Oligomer formation during gas-phase ozonolysis of small alkenes and enol ethers

33 Pages·2008·1.81 MB·English

Atmos. Chem. Phys.,8,2667–2699,2008
Atmospheric
www.atmos-chem-phys.net/8/2667/2008/
Chemistry
©Author(s)2008. Thisworkisdistributedunder
theCreativeCommonsAttribution3.0License. and Physics
Oligomer formation during gas-phase ozonolysis of small alkenes
and enol ethers: new evidence for the central role of the Criegee
Intermediate as oligomer chain unit
A.Sadezky1,2,R.Winterhalter1,B.Kanawati1,A.Ro¨mpp3,B.Spengler3,A.Mellouki2,G.LeBras2,P.Chaimbault4,
andG.K.Moortgat1
1Max-Planck-InstituteforChemistry,AtmosphericChemistryDepartment,P.O.Box3060,55020Mainz,Germany
2InstitutdeCombustionAe´rothermiqueRe´activite´ etEnvironnement,CNRS,1CAvenuedelaRechercheScientifique,45071
Orle´ansCedex2,France
3Institutfu¨rAnorganischeundAnalytischeChemie,Justus-Liebig-Universita¨t,35392Giessen,Germany
4InstitutdeChimieOrganiqueetAnalytique(ICOA),CNRSFR2708,UMR6005,Universite´ d’Orle´ans,BP6759,45067
Orle´ansCe´dex2,France
Received: 6August2007–PublishedinAtmos. Chem. Phys.Discuss.: 4October2007
Revised: 26March2008–Accepted: 9April2008–Published: 21May2008
Abstract. An important fraction of secondary organic our previous study. Analysis of the collected SOA filter
aerosol (SOA) formed by atmospheric oxidation of diverse samples reveal the presence of oligomeric compounds in
volatileorganiccompounds(VOC)hasrecentlybeenshown the mass range 200 to 800u as major constituents. The re-
to consist of high-molecular weight oligomeric species. In peatedchainunitsoftheseoligomersareshowntosystemat-
our previous study (Sadezky et al., 2006), we reported the icallyhavethesamechemicalcompositionastherespective
identificationandcharacterizationofoligomersasmaincon- mainCriegeeIntermediate(CI)formedduringozonolysisof
stituents of SOA from gas-phase ozonolysis of small enol theunsaturatedcompounds,whichisC H O (mass74)for
3 6 2
ethers.Theseoligomerscontainedrepeatedchainunitsofthe ethylbutenylether(EBE),trans-3-hexene,and2,3-dimethyl-
same chemical composition as the main Criegee Intermedi- 2-butene,andC H O (mass88)fortrans-4-octene. Analo-
4 8 2
ates(CI)formedduringtheozonolysisreaction,whichwere gous fragmentation pathways among the oligomers formed
CH O (mass46)foralkylvinylethers(AVE)andC H O by gas-phase ozonolysis of the different alkenes and enol
2 2 2 4 2
(mass 60) for ethyl propenyl ether (EPE). In the present ethers in our present and previous study, characterized by
work, we extend our previous study to another enol ether successivelossesoftherespectiveCI-likechainunitasaneu-
(ethyl butenyl ether EBE) and a variety of structurally re- tral fragment, indicate a similar principal structure. In this
latedsmallalkenes(trans-3-hexene,trans-4-octeneand2,3- work,weconfirmthebasicstructureofalinearoligoperox-
dimethyl-2-butene). ide – [CH(R)-O-O] – for all detected oligomers, with the
n
Experiments have been carried out in a 570l spherical repeated chain unit CH(R)OO corresponding to the respec-
glassreactoratatmosphericconditionsintheabsenceofseed tive major CI. The elemental compositions of parent ions,
aerosol. SOA formation was measured by a scanning mo- fragmentionsandfragmentedneutralsdeterminedbyaccu-
bility particle sizer (SMPS). SOA filter samples were col- ratemassmeasurementswiththeFTICRtechniqueallowus
lectedandchemicallycharacterizedoff-linebyESI(+)/TOF to assign a complete structure to the oligomer molecules.
MS and ESI(+)/TOF MS/MS, and elemental compositions We suggest that the formation of the oligoperoxidic chain
were determined by ESI(+)/FTICR MS and ESI(+)/FTICR units occurs through a new gas-phase reaction mechanism
MS/MS. The results for all investigated unsaturated com- observed for the first time in our present work, which in-
pounds are in excellent agreement with the observations of volves the addition of stabilized CI to organic peroxy rad-
icals. Furthermore, copolymerization of CI simultaneously
formedinthegasphasefromtwodifferentunsaturatedcom-
Correspondenceto: G.K.Moortgat
poundsisshowntooccurduringtheozonolysisofamixture
([email protected])
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
2668 A.Sadezkyetal.: OligomerandSOAformationfromgas-phasealkeneozonolysis
oftrans-3-hexeneandethylvinylether(EVE),leadingtofor- sation processes are described as part of SOA aging pro-
mation of oligomers with mixed chain units C H O (mass cessestakingplaceoverseveralhoursafterSOAformation.
3 6 2
74)andCH O (mass46). Wethereforesuggestoligoperox- Furthermore, a variety of high-molecular peroxidic com-
2 2
ideformationbyrepeatedperoxyradical-stabilizedCIaddi- pounds mainly formed as reaction products of stabilized
tion to be a general reaction pathway of small stabilized CI Criegee Intermediates have been identified as important
inthegasphase,whichrepresentsanalternativewaytohigh- SOA constituents. Among those products are secondary
molecularproductsandthuscontributestoSOAformation. ozonides, α-acyloxyalkyl hydroperoxides, cyclic geminal
diperoxides, peroxyhemiacetals and diacyl peroxides (Za-
hardis and Petrucci, 2007; Mochida et al., 2006; Reynolds
etal.,2006;Tolockaetal.,2006;Zahardisetal.,2006,2005;
Dochertyetal., 2005; Dreyfusetal., 2005; Ziemann, 2003,
1 Introduction
2002). Initial unsaturated compounds are either monoter-
penesandcyclicalkenesconsistingofsixtotencarbonatoms
Organicmaterialaccountsforasubstantialfractionofatmo- (Tolockaetal.,2006;Dochertyetal.,2005;Ziemann,2003,
spheric fine particular matter that affects the global climate 2002), cholesterol (Dreyfus et al., 2005) or the linear C18
bydirectandindirecteffectsaswellashumanhealth(Po¨schl, oleic acid and methyl oleate (Zahardis and Petrucci, 2007;
2005, and references therein). Secondary organic aerosol Mochida et al., 2006; Reynolds et al., 2006; Zahardis et
(SOA) is formed by gas-to-particle conversion of products al.,2006,2005). Formationreactionsleadingtothosehigh-
ofthetroposphericoxidationofvolatileorganiccompounds molecular peroxidic compounds were partly suggested to
(VOC),anditsglobalformationisestimatedtorangefrom12 take place in the liquid phase within the aerosol particles
to70Tgy−1(Kanakidouetal.,2005,andreferencestherein). or heterogeneously. Furthermore, formation of oligomers
Understandingthechemicalcompositionandformationpro- from peroxidic reactions products of oleic acid and choles-
cesses of SOA is required for a quantitative assessment of terolozonolysiswasreportedtoproceedviatheiradditional
itsproduction,propertiesandenvironmentaleffects(Fuzziet freecarboxylicacidandcarbonylfunctionalities,whichreact
al.,2006). withother,eventuallymultifunctionalCriegeeIntermediates
Animportantfractionoforganicaerosolconsistsofhigh- (ZahardisandPetrucci,2007;Zahardisetal.,2006;Reynolds
molecular weight organic species, as have shown several etal.,2006;Dreyfusetal.,2005).
studies of a wide range of VOC oxidation reactions. Most Other suggested pathways leading to oligomer formation
studiessuggestthatoligomerizationtakesplacethroughhet- in organic atmospheric aerosol involve aqueous-phase reac-
erogeneouscondensationreactionsofmorevolatilereaction tionsofpyruvicacid,aproductoftheatmosphericoxidation
products on the surface and within the bulk of aerosol par- of isoprene, initiated by OH radicals (Altieri et al., 2006)
ticles, producing stable oligomeric compounds. Such reac- or photolysis (Guzman et al., 2006) within cloud droplets.
tionsarealdolcondensationandgem-diolformation(Gaoet Recently,formationofhigher-molecularweightspecieswas
al., 2004; Tolockaetal., 2004), aciddehydration(Hamilton also observed for photooxidation and ozonolysis of tertiary
etal.,2006;Gaoetal.,2004)andesterification(Hamiltonet alkylamines(Murphyetal.,2007).
al., 2006; Surratt et al., 2006). Identified monomers were In our recent study (Sadezky et al., 2006) we reported
typical low-volatile reaction products formed during gas- the discovery of oligomeric compounds by chemical analy-
phase ozonolysis of cycloalkenes, such as multifunctional sisofsecondaryorganicaerosolformedduringozonolysisof
acids and diacids (Hamilton et al., 2006; Gao et al., 2004) enolethersusingtheoff-lineESI/MS-TOFtechnique. These
and 2-methylglyceric acid formed during photooxidation of oligomers were found to consist of repetitive chain units,
isopreneinthepresenceofhighNO -concentrations(Surratt which have the same elementary compositions as the main
x
etal.,2006).Aldolandgem-diolcondensationreactionshave CriegeeIntermediates(CI)formedfromtheseozonolysisre-
beenreportedtobesignificantlyenhancedbyacidicseedpar- actions: CH O (=CH OO for C -CI) of mass 46 for the
2 2 2 1
ticles providing acid catalysis (Gao et al., 2004; Tolocka et alkylvinylethers(AVE)andC H O (=CH CHOOforC -
2 4 2 3 2
al.,2004). Moreover,oligomerformationwasdetecteddur- CI)ofmass60forethylpropenylether(EPE).Itisproposed
ing dark ozonolysis of α-pinene by high-resolution FTICR thattheseoligomershavethefollowingbasicstructureofan
MS (Reinhardt et al., 2007), and during photooxidation oligoperoxide, –[CH(R)-O-O] –, where R=H for the AVE
n
of 1,3,5-trimethylbenzene and α-pinene by on-line aerosol and R=CH for the EPE. We suggested a new pathway for
3
time-of-flight(ATOF)massspectrometry(Grossetal.,2006) secondary organic aerosol and oligomer formation involv-
andoff-linematrix-assistedlaserdesorptionmassspectrom- ing gas-phase reactions involving stabilized CI, which lead
etry (Gross et al., 2006; Kalberer et al., 2004). Kalberer et to formation of oligoperoxidic chains carrying mostly three
al.(2004)attributedtheoligomersobservedduringphotoox- tofourCI-likechainunits.
idationof1,3,5-trimethylbenzenetohydration-condensation Ourpresentworkisaimedatinvestigatingthepossibility
reactions involving the main reaction products of aromatic thatthecorrelationbetweenthestructureofthemainCriegee
photooxidation, glyoxal and methylglyoxal. These conden- Intermediate formed during alkene ozonolysis and the
Atmos. Chem. Phys.,8,2667–2699,2008 www.atmos-chem-phys.net/8/2667/2008/
A.Sadezkyetal.: OligomerandSOAformationfromgas-phasealkeneozonolysis 2669
O
+ O O O
R O R' 3
enol ether R O RH'
primary ozonide
.
*
O H . O. * O H C. O O
R + C O + O
H O R' H R' R
primary carbonyl Criegee-Intermediate primary carbonyl alkoxy-substituated
compound: alkyl formate CI compound: aldehyde Criegee Intermediate
R ’ = H : C1-CI alkoxy-CI
R’ = CH : C-CI
3 2
R’ = CH : C-CI
2 5 3
20 %
80 %
Alkyl vinyl ether (AVE): R = CH, CH, CH; R’ = H
2 5 3 7 4 8
Ethyl propenyl ether (EPE): R = CH; R’ = CH
2 5 3
Ethyl butenyl ether (EBE): R = CH; R’ = CH
2 5 2 5

(a)
Fig.1.Generalmechanismofthegas-phaseozonolysisofenolethersandsymmetricalkenes
(a)Enolethers.
compositionofformedSOAandoligomersdescribedinour 2 Experimental
previouswork(Sadezkyetal.,2006)mightapplytoawider
range of unsaturated compounds. We therefore extend our Experiments in the laboratory were performed in a 570-l
previousstudytoawidervarietyofsmallunsaturatedcom- spherical glass reactor at room temperature in synthetic air
pounds, among which are another enol ether, ethyl butenyl atatotalpressureof730Torr. Adetaileddescriptionofthis
ether (EBE, C H OCH=CHC H ), and three symmetric setuphasbeendescribedinearlierpublications(Neebetal.,
2 5 2 5
hydrocarbon alkenes, trans-3-hexene (C H CH=CHC H ), 1998; Winterhalter et al., 2000). Ozone was produced by a
2 5 2 5
2,3-dimethyl-2-butene ((CH ) C=C(CH ) ), and trans-4- mercury pen-ray lamp inside the reactor, prior to the addi-
3 2 3 2
octene (C H CH=CHC H ). A gas-phase ozonolysis ex- tion of the mixture of the unsaturated compound and syn-
3 7 3 7
periment of a mixture of ethyl vinyl ether (EVE, C H O- thetic air (reaction start). The concentrations of reactants
2 5
CH=CH ) and trans-3-hexene (C H CH=CHC H ) was and reaction products were followed by Fourier Transform
2 2 5 2 5
also performed in order to investigate a possible formation infraredspectroscopy(FTIR).Theaerosolconcentrationand
ofmixedoligomersthatmightcontaincombinationsofchain sizedistributionwasmonitoredwithascanningmobilitypar-
units corresponding to the main CI formed during both re- ticlesizer(SMPS,TSI3936)andprovidesinformationofthe
actions. Gas-phase ozonolysis experiments have been per- total SOA mass M0. The SMPS consists of an electrostatic
formedinalaboratory-reactionchamberunderexperimental classifier (TSI 3080) with a long differential mobility ana-
conditions similar to those of Sadezky et al. (2006). SOA lyzer,(LDMA;TSI3081)andanultrafinecondensationpar-
formedduringthereactionshavebeenobservedbyaSMPS ticlecounter(CPC;TSI3025A)asdetector.
system and chemically characterized by a hybrid ESI(+)/Q- Experimentswereperformedwithinitialozonemixingra-
TOF and chemical composition was confirmed by accurate tios of 8ppm, and enol ether and alkene mixing ratios of
mass measurements with an ESI Fourier Transform Ion cy- 15ppm. For simultaneous ozonolysis of EVE and trans-3-
clotron resonance (FTICR) mass spectrometer. FTICR MS hexene,initialmixingratioswere8ppmofozone,8ppmof
offers ultrahigh resolution and high sensitivity for the char- EVE and 12ppm of trans-3-hexene. In order to prevent re-
acterizationofcomplexsamples(e.g.Ro¨mppetal.,2005). actions of vinyl ethers with OH radicals, which are known
tobegeneratedduringtheozonolysisofalkenes(Finlayson
et al., 1972), cyclohexane (excess, 300ppm) was added in
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2670 A.Sadezkyetal.: OligomerandSOAformationfromgas-phasealkeneozonolysis
O . *
R' + O3 O O O + H C. O O
R' H R' R'
R' R'
primary carbonyl Criegee Intermediate
primary ozonide
compound: aldehyde CI
R’ = CH : C-CI
2 5 3
Trans-3-hexene: R’ = CH R’ = CH : C-CI
2 5 3 7 4
Trans-4-octene: R’ = CH
3 7
(b)
. *
O H C . O
H3C CH3 + O + 3 C O
3 H C CH H C
H C CH 3 3 3
3 3
primary carbonyl Criegee Intermediate
compound: acetone CI
iso-C -CI
3
2,3-Dimethyl-2-butene
(c)
Fig.1.Generalmechanismofthegas-phaseozonolysisofenolethersandsymmetricalkenes
(b)Trans-3-hexeneandtrans-4-octene.(c)2,3-Dimethyl-2-butene.
some experiments. All chemicals were commercially avail- Fourier transform ion cyclotron resonance (FTICR) mass
able(purity>95%)andusedwithoutfurtherpurification. spectrometery. The instrument used was a combined lin-
eariontrapandFTICRmassspectrometer(LTQFT,Thermo
Theaerosolformedinthelaboratoryexperimentswascol-
FisherScientific,Bremen,Germany). Thesamplewasintro-
lectedduring20–25minonTeflon(PTFE)filters(45mmdi-
ameter, 0.45µm pore size), using a flow rate of 14lmin−1. duced by a nanospray source using gold-coated fused-silica
emitters (New Objective, Woburn, MA, USA) at an ioniza-
After collection the filters were extracted in a 7cm3 glass
tionpotentialof+1kV.DataanalysiswasdonewiththeXcal-
flask with 3ml pure methanol (HPLC grade), and stored
at –20◦C until analysis. LiCl solution was added to se- ibur2.0software(ThermoElectron,Bremen,Germany).The
massdeviationoftheFTICRmeasurementsareusuallywell
lectedsamplesinordertostudytheformationofmetalcation
adducts. The resulting Li+ ion concentration in these sam- below2ppm.
pleswasabout0.1mg/ml.
Chemical constituents were detected by a hybrid mass 3 Resultsanddiscussion
spectrometer (quadrupole and time-of-flight) QSTAR (Ap-
plied Biosystems MDS SCIEX) with an electrospray ion 3.1 Gasphasereactionmechanisms
source. The extraction solution was directly injected
(30µl/min). The electrospray ion source (TurboIonSpray) 3.1.1 Enolethers
was operated in the positive mode at 400◦C and an ioniza-
tion voltage of +3.4kV. The declustering potential was 0 to The general mechanism of the ozonolysis of enol ethers
+30V,andthefocussingpotential(focusring)was+100V. is displayed in Fig. 1a. The initial product formed is the
FortandemQ-TOFexperiments,thecollisionenergywasbe- primary ozonide (1,2,3-trioxolane), which is unstable and
tween10and30eVwithCAD(collisiongas)setto2.Instru- decomposes into a carbonyl oxide, called the Criegee in-
mentcontrol,spectratreatmentandcalculationsofelemental termediate (CI), and a primary carbonyl compound. The
compositionsweredonewiththesoftwareAnalyst(Applied ozonolysisofenolethermoleculesproducesCIsofthetype
BiosystemsMDSSCIEX).Moreover,theelementalcompo- CH OO (C -CI, R’=H) for the alkyl vinyl ethers (AVE),
2 1
sition of the analytes was determined by nanoelectrospray CH CHOO(C -CI,R’=CH )forethylpropenylether(EPE),
3 2 3
Atmos. Chem. Phys.,8,2667–2699,2008 www.atmos-chem-phys.net/8/2667/2008/
A.Sadezkyetal.: OligomerandSOAformationfromgas-phasealkeneozonolysis 2671

nnn === 222
OOOllliiigggooommmeeerrr (((aaa))) OOOllliiigggooommmeeerrr (((bbb)))
OOOllliiigggooommmeeerrr (((BBB))) OOOllliiigggooommmeeerrr (((EEE)))
OOOllliiigggooommmeeerrr (((FFF)))
OOOllliiigggooommmeeerrr wwwiiittthhh mmm///zzz iiidddeeennntttiiicccaaalll tttooo
ooollliiigggooommmeeerrr (((aaa))) fffooorrrmmmeeeddd ddduuurrriiinnnggg
nnn === 333 ooozzzooonnnooolllyyysssiiisss ooofff tttrrraaannnsss---333---hhheeexxxeeennneee
FFFrrraaagggmmmeeennnttt iiiooonnnsss 111555999,,, 222333333,,, 222777555,,, 333000777
CCCHHH
222 555 +++ OOO
333
CCCHHH OOO
222 555
EEEBBBEEE
nnn === 444
222000111 nnn === 111
111888777 555000111 nnn === 555
666333333 nnn === 666
770077
Fig.2a.ESI(+)/TOFMSmassspectrumofSOAformedduringthegasphaseozonolysisofEBE(initialmixingratios:8ppmozone,15ppm
EBE).
and C H CHOO (C -CI, R’=C H ) for ethyl butenyl ether, Theprimarycarbonylcompoundssimultaneouslyformed
2 5 3 2 5
togetherwithROCHOO(alkoxy-substitutedCI). are the corresponding aldehydes propanal (trans-3-hexene,
The corresponding primary carbonyl compounds consist R’=C2H5)andbutanal(trans-4-octene,R’=C3H7).
of an alkyl formate ROC(O)H and formaldehyde (AVE, 2,3-Dimethyl-2-butene produces CIs of the type
R’=H), acetaldehyde (EPE, R’=CH ) or propanal (EBE, (CH ) COO (iso-C -CI), which is an isomer of the
3 3 2 3
R’=C H ). C -CI formed from the ozonolyses of trans-3-hexene and
2 5 3
Previousstudiesofthegas-phaseozonolysesofethylvinyl EBE. The corresponding primary carbonyl compound is
ether (EVE, C H O-CH=CH ) and ethyl propenyl ether acetone(Fig.1c).
2 5 2
(EPE, C H O-CH=CHCH ) by FTIR spectroscopy showed The CI formed from the decomposition of the primary
2 5 3
that the branching ratios of the splitting of the primary ozonide are formed in excited states, which then either de-
ozonide into both pathways was (71±13)% for the “ethyl composeintovariousproductsorbecomecollisionallystabi-
formate +C -CI” channel for EVE, and (83±13)% for the lized.
1
“ethyl formate +C2-CI” channel for EPE (Sadezky, 2005). About 50–60% of the excited C1-CI are stabilized, while
Inthiswork,thebranchingratioforthe“ethylformate+C3- the yields of stabilized C2-CI and C3-CI are estimated to
CI”channelduringozonolysisofEBEwasalsodetermined be between 20 and 40% per reacted alkene or enol ether
tobecloseto80%. (Sadezky,2005;Krolletal.,2002). Thestabilizationrateof
theexcitediso-C -CI,however,isverylow,asthistypeofCI
3
3.1.2 Symmetricalkenes decomposesbynearly100%viathehydroperoxidechannel
(e.g.Rickardetal.,1999).
Symmetric alkenes form only one type of primary carbonyl
compoundandCriegeeIntermediateupontheirreactionwith 3.2 Formationofsecondaryorganicaerosol(SOA)
ozone(Fig.1b).Thesymmetricalkenetrans-3-hexenethere-
fore produces only CIs of the type C H CHOO (C -CI, Total SOA masses M (µg/m3) measured by SMPS after
2 5 3 0
R’=C H ),whichisalsothemajorCIformedduringozonol- completionofthereaction,beforethebeginningofthefilter
2 5
ysis of ethyl butenyl ether (EBE). Trans-4-octene forms the sampling, are given in Table 1. Initial mixing ratios of re-
analogousCIsofthetypeC H CHOO(C -CI,R’=C H ). actantsandofcyclohexane(C H )addedasOHscavenger,
3 7 4 3 7 6 12
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2672 A.Sadezkyetal.: OligomerandSOAformationfromgas-phasealkeneozonolysis

nn == 22
OOlliiggoommeerr ((aa))
OOlliiggoommeerr ((bb))
FFrraaggmmeenntt iioonnss 115599,, 118855,, 223333,, 225599
CC HH
22 55 ++ OO
33
CC HH
nn == 11 22 55
ttrraannss--33--hheexxeennee
nn == 33
nn == 44
Fig.2b. ESI(+)/TOFMSmassspectrumofSOAformedduringthegasphaseozonolysisoftrans-3-hexene(initialmixingratios: 8ppm
ozone,15ppmtrans-3-hexene).
Table1. TotalSOAmassesM0(µg/m3)formed,initialmixingratiosofreactantsandcyclohexane(C6H12),andtypesofmajorCIformed
inthegas-phaseozonolysisreactionsoftheunsaturatedcompoundsstudiedinthiswork.
Alkene [alkene]0 [ozone]0 [C6H12]0 Typeof M0(SOA)
[ppm] [ppm] [ppm] CI [µg/m3]
EBE(C2H5O-CH=CHC2H5) 15 8 – C3-CI 250
trans-3-hexene(C2H5CH=CHC2H5) 15 8 – C3-CI 160
“” 15 8 300 C3-CI 400
trans-4-octene(C3H7CH=CHC3H7) 15 8 – C4-CI 300
2,3-dimethyl-2-butene((CH3)2C=C(CH3)2) 15 8 300 iso-C3-CI 40
“” 15 8 – iso-C3-CI 3
EVE(C2H5O-CH=CH2) 6 8 – C1-CI **
+ + +
trans-3-hexene(C2H5CH=CHC2H5) 8 C3-CI
**notmeasured
andthemaintypesofCIformedfromtheseozonolysisreac- to 40% of stabilized CI) (Kroll et al., 2002; Rickard et al.,
tions,asdiscussedintheprevioussection,arealsogiven. 1999). A correlation between the amount of SOA formed
MeasuredtotalSOAmassesM formedduringgasphase and the stabilization rate of the CI, independently of their
0
ozonolysis amount to several hundreds of µg/m3 for most numberofcarbonatoms–asthedisubstitutedC3-CIandthe
unsaturated compounds, except the 2,3-dimethyl-2-butene, monosubstituted iso-C3-CI are isomers –, might indicate a
which forms much lower amounts of SOA. The reason for keyroleofstabilizedCIinSOAformation.
this difference might be the very low stabilization rate (less
than 1% of stabilized CI) of the disubstituted iso-C -CI in
3
comparisonwiththemonosubstitutedC -CIandC -CI(20%
3 4
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A.Sadezky etal.: OligomerandSOAformationfromgas-phasealkeneozonolysis 2673
nn == 11
OOlliiggoommeerr ((aa)) OOlliiggoommeerr ((bb))
OOlliiggoommeerr ((BB))
nn == 22
FFrraaggmmeenntt iioonnss 118855,, 225599
HHCC CCHH
33 33
++ OO
33
HHCC CCHH
33 33
22,,33--ddiimmeetthhyyll--22--bbuutteennee
MMSS
nn == 00
nn == 33
Fig.2c. ESI(+)/TOFMSmassspectrumofSOAformedduringthegasphaseozonolysisof2,3-dimethyl-2-butene(initialmixingratios:
8ppmozone,15ppm2,3-dimethyl-2-butene,300ppmcyclohexane).
A significant increase of the total SOA mass M upon ethylbutenylether(EBE),trans-3-hexeneand2,3-dimethyl-
0
addition of an excess of cyclohexane (C H ) as an OH 2-butene (Fig. 2a–c), and of 1 m/z=88 for trans-4-octene
6 12
radical scavenger is observed for the two alkenes trans-3- (Fig. 2d). The results are consistent with those from analo-
hexene and 2,3-dimethyl-2-butene. A similar influence of gousstudies(Sadezkyetal.,2006;Sadezky,2005),revealing
C H on SOA yields has been observed by Docherty and oligomerionswithregularmassdifferencesof1m/z=46for
6 12
Ziemann(2003)fortheozonolysisofβ-pinene,whilethere- alkylvinylethers(AVE)and1m/z=60fortheethylpropenyl
verseeffectwasfoundfortheozonolysisofalkylvinylethers ether (EPE). The pseudomolecular ions carry a single posi-
(Sadezkyetal.,2006). Inallcases,however,thequalitative tivecharge.
resultsobtainedfromchemicalanalysisoftheSOAbymass
Thedifferentionseriesforthevariousunsaturatedethers
spectrometry do not change in the presence of an OH scav-
are presented in Table 2 and correspond to different types
enger.
ofoligomersdesignated(a),(b),(B),(C),(D),(E),(F),(G).
Inorder tobetterdistinguishthe series, thepeaksof these-
3.3 ChemicalanalysisoftheSOA:identificationandchar-
riesareidentifiedbydifferentcolours, correspondingtothe
acterizationofoligomers
colours of the peaks of Fig. 2a–d. Ion series observed for
ethyl vinyl ether (EVE) and ethyl propenyl ether (EPE) are
3.3.1 IdentificationofoligomersintheSOA
alsogivenforcomparison(Sadezkyetal.,2006).
Deploying the smooth ionisation of the electrospray tech- Themostintenseseriesobservedforeachetheriscoloured
nique, oligomeric products were detected in the SOA filter inred,andisassignedasoligomeroftype(a). Theotherob-
samples for all compounds studied. Figure 2a–d show the servedoligomerseriesusuallyappearwithmuchlowerinten-
massspectraoftheaerosolsamplesobtained. sitiesanddifferfromthemajoroligomerseriesoftype(a)by
The spectra show the presence of ions in the mass range multiples and sums of 1 m/z=16 and 1 m/z=14. Oligomer
betweenm/z200and800fortheenoletherEBEandbetween series with similar differences of 1 m/z towards the main
m/z200and600forthethreealkeneswiththetypicalregular oligomer series of type (a) are labelled with similar letters
structures of oligomers. The ion peaks could be grouped in and colours in Table 2 and in Fig. 2a–d for the different
serieswhoseionsdisplayregulardifferencesof1m/z=74for alkenes and enol ethers. For example, for most enol ethers
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2674 A.Sadezkyetal.: OligomerandSOAformationfromgas-phasealkeneozonolysis

nnn === 111
OOOllliiigggooommmeeerrr (((aaa))) OOOllliiigggooommmeeerrr (((bbb)))
OOOllliiigggooommmeeerrr (((CCC))) OOOllliiigggooommmeeerrr (((EEE)))
OOOllliiigggooommmeeerrr (((FFF))) OOOllliiigggooommmeeerrr (((GGG)))
nnn === 222
FFFrrraaagggmmmeeennnttt iiiooonnnsss 111777333,,, 222111333,,, 222666111
CCC HHH
333 777
+++ OOO
333
CCC HHH
333 777
tttrrraaannnsss---444---ooocccttteeennneee
MMSS
nnn === 000 nnn === 333
444777999
Fig.2d. ESI(+)/TOFMSmassspectrumofSOAformedduringthegasphaseozonolysisoftrans-4-octene(initialmixingratios: 8ppm
ozone,15ppmtrans-4-octene).
dimethyl-2-buteneandethylpropenylether(EPE).Massdif-
ferences of these oligomer series towards the oligomers of
HHH HHH
type (a) are, for example, 1 m/z=14 for oligomers (C) and
(D)(with1m/z14possiblycorrespondingtoaCH group),
2
CCC CCC 1m/z=42foroligomer(B)(1m/z42=3×1m/z14,possi-
bly corresponding to three CH groups), and 1 m/z=30 for
2
RRROOO RRR‘‘‘ oligomer (E) (1 m/z 30=1 m/z 14 + 1 m/z 16, eventually
accountingintotalforaformaldehyde-likeunitCH O).
2
EEnnoolliicc ssiiddee VViinnyylliicc ssiiddee As described in a later section of this work and in our
previous study (Sadezky et al., 2006), MS/MS experiments
allowed to fragment the pseudomolecular ions and thus to
Fig.3.Schematicstructureofanenolether. determine the minimum number n of fragmented repetitive
chain units 46, 60, 74 or 88 contained in the molecular
andalkenes,oligomerionsoftype(b)areobserved,coloured species. In Fig. 2a–d, n are given for the pseudomolecular
in green in Table 2 and Fig. 2a–d. They differ from the ionsofthemostintensiveoligomerseriesdesignatedastype
series of type (a) ions by an additional 1 m/z=16. Exact (a). Ionsoftype(a)andionsofweakeroligomerseriessug-
mass measurements by FTICR have revealed in our present gestedtocarrysimilarnumbersofchainunits, arearranged
work(Sect.3.3.3.) thattheionsoftype(a)andtype(b)re- inverticalcolumnsinTable2.
spectively represent the Na+ and K+ adducts of the same Some ions between m/z 150 and 300, which are also ob-
oligomer molecule. The difference 1 m/z=16 between the servedasfragmentionsintheMS/MSspectra(Sect.3.3.3.)
type(a)and(b)oligomerionsthuscorrespondstothediffer- ofparentoligomerions,arelistedseparatelyonFigs.2a–d.
encebetweenNa+ andK+ ratherthantoanadditionaloxy- OligomerionsobservedforEBEinthepresentworkcor-
genatom, assuggestedbySadezkyetal.(2006). Whilefor roborate the dependence of their chain unit on the vinylic
trans-3-hexeneandethylvinylether(EVE),onlyoligomers side =CHR’ of the double bond of the initial enol ether de-
oftype(a)and(b)areobserved,avarietyofotherionsseries rivedfromtheanalogousstudyofAVEandEPE(Sadezkyet
appear for ethyl butenyl ether (EBE), trans-4-octene, 2,3- al.,2006). Theschematicstructureofanenolethershownin
Atmos. Chem. Phys.,8,2667–2699,2008 www.atmos-chem-phys.net/8/2667/2008/
18 A. Sadezky et al.: Oligomer and SOA formation from gas-phase alkene ozonolysis
Table 2. Oligomer pseudomolecular ion series detected by
ESI(+)/TOF MS in the SOA formed during ozonolysis of enol
ethers and alkenes (n: minimum number of repetitive chain units
A.Sadezkyetal.: OligomerandSOAformationfromgas-phasealkeneozonolysis 2675
which are directly identified as neutral fragments in the MS/MS
spectra) (MW: molar weight [g/mol]).
Table2.OligomerpseudomolecularionseriesdetectedbyESI(+)/TOFMSintheSOAformedduringozonolysisofenolethersandalkenes
(n: minimumnumberofrepetitivechainunitswhicharedirectlyidentifiedasneutralfragmentsintheMS/MSspectra)(MW:molarweight
[g/mol]).

Ether/Alkene Ion Series (m/z)
n n=0 n=1 n=2 n=3 n=4 n=5 n=6 n=7 n=8 n=9
Ethyl propenyl ether 339 399 459 519 579 639 699 759 (a) [M+Na]+
(MW 86 g/mol) 357 417 477 (B)
EPE, CHOCH=CHCH 265 325 385 445 (C)
2 5 3
(Sadezky et al., 2006) 293 353 413 473 533 (D)
489 549 609 669 729 789 (E)

Ethyl vinyl ether 283 329 375 421 467 513 (a) [M+Na]+
(MW 72g/mol) 345 391 437 483 (b) [M+K]+
EVE, CHOCH=CH
2 5 2
(Sadezky et al., 2006)
Ethyl butenyl ether 321 395 469 543 617 691 (a) [M+Na]+
(MW 100 g/mol) 411 485 559 633 707 (b) [M+K]+
EBE, CHOCH=CHCH 279 353 427 501 (B)
2 5 2 5
(this work) 647 721 795 (E)
437 511 585 (F)
305 379 453 527 601 ∗

trans-3-hexene 305 379 453 527 (a) [M+Na]+
(MW 84 g/mol) 321 395 469 (b) [M+K]+
CHCH=CHCH
2 5 2 5
(this work)

2,3-dimethyl-2-butene 231 305 379 453 (a) [M+Na]+
(MW 84 g/mol) 245 321 395 (b) [M+K]+
(CH) C=C(CH) 263 337 411 495 559 (B)
3 2 32
(this work)

trans-4-octene 273 361 449 537 (a) [M+Na]+
(MW 112 g/mol) 289 377 465 553 (b) [M+K]+
CHCH=CHCH 347 435 (C)
3 7 3 7
(this work) 391 479 (E)
417 505 593 (F)
401 489 (G)

*Oligomerwithm/zidenticaltotheoligomerion(a)formedduringozonolysisoftrans-3-hexene
*O ligomerwithm/zidenticaltotheoligomerion(a)formedduringozonolysisof
trans-3-hexene
Fig. 3 displays the two alkyl substituents, R’ on the vinylic As discussed in the Sect. 3.1.1, the major Criegee Inter-
side =CHR’ of the double bond and R on its enolic side mediate (CI) formed with yields around 80% during enol
=CHOR. etherozonolysisoriginatesfromthevinylicsideoftheenol
ethers, thus carrying R’ as substituent. The major CI pro-
R’=C H for EBE (C H OCH=CHC H ), while
2 5 2 5 2 5
ducedfromEBEisthereforetheC -CIwithR’=C H ,which
R’=H for AVE (ROCH=CH ) and R’=CH for EPE 3 2 5
2 3
isidenticaltotheC -CIformedduringozonolysisoftrans-
(C H OCH=CHCH ). R’ differs thus by mass 14 (CH ) 3
2 5 3 2
3-hexene(Sect.3.1.2). TheresultsgiveninFig.2aandband
betweentheAVEandEPEaswellasbetweenEPEandEBE,
in Table 2 show that the oligomers obtained from ozonoly-
a difference that is reflected in the masses of the observed
sis of EBE and trans-3-hexene consist of chain units with
oligomerchainunits,whichareof1m/z=46fortheAVE,1
identical mass 74. Moreover, chain units with similar mass
m/z=60=46+14forEPEand1m/z=74=60+14forEBE.
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Atmos. Chem. Phys., 8, 1–44, 2008 www.atmos-chem-phys.net/8/1/2008/
2676 A.Sadezkyetal.: OligomerandSOAformationfromgas-phasealkeneozonolysis
Table3a.ESI(+)/FTICRMS/MSmeasurements:
Calculatedelementalcompositionsofparentions,fragmentionsandfragmentedneutralmoleculesforthered-markedfragmentationpathway
ofthepseudomolecularoligomerion379formedfromtrans-3-hexene*.
Accuratemass Elemental com- Exactmass Absolutemasser- Relative mass er-
(u) position (u) ror(mDa) ror(ppm)
parention
379.19384
+
[X-[74]2-Y+H] C17H31O9 379.1962 –2.4194 –6.3804
+ +
[X-[74]2-Y+Na] C15H32O9Na 379.1938 –0.0141 –0.0373
initiallyfragmented
neutralX C8H18O2 146.1306 –0.0499 –0.3421
146.13063
fragmention
233.06321
+
[Y-[74]2+H] C9H13O7 233.0655 –2.3694 –10.1664
+ +
[Y-[74]2+Na] C7H14O7Na 233.0631 0.0358 0.1537
neutral
chainunitCI C3H6O2 74.0367 0.0604 0.8168
74.03684
fragmention
159.02637
+
[Y-[74]+H] C6H7O5 159.0287 –2.4299 –15.2798
+ +
[Y-[74]+Na] C4H8O5Na 159.0263 –0.0246 –0.1549
*Theion85appearingintheESI(+)/TOFMS/MSspectrainFig.4aisnotvisibleintheESI(+)/FTICRMS/MSspectraduetothestrong
decreaseinsensitivityoftheFTICRanalyzerforionswithm/zbelow100u.
74 are also found for oligomers formed from 2,3-dimethyl- (Table 2 and Figs. 2a and b), which might account for the
2-butene (Fig. 2c and Table 2), which produce the iso-C - enolic O atom of EBE. Moreover, Table 2 and Fig. 2b and
3
CI, an isomer of the C -CI. Finally, trans-4-octene forms c show that trans-3-hexene and 2,3-dimethyl-2-butene form
3
the C -CI, which differs by a CH group of mass 14 from oligomer ions of type (a) and (b) with identical m/z. Both
4 2
theC -CIproducedbyEBEandtrans-3-hexene. Thechain alkenes,liketheirCI,areisomers,carryingthesamenumber
3
units of the oligomers obtained from ozonolysis of trans-4- ofcarbonandhydrogenatomsasalkylsubstituentsoneach
octene(Fig.2dandTable2)reflectthisdifferencebytheir1 sideoftheirdoublebonds.
m/z=88=74+14. These results point towards a decisive role TheresultspresentedinTable2andinFig.2a–dfurther-
oftheCIintheformationoftheoligomersobservedaschem- moreshowthatthedegreeofoligomerizationdecreaseswith
ical constituents of the SOA from the different unsaturated increasing size of the chain unit. For the most abundant
compounds. oligomers,nis3withthechainunitsofmass46(AVE)and
Inourpreviousstudy(Sadezkyetal.,2006),wesuggested mass60(EPE)(Sadezkyetal., 2006), whilenis2withthe
furthermore that the alkoxy group OR on the enolic side chainunitofmass74(EBEandtrans-3-hexene,Fig.2aand
of the initial ether is contained once in each oligomer ion. b),andnis1withthelargestchainunitofmass88(trans-4-
A comparison of the initial structures of EBE and trans-3- octene,Fig.2d).
hexene shows that each symmetric side of the double bond Althoughthechainunitsareofsimilarmass74,lowerde-
oftrans-3-hexeneisidenticaltothevinylicsideofEBE.The greesofoligomerizationareobservedforoligomersformed
enolicsideofEBEdiffersfromthevinylicsideonlybythe from2,3-dimethyl-2-butene,withn=1forthemostabundant
enolicOatomdirectlylinkedtothedoublebond.Indeed,the oligomer molecule (Fig. 2c), than for those produced from
Na+ andK+ adductsoftheoligomers(oligomeriontypesa EBEandtrans-3-hexene. Thisobservationisconsistentwith
and b) from ozonolysis of EBE carry an additional 1 m/z the low total SOA masses formed during ozonolysis of this
16 in comparison with those formed from trans-3-hexene compound,andmightbecorrelatedwiththelowstabilization
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