The reactions of ozone with alkenes: An ... - Wiley Online Library

GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 25, PAGES 3727-3730, DECEMBER 15, 1996

The reactionsof ozonewith alkenes:An importantsourceof HOx in the boundary layer Suzanne E. Paulson Departmentof AtmosphericSciences, Universityof California,LosAngeles,LosAngeles,CA

John J. Orlando AtmosphericChemistryDivision,NationalCenterfor AtmosphericResearch,Boulder,CO Abstract.

The reactions

of ozone with

alkenes have been

At night, whenNO levelsare suppressed, the efficiencyof HOx cyclingis reduced. Other primary sourcesof HOx radicals have also been

shownrecentlyto leadto the directproductionof OH radicals. Organicperoxyradicals(RO2)probablyaccompany the productionof OH. In thispaper,we draw attentionto the potentialimportanceof thesereactionsin the primary productionof HOx (HOx = OH, HO2 and RO2) radicalsin variousregionsof the boundarylayer. The reactionsof ozone with anthropogenicalkenesare shown to be the most important sourceof HOx in many urban settings during the day and evening,and a significantsourceat night. The majorityof HO x comesfrom tracequantitiesof alkeneswith internal double bonds. Reactionof 03 with isopreneand terpenescanbe an importantsourceof HOx in forestedregions;we show that thesereactionsare the dominant

radical source in the late afternoon

identified. Photolysisof carbonyl species,particularly formaldehyde, leadsto netproduction of HOx: CH20 + hv --> HCO + H (+ 2 02) --> HO2+ HO2+CO (11a) --> CO + H 2

(1 lb)

Nighttimeproduction of HOx radicalsis usuallythoughtto be drivenby NO3 chemistry,throughits reactionswith aldehydes andalkenes;for example:

NO3 +CH20

-•

HCO+ HNO3

(12)

and into the

O2NO-CH2CH202 (13) night. This additionalHOx sourceis expectedto increase NO3+CH2----CH2(+O2)--> predictedOH concentrations comparedto thosecalculated Total radical yields from the NO3 reactionswith alkenesare by modelsthat do not includethe O3-alkenesource. uncertain dueto an incomplete understanding of thechemistry of thenitratoalkylperoxy radicalsgenerated in reaction(13). Lessattentionhasbeenpaidto the contribution of O3-alkene reactions in thegeneration of HOx, although thesereactions occur The hydroxyl radical has been shownto be a key speciesin during both day and night. Small amounts of radicals attributed initiatingthe oxidationof a varietyof speciesin the atmosphere. to the reactions of ozone with ethene and a few other terminal The dominantproductionof OH in the daytime is generally believed tobefromO3photolysis, followed byreaction of0( 1D) alkenes,havebeenincludedin manymodels(e.g.,McKeenet al., Introduction

1991;Gery et al., 1989;MadronichandCalvert,1990). O3

with water:

reactionswith larger alkenesare very complexand the overall

03 +hv (< 320nm) O(1D) + H 20

--> -->

O(1D)+O2 OH+ OH

(1) (2)

Over mostcontinentalregions(whereNO > 20 ppt), subsequent reactionsof OH with hydrocarbons in the troposphere leadsprimarily to the interconversion of OH with HO2 andRO2 radicals, withno netlossor gainof totalHOx (HOx = OH + RO2 + HO2): OH + RH

-->

R + H20

(3)

R + 02 + M

-->

RO2+ M

(4)

RO2 + NO

-->

RO + NO2

(5)

RO + O 2

-->

R'= O + HO2

(6)

HO2 + NO

-->

OH + NO 2

(7)

mechanismis still uncertain,however it is now clear that the rad-

ical yieldsfromthesereactions arequitehigh,andin mostcases lead to the directproductionof OH (Tab. 1). This sourceis sev-

eraltimeslargerthantheradicalproduction fromO 3 withsmall alkenes(below). A few indications of the importance of these reactionshaveappeared in theliterature.Hu andStedman(1995)

notedthe importance of the reactionof 0 3 withanthropogenic alkenesin explainingobservedlevelsof RO2 in downtown Denver. MadronichandCalvert(1990) suggested the potential importanceof radicalformationfrom the reactionof biogenic alkenes withO 3 duringnighttime in theAmazonboundary layer. Carter(1995) hasshownthattheadditionof HOx production to theO3-alkenemechanisms significantly improves theagreement Table 1. Measured OH yields from the reaction of ozonewith

Destructionof HOx occursthroughreactionsamongthemselves, or reactionwith NOx, e.g.:

OH + NO2 + M

-->

HNO3 + M

(8)

HO2 + HO2 + M

-•

H202 +0 2 + M

(9)

RO2 + HO2

-->

ROOH + 0 2

several alkenes. A!kene

OH Y!d.*

Ref.

ethene

0.12, 0.18,

a, b, 2,3-dimethyl-2 1.0, 0.7

0.2

propene 1-butene cis-2-butene

(10)

0.33,0.37 0.41,0.3 0.41

c

A!kene

OH Y!d.

a, d

butene

a, c isoprene a, c c•-pinene a

Ref.

limonene

trans-2-butene 0.64,0.56 a,e A3.carene 2-methyl propene 0.84 a [I-pinene

0.27 0.83

a f

0.86

f

1.06 0.35

f f

Papernumber96GL03477.

a: (Atkinson and Aschmann, 1993), b: (Thomaset al., 1993), c: (Paulsonet al., Pers.Comm., 1996), d: (Niki et al., 1987), e: (Horie and Moongat, 1991), f: (Atkinsonet al., 1992b). *Uncertaintiesfor OH yield measurements exceptthosein boldare

0094-8534/96/96GL-03477505.00

a factor of 1.5 or more. Those in bold are _+25% or better..

Copyright1996 by the AmericanGeophysical Union.

3727

3728

PAULSON

AND ORLANDO:

REACTIONS

betweenmodelsand chamberexperimentsinvestigatingthe reactivity of propene,2-methylpropene, and trans-2-butene. In this paper,we use measuredambientdatato calculatethe rateof production of H Ox form variousphotochemical sources at two sites--one urban (Los Angeles) and one rural (western Alabama). Thesecalculations clearlyhighlightthe importanceof

OF OZONE WITH ALKENES

Rate constants for the reactionsof NO 3 with alkeneswere obtainedfrom recentreviews(Wayne et al., 1991; Atkinson,1994). Radicalformationfrom the NO3 reactionswith alkenesis highly uncertain. Several models have assumedRO2 yields of zero (e.g., McKeen et al., 1991), howeverrecentproductstudiesindicatethat RO2 yields may be closerto 0.8 or 0.9 (e.g., Paulson, 1996). To be certainthat we have not underestimatedthis source,

thealkene-O 3reactions assources of tropospheric HO x.

Mechanism of the Reaction of 03 With Alkenes

RO2 yieldsof 1.0 havebeenassumed for thesecalculations. For the reactionsof 03 with alkenes,the radicalyield was assumedto be doublethe OH yield, consistent with reactions(16-

17). It shouldbe stressedthat the full details of the mechanism have not been proven. If OH is somehowproducedwithout concomitantRO2, then the radical yields from 03 alkene reactionswill be half thoseassumed. However, if the proposed mechanismis correct,then the total radical yield is likely greater CH2-----CH2+ 03 --> H2CO + [H2COO]$ (14) than twice the OH yield, sinceadditionalRO2 radicalsprobably The Criegeeintermediateis likely producedwith considerable form in other reactionsof the Criegeeintermediates,even when OH is notproduced.To derivethe HOx productionratefrom O 3 internalenergyanddecomposes via a numberof pathways,or is reactionswith alkenes,the radicalyields(i.e. twice the OH yield) thermalizedby collisionswith the surrounding gas. In the caseof fromtheliteraethene,H atom, HCO and OH are producedwith a total radical weremultipliedby the 03 reactionrateconstants ture (corrected for temperature using an average activation yield of approximately0.45. There is a growing body of evidencethat OH is produced energyof EA/R=2000 K) (Atkinson,1994), and literaturevalues directly in high yield from 03 reactionswith larger alkenes, for ambient0 3 and alkene concentrations.Contributionsof carbonyl products to radical formation from these and other althoughthe chemistryinvolvedis probablydifferentthanthatof reactions is accounted for by using measured carbonyl CH2OO. The OH formationyield is dependenton the structure concentrations. Since many hydrocarbonoxidation reactions of the alkene (see Tab. 1), and has been postulatedto occurvia the formation of a vibrationally excited unsaturated producealdehydesandketones,we havenot attemptedto ascribe thisrelativelysmallHOx sourceto particularreactions. hydroperoxide(Niki et al., 1987). For 2-butene:

The reactionof 03 with alkenesis believedto occurvia the formationof a five-memberedring intermediatethat decomposes to producea carbonylcompoundandthe so-called"Criegeeintermediate"(e.g.,Finlayson-Pitts andPitts,1986). For ethene:

CH3HC-----CHCH 3 + 03

H

I-I,tI-I . •

k••O H •

H•

.

--> CH3CHO+CH3HCOO*

.•

'C• CH2 OOH

(15)

.72+ OH H•C XXO

(•6)

HOx Production in Urban Air Los Angeles was chosenas an urban case studybecausea fairly complete,averageddata setof speciatedhydrocarbons and oxygenatedorganics(VOC' s) hasbeendeveloped(Lurmannand Main, 1992). In additionseveraldaysof simultaneous total nonmethanehydrocarbons(NMHC), 0 3 and NOx measurements

The co-productof OH formedthroughthismechanism is an alkyl radicalthatrapidlygenerates HO2: 30

2

.O

7H2

.07H2

O 02 H•Cxx+ • H•xx -• H•Cxx O+02• O

NO2

ßI-

+ H2CO

I

-&-03Photol.

HO 2+CO

I

I

I

ß-•-- CH20Photol./•'. X •h 25 ß-.&--OtherCar!• • • Total Photol. [1 '• • • (17) - 20 bonyl ¸

If NO concentrations are sufficientlylow, the RO2 radicalwill react with HO 2 (reaction10), resultingin an organicperoxide thatis eventuallyphotolyzedto producetheRO radical,or it will reactwith otherRO 2 radicalsto producea mixtureof RO plus alcoholsandcarbonyls(Lightfoot,et al., 1992).

HOx Production Rates HOx productionratesfrom varioussourcesfor an urbansite (Los Angeles)and a rural site (Southeastern USA) were calculatedusingavailableNO x, volatileorganiccarbon(VOC) andO 3 mixing ratio data. Radical productionfrom 03 photolysiswas estimatedby multiplying[03 ] by the photolysisrate (J1) for re-

action(1), andthefraction(f) of O( 1D) produced thatreactwith H20 beforebeing collisionallyquenched.For simplicity,90% humidityat 300-310 K wasassumed,yieldingf= 0.2. This relative humidityis higherthananticipatedfor Los Angeles,but provides a conservativeestimateof the relative strengthof the HOx sourceform O3-alkenereactions.J1 wascalculatedfrom quantum yield and crosssectiondata (DeMote et al., 1994) and solar flux values for 34 øN (Finlayson-Pittsand Pitts, 1986). HOx from photolysisof formaldehydeandothercarbonylswascalculated by doublingthe productof the aldehydeconcentration and J value for each radical forming pathway (e.g., reaction 1l a) (Atkinson et al., 1992b; DeMote et al., 1994; Martinez et al., 1992; RaberandMoortgat, 1996).

'" Alkene-O3 l/ •'•

--

-•-HONO Photol][ / X•

'

ß... ,•

15

¸

/, /'

50

•_

40

•

30

,

=

10

•

5

20

•

•0 •

¸

--"'---.-.%:_.-'7_'_: 0

0

5

10

15

20

Time (Hour, PST))

Figure 1. HOx formationratesfor an averageof two moderately pollutedLos Angelesstations,usingan averagespeciated mix for VOC' s. HourlyNMHC, 03 , andNO x measurements for central Los Angelesand ResedaduringAugust26-29, 1987 were averaged. The maximumNMHC and 03 valuesfor this data set, 1 ppmC at 6:00 AM and 112 ppb at 2:00 PM respectively,are fairly representativevalues for the Los Angeles Air Basin in summereven today (Mackay, 1994). Estimatesof NO 3 and HONO were madeby averagingtime resolvedNO 3 and HONO measurements collectedat Claremont(also in the Los Angeles Air Basin) for the summersof 1987 and 1993 (Winer et al., 1989;

Mackay, 1994). It shouldbe noted that while HONO is frequently undetectable,it is sometimesobservedat 40 times the peakconcentrations usedhere(Winer andBiermann,1994).

PAULSON AND ORLANDO: REACTIONS OF OZONE WITH ALKENES other

alkenes

propene

4%

ethene 8•

It is worth notingthat the ozonereactionswith alkenesmay be the unidentifiedadditionalsourceof HOx implicatedby Tonnesenß and Jeffries (1994) in their recent studiesof low NO x and VOC/NOx ratio smog chamberexperiments.If such a radical sourceis attributedto reactivityof the chamberwall, thenthe resultingphotochemicalmodel will have significantlyunderestimated reactivity when applied to urban air. Further, since the majority of aromaticphotooxidationproductsare alkenes(e.g., Atkinson,1994), O3-alkenereactionsmay alsoaccountfor part of the unexplainedreactivityof aromatics(Carter,1995).

trans-2-

cis-2-2%

butene /

3729

pentene

:.......................•......•••• / "-'"""""'•"'""'"•'•'•:':'"••:•••:•••••:• .................. 34%

6%

trans-2-•:•

'-"'•':" ':•':'••••ii :•::' '

HOx Production at a Rural Site '"i:"'¾':"'"""'"'"•i!! The Rural Oxidants in the SouthernEnvironment (ROSE) experiment was conducted at a forested site in west-central

Alabama.Production ratesof HO x radicalsfor thesiteareshown

inFig.3,based oncalculated J (03)andNO3, andmeasured NOx cis -2-

2-meth¾]-2-

pentene

pentene

12%'

3-methyl-2pentene

15%

2%

and organics(Canttell et al., 1992; Goldan et al., 1995). The mostimportantbiogenicspeciesin termsof radicalproduction areisoprene(6.3 ppbaveragedaytimemixingratio)and cz-pinene (0.3 ppb), with approximatelyequalcontributions from each. At midday,03 photolysisis the dominantHOx source,but the 0 3-

alkenereactions account fora non-negligible fraction (10-15%) Figure 2. HOx formationrates for the reactionsof 03 with alkenesby typefor thehydrocarbon mix assumed in Fig. 1.

were available(B. Croes,pers.comm., 1996). Somemeasure-

ments of HONOandNO3 havealsobeenreported (Wineret al., 1989;Mackay, 1994). The speciation of the VOC mixturewas assumedto remainthe samethroughoutday andnight,andthe totalis scaledto the hourlyNMHC data. Figure1 showsthe diurnalvariationof radicalsources in Los Angeles. In the early morning,beforeO3 builds up (or is transported downfromaloft),photolysis of carbonyls andHONO dominateradical production.After about8:00 AM the reaction of O3 with alkenesis the largestsingleHOx' source,andit is

especially significant afterabout4:00PM whentheO3 levelsare still highandphotolysis hasslowed.This is consistent with the explanation of Hu and Stedman(1995) for the observedROx levelsin Denver. Duringthe night,NO3 reactionswith alkenes arethe dominantHOx source.However,the importanceof this sourcemay have beensignificantlyoverestimated, sincerapid interconversion between RO2, HO2 andOH doesnottakeplace without NO, and very low levels of NO are often observedto accompany highervaluesof [NO3] (Mackay,1994).

Figure2 showsa detailedbreakdown of thecontributions to HOx productionfrom the alkenemix assumed for Fig. 1. The

of the total radical production. Over a full diurnal cycle, 03alkenereactionsaccountfor 20-25% of totalHO x. As is the case for the urban situation((Hu and Stealman,1995) and above), the importanceof the O 3-alkenereactionsis particularlylargein the late afternoonandearlyeveningdueto theenhanced O 3 levelsat thesetimes. Further,RO 2 measurements (Canttellet al., 1992)in the 6:00 PM to midnightperiodwere generallyhigherthanthose calculated from a model that included only NO3-initiated chemistry. It is likely that most of the observedRO2 radicals duringthistime periodoriginatedfrom the O 3-initiatedoxidation of thebiogenicspecies,ratherthanfromNO3 chemistry.

Thedirectproductioh OfOH in thereaction of thebiogenic specieswith 03 may be of particularsignificancein nighttime chemistry,since[NO] may not alwaysbe high enoughto allow efficient cycling of RO2 and HO2 to OH (measurednighttime [NO] were usually 10-20 ppt (Goldan et al., 1995)). A simple steady-state analysis,assuming[NO] = 0, indicatesthatthelower

limitof [OH]is (1-3)x 105moleccm-3in the7 PM tomidnight time periodat the ROSE site. Goldanet al. observednighttime lossof isoprenewith a time constantof 4.4 hrs.,whichcouldnot be explainedfrom reactionwith NO 3 ([NO3] = 0.2 ppt,x = 100

hrs.),or03 (x = 20 hrs.).However, with[OH]= 2 x 105molec crn-3 chemical lossof isoprene (mostly duetoreaction withOH) would account for about half the isoprene loss observed by Goldan et al. (1995).

mostactivealkenesincludeseveralspecies thatareobserved only at low concentrations--in particularthe internalalkenes.For example,thetop4 contributors are trans-2-butene, 2-methyl-2-pentene•and c/s-and trans-2-pentene.Theseaccountfor 77% of the HOx from alkenesyet areonly 5% of the total alkenes,andless than2% of the total VOC's (on a carbonbasis). Our calculations of HOx productionfor othercitiesshowthatinternalalkenesare againthe dominantcontributors.Our analysissuggests that for both the U.S. EPA 29 city averagespeciatedhydrocarbonmix

-&- O3Photol. •- Alkene-O 3 ß-1-.CH20Photol. •- Alkenes-NO 3

Total

20 • 15

.

(Jeffries, 1995) and the mix observedin London (Blake et al., 1993), internalalkenesaccountfor 80% or moreof the HOx pro-

lO

•

ductionfrom O3 reactionswith alkenes. The EPA data set is uniquein includingalkeneswith 6-8 carbons,and thesealone contribute25% of the source,indicatingthattracespeciesthatare not routinelymeasuredcan be significantcontributors.By contrast,etheneand propene,which are commonlyincludedin the chemistrymoduleof airshedmodels,accountfor lessthan 10% of the new HO x arising from O3-alkenereactions. Radical formation from internal alkenes is high (Tab. 1), but more importantly, internalalkenesreactmorerapidly(by a factorof 10 or more)with O3 thando terminalalkenes.

5

•

0 0

5

10

15

z.

20

Time (Hour, EST)

Figure 3. Sourcesof new HOx for a rural forestedsite in the SoutheasternUSA. Productionrates of HOx radicals were calculatedusing publisheddata for the HOx precursorspecies (e.g., Cantrellet al., 1992; Goldanet al., 1995).

3730

PAULSON AND ORLANDO: REACTIONS

HO x productionfrom reactionof ozonewith biogenicspecies is not uniqueto the ROSE measurementsite.Levels of biogenic speciesmade at other forestedsites--e.g.,Brazil (Greenbergand Zimmerman, 1984) and Pennsylvania(Trainer et al., 1991)--are comparableto thoseat the ROSE site and HOx productionfrom ozone reactionswith the biogenicsis likely to be importantat thesesitesalso. Finally, it is worth notingthat certainterpenes (e.g., {x-terpinene,d-limonene) as well as sesquiterpenes (Shu and Atkinson, 1994) react much more rapidly with 0 3 than apineneor isoprene. Hence, even ppt levels of thesespecies(as are often observed,e.g., Greenbergand Zimmerman,1984) can imply large HOx radical production rates, and the overall contributionof the reactionsof ozonewith biogenicspeciesmay be largerthanestimatedabove.

Greenberg,J.P. andP. R., Zimmerman,Non-methane hydrocarbons in remotetropical,continental,and marineatmospheres, J. Geophys. Res., 89, 4767-78, 1984.

Horie, O. andG. K. Moortgat,Decomposition pathwaysof the excited Cregee intermediates in the ozonolysisof Simplealkenes,Atmos. Environ., 25A(9), 1881-96, 1991.

Hu, J. andD. H. Stedman,Atmospheric ROx radicalsat an urbansite:

comparison toa simple theoretical modelEnvi•.Sci.Tech.,29,16559, 1995.

Jeffries,H. E., Photochemical air pollutionin: Composition, Chemistry andClimateof the Atmosphere Ed?'H. B. Singh.New York, Van Nostrom Reinhold, 1995.

\

Lightfoot, P.D.,et al.,Organic peroxY• radi. cals,. Atmos. Environ., 26A(10), 1805-1961, 1992. Lurmann, F. W. and H. H. Main, Analysis of the ambientVOC data

collected in theSo.CA Air QualityStudy.CA. Air Res.Board,1992.

Mackay,G.I., Claremont atmospheric freeradicalstudy,, California Air Resources Board,.Reportno.A92-327, 1994.

Madronich, S. andJ. G. Calvert,Permutation reaction of organicperoxy

Conclusion It has been shown that the reactions

OF OZONE WITH ALKENES

of ozone with alkenes

make a significantcontributionto the productionof HOx in both urbanand rural forestedsettings.The full mechanisticdetailsof thesereactionsare not yet fully understood,and clearly warrant furtherstudy. The relationshipbetweenHOx productionand ambient OH levelsis complexandnon-linear.In general,an additionalsource of HOx will increase the ambient OH concentration; the magnitudeof the increasedependson severalfactors,including availableNO x, andtheconcentration of HOx species. Acknowledgments. We thank Dr. Bart Croes and Dr. Rong Lu for making Los Angelesdata available,and ProfessorHarvey Jeffries,Dr. GeoffreyTyndall, Dr. SusanSolomon,andDr. BrianRidley for helpful

radicalsin thetroposphere,J. Geophys. Res.,95, 5697-5715,1990. Martinez,R. D., et al., The near UV absorptionspectraof several aliphatic aldehydesand ketonesat 300 K. Atmos. Environ. 26A: 785-92, 1992.

McKeen,S. A., et al., A regionalmodelstudyof theozonebudgetin the EasternUnitedStates,J. Geophys. Res.,96(D6), 10809-45,1991.

Niki, H., et al.,FTIR Spectroscopic Studyof themechanism forthegasphasereactionbetweenozoneand tetramethylethylene, J. Amer. Chem. Soc., 91,941-946, 1987.

Paulson, S. E., Thetropospheric oxidation of organiccompounds: recent developments in OH, 0 3, andNO3 reactions withisoprene andother compounds.Current Problems and Progress in Atmospheric Chemistry,Ed. J. Barker.World Scientific,.111-43, 1996.

Paulson,S. E., et al., Atmospheric photooxidation of isoprene. Part2: The isoprene- ozonereaction,Intl. J. Chem.Kinet., 24, 103-25,1992.

Raber,W. H. And G. K. Moortgat,Photooxidation of selectedcompoundsin air: methylethylketone, methylvinylketone, methacrolein andmethylglyoxal,Currentproblemsandprogressin atmospheric chemistry.Ed., J. Barker,World Scientific, 318-73, 1996.

Shu,Y. andR. Atkinson, Rateconstants forthegas-phase reactions ofO3 witha seriesof terpenes andOH radicalformation fromthe0 3

discussions.

reactions with Sesquiterpenes at 296 + 2 K, Intl. J. Chem.Kinet., 26,

References

1193-1205, 1994.

Thomas,W., et at., A mechanistic studyon the ozonolysis of ethene. Atkinson, R., Gas-Phase Tropospheric Chemistry of Organic Physio-Chem. Behav.Atrnos.Pollutants, Varese,Italy, 1993. Compounds."J. Phys.Chem.Ref. Data, 1-216, 1994. Tonnesen, S. andH. E. Jeffries,Inhibitionof oddoxygenproduction in Atkinson,R. and S. Aschmann,OH radical productionfrom the gasthe carbonbondfour and genericreactionsetmechanisms, Atmos. phasereactionsof 0 3 with a seriesof alkenesunderatmospheric Environ., 28(7), 1339-49, 1994.

conditions,Environ. Sci. Tech. 27, 1357-63, 1993.

Atkinson,R., et al., Formationof OH radicalsin thegasphasereactions of 03 witha seriesof terpenes,J. Geophys. Res97, 6065-73,1992a. Atkinson, R., et al., Evaluated kinetic and photochemicaldata for atmospheric chemistry.SupplementIV, J. Phys.Chem.Ref.Data, 21, 1125-1569, 1992b.

•'

Blake, N.J., et al., Estimates of atmospherichydroxyl radical concentrationsfrom the observeddecay of reactivehydrocarbonsin well-definedurbanplumes,J. Geophys. Res.,98: 2851-64, 1993. Cantrell, C. A., et al., Peroxy radicals in the ROSE experiment: measurement andtheory, J. Geophys.Res.,97, 20671-86, 1992. Carter, W. P. L., Computer modeling of environmental chamber measurements of maximumincrementalreactivitiesof volatileorganic compounds,Atmos.Environ.29(18), 2513-27, 1995.

Trainer,M., et al., Observations andmodeling of thereactive nitrogen photochemistry at a ruralsite,J. Geophys. Res..96, 3045-3063,1991.

Wayne, R., et al., The nitrateradical:physics,chemistryand the atmosphere. Atmos.Environ.,25A, 1-206, 1991.

Winer,A.M. AndH. W. Biermann, Longpathlength differential optical absorption spectroscopy (DOAS)measurements of gaseous HONO, NO 2, andHCHO in the CaliforniaSouthCoastAir Basin. Res.Chem.

Interreed., 20, 423-45, 1994.

Winer, A.M., et al., Measurementsof nitrous acid, nitrate radicals, formaldehyde, and nitrogendioxidefor the southernCaliforniaair

qualitystudyby differential opticalabsorption spectroscopy. CA. Air Res.Board.,ReportA6-146-32, 1989.

DeMore, W. B., et at., Kinetics and Photochemical Data for use in

Stratospheric Modeling,Evaluation#11. Jet PropulsionLaboratory Publication 94-26, 1994. J. J. Orlando,AtmosphericChemistryDivision,NationalCenterfor Finlayson-Pitts, B. J. andJ. Pitts, Atmospheric Chemistry: Fundamentals AtmosphericResearch,Boulder,CO 80303 andExperimental Techniques. New York, Wiley-Interscience, 1986. S. E. Paulson,Departmentof Atmospheric SciencesUniversityof Gery, M. W., et al., A photochemicalkineticsmechanismfor urbanand California, LosAngeles, LosAngeles, CA 90095-1565 regionalscalemodeling,J. Geophys. Res.,94, 12925-56,1989. Goldan, P. D., et al., Hydrocarbonmeasurements in the southeastern (ReceivedAugust7, 1996;revisedOctober3, 1996; United States: The Rural Oxidants in the Southern Environment (ROSE) Program1990.J. Geophys.Res.,100, 25945-63, 1995. accepted October22, 1996.)