GEOPHYSICAL RESEARCH LETTERS, VOL. 18, NO. 1, PAGES 13-16, JANUARY 1991

ISOTOPICEXCHANGE BETWEEN CARBONDIOXIDE AND OZONEVIA O(!D) IN THE STRATOSPHERE

Yuk L. Yung, W.B. DoMore,and Joseph P. Pinto

Abstract. We proposea novelmechanism for isotopic Table 1 exchangebetween CO 2 and0 3 vi•aO(ID) + CO2 -->CO_•* followedby CO3' --> CO2 + O(aP). A one-•ensior•al List of essentialreactions used in the photochemicalmodel. model calculation shows that this mechanism can account for The units for photodissociationcoefficients and two-body theenrichment in 180 in thestratospheric CO2 observed by rateconstants are s -1 andcm 3 s-l, respectively.The coeffi- Gatnoeta!. [1989],using the heavy 0 3 profile-observed by cientsfor photolysisrefer to nfid-latitudedimally averaged Mauersberger[1981]. The implicationsof this mechanism values at 30 kan. All molecular kinetic data are taken frmn for otherstratospheric species and as a sourceof isotopically DoMore etal. [1990], except otherwise stated. Rate heavyCO 2 in thetroposphere are briefly discussed. constantsforisotopic species are estimated byus. Q = 180. h•troduction Rla O3+ bY-->O2 + O(1D) Jla=7.4x10 -5 RecentlyGamo et at. [1989] reportedmeasurements of Rib O2Q+ hv-->O2 + Q(!D) Jlb=1/2 J la heavyisotopes of CO2 in the stratosphereover Japan. Let O R2a O(ID)+ M--->O+ M k2a= 2.0x 10-11 e ll0/T = 160 andQ = 180.The fiQ of stratosphericCO2, as defined R2b Q(1D) + M --->Q + M k2b= k2a in Gatno et at., was found to be 2O/oo(two parts in a R3 O(ID)+ CO2---->CO2 + O k3 = 7.4x 10-11 e 120/T thousand)greater than that of troposphericCO 2 at 19 km, R4a Q(1D)+ CO2-->CO Q + O k4a= 2/3k 3 and appearsto continueto increasewith altitudeup to the R4b O(ID)+ COQ-->CO2 + Q k4b= 1/3k 3 highestaltitude (25 km) where sampleswere taken. The data are reproducedin our Figure 2. The authorsproposed R5 O(1D)+ CO-+CO+ O k5 = 4.6x 10-11 (a) two possibleexplanations, one dynamical and one chemical. R6a Q(1D)+ CO-->CQ+ O k6a= 1/2k 5 The preferred mechanism was chemical and involved R6b O(ID)+ CQ-->CO + Q k6b= 1/2k 5 transferringa heavyoxygen atom from 0 3, which was first R7 QH+ CO2---> COQ + OH k7 < 1x 10-17 Co) measuredto be isotopically enriched by Mauersberger R8 Q+ CO2-->COQ+ O k8 < 1x !0-18 (c) [I981], to carbon dioxide. However, the authors did not proposea detailedchemical scheme. The direct thermal exchangereaction CO 2 + O2Q --> COQ + 03 is inefficient (a) Cvetanovic [ 1975]. (J.-S.Wen, privatecommunication, 1990). (b) Greenblatt and Howard [1989]. We propose,instead• that the transferof Q from 0 3 to (c) Wen and Thiemens [1990]. CO2 is initiatedby O(ID) reactions. We will use a one- dimensionalphotochemical model 'mcorporatingreactions for exchangeof Q to simulatethe vertical profile of COQ Caltech-JPLmodel [Froidevauxet at., 1985], a typical observedby Gamo etal. The factors controlling the diumally-averagedone-dimensional photochemical model, COQ/CO2 ratio in the tropospherehave been studiedby alongwith our 0 3 proErie. Keeling [1961], Bottinga and Craig [1969], Francey and Considerthe reaction between O(1D) and CO 2 Tans [1987], and Fried!i etal. [1987]. We will attemptto O(!D)+ CO2 --> CO3' assessthe importanceof exchangewith the stratosphere relative to these other factors. CO3' ---->CO 2 + O Theexistence of the CO3' complex was first proposed by Chemical Kinetics Katakis and Taube [1962] in a study of the exchange Photolysisof 03 belowabout 305 nm produces O(1D), an betweenO(1D) and CO 2. Jacoxand Milligan [1971] favored excited state of the oxygen atom, by R1 (see listing and a three-memberring structurewith an O-C-O angleof 65'. numberingof reactionsin Table !). The primary fate of The complexrapidly predissociates to CO 2 + 0 [DeMote O(ID) in the atmosphereis quenchingby the ambient atmosphericgases (R2a) Becauseof the efficiencyof the quenching reactions, the abundance of O(tD) in the 8o atmosphereis very small and it has not been directly detectedin the atmosphere.Nevertheless, this excited atom plays fundamentalroles in the chemistryof HOx radicals [Levy,1971; Wofsy etal., 1972]and NO x species[McElroy and McConnell, 1971; Nicolet and Peetermans, 1972]. Figure1 presentsthe O(1D) concentration predicted by the .• 40

1Divisionof Geologicaland Planetary Sciences; Califomia Instituteof Technology,Pasadena, California 91125. 20 2Jet PropulsionLaboratory, 4800 Oak Grove Drive, Pasadena,California 91109. 3AREAL,United States Environmental Protection Agency, ResearchTriangle Park, North Carolina 27711. 0 0 5 10 15 20 25 •Q - •Qo(%ø) Copyright 1991 by the American Geophysical Union.

Paper number 90GL02478 ri•;i•one-dimensionalConcentrations ofphotochemicalO(ID) and03 modelfrom the[Froidevaux Caltech- 0094-8534/90/90GL-02478503.00 et al., 1985].

13 14 Yung et al.' CarbonDioxide and Ozone andDede, 1970]. There has been great interest in CO3'in enrichmentproduced by Q(1D). However,we believethat comxectionto the CO2 stabilityproblem on Mars [McElroy the results of Jaffe and Klein are in error. First, it is and Hunten, 1970; Noxon, 1970], which was subsequently extremelyimprobable that k 8 > 100 k7, sinceOH is gener- solved by a different scheme [Parkinson'and Hunten, 1972; ally morereactive than O. Second,a qualitativeupper lhnit McElroyand Donahue, 1972].. for k8 may be obtainedby modelingan experimentper- The rapid quenchingof O(tD) by CO9 [DeMote eta!., formedby Wen and Thiemens[1990]. Wen (1990, private 1990]with rate coefficient, k3 = 1.1x I0-'10 c•m 3 s -1 atT = conununication)deduced k8 < 1.5x 10-17cm 3 s-1 at T = 298 K, undoubtedlyproceeds via the CO3* intermediate 363K. Weconclude that k 8 < 10-18 cm 3 s-! atstratospheric [DeMote and Dede, 1970]. For comparisonwe notethat the temperatures,and hence R8 is nothrtportant. correspondingrate constants for quenchingof O(ID) by Ar andKr are7 x 10-13 and8 x 10-12 cmTs-1, respectively PhotochemicalModeling [Cvetanovic,1975]. The existenceof CO3' wouldbe The Caltech-$PLone-dimensional photochemical model expectedto result in an exchm•gebetween the incidentand is describedelsewhere [Allen et ai., !981; Froidevauxet al., emergentO atoms. Indeed,in an isotopicallylabelled study usingCO 2 photolysisat 147nm asthe source of O(ID), 1985;Michelangeli etal., 1989] i In the present investigation Baulch and Breckenridge[1966] showedthat the efficiency we fix the concentrationsof O(D) and0 3 (seeFigure 1) to the values in Froidevauxet al. [1985]. This is justified yield for isotopicexchange is 0.69, closeto the expected valueof 2/3ff theejection of anO atomfrom CO3' were a becauseCO 2 has negligibleinfluence on oxygen.photo- purely statisticalprocess. Other kinetic studies[Yamazaki chemistry.The relevantphotochemistry of O, O(XD),03, and Cvetanovic, 1964; Weissberger et al., 1967] have CO2, CO, andtheir isotopes is summarizedin Table !. The generally confim•edthis result. rate coefficientsfor the isotopicspecies are for the mostpart The starflingdiscovery of an 180 enrictunentin strato- unknown, and we have to make reasonableguesses to arrive spheric O by Mauersberger[1981] has been confmxted at thepreferred values given in Table !.. Verticaltransport is [Carlieta•., 1982; deZafra etal., 1984; Mauersberger, 1983, parameterized by eddydiffusion. 1987; Rinsland et al., 1985; Goldman et aI., 1989], and Let thestandard Q/O ratiobe f. Sincestratospheric 0 3 is studiedin the laboratory [Heidenreichand Thiemens, 1986; enrichedin Q, we have [O.2Q]/[O3] = 2 f E where E = Thiemensand Jackson,1987; Yang and Epstein, 1987a,b; enrichmentfactor. SinceO(XD) is in photochemicalequilib- Morton et al., 1989; Andersoneta!., 1989]. The theory of rium with 03, we have, assumingthe reactionsof Table 1, isotopicenrichment for 0 3 was initiatedeven before the [Q(ID)]/[O(ID)]= f E. Thechoice of E in ourmodel repre- atmosphericobservations [Cicerone and McCmmb, 1980], sentssome difficulty. The observedenric!maent factors axe but this and all subsequenttheories [Kaye and Strobel,1983; not reproducible,either due to uncertaintiesin the experi- Blake et aI., 1984; Kaye, 1986, 1987] completelyfail to ments or a possible dependenceon the solar cycle accountfor either the atmosphericor laboratorymeasure- (Mauersberger,1990, private communication). It must be ments. The suggestionsof Valentini et al. [1987] and Bates emphasizedthat there is no theoreticaljustification for a [ 1986]are not widely accepted.It is not thepurpose of this solar cycle dependence. In case A, we set the enrichment factorE A to be the measurementsof Mauersberger[1981]. paperthatstratospheric toenter this great0 3 is debate. enhanced Wein acce•,t ! Oas andempirical pursue factits To set a lower bound for E we also use the later measure- mentsto deriveanother profile E B = 1 + (EA - !)/3. All implicationsfor the isotopicenrichment of CO2 and other otheruncertahaties in the model are probablysmaller than species. that of E. Considerthe followingsequence of reactionsinitiated by The photochemicalmodel was mn to steadystate in the the photolysisof a heavyozone molecule, diurna/lyaveraged mode. We adoptf = 1/500 [Kaye, 1987] The lowerboundary conditions at the groundare [CO2] = O2Q+hv--> 0 2 +Q(!D) 340 ppmmad [COQ] = 1.36ppm. At theupper boundary (80 km), the fluxes of CO2 and COQ are zero. In the standard Q(1D)+ CO2 --> CO2Q* runE = 1.0. In runsA andB we useas input the profries E^ CO2Q*--> COQ+ O madE B. Theresults for •JQ - ;SQofor CO 2 are.shown in Fig- ure 2 (bQo= troposphericvalue = 40.7O/oorelative to The net result is the transfer of Q from ozone to carbon SMOW at 24øC),along with Gamoet al.'s measurements.It dioxide.The reverseprocess can also readfly occur via R4b, is clear that our mechanismyields the correcttrend with leading to the transferof Q from carbondioxide back to altitudeas well as the magnitudeof isotopicenrichment ozone. Therefore, ozone and carbon dioxide may set up below 30 km. Model B appearsto be closer to the somekind of kineticisotopic equilibrium in the stratosphere. observationsof Gamoet al. [1989], althoughdata in the Under the assumptionsof photochemicalequilibrium and higherregions were not available. The profile of bQ- bQo usingthe somewhatidealized chemical scheme summarized is generallywhat we expected.The maximumeffect occurs in Table 1, it can be shown that [COQ]/[CO2] = at 40 kanwhere O(!D) concentrations arehighest. In the 2([Q(1D)]/[O(1D)])= [O2Q]/[O3].On theother hand, this tropospherethe effectis overwhelmedby the oceanand the ratiowould be equalto thatat the groundif the-atmosphere biosphere.The resultsabove 60 km may not be reliable werewell-mixed (i.e., transporttime << chemicaltime). In becausewe have completelyignored possible isotopic otherwords, the ratio COQ/CO 2 in theatmosphere reflects a effectsin CO2 photolysisin the mesosphere[Allen et al., "tug-of-war"between photochemical equilibrium driven by 1981]. UV radiationin the upperatmosphere, and reactions involv- To testthe sensitivityof our modelto transport,we ing exchangewith H20 in the biosphere,the oceans,and doublethe eddydiffusion coefficients in modelC, whichis clouds. otherwisethe same as model A. Thisresult, shown in Figure For our theory to be correct we must prove that other 2, demonstratesthe greaterinfluence of the troposphere isotopicexchange reactions such as R7 withQH andR8 with when mixing is increased. Similar runs were made for CO. Q arenot important. Evidence for R7 wasfound by Kurylo Thesource of CO is assumedto beisotopically normal. The and Laufer [1979], but Greenblatt and Howard [!989] exchangeof Q betweenozone and carbonmonoxide is less reportedan upper limit of k < ! x 10-17 cm 3 s-i, makingit efficiem than the loss of CO via OH. We obtainedan iso- unimportantfor the stratosphere.Jaffe and Klein [!966] topicenrichment of CQ of theorder of ! ø7oo.Unfortunately, reportedk8 = 1.1x 10-!2 e-1750/r cm 3 s-1 andtiffs would thereare no observationsof CQ in the stratosphereto suggestthat R8 is fast enoughto neutralizethe isotopic colnparewith. Yunget al.: CarbonDioxide and Ozone 15

at., 1987; Francey and Tans, 1987]. An even greater enhancementin COQ should result in the high latitude Northern Hemisphereby this mechanismsince the stratosphere-troposphereexchange in the northemhemi- sphereis twiceas largeas in the SouthemHemisp. here. However,COQ rapidlyexchanges with liquidwater m the • 40 terrestrialbiosphere, resulting in even lower COQ abun- dancesat high northernlatitudes compared to the tropics. -__= The effectsof stratosphericintrusions therefore may not be visiblein the troposphereof thenorthern hemisphere except for shortperiods following injection. Measurementsof the decayof excessstratospheric COQ in the tropospheremay give additionalinformation regarding rates of exchangewith 0 IItl I I I I1111 I I fill! I I IllIll I I I t Ill ! I Ill, theterrestrial biosphere compared with 13CO2, which is cur- 10'a 0.0! 0.1 1 10 100 1000 rent!yused. In view of thein•portance of CO2 in controlling Number Densities (cm-a) clhuate, the presence of another isotopomer must be welcome. Fig.2. Computed•Q - •Qo- CasesA •8 B •ere computed usingenric•nent factors E A •d EB for O3 described• Otherstratospheric species such as CO andN20 mayalso the text. C•e C is s•e • caseA, exceptthin the eddy exchangeQ with 0 3. However,in the caseof CO, the cal- dff•sivity profile is doubled. •e dma •e •om Gmno et culatedenriclunent by thismechanism is smalk The isotopic al. [19891. compositionof stratosphericCO may be determinedmainly with reactionsin the methaneoxidation chain. O(ID) quenchingby N20 primarilyresults in a reaction.The upper limit for regeneratingan oxygen atom is 4% [DeMore et al., Discussion and Conclusion 1990]. Therefore,we expectnegligible exchange of Q via Thediscovery of the•5180 anomalies in stratospheric 0 3 this mechanism. andCO 2 opensa new and excitingchapter in atmospheric A numberof importantstudies remain •o be done. The chemistry. For the first time we encounterchemical chemicalkinetics of isotopicspecies, surmuaarized in Table 1, phenomenathat are the resultof subfiequantum kinetic urgently needs to be refined by laboratory work. When effectswhich axe still not fully understood. We now have properlymodeled, the measurement of co ll•o in thestrato- further evidence on how the "heaviness" in ozone is trans- spheremay offer the best estimatefor the averagevalue of mittedto anotherspecies, CO 2. We believethat similar O(ID) concentrationsin the stratosphere. Further laboratory transmissionsof 180 anomalycould occur to otherstrato- studiesare alsorequired to clarifythe mysteryof !80 sphericspecies that are connected to 03, O, andO(1D). enrichmentin 0 3 itself. However,CO 2 is somewhatunique for enrichmentby the presentmechanism because ofits rapid exchange with O(1D) Acknowledgement.We thank M. Allen, S. Epstein,R. andits long lifetime. Friedl, K. Mauersberger,D. Michelangeli, and S. Sanderfor It is of interestto note that not all stratosphericspecies useful discussions,and M. Gunson and M. Thiemens for cancommunicate isotopically. In a perceptiveanalysis Kaye conm•unicationof resultsprior to publication. One of us [1990]pointed out that the enhancement of lSo in strato- (YLY) wishesto thankJ.-S. Wen for drawinghis attentionto spheric0 3 is not expectedto be reflectedin stratospheric the work of Gamo et al., and sharingwith hhn his experi- H20, thuscasting doubts on theclaims of observationsof mental insights. This researchwas supportedby NASA enhancementsof180 in H20 in theupper stratosphere [Guo grant NAGW-413 to CaliforniaInstitute of Technologyand et al., 1989]. Recentaccurate measurements of H2Q in the EPA grant 9D4 25 NALX to CaliforniaInstitute of Technol- stratosphereby theATMOS experiment(M. Gunson,private ogy, and by the Jet PropulsionLaboratory, California Insti- cormnunication1990) have brilliantly confirmedKaye's tute of Technologyunder contract with NASA. Contribution insight. number 4889 from the Division of Geological and Planetary As shownin themodeling section, bQ qf COQ is deter- Sciences, California Institute of Technology, Pasadena, nilnedby transportand the abundance of O(XD).Thus, accu- California 91125. ratesimultaneous measurements of O2Q and COQ, together with precisedetemfination of appropriaterate constantsmay providea reliablemethod for estimatingthe meanconcen- References trationof O(ID) in the stratosphere,a species that is other- wiseexceedingly difficult to measuredue to its low concen- Abbas,M. M., J. Ouo,B. Carli, F. Mencarag!ia,M. Carlotti,and I.O. trations.This idea is similarto that of usingmethyl chloro- No!t, Heavyozone distribution in thestratosphere from fax-infrared formto estimatea meanglobal OH concentrationin the tro- observations,J. Geophy•. Res., 92, 13,231-13,239,1987. posphere[Singh, 1977; WMO 1986, Chapter4]. The Allen,M., Y. L. Yung,and J. Waters,Vertical tramport and photochem- downwardtransport of COQ mayhave implications for the istryin the terrestrialmesosphere and lower thermosphere (50-120 interpretationof its abundance and latitudinal gradient in the kan).J. Geophys.Res., 86, 3617--'3627,1981. troposphere.CO 2 is observedto be heavierby a few tenths Anderson,S., andK. Mauersberger,Calibration of a rn•ssspectrometer per mil at the SouthPole, comparedto the tropicsand experimentfor ozone, Rev. Sci. !nstrum., 52, 1025-1028,198!. decreaseseven more rapidly towardsthe North Pole Anderson,S. M., J. Morton, and K. Mauersberger,Laboratory measure- [Franceyand Tans, 1987]. Assumingthat stratospheric air mentsof ozoneisotopomers by tunablediode laser absorption spec- enteringthe troposphere in tropopause folds in theSouthern troscopy,Chem. Phys. Letts., 156, 175-180,1989. Hemisphereisenic r h edin 18O by about 19700 andscalm ' g. the Bates,D. R., Heavy ozonein the stratosphere,Geophys. Res. Lett., 13, downward transvortof COQ to that for Oa given by 664-666, !986. Mahhnanetal. [1'980],we calculate anenrichme•at in 180 in Baulch,D.L., and W. H. Breckenridge, Isotopic exchange ofO(ID) with CO2 of 0.25to 0.5ø/,,.,in goingfrom the tropics to theSouth carbondioxide, Trans. Faraday Soc., 62, 2768-2773,1966. Pole.This calculati(•a assumes that the isotopic equilibration Blake, A.J., S.T. Gibson, and D.G. M½Coy, Photodissodafionof time with the oceansand cloudsis much longerthan the 160180inthe atmosphere, J.Geophys. Res,,89, 7277-7284, 1984. intrahemisphericmixing time of about3--6 months [Friedli et Bottinga,Y., •mdH. Craig,Oxygen isotope fractionafion between CO 2 16 Yunget al.: CarbonDioxide and Ozone

andwater, and the isotopic composition of marine atmospheric CO2, Mahlman,J. D., H. Levy H, andW. J. Moxim, Three-dimensionaltracer Earth Planet. Sci. Lett., 5, 285-295, 1969. structureand behavioras simulatedin two ozoneprecursor expefi. Carli,B., F. Mencaraglia,and A. Bonetti,New assignments in the sub- ments,J. Atmos.Sci., 37, 655. millimeteremission spectrum of thestratosphere, !nt. ]. Infrared Mauersberger,K., Measurementof heavy ozonein the stratosphere, Millimeter Waves,3, 385-394, 1982. Getphys.Res. Lett., 8, 935-937, 1981. Cicerone,R J., and J.L. McCmmb,Photodissociation of isotopically Mauersberger,K., Massspectrometry in the stratosphere,Adv. Space heavy0 2 asa sourceof atmospheric0 3, Getphys. Res. Lett., 7• Res.,2, 287-290, 1983. 251-254, 1980. Mauersberger,K., Ozone isotopemeasurements in the stratosphere, Cvetanovic,RJ., Reactions of O(1D2)-atoms, in The Natural Straw- Getphys.Res. Lett., 14, 80-83, !987. sphereof 1974CIAP Monograph, 1, NTIS PB246318, institute of McElroy, M.B., The upperatmosphere of Mars, Astrophys.J., 150, DefenseAnalyses Science and Technology Division, 400 Axmy-Navy 1125-1125, 1967. Dr., Arlington,VA 22202,p. 5-92. 1975. McElroy,M. B., andJ. C. McConnell,Nitrous oxide: A naturalsource DeMote,W. B., and C. Dede, Pressure del?ndence ofcarbon trioxide of stratosphericNO, J. Atrnos.Sci., 28, 1095-1098, 1971 formationin the gas-phasereaction O(•D) withcarbon dioxide, J. McElroy,M. B., andD. M. Hunten,Photochemistry of CO 2 in theatmo- Phys.Chem., 74, 2621-2625,1970. sphereoœMars, J. Geophys.Res. 75, 1188-1201,1970. DeMote, W.B., S.P. Sander, D.M. Golden,.M.J. Molina, R.F. Michelangeli,D.V., M. Allen, and Y.L.. Yung, El Chichonvolcanic Hampson,M. J. Kurylo,C. J. Howard, A. R. Ravishankara,Chemical aerosols:Impact of radiative,thermal and chemical perturbations, J. io•eficsand photochemical data for usein stratospheficmodeling, Getphys.Res., 94, 18,429-18,443,1989. Evaluationnumber 9, JPLpublication 90-1, I990. Morton, J, B. Schueler,and K. Mauersberger,Oxygen fracfionationof deZafra, R.L., A. Parrish,P.M. Solomon,and J.W. Barrett,A ozoneisotopes 4803 through 5403, Chem. Phys. Lett., 154, 143-145, measurementof stratospheric HO2 byground-based millimeter-wave 1989. spectroscopy,J. Geophys. Res., 89, 1321-1326, 1984. Nicolet,M., andW. Peetermans,The productionof nitric oxidein the Francey,R.J., andP.P. Tans,Latitudinal variation in oxygen-18of stratosphereby oxidationof nitrousoxide, Ann. Getphys.,28, atmosphericCO2, Nature,327, 495-497, 1987. 751-762, !972. Friedli,H., U. Siegenthaler,D. Rauber, and H. Oeschger,Measurements Noxon,J.F., Optical emission fromO(ID) and O2(bll;g) inultraviolet of concentration,13C/12C and 180/160 ratios of tropospheric photolysisof0 2 andCO2, J. Chem.Phys., 52, 1852-[873, 1970. carbondioxide over Switzerland, Tellus, 39B, 80-88, 1987. Parkinson,T. D., andD. M. Hunten,Spectroscopy and aeronomy of 02 Froidevaux,L., M. Allen,and Y. L. Yung,A criticalanalysis of CIOand on Mars, J. Atrnos.Sci., 29, !380-1390, 1974. 03 in the mid-latitudestratosphere, J. Getphys. Res., 90, Rinsland,C. P., V. Malathy Devi, J.-M. Flaud, C. Camy-Peyret,M. A. Fl. 12,999-13,029, 1985. Smith,and G.M. Stokes,Identification of 18t-isotopic lines of Gamo,T., et al.,Carbon and oxygen isotopic ratios of carbondioxide of ozonein infraredground-based solar absorption spectra, J. Getphys. a stratosphericprofile over Japan, Tellus 4lB, 127-133,1989. Res.,90,10,719-10,725, 1985. Goldman,A, et al., Isotopicabundances of stratosphericozone from Singh,H. B., Atmospherichalocarbons: Evidence in favourof reduced balloon-bornehigh-resolution infrared solar spectra,J. Getphys. averagehydroxyl radical concentration in the troposphere,Getphys. Res.,94, 8467-8473, 1989. Res.Lett., 59, 1203-1208, 1977. Greenblatt,G. D., andC. $. Howard,Oxygen atom exchange in theinter- Thiemens,M.H., and T. Jackson,New experimentalevidence for the actionof !8OHwith several small molecules, J. Phys. Chem., 93, mechanismfor productionof isotopicallyheavy 0 3, Getphys.Res. 1035-1042, 1989. Lea., 15, 639-642, 1988. Gut,J., M. M. Abbas, and I. {3.Nolt, Stratosphefic H2180 distribution Valentini, J.J., Mass-independentisotopic fractionation in nonadiabatic fromfar infraredobservations, Getphys. Res. Lett., 16, 1277-1280, molecularcollisions, J. Chem.Phys., 86, 6757-6765, 1987. 1989. WMO 1986, Atmosphericozone 1985: An assessmentof our under- Heidenreich,J.E., III, and M H. Thiemens,A non-mass-dependent standingof theprocesses controlling present distribution and change oxygenisotope effect in the productionof ozonefrom molecular Rep. 16, Global ozoneResearch and MonitoringProject, World oxygen:The role of molecularsymmetry in isotopechemistry, J. Meteorol. Organ., Geneva. Chem.Phys., 84, 2129-2136,1986. Weissberger,E.,W. H. Breckemidge,andH. Taube,Reaction ofO(ID) Jacox,M. E., andD. E. Milligan,Infrared spectrum and structure of the withCO 2 atlowtemperatures,./. Chem. Phys., 47, 1764-1769,1967. speciesCO 3, J. Chetn.Phys., $4, 9!9-926, 1971. Wen,J., and M. H. Thiemens,First multi-isotopic study of the O(ID) + Jaffe,S., andF. S. Klein,Isotopic exchange xeactions of atomicoxygen CO2 exchangeand stratosphericconsequences, submitted to producedby the photolysisof NO2, Trans.Farad. Soc., 62, Getphys.Res. Lett., 1990. 3135-3141, 1966. Wofsy,S.C., J. C. McConnell,and M. B. McElroy,Atmospheric CH 1, Katalds,D., andH. Taube,Some photochemical reactions of 0 3 in the COand CO2, J. Geophys.Res., 77, 4477--4493, 1972. gasphase, •r. Chem. Phys., 36, 416-422, 1962. Yamazaki,H., and R. J. Cvetanovic,Isotopic exchange of the excited Kaye,J. A., Theoretical analysis ofisotope effects on ozone formation in oxygenatoms with CO218 and their collisional tieactivation, J. oxygenphotochemistry, J. Getphys. Res., 91, 7865-7874, 1986. Chem.Phys., 40, 582-585, 1964. Kaye,J.A., Mechanismsand observations forLsotope fracfionation of Yang,J., andS. Epstein,The effectof pressureand excitation energy on molecular.species in planetaryatmospheres, Rev. Getphys.,25, the isotopicfractionafion in the formationof ozoneby dischargeof 1609-1658, 1987. 0 2, Geochim.Acta, 51, 20!9-2024, 1987. Kaye,I. A., Analysis ofthe origins and implications ofthe 18 0 content Yang, 1., and S. Epstein,The effect of the isotopiccomposition of ofstratospheric water vapor, J. Atmos. Chem. 10, 39-57, 1990. oxygenon the non-mass-dependentisoiopic .fractionation in tl• Kaye,J.A., andD.F. Strobel,Enhancement of heavy ozone in the formationof ozoneby dischargeof 0 2, Geochim.Acta, Earth'satmosphere?, J. Getphys. Res., 88, 8447-8452,!983. 2011-2017, 1987. Keeling,C.D., The concentrationand isotopic abundances of carbon dioxide in rural and marine air, Geochim.Cosrnochim. Acta, 24, W.B. DeMote, Jet PropulsionLaboratory, 4800 Oak 277-298, 1961. Grove Drive, Pasadena,California 91109. Keeling,C. D., A. F. Carter,and W. 13.Mook, Seasonal, latitudinal, and I. P. Pinto, AREAL, United States Environmental Protec- secularvariations in theabundance and isotopic ratios of atmospheric tion Agency,Research Triangle Park, North Carolina 27711. CO2, 2. Resultsfrom oceanographic' cruises in thetropical Pacific Y.L. Yung, Division of Geological and. Planetary Ocean,J. Getphys.Res., 89, 4615-4628, 1984. Sciences, Ca!'ffomia Institute of Teclmology, Pasadena, Kurylo,M.J., andA.H. Laufer,Evidence for' atom-exchange in OH California 91125. reactionswithcad>onyl compounds: 180+ CO 2 --->18OCO + OH; 18OH+ CO---> 18OC + OH,ar. Chem. Phys., 70, 2032-2033, 1979. (ReceivedAugust 6, 1990; LevyH, H., Normalatmosphere: Large radical and formaldehyde con- revisedOctober 9, 1990; centrationspredicted, Science, 173, 141-143,1971. acceptedNovember 5, 1990.)