GEOPHYSICALRESEARCHLETTERS,VOL. 24, NO. 11,PAGES1375-1378,JUNE 1, 1997
Observationsof anthropogenicinputs of the isopreneoxidation products methyl vinyl ketone and methacrolein to the atmosphere Thomas A. Biesenthal andPaulB. Shepson • Departmentof ChemistryandCentrefor AtmosphericChemistry,York University,North York, Ontario,Canada
Abstract. The isopreneatmosphericoxidationproductsmethyl vinyl ketone(MVK) andmethacrolein(MACR), alongwith CO,
benefitfrom concurrentmeasurements of its oxidationproducts, since
useful
model
constraints
are
introduced
when
concentrations of both reactantand productspeciesare known. 1996. Thesecarbonylcompoundswere highly correlatedwith However, in order to properly interpret both ambient CO in the winter, indicatingthat they are producedin significant measurements and model resultsit is necessaryto correctly amountsby automotivesources. Regressionof the observed apportionthe sourcesof thesecompounds. MVK and MACR concentrations againstCO leadsto emission Thereis growingevidencein the literatureto supportthe idea factorsrelativeto CO of 1.4(+0.3)x10 '4 and 7.3(+1.6)x10 -5 that MVK and MACR are significantcomponentsof vehicle (mole/mole),respectively. Emissioninventoriesfor CO and exhaust. Laboratorystudiesindicatea small exhaustemission isopreneallow us to estimatethat, for Torontoin the summer,as sourcefor MACR (Kaiserand Siegl, 1994; Kaiseret al.., 1993), and both MACR and MVK have been observed as vehicle much as 48(+30)% of the MVK and 36(+23)% of the MACR input into the atmosphereis derived from mobile source emissioncomponentsand in wintertimeurban air samplesby by our emissions. This source also has a strong influence on the Jonssonet al. (1985). Furthermore,recentmeasurements a directsourceof MVK and MVK/MACR ratio, which in this environmentremainsrelatively researchgrouphave also suggested constant,with values typically ranging between 2.0-2.5. The MACR in the absence of local isoprene photochemistryin impactof this is that MVK and MACR mixing ratiosare not Vancouverin the summer(Biesenthalet al., 1997). Althoughit unambiguous indicatorsfor isoprenechemistry'and its impacton appearsclear that there is a significantmobile sourceinput for thesespecies,the magnitudeof thissourceis asyet unquantified. ozoneproductionfor urbanenvironments. In an effort to further our understandingof this issue, were measuredin Toronto, Ontario in the winter and summer of
measurements of MVK
and MACR
were made in the winter in
Toronto,when biogenicisopreneemissionsare inactive. Carbon monoxide was also monitored, as a useful automotive
Introduction
Vegetativeemissionshave gainedrecognitionfor their large impact on both global and local troposphericchemistry (Zimmermanet al., 1978; Fehsenfeldet al., 1992). Of the myriad of compoundsof biogenicorigin, isopreneis one of the most importantwith respectto ozoneproduction,and certainly one of the most studied. The importanceof isopreneto regional tropospheric chemistryliesin its highreactivityto OH andhence in its potentialfor ozone formation,which is amplifiedwhen isopreneemissionsare injected into urban air masseswith relativelyhigh NOx concentrations.The model simulationsof Chameideset al. (1988) for Atlanta have shown that natural hydrocarbons(modeled as isoprene)can be as important as anthropogenichydrocarbonswith regard to local urban area ozoneproduction,illustratingthe necessityof includingisoprene chemistryin any descriptionof the atmospheric chemistryof an urbanenvironmentandthe impactof urbanemissionson regional scaletropospheric chemistry. The OH-initiated oxidation of isoprenehas been found to yield methyl vinyl ketone(MVK), methacrolein(MACR), and formaldehydeas major products(Tuazon and Atkinson, 1990; Paulsonet al., 1992; Miyoshi et al., 1994). Theseproductsare quite reactivein their own right and can contributeto ozone productionas well as providea sourceof free radicalsthrough their photolysis. Model simulationsof isopreneoxidationalso
combustion
source tracer.
Summer-time
measurements of these
three specieswere also undertakenat the same site to enable evaluationof the impact of direct emissionswhen biogenic isoprenechemistrywasactive.
Experimental
Air sampleswereobtainedat the campusof York University, which is situatedon the northernedgeof metropolitanToronto and is thus significantly impacted by the urban area anthropogenicsources. Measurementswere made every 45 minutesin the late winter and early spring (16-22 and 27-29 March96) aswell asin the summer(6-14 August96). MVK and MACR were measured using an automated adsorbent-based preconcentration systemcoupleddirectlyto an HP5890 Series II Gas Chromatographequipped with an HP5972A Mass Selective Detector (Hewlett Packard). The preconcentrationsystemhas been describedin detail elsewhere (Biesenthalet al., 1997), but severalchangesweremadefor this study.Essentiallythe systemis a networkof threesix-portvalves (Valco) and stainlesssteel tubing, all maintainedat 160øC to minimize adsorptivelosses. Switching of the valves allowed eitherambientair, standardair, or zero air to be drawnthrough the adsorbenttrap. The incomingair was first passedthrougha trap containing100 mg crystallinepotassiumiodide to remove •Departments of Chemistry, andEarthandAtmospheric Sciences,ambient ozone, which is an interferencewith this technique. PurdueUniversity,West Lafayette,Indiana,USA MVK and MACR were found to be quantitativelytransmitted throughthis trap. The adsorbenttrap consistedof a bed of Tenax Copyright 1997bytheAmerican Geophysical Union. TA (150 mg) followed by Carboxen569 (100 rag) contained within a 5 cm length of ¬" fused silica-lined stainlesssteel Papernumber97GL01337. 0094-8534/97/97GL-01337505.00 tubing. During sampling,this trap was cooled to 5øC by 1375
1376
BIESENTHAL & SHEPSON:ANTHROPOGENICISOPRENEOXIDATION PRODUCTS
divertingwater from a recirculatingcoolantbath throughan aluminumblockwhichencased thetrap. Air wasdrawnthrough thistrapfor 5-10 minutes,resultingin a totalsamplevolumeof
0.3
MVK
0.2
0.75-3.5L, depending onthesampling flowratemeasured using a calibrated massflow controller.After sampling, thetrapwas heatedto 250øC for 10 minutes,and the thermallydesorbed species were carried under helium flow to the head of the
0.1
chromatographic column(Supel-Q-Plot,30 m x 0.32 mm i.d., Supelco). The massselectivedetectorwas operatedin scan mode,from28-200a.m.u.Quantitation of eachspecies wasbased uponthe chromatographic peakareaof a singlemassfragment, chosento optimizeselectivityandsensitivity. The systemwascalibratedseveraltimesdaily with a certified gas-phase standardof MVK andMACR (ScottSpecialtyGases). The standard wasdynamicallydilutedfrom~ 1 ppmvto 300-1000 pptv usingcalibratedmassflow controllers(Tylan) and wasthen sampled and analyzed in the same manner as for ambient samples. Blank samplesof ultra-zero air were also obtained regularly, and revealed no detectableamountsof MVK or MACR. The estimateduncertaintyof the MVK and MACR measurementsis -+15%; uncertaintyin the calibration standard concentrationswas by far the dominant contribution to the overall uncertainty. The detectionlimit of the instrumentwas 510 pptv, dependinguponthe samplevolumeused. Carbonmonoxidewas measuredusinga TECO Model 48 Gas Filter Correlation CO Analyzer (Thermo Electron Instruments). The instrumentwas zeroed for 5 minutesevery 50 minutesby drawingambientair througha trap of Pt-coatedaluminapellets heated to 250øC. Weekly multi-point (0.2 to 2.0 ppmv) calibrationsof the instrumentwere performed,usinga certified CO standardof 48.6 ppmv (Scott SpecialtyGases). The overall uncertaintyof the CO measurementsis estimatedto be _+10%, derivedlargely from uncertaintyin the calibrationstandardand variability in the instrumentzero. The variability in the zero resultedin a detectionlimit of 0.050 ppmvfor CO. The instruments shared a common inlet line of 75' of •" PFA
Teflon tubing leading from the laboratoryto the PFA-Teflonfilteredinlet on the buildingroof;the inletwasapprox-imately 15 m above ground level. The continuous-sampling CO analyzer servedto maintaina flow of 500 mL/min throughthe inlet line at all times. Experimentshaveshownthat thereareno lossesof the analytesof interestin this lengthof PFA tubing. Results
and Discussion
The time seriesof the winter measurements of MVK, MACR, and CO are shownin Figure 1. As shownin the Figure, we observedsubstantial concentrations of MVK andMACR during winter sampling,i.e. maximum concentrationsof 0.26 and 0.14 ppb, respectively. That MVK and MACR are well correlated with eachotherand with CO is evidentfrom inspectionof the data in Figure 1. The frequent nighttime maxima in the concentrations are likely a result of emissionsbeneaththe much shallowernocturnalboundarylayer. In addition,thereare peaks associatedwith the morningand eveningrushhours,which can be seento someextenton most days,with the exceptionof the weekendperiod(16-17 March). To investigatethe correlation, the carbonyl compound concentrations were plottedagainstCO concentrations as shown in Figure2, wherethe CO datawas averagedover the GC/MS sampling period of 5-10 minutes. In order to minimize the effectsof atmospheric oxidationof MVK andMACR by OH on the relationship,only samplestakenwhen CO was increasing
MACR
o.o --- - CO 1.2
0.8
0.4
0,0
16
I
I
I
I
I
17
18
19
20
21
•/
I
I
28
29
30
Day in March Figure 1. Time seriesof MVK, MACR, and carbonmonoxide
measurements duringthe latewinterandearlyspringin Toronto. The tick marksindicatemidnight,easternstandard time.
rapidly(i.e. whenthe sampledair had likely beenimpactedby the abundant vehicleemissionsources in the surrounding urban area)wereincluded.A linearleast-squares fit to thedatayielded
a slope of 1.4(+0.2)x10 -4moles MVK/mole COwithanr2value of 0.78,anda slopeof 7.3 (+0.9)x10 '5 molesMACR/mole CO withr2 of 0.83;theuncertainties in theslopes arethe95 % confidence limits.
The actual uncertainties in the determination
of the emission factors are +22%, given the measurement uncertainties for the componentanalytes. Jonssonet al. (1985) have previouslymade measurements of MVK
and MACR
in both vehicle exhaust and in urban air
samplesin Stockholm. Their measurements at a densetraffic site in autumnindicateda strongcorrelationof MVK, MACR, and CO with benzene, suggestinga common exhaust source.
Although they did not explicitly determineemissionratios relativeto CO, suchratioscanbe calculatedfromthe datathey presented. From their ambientmeasurementdata, we calculate emissionratiosfor MVK and MACR (relativeto CO) of 3.9 and
6.6x10 -5(m/m),respectively. Although ouremission factorfor MACR agreeswell with that observedby Jonssonet al., this is not the casefor MVK, for which their value is 3.5 times smaller
thanours. This differencemay reflectdifferencesin the emission standards of the automotive fleets or differences in the fuel used in Sweden at that time.
We assumethat our calculatedemissionratios closelyreflect the automotiveemissionratios. However, this assumesthat the observedconcentrations derive entirelyfrom automobileexhaust emissions.For MVK andMACR, this is likely to be the case,as they have been directly observedin automobileexhaust,and it
seemsunlikelythat otherfossilfuels usedin this region(natural gas and coal) resultin emissionsof thesegases. However,the CO emissionsinventory for the Toronto region estimatesthat 82% of the CO emissionsare derived from gasoline use. Therefore, the actual automotive emission factors may be
somewhat largerthan thosederivedfromour ambient measurements.Although it is conceivablethat there are other
BIESENTHAL
& SHEPSON: ANTHROPOGENIC
300
MAC . ß
•.
200
1377
Scatterplotsfor MVK and MACR versusCO are shown in 120 • 90
,--:, 150
PRODUCTS
of the compound which are independentof the tracer source.
150
250
ISOPRENE OXIDATION
,_:,
Figure3 for the entire summerdata set. Linear fits to the lower edgedefinedby the filled symbolsyield summeremissionratios
relative to COof 1.8(+0.3)x10-4and1.2(+0.2)x10-4forMVK
• lOO
60 30
50
0.0
0.4
0.8
1.2
[CO], ppmv
1.6 0.0
0.4
0.8
1.2
1.6
[CO], ppmv
Figure2. Regressions of MVK andMACR against CO during periodsof increasing CO concentration. influences on the MVK and MACR concentrations,this seems
unlikely. Isopreneconcentrations in thisareaduringpastwinters were typically< 100 pptv (McLaren et al., 1996), and basedon this amount one cannot account for the observed increases of
MVK and MACR throughisopreneoxidation,duringperiodsof rapid CO increase.Consideringthat the majorityof the samples used for Figure 2 were obtainedat night, and given isoprene's relatively long lifetime with respectto the ozonereaction(56 h for 30 ppbv 03 at 5øC), isoprenereactionwith OH or 03 is not expectedto be importantin thesecases. Althoughreactionof isoprenewith the NO3 radicalat night can be fast, the yield of MVK and MACR from NO3 reactionwith isopreneis only 3.3% each(Kwok et al., 1996), suggesting that 1-5 ppbv of isoprene would have to be consumed to account for all of the observed
and MACR respectively,which agree reasonablywell with the winter emissionratiosas shownin the Figure. This treatmentis subjectto perturbationsdue to other sourcesand sinksof MVK andMACR. It is notable,however,that the observedlower edge is comprisedentirely of night and early morning sampledata, when photochemicalproductionand destructionof MVK and MACR is suppressed. The relativeimportanceof the automotivesourceof MVK and MACR can be estimatedthroughthe useof an isopreneemission inventory,and the fact that isoprenewill be rapidly convertedto products,includingMVK and MACR, in the summermonths. The programCANBEIS2 (similarto BEIS2, an upgradedversion of the Biogenic Emissions Inventory System, as described originallyby PierceandWaldruff, 1991) hasbeenusedto obtain an estimatedisopreneemissioninventoryfor an oval region (25 km x 57 km) which encompassesmetropolitan Toronto (PollutionData Branch,EnvironmentCanada). For this area,the
estimated total isopreneemission flux is 2.00x106moles, integratedover the period June, July and August (for 1990). Over this sameperiod and area, it is estimated(Pollution Data Branch, Environment Canada) that the total CO emitted from
gasoline-powered carsandlight-tracks is3.95xl09moles.If we
then multiply by our emission factors, we obtain total increaseof its oxidationproductson manynights. Additionally, anthropogenic emissionsof MVK andMACR for this regionand NO3 in this urbanenvironmentwould tend to be suppressed by period of 5.5xl05and2.9xl05moles, respectively. These values its rapidreactionwith NO, especiallyfor conditionsof increasing canthenbe comparedto thoseproducedthroughthe atmospheric CO, where [NO] would be expectedto be high. It thus seems oxidation of the naturally emitted isoprene. The dominantloss clear that for this data set, direct emissionis the only plausible processfor isoprenein the atmosphereis its reactionwith OH, source of MVK and MACR. leadingto an isoprenelifetimeof 2.8 hours(for a typical24 hour Apart from othersourcesof MVK andMACR, lossesbetween avg.[OH]=I x 106).Duringdaytime, whenisoprene is emitted, emissionfrom vehiclesand the time of samplingwould also [OH] will be significantlylarger,with a correspondingly smaller affectthe determinedemissionratios. Reactionsof 03 and NO 3 isoprenelifetime. Thus we can assumethat isopreneemitted with MVK and MACR at nightwouldhavelittle impactbecause within the definedoval Torontoregionwill be largelyconverted they proceedtoo slowly, but dry depositionis a potentialsink. to productswithin that sameregion for typical wind speedsof The depositionvelocitiesof these compoundshave not been 10-15 km/hr. Isoprenereactionwith OH producesMVK and measured, but it is noteworthy thatan estimate of 0.2 cm s'• MACR in yields of 32 and 23 percent(Tuazon and Atkinson, would lead to a lifetime of only 14 hoursin a 100 m nocturnal 1990; Miyoshi et al., 1994). If we applytheseMVK and MACR boundarylayer and thus could constitutea significanteffect on yields to the biogenicisopreneemissionsfor the Toronto area the determinedemissionratios. Becauseof this potentialsink for over the June-August1990 period (ignoringthe much smaller MVK and MACR, the emissionratios could be a lower limit. However, it shouldbe noted that when the entire winter data set (i.e. whenCO is moreslowlyvarying,or evendecreasing) is used for the regressions, the slopesobtainedare nearly identicalto MACR • • MVK ø o o 0.6 o v v 0.3 thoseof Figure2. This supports thebeliefthatvehicleexhaustis o o o the predominantwinter-timesourceof these compoundsand o o ¸ other sourcesand sinks have relatively little effect on our vv vv r• determined emission ratios.
Determiningthe vehicleemissionratiosof MVK and MACR to CO from summer-time measurements is difficult
0.2
v •vv•vvv• 0.2 •-•
•%'•E•o ½o ø OøooI
because of
theirrapidproductionfromisopreneoxidationduringthe day. A simpleprocedurecalledGRACE (GraphicalRatio Analysisfor CompositionEstimates,Henryet al., 1994) wasusedto estimate the emissionratios using the summerdata set. The method involvesmaking a scatterplotof a given compoundagainsta tracerspecies.Suchplotsoftenexhibita distinctlower edgeto the scatter,and that loweredgecanbe usedas an estimateof the ratio of the compoundto the tracerspeciesin the tracerspecies' source.The scatterabovethe lower edgeindicatesfurtherinputs
,
0.0 0.0
0.2
0.4
0.6
[CO], ppmv
0.8
1.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
[CO], ppmv
Figure 3. Scatterplotsof summerMVK and MACR concentrations againstCO. The filled symbolsshowthe loweredges of the scatterplots, and the solid linesare the linear fits to those data, which define the approximate emission ratios in the summer. ratios.
The dotted lines are the determined winter emission
1378
BIESENTHAL & SHEPSON: ANTHROPOGENIC ISOPRENE OXIDATION PRODUCTS
productionof MVK and MACR from isopreneozonolysis),we can estimatethe effective biogenicallyproducedMVK and
products, and ozone, in the Lower Fraser Valley, BC, Atrnos. Environ., in press,1997. W. L., R. W. Lindsay,J. Richardson,and C. S. Kiang, The MACRforthisregion;specifically, weobtainvalues of 6.4x105 Chameides, role of biogenichydrocarbons in urbanphotochemical smog:Atlanta molesMVK, and4.6x105 molesMACR. Usingtheestimated as a casestudy,Science,241, 1473-1475, 1988. automotiveemissionsof thesetwo productscitedabove,we then Fehsenfeld, F., J. Calvert, R. Fall, P. Goldan, A. B. Guenther, C. N. estimate that the automotive source of MVK and MACR Hewitt, B. Lamb, S. Liu, M. Trainer, H. Westberg,P. Zimmerman,
contributesapproximately46(+29)% and 39(+25)% to the total atmosphericinput of these two compoundsinto this urban atmospheric environment in the summer months. The uncertaintiesare mainly due to the emissionestimates,which we considerto be accuratewithin a factor of two. Althoughthe calculatedvaluesare quite uncertain,it is clearthat evenduring summer,the automotivesourceis significantin the input of MVK and MACR to this environment,and that use of MVK and
MACR measurement data as a measureof isoprenechemistryin such an environment
should be done with this in mind.
Emissions of volatile organiccompounds from vegetationand the implications for atmospheric chemistry, GlobalBiogeochern. Cycles, 6, 389-430, 1992.
Henry, R. C., C. W. Lewis, and J. F. Collins, Vehicle-related hydrocarbon source compositions from ambient data: the GRACE/SAFERmethod,Environ.Sci. Technol.,28, 823-832, 1994. Jonsson,A., K. A. Persson,and V. Grigoriadis, Measurementsof some low molecular-weight oxygenated, aromatic, and chlorinated
hydrocarbons in ambient air and in vehicle emissions,Environ. International, ! l, 383-392, 1985.
Kaiser,E.W., W.O. Siegl, D.F. Cotton,and R.W. Anderson,Effect of fuel structureon emissions from a spark-ignited engine.3. Olefinic fuels,Environ. Sci. Technol.,27, 1440-1447, 1993.
Vehicle emissionsin the summerwill have a much greater Kaiser,E.W. and W.O. Siegl, High resolutiongas chromatographic influenceuponnight-timeMVK and MACR levelscomparedto determination of theatmospheric reactivityof engine-out hydrocarbon emissions froma spark-ignited engine,J. High Res.Chrorn.,17, 264day-timelevels,when isopreneoxidationwill dominate. One of 270, 1994. the ways this influence will manifest itself is in the ratio of
Kwok, E. S.C., S. M. Aschmann,J. Arey, and R. Atkinson, Product formationfromthereactionof theNO3 radicalwith isoprene andrate measureof isoprenechemistryin rural environments,as it is constants for the reactions of methacrolein and methylvinyl ketone driven toward a daytime maximum of-2 and a night-time with theNO3 radical, Int. J. Chern.Kin., 28, 925-934, 1996. minimumof-1 by the diurnalchangesin the ratesof OH and03- McLaren,R., D. L. Singleton,J. Y. K. Lai, B. Khouw,E. Singer,Z. Wu, and H. Niki, Analysisof motorvehiclesourcesand their contribution induced oxidation of isoprene and its oxidation products to ambienthydrocarbon distributions at urbansitesin Torontoduring (Montzka et al., 1992; Yokouchi, 1994). However, in an urban the SouthernOntario OxidantsStudy, Atrnos.Environ., $0, 2219MVK/MACR.
This ratio has been shown to be useful as a
environment,emissionsof MVK and MACR (in the ratio of 1.9 2232, 1996. determinedhere) into the shallow nocturnal boundary layer Miyoshi, A., S. Hatakeyama,and N. Washida, OH radical-initiated photooxidation of isoprene: An estimateof globalCO production,J. would have the effect of sustainingthe ratio at an elevatedlevel. Geophys.Res.,99, 18779-18787, 1994. This has been observedpreviously for Vancouver, where the Montzka, S. A., M. Trainer, P. D. Goldan,W. C. Kuster,and F. C. MVK/MACR ratio in summer remains typically near 2.0 Fehsenfeld, Isopreneand its oxidationproducts,methylvinyl ketone throughoutthe day (Biesenthalet al., 1997). For our summer and methacrolein, in the rural troposphe:e,J. Geophys. Res.,98D, 1101-1111, 1993. data in Toronto,the MVK/MACR ratio variestypicallybetween Paulson,S. E., R .C. Flagan, and J. H. Seinfeld, Atmospheric 2.0-2.5 throughoutthe day. photooxidation of isoprenePart I: The hydroxylradicaland ground stateatomicoxygenreactions,Int. J. Chern.Kinet., 24, 79-101, 1992.
Conclusions
Pierce,T. andP. Waldruff,PC-BEIS:a personal computer versionof the
BiogenicEmissionsInventorySystem,J. Air WasteMan. Assoc.,41, Isoprene'smajor oxidationproductsMVK and MACR have 937-941, 1991. been measured in a urban environment in the winter, when Tuazon, E. C., and R. Atkinson,A productstudy of the gas-phase biogenic isopreneemissionsart' excludedas a source. Their reactionof isoprenewith the OH radicalin thepresence of NOx,Int. J. Chem.Kinet., 22, 1221- 1236, 1990. concentrations were found to be correlatedwith CO, indicating Yokouchi,Y., Seasonalanddiurnalvariationof isopreneandits reaction that their source is automobile exhaust. We found that even for
summer,the automotivesourceis a significantcontributorto the total input of MVK and MACR to the atmosphere. Thus it should be clear that use of MVK
and MACR
as indicators of
isoprenechemistryin an urbanenvironmentis problematic.We
productsin a semi-ruralarea,Atrnos.Environ.,28, 2651-2658, 1994. Zimmerman,P. R., R. B. Chatfield,J. Fishman,P. J. Crutzen,and P. L. Hanst, Estimateson the productionof CO and H2 from the oxidation
of hydrocarbon emissions from vegetation,Geophys.Res.Lett., 5, 679-682, 1978.
note, however, that the automotive source would not be
importanton a globalor evenregionalbasis,in the summer.
T. A. Biesenthal, York University,Centrefor Atmospheric Chemistry, NorthYork, Ontario,CanadaM3J 1P3(
[email protected]) P. B. Shepson,PurdueUniversity,Depts.of ChemistryandEarthand Acknowledgments.We thankthe AtmosphericEnvironmentService Sciences,1393 BrownBldg.,WestLafayette,Indiana,USA of EnvironmentCanada and the Natural Sciencesand Engineering Atmospheric Research Councilof Canadafor theirsupport of thisresearch, andRobert (
[email protected]) McLaren for helpful discussions and assistance.
References
Biesenthal, T. A., Q. Wu,P. B. Shepson, H. A. Wiebe,K.G.Anlauf,and G. I. Mackay,A studyof relationships betweenisoprene,its oxidation
(Received February 28, 1997;revised April24, 1997; acceptedApril 30, 1997.)
