139,153,155,197,199,215,244,260,301,346
x 2 2
Caltechhigh-NO noneadded HONO 500 382 20.1 nonedetected
x
Caltechhigh-NO AS HONO 500 366 21.4 199,215,260,333
x
EPA-299stage1 ASb c 2500 200 29.0 nonedetected
EPA-299stage2 AAS c 2500 200 29.0 153,155,167,169,181,197,199,215,244,
260,333
EPA-299stage3 H SO only c 2500 200 29.0 153,155,157,167,169,181,197,199,215,244,
2 4
260,333
EPA-199stage1 60ppbSO c 1598 475 24.6 197,199,215,244,260,301,317,333
2
EPA-199stage2 200ppbSO c 1598 475 24.6 155,169,197,199,215,244,260,301,317,
2
333,346
aAS)15mM(NH)SO;AAS)15mM(NH)SO +15mMHSO forCaltechexperimentsand0.31mM(NH)SO +0.612mMHSO for
42 4 42 4 2 4 42 4 2 4
EPA-299experiments;HSO only)0.92mMHSO;EPA-199hadnoseednebulizedbutinsteadusedthephotooxidationofSO togenerate
2 4 2 4 2
sulfuricacidaerosol.bDuetothelowinitialinorganicseedaerosolconcentration,thisconditionismoreconducivetonucleation.cNoOHprecursor
wasused.
TABLE 2.Summary of Experimental Conditions and Sulfate Ester Formation from r-Pinene Photooxidation
initial initial initial initial
[r-pinene] [isoprene] [toluene] [NOx] SO2 averageT [M-H]-detectedsulfate
experiment (ppb) (ppb) (ppm) (ppb) (ppb) ((cid:176) C) esterions(m/z)a
EPA-220 220 b b 450 b 25.5 nonedetected
EPA-326 186 1108 b 248 287 20.5 249,265,294,310,412,426
EPA-211stage1 105 b b 378 b 24.3 nonedetected
EPA-211stage2 105 b 1.59 378 b 24.3 nonedetected
EPA-211stage3 103 820 1.58 378 b 24.3 nonedetected
EPA-211stage4 117 854 1.57 378 275 24.3 265,279,294,310,326,412,426
EPA-211stage5 115 b 1.59 378 275 24.3 249,265,279,294,310,326
EPA-211stage6 b 794 1.56 378 b 24.3 nonedetected
EPA-205 106 592 1.45 599 278 24.0 265,294,310,412,426
aIsoprenesulfateesterproductslikethoseinTable2werealsodetectedonlywhenSO andisoprenewerecopresent.Nodiscernibletoluene
2
sulfateesterproductsweredetected.bThiscompoundwasnotpresentduringtheexperiment.
low-NO experiments,onlyHO wasadded,resultinginNO followingaerosolconditionswereused: AS,AAS,andsulfuric
x 2 2 x
concentrations<1ppb.Caltechhigh-NO experimentseither acid,witheachoftheaqueoussolutionsatomizedintothe
x
usedHO andaninitialamountofNO((cid:24)800ppb),orwith chamberbyatomization.Theinitialaerosolconcentrations
2 2
HONOandNO asasideproduct.IntheCaltechexperiments, were0.1,30.0,and30.0(cid:237)g/m3,forEPA-299stage1,EPA-299
x
three initial inorganic seed aerosol conditions were used: stage2,andEPA-299stage3,respectively.Forthesecondset
(1)theabsenceofaerosolwhereSOAformationwasinitiated ofEPAexperiments,EPA-199stage1andEPA-199stage2,
bynucleation;(2)ammoniumsulfate(AS)aerosol;and(3) acidicaerosolwasgeneratedbyadding60and200ppbof
acidifiedammoniumsulfate(AAS)aerosol.Concentrations SO,respectivelytothereactantmixture.AllEPAexperiments
2
of the aqueous solutions that were introduced into the wereconductedatarelativehumidityof(cid:24)30%.Resultsfrom
chambersbyatomizationareshowninTable1.Theinitial bothEPAexperiments(i.e.,SOAyields,gas-phaseproducts,
seedaerosolconcentrationsthatresultedrangedfrom(cid:24)19- trends,etc.)willbediscussedinmoredetailinforthcoming
24(cid:237)g/m3.Teflonfilters(PALLLifeSciences,47-mmdiameter, publications;evidencefororganosulfatesisthefocushere.
1.0-(cid:237)m pore size, Teflon filters) were collected for offline r-Pinene Chamber Experiments. All R-pinene experi-
chemicalanalysisfromtheCaltechexperimentsatthepoint mentswereconductedintheEPAdynamicchamber(15).
atwhichtheaerosolvolumereacheditsmaximumvalue,as Conditions for each experiment are listed in Table 2. The
determined by the differential mobility analyzer (DMA). EPAexperimentsconsistedofoneR-pinene/NO irradiation
x
All experiments were carried out at relative humidities experimentalongwithaseriesofexperimentswheremixtures
(RHs)<9%. ofhydrocarbonscontainingR-pinenewereirradiatedinthe
InthedynamicexperimentsintheEPAchamber,reactants presenceofNO.Forsomeoftheseexperiments,SO was
x 2
suchasNO,SO,andisoprenewerecontinuouslyaddedfrom addedtothechambertogenerateacidityintheaerosol.The
2
high-pressurecylinderstothereactionchamberthrougha samecollectionprotocolwasusedhereasthatemployedin
mixingmanifold.Thesteady-statenatureofchamberopera- theEPAisopreneexperiments.
tion allows for filter sampling for extended periods for AmbientAerosolCollection.Ambientaerosolwascol-
determining the composition of the resultant SOA. Once lected from the Southeastern Aerosol Research and Char-
steady-stateconditionswereattained((cid:24)24h),samplesfor acterizationStudy(SEARCH)networkandanalyzedforsulfate
determiningthecompositionoftheSOAwerecollectedon esters.Thisnetworkcomprisesoffoururban-rural(orurban-
glassfiberfiltersprecededbyacarbonstripdenuder.Two suburban)sitepairsatlocationsacrossthesoutheastU.S.
setsofEPAexperimentswereconducted.Inthefirstset,the and was initiated in mid-1998 to carry out systematic
5189ENVIRONMENTALSCIENCE&TECHNOLOGY/VOL.41,NO.2,2007
351
FIGURE1. (-)LC/ESI-MSupfrontCIDmassspectraforselectedisoprenesulfateesterSOAproductsshowninTable2.(A)Production
massspectrumforsodiumpropylsulfatestandard(anionicmassofintactpropylsulfateester)139Da).(B)Productionmassspectrum
for a 2-methyltetrol sulfate ester detected in a Caltech high-NO HO AS seed photooxidation experiment. (C) Product ion mass
x 2 2
spectrumforahemiacetaldimersulfateesterdetectedinaCaltechlow-NO AASseedphotooxidationexperiment.(D)Productionmass
x
spectrumforaC trihydroxynitratesulfateesterdetectedinEPA-299stage2.
5
measurementsoftemporalandspatialvariabilityofPM,in lium2-(2-methoxyethoxy)ethylsulfate(Sigma-Aldrich,95%
particularPM ,gasesrelevanttosecondaryO formation, purity)wereanalyzedontheLC/MSinstrumenttodetermine
2.5 3
and surface meteorology (22). Twenty-four h composite common product ions associated with sulfate esters. Ad-
quartz filters were taken on 4 days at four different sites ditionalsampleswerecollectedwiththeparticle-into-liquid
during June 2004: Birmingham, AL (BHM, urban site), sampler (PILS) with subsequent offline analysis by ion
Centerville,AL(CTR,ruralsiteoutsideofBHM),Jefferson chromatography(IC)(31).ThePILS/ICtechniqueallowsfor
Street(JST,neardowntownAtlanta,GA),andPensacola,FL thequantitativemeasurementofwater-solubleinorganicions
(PS, marine influenced urban site). Details of these sites inaerosol;noICpeakscouldbeattributedtoorganosulfates,
(terrain,vegetation,transportation,andindustrialsources), sofortheseexperimentsonlyinorganicionsaremeasured.
sample collection and handling procedures, and specific
aerosol and gas-phase measurements obtained are given Results
elsewhere(22,30).
SulfateEsterStandards.(-)ESI-MSstudieshaveshownthat
Filter Extraction and Chemical Analyses. Detailed ex-
sulfate esters produce abundant [M-H]- ions, and upon
tractionproceduresforTeflonandquartzfiltersaredescribed
collisionalactivationoftheseions,yieldm/z97(HSO-)and
elsewhere (9, 22). Glass-fiber filters were extracted in the 4
80(SO-(cid:226))productions(21,24,25).Inconjunctionwiththe
samemannerasTeflonfilters(9),exceptresultantextracts 3
knownisotopicdistributionofsulfur,whichcontains34Swith
werefilteredthroughaPALLLifeSciencesAcrodiscCR25
mm syringe filter (PTFE membrane, 0.2 (cid:237)m pore size) to anaturalabundanceof4.2%,theseproductionscanbeused
toidentifysulfateesters.Asconfirmation,authenticstandards
removefilterfibers.Allsampleextractswereanalyzedbya
ofsodiumpropylsulfate(anionicmass)139Da),sodium
Hewlett-Packard1100seriesHPLCinstrument,coupledwith
lauryl sulfate (anionic mass ) 265 Da), and 1-butyl-3-
aquadrupolemassspectrometer,andbydirectinfusiononto
methylimidazolium 2-(2-methoxyethoxy)ethyl sulfate (an-
aThermoElectronLCQiontrapmassspectrometer(ITMS),
ionicmass)199Da)wereanalyzedbythe(-)LC/ESI-MS
bothequippedwithanESIsourceoperatedinthenegative
(-)ionizationmode.Detailsoftheoperatingconditionsfor techniqueinthefullscanmodeofanalysisfollowedbythe
upfrontCIDmodeofanalysistogenerateMSandMS/MS
these instruments are described elsewhere (9). Briefly, all
data,respectively.Asshownforthesodiumpropylsulfate
sampleswereanalyzedontheLC/MSinstrumentinthefull
esterstandardinFigure1A,theseauthenticstandardsyielded
scanmodeandupfrontcollision-induceddissociation(CID)
m/z97and80productions.
modeofanalysis.Comparisonoftheresultingmassspectra
producedfromthesetwomodesofanalysesontheLC/MS Sulfate Esters from Isoprene Oxidation. Previously
instrument allows for some structural information to be characterized(9,10)isopreneSOAproductswereobserved
obtainedonthedetectedSOAcomponents.Sampleswere intheseexperimentsandareshowninTable3;however,the
alsoanalyzedontheThermoElectronLCQITMSinstrument focus here will be on the identification of sulfate esters.
toconfirmtheseresults,andinsomecases,providefurther Comparisonof(-)ESI-MSdatacollectedfromexperiments
structural elucidation. Sulfate standards of sodium propyl employingnosulfateaerosoltothosewithsulfateaerosol
sulfate (City Chemical, 98% purity), sodium lauryl sulfate showedthatnumerouscompoundsweredetectedonlywhen
(CityChemical,98%purity),and1-butyl-3-methylimidazo- sulfate aerosol was present. To understand the nature of
VOL.41,NO.2,2007/ENVIRONMENTALSCIENCE&TECHNOLOGY9519
352
TABLE 3.Proposed Isoprene Sulfate Ester SOA Products
aPositionalisomerscontainingnitrateorsulfategroupsatotherhydroxylatedpositionsarelikely.bIsopreneSOAproductspreviouslyidentified
inpriorstudiesbySurrattetal.(9)and/orSzmigielskietal.(10)and/orEdneyetal.(15).cDetectedinambientaerosolcollectedfromSEARCH
network(summer2004)forfirsttime.dConsideredmajorproductduetolargeMSabundanceinchamberstudies.eCompoundslistedinparentheses
areneutrallossesobservedupon(-)ESI-MS/MS.fInferredprecursorduetotheMS/MSfragmentationofitsrespectiveorganosulfateproduct;
thisparentisopreneproductgoesundetectedby(-)ESI-MSandGC/MSmethods.gSomeevidenceforitsexistenceinfirst-ordermassspectra.
hDetectedinambientaerosolbyMatsunagaetal.(32).
5209ENVIRONMENTALSCIENCE&TECHNOLOGY/VOL.41,NO.2,2007
353
FIGURE2. TimeevolutionoftheSO2-andNH+aerosolmassconcentrationsfromthePILS/ICanalysis.(A)Caltechhigh-NO HO isoprene
4 4 x 2 2
experimentwithASseedaerosol.(B)Caltechlow-NO isopreneexperimentwithAASseedaerosol.Acontrolexperimentwasconducted
x
inwhichseedaerosolisatomizedfromasolutionof0.015MASintotheCaltechexperimentalchamber,andnootherreactantssuch
asVOCsorNO werepresent.ThiscontrolexperimentproducedasimilarresulttothatofFigure2A(althoughnotevidentfromthetime
x
scalepresented,SO2-andNH+decayby(cid:24)20and14%,respectively,over9h),indicatingthattheonlylossmechanismforsulfatein
4 4
this case was wall loss. Of the Caltech isoprene experiments, only the low-NO AAS seed aerosol experiment showed a significant
x
decreaseintheSO2-aerosolmassconcentration,indicatingthatitwaslikelylosttoreaction.
4
thesecompounds,tandemMStechniqueswereemployed. experiments.Forexample,inthelow-NO experiments,the
x
Figure 1B-D shows the LC/ESI-MS upfront CID mass LC/MSpeakareaforthem/z215sulfateesterwasfoundto
spectracollectedforlargechromatographicpeakscommon doublewhenAASaerosolwasusedratherthannonacidicAS
tomanyofthesulfateaerosolexperimentslistedinTable1. aerosol.Althoughquantitativedatacouldnotbeobtained
The [M-H]- ions associated with these chromatographic for either the sulfate ester concentrations or the effective
peaksincludem/z215,333,and260,respectively.Aswasthe acidityofthereactionsystem,theseresultssuggestthatsulfate
caseforthesulfateesterstandards,thecollisionalactivation ester formation is enhanced by the presence of an acidic
ofthese[M-H]-ionsyieldedm/z97and80productions. sulfateaerosol,andthatthisenhancedsulfateesterformation
In addition, these ions also had an isotopic distribution may be contributing to the increased SOA mass detected
commontosulfur,andasaresult,thesecompoundswere previouslyunderacidicconditions.Furtherworkisneeded
identifiedassulfateesters.Theotherproductions,observed in order to accurately quantify these sulfate esters. It was
inthespectrashowninFigure1,providedfurtherinformation foundthat(-)LC/ESI-MScalibrationcurvesgeneratedby
onthechemicalstructuresoftheidentifiedsulfateesters. surrogate standards lacking sulfate groups (such as meso-
Proposedsulfateesterstructuresfortheseionsandallother erythritol) were not suitable for quantifying the identified
ionslistedinTable1aregiveninTable3.Manyofthesulfate sulfateesters,resultingfromthesestandardshavinglower
esters shown in Table 3 were formed from previously (-)ESI-MSsensitivities.Thesulfateesterstandardslistedin
identified isoprene SOA products, including aldehydes, the experimental section were also not suitable for quan-
dicarbonyls,hydroxycarbonyls,alcohols,andacidscontain- tification because these compounds had retention times
inganalcoholmoiety(9,10,32).Sulfateestersformedfrom much greater than (1 min of the retention times for the
smallvolatileoxidationproducts,suchasglyoxal,hydroxy- isoprene sulfate esters. Also, these standards lack many
acetone,andglycolaldehyde,wereonlydetectedinexperi- structuralfeaturescommontotheidentifiedsulfateesters;
mentsinvolvingthehighestaerosolacidity.Allsulfateesters therefore, synthesis of more representative standards is
listedinTable3elutedfromthereverse-phaseLCcolumn neededinordertoquantifysulfateestersby(-)LC/ESI-MS.
within 3 min, indicating their high water solubility. For Sulfate ester aerosol was atomized from a standard
example,them/z215sulfateesterhadaretentiontimeof solutionofsodiumpropylsulfateandanalyzeddirectlyby
(cid:24)1.4min,closetothatoftheinorganicsulfate(firstpeakto the PILS/IC technique; no significant levels of inorganic
elute).Ontheotherhand,them/z260sulfateesterwasfound sulfate were detected, suggesting that organosulfates are
tobeslightlylesspolarwithisomerselutingat2.4,2.7,and thermallystableattheoperatingconditionsofthisinstru-
2.9min.Thepresenceofanitrategroupwasconfirmedby ment.Inaddition,nochromatographicpeakintheICdata
itseven-mass[M-H]-ionandtheobservationofa63Da couldbeattributedtothesulfateesterstandard.Theseresults
(HNO)neutrallossshowninFigure1D.Sulfateesterproducts suggestthatthePILS/ICtechniquewillobservedecreasesin
3
given in Table 3 containing nitrate groups were detected inorganicsulfateifsulfateesterformationoccurs.
onlyinexperimentscontainingNO. ThetimeevolutionoftheSO2-andNH+aerosolmass
x 4 4
Inseveralpreviousstudies,thepresenceofacidicsulfate concentrationsobtainedusingthePILS/ICtechniquefora
aerosolwasfoundtohaveapronouncedeffectonthequantity Caltechlow-NO isopreneAASseedaerosolexperimentis
x
ofSOAformedbythephotooxidationofisoprene(9,14,15). compared to that of a Caltech high-NO AS seed aerosol
x
Inthepresentstudy,itappearsthatsulfateesterformation experimentinFigure2.AsshowninFigure2A(althoughnot
maybesimilarlyenhancedbytheintroductionofanacidic evidentfromthetimescalepresented,SO2-andNH+decay
4 4
sulfateaerosol.Intheabsenceofsignificantlevelsofinorganic by(cid:24)20and14%,respectively,over9h),atypicalprofilefor
sulfate, no sulfate esters were detected by LC/ESI-MS in mostCaltechisopreneexperimentsinTable1,ammonium
anyoftheisoprenesystemsconsideredhere.Whenexperi- andsulfatetypicallydecreasedslowlywithtimeduetowall-
mentswerecarriedoutinthepresenceofASaerosols,afew lossprocesses.However,intheexperimentshowninFigure
sulfateestersweredetected,including[M-H]-ionsatm/z 2B,inwhichsulfateesterconcentrationswereexceedingly
199, 215, 244, 260, or 333. Experiments carried out under high,theSO2-aerosolmassconcentrationdecreasedmuch
4
acidicconditionswithAASaerosolproducedaconsiderably faster(i.e.,SO2-decayedby(cid:24)60%over6h)thanwallloss,
4
wider array of detectable sulfate ester compounds. In suggestinganextralossprocess,mostlikelyduetochemical
addition,thepeakareasofseveralionsobservedinboththe reaction.ItshouldbenotedthattheinitialNH:SO molar
4 4
ASandAASexperimentswerefoundtobelargerintheAAS ratioinFigure2Awasnotexactlytwoduetoaknownsource
VOL.41,NO.2,2007/ENVIRONMENTALSCIENCE&TECHNOLOGY9521
354
FIGURE3. (-)ESI-ITMSproductionmassspectraforsulfateestersofr-pineneoxidationproducts.(A)Productionmassspectrumfor
m/z294detectedinEPA-211stage5.(B)Productionmassspectrumform/z265detectedinEPA-211stage4.(C)Productionmassspectrum
form/z310detectedinEPA-211stage5.(D)Productionmassspectrumform/z326detectedinEPA-211stage5.Thesesulfateesterswere
alwayspresentwhenr-pinenewasphotooxidizedinthepresenceofSO.
2
ofammoniumvolatilizationpreviouslycharacterized(31). thussupportingthatthe[M-H]-ionsatm/z294,310,and
The significant decrease in the SO2- mass concentration 326containasulfategroup.Theseresultswereconfirmed
4
observedfortheCaltechlow-NO AASseedaerosolexperi- ontheLC/MSinstrumentoperatedintheupfrontCIDmode
x
mentisconsistentwithpreviouslyobservedincreasesinSOA ofanalysis.
yields (9), strongly suggesting that particle-phase sulfate SulfateestersfromR-pinenewerefoundtoelutefromthe
esterificationisatleastpartlyresponsibleforthis“acid-effect.” reverse-phaseLCcolumnatmuchlaterRTs((cid:24)10-26min)
FiltersamplingformostCaltechisopreneexperimentsbegan thanthoseformedinisopreneoxidation,indicatingdiffer-
(cid:24)5-7haftertheexperimentwasinitiated;sulfateestersare encesinwatersolubility.IdentifiedR-pinenesulfateesters
formed by this point in the experiments as shown in listed in Table 4 were formed from the reactive uptake of
Figure2. previouslyidentifiedgas-phaseoxidationproducts(33,34),
Sulfate Esters from r-Pinene Oxidation. As in the consistentwithpreviouswork(17,18).Exceptforpinonal-
isopreneexperiments,R-pinenesulfateesterswerefoundto dehyde,nosulfateestershavebeenidentifiedtoformfrom
produceabundant[M-H]-ions,corresponding34Sisotopic previously identified R-pinene SOA products; however,
ions,m/z97and80productions,andwerenotobservedin furtherinvestigationiswarranted.Forqualitycontrolpur-
experimentswithoutSO (Table2).Isoprenesulfateesters poses,solid-phaseextraction(SPE)wasusedonduplicate
2
wereformedalsoinexperimentsinvolvingisopreneandSO filters collected from selected experiments (EPA-211) to
2
photooxidation;forsimplicity,theseestersarenotlistedin remove excess inorganic sulfate; it was found that the
Table2.ShowninFigure3A-Darethe(-)ESI-ITMSproduct R-pinenesulfateesterswerestilldetected,andinsomecases
ionspectraforrepresentativeR-pinenesulfateesters;these at higher [M-H]- ion abundances, indicating that these
includeesterscontaininga[M-H]-ionatm/z294,265,310, sulfateestersarenotaresultofinorganicsulfateclustersin
and 326, respectively. Analogous to some of the isoprene themassspectrometer.
sulfate esters, the m/z 294, 310, and 326 R-pinene sulfate SulfateEstersinAmbientAerosol.Figures4A-Ccompare
estersalsocontainnitrategroupsassuggestedbytheireven- theLC/MSextractedionchromatograms(EICs)ofm/z215
mass[M-H]-ionsandobservedneutrallossesof63(HNO) obtainedfromaCaltechlow-NO isopreneAASexperiment
3 x
and/or47Da(HONO).TheMS/MSspectraofthe[M-H]- to that of two SEARCH field samples (JST and BHM,
ions for the four R-pinene sulfate esters yielded m/z 97 respectively). Both the RTs and mass spectra of the chro-
productions.However,them/z80productionwasobserved matographic peaks shown in the EICs of m/z 215 are the
only for the m/z 265 ion due to mass range limits on the sameinallsamples,stronglysuggestingthatthisisoprene
massspectrometer.ItshouldbenotedthattheMS3spectra sulfate ester is present in ambient aerosol. In addition,
ofhigh-massproductionsshowninFigure3(e.g.,m/z250 ambient aerosol recently collected at K-puszta, Hungary
inFigure1A)didyieldthem/z97andm/z80productions, indicatesthatthem/z199and260isoprenesulfateesters
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TABLE 4.Proposed r-Pinene Sulfate Ester SOA Products
aPositionalisomerscontainingnitrateorsulfategroupsatotherhydroxylatedpositionsarepossible.bCompoundslistedinparenthesesare
neutrallossesobserveduponESI-MS/MS.cPreviouslydetectedR-pineneoxidationproductbyAschmannetal.(33,34).dProposedbyLiggioet
al.(17)toformfrompinonaldehydereactiveuptakeontoacidicseedparticles;however,structurewasnotconfirmed.Inthecurrentstudy,we
confirmitsstructurewith(-)ESI-MS.eDetectedinambientaerosolcollectedfromSEARCHnetworkJune2004forfirsttime.fESI-MScannot
differentiatebetweenwhichproductisbeingdetected;forcompletenessbothstructuresareshownhere.gObservedinanambientstudybyGao
etal.(22).hNostructuralinformationwasprovidedinthepreviousstudybyGaoetal.(22).
arepresent(M.Claeys,unpublishedresults);however,these to the lack of detailed connectivity of specific functional
compoundsareonlyweaklydetectedonsomedaysanalyzed groups(e.g.,sulfateestersandhydroxyls)providedbyESI-
fromtheSEARCHnetwork. MS/MSmethods.
Figure 5A-C compares the LC/MS EICs of m/z 294 Theresultsabovesuggestthatthechemistryoccurringin
obtainedfromtwoR-pineneexperiments(EPA-205andEPA- ourlaboratoryexperimentsarerelevanttotheconditionsin
326,respectively)andwithaSEARCHfieldsamplecollected thesoutheasternU.S.,eventhoughthelaboratoryaerosol
attheBHMfieldsiteinJune2004.Thisfigureindicatesthat wasgeneratedfrommuchhigherVOCmixingratios,lower
theR-pinenem/z294sulfateesterisaconstituentofambient RHs, and likely higher aerosol acidities observed in the
aerosol,consistentwithpreviouswork(22).OtherR-pinene southeasternU.S.
sulfateestersidentifiedinthisstudyhavebeenobservedin ESI-MSQualityControlTests.Toensurethatthesulfate
ambientaerosolinthesoutheasternU.S(22).Itispossible esterselucidatedinthisstudywereformedonlyduringSOA
thatthem/z294sulfateesterintheambientaerosolcould formationandnotonthefilterorduringtheESIprocess,
alsoresultfromtheoxidationofothermonoterpenesowing several quality control tests were conducted. First, a filter
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FIGURE4. (-)LC/ESI-MSextractedionchromatogramsform/z215.Theretentiontimesofthem/z215EICsarethesameaswellasthe
massspectraassociatedwitheachchromatographicpeak;therefore,thecomparisonoftheseEICssuggeststhatthephotooxidationof
isopreneinthepresenceofacidseedproducesthesesulfateestersobservedintheambientaerosol.Inallchamberexperimentsinvolving
isopreneinthepresenceofASseedaerosol,AASseedaerosol,orSO,them/z215ionwasdetected.
2
extractfromaCaltechlow-NO isoprenenucleation(i.e.,no aldehydes(e.g.,formaldehyde)intheuppertroposphereand
x
inorganicseedaerosolpresent)experimentwasdividedinto lower stratosphere have been suggested to occur in the
twoparts.Onepartwasspikedwithahighconcentrationof presenceofsulfateaerosols(35-37),whereincreasedacidity
(NH)SO andtheotherpartwasspikedwithpureHSO. wasfoundtoincreasetheiruptake.Someofthesestudies
42 4 2 4
Bothofthesesampleswereanalyzedby(-)LC/ESI-MS.The proposed that the observed uptake of the alcohols and
sulfateesterslistedinTable1werenotdetectedinthesetwo aldehydeslikelyoccurredbysulfateesterformation,although
testsamples,demonstratingthatthesulfateestersdetected noproductstudieswereconducted.Ithasalsobeenshown
inthisstudyarelikelynotartifactsformedintheESIinterface. that reactive uptake of butanol and ethanol onto sulfate
The use of reverse phase chromatography allowed for aerosolsoccursatroomtemperature(38,39).Esterification
inorganic sulfate not to be confused with organosulfates, was recently shown to occur from the photooxidation of
whereinorganicsulfatewastheveryfirstpeaktoelutefrom isopreneinthepresenceofNO fromcondensationreactions
x
the column. In the two test samples discussed above, the oforganicacidswithalcohols(9,10).Thelargeamountsof
inorganicsulfatepeakwasfoundtohavesometailing,which organic acids formed during the photooxidation were
wasverysimilartotheseededexperimentslistedinTable proposedtodrivethesereactions.
1; however, this tailing seems to have no effect on the Figure 6 shows the general reactions proposed for the
formationofsulfateesters. formationofsulfateestersfromalcohols(2-methyltetrolused
As a second test, a meso-erythritol (a surrogate for the asmodelcompound)andsulfatederivativesfromcarbonyl
2-methyltetrolsproducedfromisopreneoxidation)standard compounds(pinonaldehydeusedasmodelcompound).In
wasdividedintotwoparts,whereonewasspikedwith(NH4)2- thecaseofsulfateesterformationfromalcohols,theproposed
SO4andtheotherwithpureH2SO4.Asforthefirstquality reactionslikelyinvolvenucleophilicsubstitution(SN1),where
controltestabove,thesetwosamplesproducednosulfate thesulfuricacidprotonatesthealcoholicgroup,makingwater
estersin(-)LC/ESI-MS. the leaving group. The resulting carbocation becomes a
Last, a filter extract from an EPA R-pinene experiment nucleophilicsitefortheunsharedpairofelectronsonone
conductedwithoutSO2(thusnosulfateaerosolpresent)was oftheoxygenatomsofthesulfate(40).Duetothelowrelative
spiked with pure H2SO4. Again, no sulfate esters were humiditiesoftheseexperiments,thislikelyshiftstheequi-
detected.Theseresultsstronglysuggesttheorganosulfates librium in favor of sulfate ester formation. In the case of
(sulfateesters)wereformedintheaerosolphaseandarenot sulfateesterformationfromaldehydes,theproposedreaction
anartifactofsamplingormeasurement. likely involves the electron pair of the carbonyl oxygen
acceptingaprotonfromsulfuricacid,producingtheoxonium
Discussion
ion,andmakingitmoresusceptibletonucleophilicattack
SulfateEsterificationReactionMechanism.Reactiveuptake fromanunsharedpairofelectronsfromoneoftheoxygen
of gas-phase alcohols (e.g., methanol and ethanol) and atomsonsulfate.Itshouldbestressedthatotherreaction
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357
FIGURE5. (-)LC/ESI-MSextractedionchromatogramsform/z294.Theretentiontimesofthem/z294compoundswerethesameaswell
asthemassspectraassociatedwitheachchromatographicpeak;therefore,thecomparisonoftheseEICssuggeststhatthephotooxidation
ofr-pineneinthepresenceofNO andacidseedproducesthesesulfateestersinambientaerosol.Nom/z294compoundsweredetected
x
inexperimentsinvolvingonlyisopreneandacidseed(orSO).
2
FIGURE6. Proposedreactionsfortheformationofsulfateestersfrom2-methyltetrolandpinonaldehyde,arepresentativealcoholand
aldehydegeneratedbythephotooxidationofisopreneandr-pinene,respectively.Solidboxesindicate(-)ESI-MSdetectedspecies.
Dashedboxesindicateotherproposedproductspossiblyformed.
mechanisms are also possible, including: electrophilic place in the case of polyols; however, (-)ESI-MS is not
additionofHSO toanaliphaticdoublebond,additionof sensitivetosuchneutralspecies,nosuchproductshaveyet
2 4
SO throughsomeradicalprocess,orbysomeotherunknown been identified. In prior work (9) we reported oligomeric
3
mechanismcurrentlynotunderstood.Sulfatediester(ROSOR) signatures(14,16,18Dadifferences)andcompoundswith
3
formationandsulfateesteroligomerizationcouldalsotake masses up to(cid:24)620 Da in matrix-assisted laser desorption
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