Hydroxy nitrate production in the OH-initiated oxidation of alkenes

20 Pages·2015·1.53 MB·English

Atmos.Chem.Phys.,15,4297–4316,2015
www.atmos-chem-phys.net/15/4297/2015/
doi:10.5194/acp-15-4297-2015
©Author(s)2015.CCAttribution3.0License.
Hydroxy nitrate production in the OH-initiated oxidation of alkenes
A.P.Teng1,J.D.Crounse1,L.Lee2,J.M.St.Clair1,R.C.Cohen2,3,andP.O.Wennberg1,4
1DivisionofGeologicalandPlanetarySciences,CaliforniaInstituteofTechnology,Pasadena,CA,USA
2DepartmentofChemistry,UniversityofCaliforniaBerkeley,Berkeley,CA,USA
3DepartmentofEarthandPlanetarySciences,UniversityofCaliforniaBerkeley,Berkeley,CA,USA
4DivisionofEngineeringandAppliedScience,CaliforniaInstituteofTechnology,Pasadena,CA,USA
Correspondenceto:A.P.Teng([email protected])
Received:10February2014–PublishedinAtmos.Chem.Phys.Discuss.:13March2014
Revised:13March2015–Accepted:25March2015–Published:28April2015
Abstract. Alkenes are oxidized rapidly in the atmosphere ingdaytime,alkylnitratesformviaaradicalchainterminat-
by addition of OH and subsequently O leading to the for- ingbranchinthereactionofalkylperoxyradicalswithNO.
2
mationofβ-hydroxyperoxyradicals.Theseperoxyradicals The major branch in this chemistry recycles HO and pro-
x
react with NO to form β-hydroxy nitrates with a branch- ducesozone.Thefateofalkylnitratesisthoughttobedeter-
ing ratio α. We quantify α for C –C alkenes at 295K±3 minedbyeither(1)depositionleadingtolossofatmospheric
2 8
and 993hPa. The branching ratio can be expressed as α= NO or(2)furtherreactionsthatleadtorecyclingofNO or
x x
(0.045±0.016)×N−(0.11±0.05)whereN isthenumber conversion of the organic nitrates to HNO . Thus, RONO
3 2
ofheavyatoms(excludingtheperoxymoiety),andlisteder- canserveeitherasapermanentsinkorasatransportmecha-
rorsare2σ.Thesebranchingratiosarelargerthanpreviously nismforNO .
x
reportedandaresimilartothoseforperoxyradicalsformed Alkyl nitrates also play an important role in organic
from H abstraction from alkanes. We find the isomer distri- aerosolformation(Rollinsetal.,2012;Brownetal.,2009).
butions of β-hydroxy nitrates formed under NO-dominated Aerosol nitrates have been observed to form as a result of
peroxyradicalchemistrytobedifferentthantheisomerdis- NO chemistry, though our understanding of the gas phase
3
tribution of hydroxy hydroperoxides produced under HO - mechanismsleadingtoaerosolnitrateremainsincomplete.
2
dominated peroxy radical chemistry. Assuming unity yield Knowledge of the branching ratio of RO +NO to form
2
for the hydroperoxides implies that the branching ratio to alkyl nitrates from RO derived from specific volatile or-
2
formβ-hydroxynitratesincreaseswithsubstitutionofRO . ganic compounds (VOCs) is important for diagnosing the
2
Deuteriumsubstitutionenhancesthebranchingratiotoform role of individual VOCs in ozone and aerosol formation.
hydroxy nitrates in both propene and isoprene by a factor Thisknowledgecanthenguidespecificcontrolstrategiesto
of ∼1.5. The role of alkene chemistry in the Houston re- mitigatepollution(Ryersonetal.,2003;Rosenetal.,2004;
gion is re-evaluated using the RONO branching ratios re- Farmeretal.,2011).
2
ported here. Small alkenes are found to play a significant Many previous studies have reported VOC-specific
role in present-day oxidant formation more than a decade branching ratios to form alkyl nitrates. These studies sug-
(2013) after the 2000 Texas Air Quality Study identified gest that the branching ratios increase with increasing car-
these compounds as major contributors to photochemical bonnumber,increasingpressure,anddecreasingtemperature
smoginHouston. (OrlandoandTyndall,2012,andreferencestherein).Thisbe-
havior has been interpreted as evidence that the lifetime of
theO–ONOintermediatecontrolsthefractionofthenascent
complexthatisomerizesontotheONO surface.Thedynam-
2
1 Introduction
ics that lead from the peroxynitrite (ROONO) to the nitrate
(RONO )is,however,notwellunderstood(Lohretal.,2003;
2
Theformationofalkylnitratesisanimportantprocesscon-
Barkeretal.,2003;Zhangetal.,2002).
trollingtroposphericoxidantsandthelifetimeofNO .Dur-
x
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
4298 A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes
Alkenes react rapidly by addition of OH and O to form eson. Nitrogen dioxide (NO ) (5ppmv in ultra high purity
2 2
β-hydroxyperoxyradicals.Theseperoxyradicalsreactwith N )gastankforTD-LIFcalibrationwaspreparedbyMathe-
2
NOtoformβ-hydroxynitrates. son.Methylnitrite(CH ONO)wassynthesized,purified,and
3
Previousstudieshavesuggestedthatthebranchingratioto storedusingmethodssimilartothosedescribedbyTayloret
formβ-hydroxynitratesfromreactionofβ-hydroxyperoxy al.(1980).
radicalswithNOislowerthanforperoxyradicalsproduced
from reactions of alkanes of the same carbon number with
2.1 Environmentalchamberexperiments
OH (O’Brien et al., 1998). The lower nitrate branching ra-
tios for β-hydroxy peroxy radicals have been attributed to
the β-hydroxy group weakening the O–ONO bond, short- The CIMS and thermal dissociation laser-induced fluores-
ening the lifetime of the OONO complex toward decom- cenceinstrument(TD-LIF)instrumentsandtheTeflonreac-
position to NO and thereby reducing the time available to tionchamberhavebeendescribedpreviously(Crounseetal.,
2
sample the crossing to the nitrate surface (RONO ) (Muth- 2013; Lee et al., 2014). Briefly, photochemical experiments
2
muruthma et al., 1993; O’Brien et al., 1998; Matsunga and were conducted in a 1m3 enclosure composed of fluori-
Ziemann, 2009, 2010). Patchen et al. (2007), however, re- nated ethylene propylene copolymer (Teflon FEP, Dupont).
ported the branching ratio to form hydroxy nitrates derived UV photolysis of hydrogen peroxide (H O ) or methyl ni-
2 2
from 1- and 2-butene as larger than previously reported by trite (CH ONO) provided the primary HO source. Exper-
3 x
O’Brien et al. (1998). This study was conducted at 100 iments to determine the hydroxy nitrate yields were typi-
torr where RONO yields should be smaller than at atmo- callyconductedwithinitialmixingratiosof0.08–2ppmvof
2
sphericpressure.Additionalstudiesconductedonalkenesus- alkene, 0.2–2ppmv (±10%) of hydrogen peroxide or 40–
inglong-pathFT-IRhavedeterminedtotalalkylnitrateyields 200ppbvmethylnitrite,and0.5–4ppmv(±5%)ofNO.Ex-
similar to those determined for n-alkanes. However, these perimentstodeterminehydroxyhydroperoxideisomericdis-
studies provide only upper bounds for branching ratios to tributions were conducted with initial mixing ratios of 2–
RONO duetothepossibleformationoforganicnitratefrom 30ppbv alkene and 2–20ppmv of hydrogen peroxide. All
2
RO+NO chemistry (Atkinson et al., 1985; Tuazon et al., experiments were performed at ambient pressure, approxi-
2
1998;Aschmannetal.,2010). mately 993hPa. Table 1 provides a complete list of experi-
In this study, we use CF O− CIMS (chemical ionization ments.
3
massspectrometry)toquantifythehydroxynitratesyield.In AlkeneorCH ONOadditiontotheenvironmentalcham-
3
addition, we utilize gas chromatography with both CF O− berwasaccomplishedbyfirstflushinga500cm3 glassbulb
3
CIMS and thermal dissociation NO laser-induced fluores- withthecompoundandthenfillingittothedesiredpressure
2
cence (TD-LIF) to resolve and quantify isomeric distribu- (1–20hPa).ThebulbwasthenfilledwithN gasto993hPa.
2
tionsofthesehydroxynitrates.TheTD-LIFinstrumentpro- If required, the compound was serially diluted by pumping
videsindependentconfirmationthattheobservedsignalsare thebulbdowntothedesiredpressure(5–400hPa)andback-
alkyl nitrates and enables secondary calibration of CF O− filling again with N to atmospheric pressure. The concen-
3 2
CIMS sensitivity by the TD-LIF for individual β-hydroxy trations of ethene, propene, 1-butene, 2-methyl propene, 2-
nitrates. methyl 2-butene, 2-methyl 1-butene, 1-hexene and 1-octene
weredeterminedwithinthebulbbyFT-IRspectroscopy.FT-
IR cross sections were obtained from the PNNL (Pacific
2 Materialsandmethods Northwest National Laboratory) database (Johnson et al.,
2002; Sharpe et al., 2004) for all compounds except cis-
1-propene (propene) (>99%), d -propene (>99%), 1- 2-butene and d -propene. Determinations of the concentra-
6 6
butene (but-1-ene) (>99%), cis-2-butene (cis-but-2-ene) tionsofd -propeneandcis-2-butenewerebasedonmanome-
6
(>99%), methylpropene (2-methylpropene, isobutylene, try,andcheckedagainstGC-FID(gaschromatographyflame
isobutene) (>99%), 1-pentene (pent-1-ene) (>98%), 2- ionizationdetector)measurementsrelativetotheothergases
methyl 1-butene (2-methyl but-1-ene) (>98%), 2-methyl addedinthesameexperimentassumingequivalentFIDsig-
2-butene (2-methyl but-2-ene) (>99%), isopropyl nitrate nal per carbon atom. These independent methods agreed to
(nitric acid, 1-methylethyl ester) (>99%), 1-hexene (hex- within3%.
1-ene) (>99%), 1-octene (oct-1-ene) (>98%), and 1,2- NO addition was accomplished by evacuating a 500cm3
butanediol(butane1,2diol)(>98%)puritywerepurchased glass bulb, and filling from the standard tank to the desired
from Sigma Aldrich and used without further purification. pressure. NO was added to the enclosure only after at least
Hydrogen peroxide (30 and 50% by weight in water) was 0.25m3ofairwasaddedtolessenconversionofNOtoNO
2
purchased from Sigma Aldrich. Ethene (ethylene) (>99%) from the reaction of 2NO+O . All pressure measurements
2
was purchased from Scott Specialty Gases. A nitric oxide were obtained using 13.3 or 1333.3hPa full scale absolute
(NO) (1994± 20ppmv in ultra high purity N ) standard pressure gauges (MKS Baratron™). H O addition was ac-
2 2 2
gas tank for chamber experiments was prepared by Math- complished by evaporating a known mass of 30 or 50% by
Atmos.Chem.Phys.,15,4297–4316,2015 www.atmos-chem-phys.net/15/4297/2015/
A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes 4299
Table1.Experimentlist.
Experiment.# H2O2 NO VOC,ppbv Absoluteα Relativeα ROOH TD-LIF
ppmv ppbv yield measurement
1 2 500 propene,96 ISOPN,50 x
2 2 500 propene,93 ISOPN,38 x
3 2 500 propene,116 ISOPOOH,38 x
4 2 500 propene,164 ISOPOOH,50 x
5 500 propene,143 ISOPOOH,44 CH3ONO,350 x
6 500 propene,156 ISOPOOH,20 CH3ONO,370 x
7 2.5 1200 propene,262 1-butene,183 1-pentene,238 x
8 2.5 1100 propene,270 2-methyl1-butene,185 1-hexene,180 x
9 2.5 2300 propene,235 1-hexene,210 x
10 2.5 1200 propene,408 1-hexene,390 x
11 2.5 600 propene,133 1,2butanediol,80 x
12 1.0 600 propene,162 1,2butanediol,490 x
13 1.0 700 propene,164 1,2butanediol,70 x
14 600 propene,122, 1,2butanediol,70 CH3ONO,60 x
15 2 500 1-butene,131 1-hexene,105 x x
16 2 500 1-butene,15 cis-2-butene,120 x x
17 2 500 ethene,1096 cis-2-butene,115 x x
18 2 500 2-methyl2-butene,107 x x
19 2 500 methylpropene,342 x x
20 2.6 propene,9 x
21 2.6 propene,8 x
22 2.6 propene,5 x
23 10.4 propene,7 x
24 2 propene,21 x
25 2 propene,36 1-butene,17 1-hexene,33 x
26 2 propene,30 butadiene,14 2-methyl2-butene,20 x
27 2.7 1-hexene,5 x
28 2.7 2-methyl2-butene,4 x
29 2.8 methylpropene,4 x
30 2.8 1-butene,7 x
31 2 1000 d6-propene,324 1-butene,256 x
32 2 1000 d6-propene,280 propene,334.5 1-butene,102 x
33 2 1140 ethene,963 propene,426 methylpropene,261 x
2-methyl2-butene,242 1-hexene,254 1-octene,326
34 0.2 1930 ethene,2983 propene,608 methylpropene,309 x
2-methyl2-butene,339 1-hexene,263 1-octene,314
35 0.2 950 propene,1418 1-butene,85 methacrolein,902 x
2,3dimethyl,2-butene,1096
36 0.2 950 propene,1380 methylvinylketone,1258 cis-2-butene,674 x
2,3-dimethyl2-butene,732
37 20 methylpropene,181 x
weight H O in water. Concentrations were confirmed by Reaction products were monitored using CF O− chem-
2 2 3
CIMSmeasurementofhydrogenperoxideinthegasphase. ical ionization mass spectrometry (CIMS) methods. Mul-
The composition of the chamber was monitored by sam- tifunctional products from alkene oxidation were detected
pling from the enclosure at ∼2000 sccm through a sin- using CF O− cluster ion signals observed at m/z M + 85
3
gle 4mm ID perfluoroalkoxy line with instruments sam- product. The following m/z were used for hydroxy ni-
plinginseries:(1)ToF-CIMS(Tofwerk,Caltech),(2)Triple trate determination: ethene, 192; propene, 206; d -propene,
6
Quadripole MS-MS CIMS (Varian, Caltech), (3) GC-FID 212;1-butene/cis-2-butene/methylpropene,220;2-methyl,2-
(HP 5890 II), (4) NO Monitor (Teledyne 200EU), (5) O butene/1-pentene/2-methyl 1-butene, 234; 1-hexene 248;
x 3
Monitor (Teledyne 400E). Sampling conducted this way 1-octene, 276. For hydroxy hydroperoxides, the follow-
minimizes surface interactions by lowering the residence ing m/z were used for quantification: propene, 177; 1-
time of chamber air in the sampling line to <0.2s. The butene/methylpropene, 191; 2-methyl 2-butene, 205; 1-
sampling configuration in which chamber air passes only hexene, 219. 1,2 butanediol was monitored using signal at
through a Teflon sampling line without first entering a gas m/z 175, and the resulting hydroxycarbonyls were moni-
chromatographisreferredtohereas“direct”sampling.The toredm/z173.
specifics of the CIMS have been described in detail else- The alkene concentrations were monitored using an GC-
where(Crounseetal.,2006,2011,2012,2013;Paulotetal., FID (Agilent 5890). Chamber air was sampled into a 10
2009;St.Clairetal.,2010). cm3 stainless steel sample loop or a 30cm3 PFA (perfluo-
roalkoxy) sample loop using a six-port valve. The sample
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4300 A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes
was transferred to the head of the column in the oven at Thedeterminationofthesplitratio(approximately10:1)
temperaturesbetween308and373K,dependingonthehy- between the CIMS and TD-LIF NO instrument was per-
2
drocarbon. In the case of ethene, samples were cryotrapped formed using an isopropyl nitrate standard (80ppbv in air)
withliquidnitrogenontheheadofthecolumn.Amegabore preparedinthesamefashionasthealkenesdescribedabove.
(0.53mm)30mPlot-Qcolumn(AgilentJ&Wcolumns)was Thegasstandardwasbothdirectlysampledfromthecham-
used to separate compounds using 7–9 standard cm3min−1 ber and following cryo-collecting ∼200cm3 on an HP 612
N carriergas.Asuitabletemperaturerampwasselectedfor column and eluting the peak in the usual GC configuration.
2
eachcompound. The signal level in the TD-LIF instrument was recorded as
theGCrampedthroughitsusualtemperatureprogram.Sig-
2.1.1 GC-CIMS/LIFchromatography nal levels were also compared between direct measurement
of ∼80ppbv isopropyl nitrate within a Teflon bag into the
After oxidation, the chamber air was analyzed using GC- TD-LIF system and collected for 4min on a cooled sam-
CIMS/LIF (gas chromatography CF O− CIMS/TD-LIF). pleloop.Aseparatecheckwasalsoconductedwithprepared
3
Chamber air is pulled through a Teflon sampling line, standardsofisoprenehydroxynitratesinwhichthestandard
through a Teflon three-port valve, and cryofocused at 240– wasdirectlysampledfromthechamberandthroughtheGC
280K on the head of a 4m megabore HP 612, a 4m system.Thismeasurementwasproblematicduetolongequi-
megabore RTX-1701, or 1m megabore RTX-1701 column. librationtimes(>3h)resultingfromlowsamplingflowlim-
ColumnswereheldinsideaVarianGCoven(CP-3800).Fol- ited by the small diameter tubing in the TD-LIF optimized
lowing the GC, the column effluent was split between the for GC use. The TD-LIF was also calibrated at the end of
ToF-CIMSinstrumentandtheTD-LIFsystemfortheexper- eachphotochemistryexperimentwithgasstandardof5ppmv
imentslistedinTable1.Forallotherexperimentswherethe NO inN (Matheson)undermatchingpressureconditions.
2 2
TD-LIFinstrumentwasnotused,thecolumneffluentflowed
directly to the ToF-CIMS. After a measured flow of cham- 2.1.2 Experimentstodeterminehydroxynitrate
ber air ranging from 30 to 200standard cm3 was cryofo- branchingratiosrelativetopropene
cused over a length of time ranging from 2 to 12min by
placing the GC column in a cooled (−20◦C) isopropanol Experiments 31–36 (Table 1) were conducted to determine
bath, the three-port valve was switched to allow carrier gas the β-hydroxy nitrate yields from alkenes relative to the
(N )toflowthroughtheGCcolumn.Thevolumeofcham- β-hydroxy nitrate yield from propene. These experiments
2
ber air cryofocused in this manner was determined by the involved simultaneous oxidation of up to six alkenes. Ini-
collection time and the flow rate (inferred from manome- tial alkene concentrations were determined by FT-IR spec-
try). Carrier gas flow was controlled by a mass flow con- troscopy.Thechamberwasmonitoredforatleast20minto
troller (MKS) at 8.7standard cm3min−1 N . Temperature determinethebackgroundsignals.UVlightswereturnedon
2
program (30◦C, hold 0.1min, 3◦Cmin−1 from 30–60◦C, foraperiod(<10min)sufficienttoachieveahydroxynitrate
10◦Cmin−1to130◦C,hold3min)startedapproximately2– concentrationhighenoughforquantificationwhileminimiz-
3min after cryofocusing. All wetted surfaces in the analyt- ingsecondaryOHlossesofhydroxynitrates.Lessthan10%
ical setup were comprised of Teflon, PEEK, or GC column ofeachalkenespecieswasoxidizedineachexperiment.Ex-
materialstolimitsurfaceinteractions. perimentswereinitiatedbetween292and293Kandthetem-
The TD-LIF system sampled a portion of the GC carrier peraturerisewasnomorethan1Koverthecourseoftheex-
gasinto400◦Coven.PureO isaddedtothisflowupstream periments.Hydroxynitratebranchingratiosweredetermined
2
of the TD oven to ensure complete conversion of RONO relativetothoseofpropenewiththeexceptionofExperiment
2
to NO . NO is measured using laser-induced fluorescence 31, which measured d -propene hydroxy nitrate branching
2 2 6
(Leeetal.,2014).Thesystemwascalibratedatthesameop- ratios relative to 1-butene. Accurate GC-FID quantification
eratingpressurewithastandardtankofNO .Theconversion intheseexperimentswasnotpossibleduetothesmallchange
2
efficiencyoftheTD-LIFwasevaluatedwithisopropylnitrate inconcentrationofeachcompound.
and found to be 100% (see Appendix B). We assume here
thatconversionofotherRONO isalso100%.Totheextent 2.1.3 Experimentstodeterminehydroxynitrate
2
theconversionofthesehydroxynitratestoNO islessthan branchingratiosinanabsolutemanner
2
100%,thereportedbranchingratiosarebiasedlow.
TheToF-CIMSinstrumentwasoperatedinthesameman- Experiments 1–19 (Table 1) were conducted to determine
nerasduringthephotochemistrywithdilutedcolumnefflu- theabsolutebranchingratiostoformβ-hydroxynitrates.Ex-
entsubstitutingfortheambientflow.Withthissplitflowcon- periments involved addition of 1–3 alkenes, NO, and H O
2 2
figuration,theconcurrentelutionofalkylnitrateswasmon- or CH ONO into the chamber. Initial alkene concentrations
3
itored by both the ToF-CIMS and TD-LIF instruments, en- weredeterminedbyFT-IRandconfirmedbymanometryand
abling secondary calibration of the CIMS sensitivity by the GC-FID peak areas. Isoprene hydroxy nitrates or isoprene
TD-LIFtotheindividualalkylnitrates. hydroxy hydroperoxides were also added to the chamber in
Atmos.Chem.Phys.,15,4297–4316,2015 www.atmos-chem-phys.net/15/4297/2015/
26 A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes
A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes 4301
Experiments1–7tomeasuretheirOHrateconstantsrelative
to propene as described in experiments of Lee et al. (2014)
andSt.Clairetal.(2015).TomeasureOHexposure,1,2bu-
tanediol(areferencecompoundmeasurablebyCIMS)was
addedtoaseriesofexperimentstoconfirmthealkenedecay
measuredbyGC-FID.Intheseexperiments,UVlightswere
turnedonuntilasignificantandquantifiabledecayofhydro-
carbonwasobserved(>15%)ontheGC-FID.Experiments
were initiated between 292 and 293K, and the temperature
increasedbyatmost7Kafterthelightswereturnedon.
2.1.4 Experimentstodeterminehydroxy
FFiigguurree411..RReaecatciotniopnatphwatahywsaoyfsalkoefneaslkweinthesOwH.itOhHOoHxi.daOtiHonoxidation
hydroperoxideisomerdistributions wwiitthhaalklekneensefsolfloowllsowtwsotpwaothwpaayths:wOaHysa:dOdiHtionad(fdnitiaonnd(ffnn+1a)nd fn+1)
Experiments 20–30 (Table 1) were conducted to deter- aannddHH-a-basbtrsatcrtaicotnio(fnab(,fwabh,erwehfaebre=fn(cid:80)a=lb1=fabPnl).ffaa+bnf)a.b)f=a+1.fSaubb)-=1.Sub-
mine the hydroxy hydroperoxide isomer distributions. Ex- scriptsindicatethecarbonnumberatwhichne=it1herOHadditionor
asbcsrtirpatcstioinndoicccautres.tFhoeraclakrebnoesnstnuudmiedbeinrtahtiswwhorikchateroitohmerteOmHpera-dditionor
periments involved addition of 1–3 alkenes and H2O2. Ini- aatbusretr,athcetisounmoocfcfuarns.aFnodrfaanlk+e1niessmsutuchdigerdeaitnertthhiasnwfaobr.kSaetertoexotmtemper-
tial alkene concentrations were determined by FT-IR spec- faotrufruer,ththeredsisucmussoiofnfoafneastnimdaftiaonn+sf1orisfamb.uchgreaterthanfab.Seetext
troscopy. UV lights were turned on until a measurable con- forfurtherdiscussionofestimationsforfab.
centration of hydroxy hydroperoxides was produced. Less
than 10% of each alkene species was oxidized in each ex-
perimentasindicatedbyGC-FIDsignalareas. αisisomer-averagedbranchingratio,andN isthenum-
berofheavyatomsintheperoxyradical(notcounting
2.1.5 Post-experimentGC-CIMS/TD-LIFmonitoring theperoxyradicaloxygens).
Followingeachexperiment,productswereanalyzedbyGC- 4. Theabsolutebranchingratios,α,aredeterminedusing
CIMS/TD-LIForGC-CIMS.AtleastthreereplicateGCruns absolute quantification of alkene and hydroxy nitrate
wereconducted.TransmissionthroughtheCIMSportionof concentrations.Thedependenceofα onthenumberof
the GC-CIMS/TD-LIF was measured by comparing the di- heavyatoms,N,isderived.
rect sampling measurement to the integrated chromatogram
5. The entire absolute branching ratio data set is used to
signal for a given m/z. The integrations were corrected by
placeallrelativenitratebranchingratiosonanabsolute
blankGCruns(lessthan3%signalinallcasesovertheelu-
basis.
tiontimeofthehydroxynitratepeaks).Thetransmissionwas
determined by the ratio of the direct sampling and the total
6. Isomerspecificdistributionsforalkenesarederivedus-
chromatogramsignalsaftertakingintoaccountthesampling
ingGCchromatogramsofhydroxynitrateisomers.
flow-ratedifferences.
7. The OH addition branching ratios are inferred by an-
alyzing hydroxy hydroperoxide isomer distributions of
3 Resultsanddiscussion
alkenes oxidized by OH under HO -dominated condi-
2
tions.
The following analysis procedure was used to calculate
branchingratiosfromtheexperimentaldata:
8. The dependence of the alkyl nitrate branching ratios
1. β-hydroxy nitrate CIMS sensitivities are determined on the type (i.e., primary, secondary or tertiary) of β-
fromsimultaneousmeasurementofhydroxynitratesby hydroxy RO radicals is determined by comparing (5)
2
cryofocusedgaschromatography. and(6).
2. ThefractionofthereactionofOHthatproceedsviaad- We define the branching ratio (α ) to represent the frac-
n
dition,fa,isestimatedfrompreviouslyreportedkinetic tion of RO2+NO reaction that produces RONO2, where
data on alkenes. This allows normalization of subse- the subscript n denotes the carbon alpha to the peroxy rad-
quentmeasurementsoftheyieldsofβ-hydroxynitrates, ical (Fig. 1). We define the fraction of OH that adds to car-
YβHN,toproducebranchingratios,α. bonnasfan outofthetotalOH+alkenereaction.Thetotal
fraction of OH+alkene that proceeds via addition is given
3. Branching ratios to form hydroxy nitrates, α, from
l
alkenes relative to αHN_propene are determined through as fa= Pfan. The total fraction of OH+alkene that pro-
experimental data. These results allow direct compari- n=1
son of the dependence of α on structure. α is well de- ceedsviaH-abstractionisgivenasf = Pl f ,wherethe
scribedbyalinearrelationship:α=m×N+b,where ab abn
n=1
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28 A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes
A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes 27
4302 A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes
4
o 3
ti
a
R
al
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Si
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CI 1
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0 5 10
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CI Figure43.FGiCg-uCrIeM3S./TGDC-L-CIFIMchrSo/mTaDto-gLraImFcfohlrloowminagtoEgxrpa.m15f.ollowingExp.15.The
0 Thesignalssdiguneatolsindduiveidtuoalinhdydivroixdyuanlitrhaytedsraorxeydenteitrmraitneesdabrye determined by inte-
0 5 10 integrating the alkyl nitrate peaks (black line, elevated baseline)
which co-elgurteatwinigthtthheeainlkdiyvlidnuiatlrahtyedrpoexayknsit(rabtleascokbslienrvee,debleyvatedbaseline)which
Time (minutes)
CIMS,e.g.choyd-eroluxyteniwtraittehsftrhoemi1n-dbiuvteidneuaatl2h2y0dmro/xzy(bnluietr)aatneds observed by CIMS,
from1-hexeen.eg.a,th24y8dmro/xzy.AnbitsroalutetesCfIrMomSse1n-sbituivteitnieesaarted2e2te0r-m/z (blue) and from
FiguFreig4u2.reβh2y.dβro-xhyyndirtroaxteyprnoidturacttsepprordoudcuedctfsropmrosidxuaclekednefsrom six alkemniensedbyin1te-ghreatxinegneindaitvi2d4ua8lpmea/kzs.foArbCsIoMlSutseigCnaIlManSdTsDen-LsiIFtivitiesaredetermined
durindguErixnpg.1E9.xTph.e1to9p.pTanheelsthoopwsptahneeraltisohoofwCsIMtShehyrdartoixoyonif-CIMS hydrsoigxnyalanddividing.Theassignedchemicalstructuresareshownfor
byintegratingindividualpeaksforCIMSsignalandTD-LIFsignal
traten(HitNra)tseig(nHalNsr)atsioigednatolsprtoopepnreoHpeNn(e20H6Nm/(z2)0f6or:met/hze)nefoHrNetheneHN(e1a9c2halkylnitratepeak.
anddividing.Theassignedchemicalstructuresareshownforeach
(192mm//zz,,bblluuee)),,mmetehtyhlpyrloppreonpeeHnNe(H22N0m(2/2z0,rmed/),z2,-mreedth),yl2,2--methyl,2-butene
buteneHN(234m/z,teal),1-hexeneHN(248m/z,purple),and alkylnitratepeak.
HN(234m/z,teal),1-hexeneHN(248m/z,purple),and1-octene
1-octeneHN(276m/z,gold).Thebottompanelshowstheabso-
lutesHigNnal(f2o7r6promp/enze,HgNol(d2)0.6Tmh/ez)b.oTthteolmighptsawneerlesthuronwedsotnhaetabsolutesignal
timef=or0,parnodpteunrneedHoNffa(t270m6inm(v/ezr)ti.caTlhliene)l,igahtwtshiwcherpeointutrthneed on at time =
that proceed via OH addition: α=Y /(f /(f +f ))=
oxida0t,ioannsdtoptuperdn.eFdoroaflflacotm7pmouinnds(votehretricthaalnlhinyedr)o,xaytnwitrhatiecsh point the oxi- βHN a a ab
fromd1a-toicotennset,oaprapteiodi.sFploorttaedllacsoamdapshoeudnldinseoutshinegratvhearangehdyddartoaxynitratesfrom YβHN/fa.Foralkenesstudiedhere,faisgreaterthan0.75.
fromthreeminutesafterlightsuntilthemeasurementsstopped.For
1-octene,aratioisplottedasadashedlineusingaverageddatafrom
hydroxynitratesfrom1-octene,dataafter10minisaveraged.The
3.1 DeterminationofCIMSsensitivitiesbyTD-LIFfor
time3lamgifnorahfytedrrolxiyghntistrautenstiflrotmhe1m-ocetaesnueraermiseesnftrsomstowpaplleadn.dForhydroxyni-
samptlreatleinseferqoumilib1ra-toiocnt.eTnhei,sdsuagtageastfstethra1t0themmineaasureredavyeierladgiesd.Thetimelag hydroxynitrates
alowfeorrlihmyidt.roxynitratesfrom1-octenearisesfromwallandsampleline
equilibration.Thissuggeststhatthemeasuredyieldisalowerlimit. CIMSsensitivitieswerederivedfromcryofocusedgaschro-
matography and simultaneous measurement of β-hydroxy
nitrate compounds by CF O− CIMS and TD-LIF. Discrete
3
gaussian peaks which eluted at the same time in the CIMS
subscript indicates the carbon at which H-abstraction takes
andTD-LIFwereintegrated.Thetime-integratednormalized
place.Thereforef +f =1.Theisomer-averagedbranch-
a ab ion counts from the CIMS was multiplied by the split ra-
ing ratio (α no subscript) to form β-hydroxy nitrates from
tiobetweenTD-LIFandCIMSinstrumentsthendividedby
OHadditiontoamono-alkeneisthendefinedinthispaperas the integrated NO signal (ppbv × s) measured by the TD-
2
α=fan×αn+1+fan+1×αn. LIFtodetermineasensitivityinnormalizedcountsppbvof
Yields of β-hydroxy nitrates (Y ) are defined as the
βHN RONO .SensitivitiesarelistedinTable2.Anexamplechro-
2
change in hydroxy nitrate concentration over the change
matogramofGC-CIMS/TD-LIFfromwhichsensitivitiesare
inhydrocarbon(HC):Y =−d[HN]/d[HC].Hydroxyni-
βHN derivedisshowninFig.3.
tratesproducedviaH-abstractionandsubsequentalkoxyH-
shiftisomerizationresultinmoleculeswithdifferentmolecu- 3.2 Therelativeyieldsofβ-hydroxynitratesfrom
larweightsfromhydroxynitratesproducedbyOHaddition, alkeneoxidation
and therefore only β-hydroxy nitrates produced via OH ad-
dition are counted in yields. Isomer-averaged branching ra- β-hydroxynitratebranchingratiosweremeasuredrelativeto
tios (α) to form β-hydroxy nitrates from β-hydroxy peroxy α forExperiments31–36withtheexceptionofEx-
HN_propene
radicals can be calculated from β-hydroxy nitrate yields by periment31whichmeasuredd -propenerelativeto1-butene.
6
normalizingforthefractionofthealkenereactionswithOH With minimal oxidation of total hydrocarbon (<10%), the
Atmos.Chem.Phys.,15,4297–4316,2015 www.atmos-chem-phys.net/15/4297/2015/
A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes 4303
Table 2. Absolute sensitivities for each individual isomer and determined by GC-FID due to the small fractional change
isomer-averagedsensitivities(bold)weredeterminedbysumming in the mixing ratio. Therefore, the loss was estimated itera-
peaks in both the CIMS and TD-LIF, and deriving a sensitivity, tivelybyusingthecalculatedbranchingratioforthehydroxy
expressed as normalized counts of analyte ion (normcts) per pptv nitrates(seeSect.3.3forderivationofabsolutebranchingra-
of analyte. Uncertainties (1σ) include the 10% uncertainty from
tiosusedinthiscalculation).Thedifferenceinthedetermina-
thesplitratioandabsoluteNO2determinationbytheTD-LIF.The tionofY betweenusingaveragevs.theinitialalkenecon-
βHN
measured1-hexeneHNsensitivitiesarelowerthanotherHNsmea-
centrationswaslessthan5%inallcases.CIMSsignalsfrom
sured in this study. This may reflect precision errors for this one
Expt.19areshowninFig.2.Estimatesforsecondarylossof
compound,andthusleadtoahighbiasingintheHNbranchingof
hydroxy nitrates by reactions with OH for the relative yield
1-hexeneand1-octene.
experiments(k estimatesfromTrevesandRuddich,2003;
OH
Hydroxynitratesderivedfrom MeasuredCIMS andKwokandAtkinson,1995)usingthemethoddescribed
CF3O−sensitivity by Atkinson et al. (1982) result in corrections of<3% and
(normctspptv−1),104 areneglectedinthissubsetofexperiments.Forabsoluteyield
determinations,whichinvolvedlargerOHexposure,theap-
ethene,1-OH,2-ONO2 3.5±0.4
pliedcorrectionsarelistedinTable3.
propene1-OH,2-ONO2 5.0±1.2
propene2-OH,1-ONO2 5.0±1.4 The relative branching ratio data set allows us to directly
propene,both 5.0±1.0 compare the dependence of α on structure by reducing the
1-butene1-OH,2-ONO2 3.3±0.5 uncertaintiesassociatedwiththemeasurementandanalysis.
1-butene2-OH,1-ONO2 2.9±0.4 The analysis of the relative yield experiments also reduces
1-butene,both 3.1±0.4 systematicuncertaintythroughcancellationofcorrelateder-
cis-2-butene,firstdiastereomer 2.8±0.4 rors associated with determination of total chamber volume
cis-2-butene,seconddiastereomer 2.9±0.4 andtheuseoftheGC-FID.Therelativeyielddetermination
cis-2-butene,both 2.9±0.4
relies on the ratio of OH rate constants, the ratio of initial
methylpropene1-OH,2-ONO2 3.9±0.4 alkene concentrations, the ratio of CIMS HN sensitivities,
methylpropene2-OH,1-ONO2 3.7±0.8 andtheratioofCIMSHNsignals.Ineachrelativeyieldde-
methylpropene,both 3.8±0.5
termination, only the ratio of OH rate constants and ratio
2-methyl,2-butene2-OH,3-ONO2 3.8±0.4
2-methyl,2-butene3-OH,2-ONO2 3.8±0.5 of HN sensitivities are determined outside the given exper-
2-methyl,2-butene,both 3.8±0.4 iment.
1-hexene,1-OH,2-ONO2 2.6±0.3
1-hexene,2-OH,1-ONO2 2.0±0.4 3.3 NormalizingnitrateyieldsforH-abstraction
1-hexene,both 2.4±0.3
TocalculatethebranchingratioforreactionofRO withNO
Absolutesensitivitiesforallisomersweredeterminedbysummingallpeaksin 2
boththeCIMSandTD-LIF,andderivingasensitivityfromthetotal. toformβ-hydroxynitratesfollowingadditionofOHandO2
Uncertainties(1σ)includethe10%uncertaintyfromthesplitratioandabsolute to alkenes, it is necessary to estimate the fraction of alkene
NO2determinationbytheTD-LIF.Themeasured1-hexeneHNsensitivitiesare
lowerthanotherHNsmeasuredinthisstudy.Thismayreflectprecisionerrorsfor loss, fa, that proceeds via this channel. A quantitative de-
thisonecompound,andthusleadtoahighbiasingintheHNbranchingof termination of f from our experimental data is not possi-
1-hexeneand1-octene. a
ble because CF O− CIMS is insensitive to singly function-
3
alized carbonyl or nitrate compounds formed from the OH
measured CIMS signal of each compound is used to deter-
H-abstraction channels. The H-abstraction channel by OH
minetheratiooftheyieldofβ-hydroxynitrates,whereyield
has been measured for propene and cis-2-butene to be less
isdefinedasY =fa1×α2+fa2×α1: than3%,andfor1-butenetobe8±3%(Krasnoperovetal.,
Y signal sens 2011; Loison et al., 2010, and references therein). There
HN_alkene = HN_alkene · HN_propene
Y signal sens have, however, been few studies of H-abstraction rates for
HN_propene HN_propene HN_alkene
otheralkenesatambienttemperatures.
k [propene]
· OH,propene · avg. (1) Using theoretical methods, Pfrang et al. (2006a, b) pre-
k [alkene]
OH,alkene avg dicted that the chain length should not affect the rate of
To determine the ratio of the average branching ratios OH addition to 1-alkenes, and therefore, the increasing ab-
(αalkene/αpropene), we multiply the ratio of the yields by the straction rate with size should scale with additional CH2
ratioofthefractionofOHaddition(f ): groups.Thereis,however,disagreementinexperimentalre-
a
sults about how much the abstraction rate increases with
α Y f
HN_alkene = HN_alkene · a_propene. (2) each additional CH . Aschmann and Atkinson (2008) mea-
2
α Y f
HN_propene HN_propene a_alkene sured OH rate constants for a series of 1-alkenes and
The mean concentrations throughout the experiment were found that the OH rate constant increases at a rate of 2×
calculatedbyaveragingtheinitialandfinalalkeneconcentra- 10−12cm3molec−1s−1perCH group,roughly25%higher
2
tions.Theamountofalkeneoxidizedcouldnotbeaccurately thanforn-alkanes(1.4×10−12cm3molec−1s−1,determined
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4304 A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes
Table3.Absolutehydroxynitrate(HN)yieldsfromalkenesat293Kand993hPa.
Alkene (cid:49)Alkene HN, kO1H,HN(×10−11) kw2(×10−6) F3 Ft4emp YβHN fa5 α
ppbv/% ppbv (cm3molec−1s−1) (s−1) (%) (%)
ethene 221/21(±2) 2.6±0.5 0.3 3.8 1.06 1.02 1.3±0.2 1 1.3±0.2
overallethene 1.3±0.5
propene 38/32(±3) 1.6±0.4 0.6 1.6 1.05 1.01 4.4±1 0.97 4.6±1
propene 60/37(±4) 2.4±0.6 0.6 1.6 1.07 1.01 4.3±1 0.97 4.4±1
propene 104/39(±4) 4±0.8 0.6 1.6 1.07 1.05 4.3±1 0.97 4.5±1
propene 83/31(±2) 3.7±0.8 0.6 1.6 1.05 1.05 4.7±1 0.97 4.8±1
propene 71/18(±2) 2.9±0.6 0.6 1.6 1.03 1.04 4.4±1 0.97 4.1±1
propene 68/33(±2) 2.4±0.5 0.6 1.6 1.09 1.04 4.0±1 0.97 4.1±1
propene 34/21(±3) 1.2±0.3 0.6 1.6 1.03 1.03 3.7±1 0.97 3.9±1
propene 59/36(±4) 2.1±0.5 0.6 1.6 1.05 1.06 4.0±1 0.97 4.1±1
propene 42/34(±4) 1.5±0.4 0.6 1.6 1.05 1.01 3.8±1 0.97 3.9±1
overallpropene 4.1±2
1-butene 71/55(±4) 6.8±1.2 0.7 3.7 1.1 1.06 11±2 0.92 12±2
1-butene 78/43(±3) 7.6±1.3 0.7 3.7 1.09 1.06 12±2 0.92 13±2
overall1-butene 12±5
cis-2-butene 85/74(±5) 8.4±1.3 0.6 4 1.11 1.06 12±2 0.97 12±2
overallcis--butene 12±4
methylpropene 166/49(±3) 13.7±1.9 0.5 6.5 1.05 1.05 9±1 0.97 9±1
overallmethylpropene 9±3
2-methyl1-butene 114/60(±4) 15.5±3.9 0.8 2 1.08 1.05 15±4 0.95 16±4
overall2-methyl1-butene 16±7
2-methyl2-butene 66/64(±4) 5.3±0.8 0.8 8.3 1.07 1.04 9±2 0.97 9±2
overall2-methyl2-butene 9±4
1-pentene 108/44(±3) 12±3 0.85 6.7 1.12 1.05 13±3 0.87 15±3
overall1-pentene 15±6
1-hexene 62/59(±6) 9.2±1.5 1 10 1.2 1.06 19±3 0.86 22±4
1-hexene 78/43(±4) 11.8±1.9 1 10 1.13 1.07 18±3 0.86 21±4
1-hexene 91/23(±2) 14.6±2.4 1 10 1.06 1.06 18±3 0.86 21±4
overall1-hexene 21±8
1RateconstantsarefromTrevesandRuddich(2003)orestimatedbasedonTrevesandRuddich(2003).2AlllossrateconstantsarecalculatedfrompostexperimentHNsignaldecay.
3CorrectionfactorFaccountsforlossofhydroxynitratesduetowalllossandreactionwithOH.ThisfactorwascalculatedusingamodifiedequationforFdescribedbyAtkinson
etal.(1982),wherek7=kOH,alkene×[OH]average,andk10=kOH,HN×[OH]average+kw,where[OH]averageiscalculatedusingalkenedecay.4CorrectionfactorFtempaccountsforthe
changeinαinresponsetotemperaturevariationduringtheexperiment.ThisfactorwasestimatedusingthetemperaturedependenceofαreportedbyAreyetal.(2001).5Seetextfor
moredetailsonestimatesfornormalizationforthefractionofOH+alkenethatproceedsviaOHaddition,fa.
by Kwok and Atkinson, 1995). For this analysis, we as- primary allylic CH group of 0.8×10−12cm3molec−1s−1.
3
sumethatf is8%for1-buteneand,becauseH-abstraction Assuming vinylic hydrogen abstraction rates are negligible,
ab
from non-allylic CH groups is expected to be similar to we estimate f for cis-2-butene, methylpropene, 2-methyl
2 ab
CH groups in alkanes, we assume that the abstraction rate 1-butene,2-methyl2-butenetoallbelessthan5%.
2
increases according to the parameterization suggested by
KwokandAtkinson(1995).Thisimplies15%H-abstraction
3.4 Absoluteβ-hydroxynitrateyieldsandbranching
for1-hexene,and22%for1-octene.Usingaratesimilarto
ratios
thatreportedinAschmannandAtkinson(2008),theabstrac-
tionfraction(f )for1-octenewouldbe28%.
ab The absolute yield of hydroxy nitrates for several of the
To estimate abstraction rates for methyl-
alkenes was determined in Experiments 1–19. Longer reac-
substituted alkenes, we use an overall k of
OH tion times were necessary to quantify, with sufficient preci-
31.4×10−12cm3molec−1s−1 for 1-butene (Atkinson and
sion, the amount of alkene oxidized. The concentration of
Arey,2003),KwokandAtkinson’s(1995)suggestedk for
OH hydroxynitratesattheendoftheexperimentwasdetermined
a CH group of 0.14×10−12cm3molec−1s−1, and an 8%
3 bymeasuringthetotalGC-TD-LIFpeaksignalwhichcorre-
H-abstraction(f ).Weestimateak rateconstant
ab OH,abstraction sponded to a CIMS hydroxy nitrate signal. The initial con-
forasecondaryallylicCH groupof2.4×10−12.Usingan
2 centrationofalkenewasdeterminedbyFT-IRandGC-FID,
overall k of 26.3×10−12cm3molec−1s−1 for propene
OH and total loss was determined by the decay in peak area by
(AtkinsonandArey,2003)andanupperlimitof3%abstrac-
theGC-FID.InExperiments11–14,1,2butanediolwasalso
tion,wederiveanupperlimitk rateconstantofa
OH,abstraction added as a reference compound to allow the total loss of
Atmos.Chem.Phys.,15,4297–4316,2015 www.atmos-chem-phys.net/15/4297/2015/
A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes 31
A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes 29
A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes 4305
0.4
276m/z
0.2
0.35 248m/z
234m/z
0.3 220m/z
α) 0.15
( y = 0.045*N − 0.11 206m/z
atio 0.25 gnal 192m/z
R Si
g 0.2 n 0.1
n o
hi I
c 0.15
n
ra 0.05
B
0.1
30 A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes
0.05
0
8 10 12 14 16 18
0 Time (minutes)
2 4 6 8 10
N (number of heavy atoms) FFigiugruere46.6.AAchrcohmraotmogaratpohgicrapsehpiacratsioenpaorfatihoyndrooxyf hniy-droxy nitrates
trates formed from a set of alkenes in Experiment 19. 192
formedfromasetofalkenesinExperiment19.192m/z=ethene
FFigiugruer4e4.4T.hTehiesoimseorm-aveerr-aagvederbarganecdhinbgrarnatciohsi,nαg,rdaetriivoesd,iαn,derivedinthis m/z=ethene hydroxy nitrate, 206 m/z=propene hydroxy ni-
thisstudy(blueboxes,datafromTable44),comparedtoprevi- trhayted;r2o2x0ym/nzi=tramteet,hylp2r0op6enemhy/dzro=xypnriotrpaeten;e234hmyd/zro=xy2- nitrate; 220
study(blueboxes,datafromTable4),comparedtopreviouslypub-
ouslypublishednitratebranchingratios(Areyetal.,2001withpink mmet/hzyl=2-mbuetetnheylhpyrdoropxeynneitrathey;d2r4o8xmy/zn=it1r-ahteex;ene2h3y4droxmy/z=2-methyl
elxiessh;eOd’Bnriietnraettealb.,r1a9n9c8hwinithggrraeetinosstar(sA).rAelykeenetnailt.r,ate20yi0el1dswith pink exes; n2it-rabtue;te2n76emh/yzd=ro1x-oyctennietrhaytder;ox2y4n8itramte/.zIso=m1er-hdiesxtreibnuetionhsydroxy nitrate;
fferoOOrorrom’’HrBBO-wrraii’ebBeeisgnntrrhiaetceeentdttieotfiaantall..lcd.,h,e(a11r1ni99v9n9e9e98dl88hfi)nrawovmhtehiatebhFvesieegagn.mrb4neee4oefernaimnsshasslinhtiozaoonewrrdsamn)sb.fyatohAlrfiisazlhkeystotdedurndaobcyexc.yyonTunfihnite-atrattoe yacieclodusnftrofomr wme2dleua7ertteoit6oegnrrd.ammemItnei/srn.szmueTic=dnhheebdc1ala-asbosteeaecsrs,dteeGedlnoauuentoisnnsighinaiynnptetdepegargekorrasaakxttsiayinrsngehgnapppirpteeorasenakeawtkeeta.orreaetaIruassesioleaifdnmsrgoteofmarrdnoedcmdchocirsoonct---hrirboumtiaotnosgrwamerse.
trHat-eabbrasntrcahicntgiorantiocsh(balnacnkedloittnedtlhinee)sfarommealflamsehaisournedaaslkethneiss study. The er- vTolhuteelcaot-eerluteinlugtiisnogmepresaakndstahreetrpariloinngetatiolwtaaislianssgiganenddtocoth-eelution.Insuch
yrioelrdswaseloigpehtoefd0.0fi45t de0r.0iv1e6dandfriontmerceFptigo.f 40.1is1 sh0o.0w5n(er-for hydroxy ni- laterelutingpeak.
± − ± cases,Gaussianpeaksshapeswereusedtodeconvoluteco-eluting
rotrrastaereb2rσa)n.cThhiinsgfitraagtrieoess(wbellalcwkithdothteterdelaltiinones)hifprodmerivaeldlbmyeasuredalkenes
Areyetal.,2001,whocalculatedaslopeof0.0381 0.0016and isomersandthetrailingtailwasassignedtothelaterelutingpeak.
ayniienltedrsceaptsolfope0.0o7f30.004.0509±fo0r.n0-a1lk6anaensd. inte±rceptof−0.11±0.05(er-
rors are 2σ−). This±fit agrees well with the relationship derived by
Areyetal.(2001),whocalculatedaslopeof0.0381±0.0016and
first order wall loss rate constant. Rate coefficients for
aninterceptof−0.073±0.009forn-alkanes.
OH+hydroxy nitrates were estimated based on Treves and
Ruddich(2003).Firstorderwalllossrateconstantswerede-
termined by monitoring post-oxidation dark decay over at
leastanhourandfoundtobe≤10−5s−1 forallcompounds.
Asecondcorrectionfactor,F ,wasappliedtonormalize
temp
theyieldstoasingletemperature(T =293K)toaccountfor
thedependenceofthebranchingratioontemperature.F
temp
FFigiguruer4e5.5A.lAkolxkyoHx-syhiHft-issohmiefrtiziastioomnleeardiiznagttioodnihlyedarodxiynngi-todihydroxynitrate wasestimatedusingthetemperaturedependenceonbranch-
trateformationfor1-hexene.
formationfor1-hexene. ingratiodescribedinAreyetal.(2001).
Branching ratios to form β-hydroxy nitrates were calcu-
lated by normalizing the β-hydroxy nitrate by the fraction
alkenetobedeterminedindependentoftheGC-FID.1,2bu- ofOH+alkenereactionsestimatedtoproceedviaOHaddi-
tane diol was monitored using the CIMS at signal m/z 175 tion(α=Y/f ).SeeSect.3.4formoredetails(Table3).The
a
by CIMS to determine its decay over time. The ratio of the yieldsincreaselinearlywithsizeofthemolecule.
OHrateconstantsfor1,2butanedioltopropeneisestimated We find that the branching ratios can be expressed as
to be 1.1±0.1 from relative rate information from the lit- α=(0.045± 0.016)×N−(0.11± 0.05), where N is the
erature (Atkinson et al., 1982, 1986; Bethel et al., 2001). numberofheavyatomsintheperoxyradical(notincluding
Thealkenedecayinferredfromthe1,2butanedioldecaywas the peroxy radical oxygens). The α derived for 2-methyl 2-
found to match the GC-FID alkene decay within error. The butenewasnotincludedinthefit,asitwasfoundtobesignif-
nitrateyieldiscalculatedbydividingtheamountofhydroxy icantlylowerthan1-penteneand2-methyl1-butene.Prelim-
nitratesformedbytheamountofalkenesreacted. inary data from other experiments (not reported here) indi-
For each reaction, a secondary loss correction factor, catethatαforanotherinternalalkene,2,3-dimethyl2-butene,
F, was applied to account for losses of hydroxy nitrates isalsosubstantiallylowerthansimilarcarbonnumbercom-
by OH and wall loss using equations derived in Atkin- pound1-hexene.Itisunclearwhytheseinternalalkenesex-
son et al. (1982), and substituting k by k ×OH and k hibit significantly lower branching ratios to form alkyl ni-
7 7 10
by k ×OH +k , where k is the experimentally derived trates.
10 w w
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32 A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes
4306 A.P.Tengetal.:HydroxynitrateproductionintheOH-initiatedoxidationofalkenes
e suggeststhatdestabilizationoftheO–ONObondduetothe
en 0.45 presenceoftheβ-hydroxygroupislikelysmall.
p
o Deuteration also leads to an increased branching ratio to
r
P
formnitrates,possiblyduetoincreasedO–ONOlifetimere-
H 0.4
O sulting from the lower frequency vibrational and rotational
O
− modes. The nitrate branching ratio of d6-propene is a fac-
2
H, 0.35 tor of 1.5 times higher than h6-propene. A similar increase
O in nitrate branching ratio has been observed for deuterated
−
1 isoprene(Crounseetal.,2011).
of 0.3 Themeasuredbranchingratiostoformβ-hydroxynitrates
n
o (Table4)areconsistentwithTuazonetal.(1998)determina-
cti tionsoftotalalkylnitratesformedfrommethylpropene,cis-
a 0.25
Fr 10−2 10−1 100 2-butene, and 2-methyl,2-butene. For the experimental con-
Fraction of alkene OH reactivity ditionsinTuazonetal.(1998),however,formationofmethyl
nitratefromCH O+NO maybesignificant.AstheFT-IR
3 2
Figure47.Themeasuredisomerdistributionofpropenehydroxy
Figure 7. The measured isomer distribution of propene hydroxy nitratedeterminationincludesthesumofallRONO species,
hydroperoxides (blue boxes) as a function of the initial alkene 2
OhHydreraocptievirtoyxaniddethse(inbitliualehybdoroxgeesn)pearosxiadefcuonnccetnitorantioon.fThthee initial alkene these results represent an upper limit to the alkene-derived
rOedHlinreearceptirvesietnytsatnhde athlkeenieniOtiHalrehayctdivriotygerengimpeeroovxeirdwehcicohncentration.The
HNyield.Branchingratiosreportedherearealsoconsistent
hryeddroxliynheydrreopperreosxeidnetsisotmheerdailsktreibnuetioOnsHwerreearecptoivrteitdyforreagllime over which
alkenesotherthanpropene.Thedashedblacklinerepresentsaki- withthosereportedbyPatchenetal.(2007)for1-buteneand
hydroxy hydroperoxide isomer distributions were reported for all
neticboxmodelsimulationdesignedtostudythemaximumimpact 2-butene determined by CIMS and calibrated using synthe-
ARanpOlekpt2eeic+nndebRixsoOBxo2ftmcohhreeofrmudritetshhtleraysrnidmmepitgaurhiolltsaphoteainvonetnehoe.dnkTetihnhseeietgiiscndommeadsoehdrtedeoli.dsstrtbiubludatcyiokntshl.ieSnemeearexpimreusemntismapakci-t sizedstandards.
RO2+RO2 chemistrymighthaveontheisomerdistributions.See The measurements of the branching ratios reported here
AppendixBforfurtherdetailsonthekineticmodel. are significantly higher than those determined by O’Brien
et al. (1998) using gas chromatography with calibration us-
ingauthenticstandards.O’Brienetal.(1998)usedasimilar
3.5 Hydroxynitratebranchingratiosfromrelative GCseparationtechniquefollowedbythermaldissociationof
measurements alkylnitrateswithdetectionofNO2 bychemiluminescence.
The experimental conditions were quite different than the
Toplacetherelativenitrateyieldsofthealkenes(3.2)onan currentstudy.Initialalkeneandoxidantconcentrationswere
absolute basis, we scale the slope of the error weighted fit 2–3 orders of magnitude higher for most alkenes studied.
oftherelativebranchingratiostomatchthatoftheslopeof BasedonsimulationsoftheexperimentsreportedinO’Brien
theerrorweightedfitderivedfromtheobserveddependence etal.(1998),thehighinitialNOconcentrationsledtorapid
of the absolute branching ratios on N derived in Sect. 3.3. productionofcopiousamountsofNO ,which,uponUVillu-
2
Branchingratiostoformβ-hydroxynitratescalculatedusing minationformssignificantlevelsofO3Pforallexperiments
thismethodarelistedinTable4andshowninFig.4. withinitialNO concentrations>100ppmv.Oursimulations
x
Themethodofplacingtherelativebranchingratiosonan suggestthatsignificantalkenelossintheirstudywasdueto
absolute basis are prone to correlated errors if the two data oxidationbyO3P.Thisimpliesasignificantunderestimation
sets are not independent of each other. We believe the data ofthebranchingratiosintheO’Brienetal.(1998)studyfor
sets are sufficiently independent for the following reasons: allcompoundsderivedfromhighNO experiments.Seethe
x
(1) the absolute nitrate yields require a correction for OH, Supplementforfurtherdetailsonthisanalysis.
temperature,andwallloss,whereastherelativesetdoesnot,
(2) the determination of (cid:49)alkene relies on GC-FID for the 3.6 Nitrateyieldsfromalkoxyisomerization
absolutedatasetwhileonlyinitialreactantconcentrationand
literaturek rateconstantsfortherelativeset,(3)thedevi- Inadditiontotheβ-hydroxynitrates,dihydroxynitratesare
OH
ationsofthedatafromthebestfitrelationshipsarenotwell formed from β-hydroxy alkoxy radicals that are able to un-
correlated. The two data sets, however, are prone to similar dergo1,5H-shiftchemistry(Fig.5).TheCIMSsensitivities
biasesinthefollowingways:(1)initialalkeneconcentrations forthesenitratescouldnotbeobtainedbecausetheyhadlow
weremeasuredbyFT-IRandwiththesamereferencespec- transmissionthroughthegaschromatograph.Fromambient
trainbothsetsofexperiments,(2)sensitivitiesforβ-hydroxy sampling, the CIMS signal for the dihydroxy nitrates, rela-
nitratesandabsoluteβ-hydroxynitrateyieldsbothrelyupon tivetotheβ-hydroxynitrates,are1-butene,<2%,1-hexene,
GC-TD-LIFdata. >10%,1-octene,>5%.For1-butene,anupperlimitispro-
As shown in Fig. 4, the dependence of the hydroxy ni- videdduetothesmallamountformed.Forthedihydroxyni-
tratebranchingratiosfromβ-hydroxyperoxyradicalsonthe tratesfrom1-hexeneand1-octene,onlylowerlimitsarere-
numberofheavyatomsaresimilartothoseobservedforper- portedassignificantuptaketothewallsofthechamberwas
oxyradicalsderivedfromn-alkanes(Areyetal.,2001).This observed.
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