Document Jx0k7Z6md3wM23Jq5k8KLpze

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. D21, PAGES 28,181-28,186, NOVEMBER 20, 1998 Lifetimes and global warming potentials for dimethyl ether and for fiuorinated ethers: CH3OCF3 (E143a), CHF2OCHF 2 (E134), CHF2OCF 3 (E125) D. A. Good and J. S. Francisco Department of Chemistryand Department of Earth and AtmosphericScience,Purdue University West Lafayette, Indiana A. K. Jain and D. J. Wuebbles Departmentof AtmosphericScience,Universityof Illinois Urbana-Champaign,Urbana Abstract. Using recentkinetic data, two-dimensional(2-D) chemical-transpormt odeling of the atmosphericlifetimesof dimethylether and fluorinatedethersCH3OCF3 (E143a), CHF2OCHF2 (E134), and CHF2OCF3 (E125) showsthat E134 and E125 have substantiallylarger lifetimesthan previouslyestimated.Dimethyl ether has a short atmosphericlifetime of 5.1 daysand a relativelyinsignificantradiative forcing leadingto a relativelylow globalwarmingpotential.Increasingfluorinationis accompaniedby slower rates of reactionwith hydroxylradical and ultimately longer lifetimes.E143a, E134, and E125 were found to have lifetimesof 5.7, 29.7, and 165 years,respectively.In addition, our work usesab initio methodologyto determineIR absorptioncrosssectionsfor each ether. Our studyfindsthat E134 and E125 have significantinfrared absorptionand thus relativelyhigh radiativeforcingvalues.These two propertiestogetheryield globalwarming potentialsfor E134 and E125 of 5720 and 14,000,respectively,integratedover a 100 year period. 1. Introduction are integrated over a range of time periods (rather than to Dimethyl ether is a proposeddiesel fuel substitutewhile ttuorinatedethers are proposedCFC/HCFC replacementsin foam blowing and refrigerationapplications[Sehestedet el., 1996;Zhang et el., 1992;Hsu and Demote, 1995]. Attractive features of thesc ethers include the absence of chlorine and the steadystate)due to the complexityof the carboncycleeffects on CO2 concentrations.Previousexperimentaland theoretical studiesof theseetherssuggesst horterlifetimesthan analogous fluorinated hydrocarbons[Cooperet el., 1992; Zhang et el., 1992]. Shorter atmosphericlifetimes generallytranslateinto lower GWPs dependingon where and how stronglythe mole- incorporationof hydrogen.Chlorine is known to catalytically cule absorbsinfraredradiation.Below7 /m (1400cm-1), degrade ozone in the stratosphere,thus its removal in the water in the atmosphere absorbs infrared radiation, while tropospherelowersthe molecule'sozone depletionpotential above12 /m (800 cm 1), bothwaterand CO2 effectively (ODP) (a measureof a molecule'spotentialeffectson ozone absorb. This leaves a window between 7 and 12 /xm where defined as the changein total ozone concentrationper unit there is less infrared radiation absorption.CFCs and other massemissionof the gas)[Wuebbles1, 995].The incorporation anthropogenicsubstanceswith absorptionfeaturesin this re- of hydrogenallowsremovalof the ether via hydrogenabstrac- gion can absorb, effectivelyclosingthis window at specific tion by tropospherichydroxylradical. In addition to affecting wavelengthsand can significantlyaffectglobalclimate [IPCC, its influence on ozone, the rate of removal determines the 1990;Ramanethanetel., 1985].Informationon whereand how molecule's atmospheric lifetime, which in turn influences globalwarmingproperties.A molecule'srelative ability to affect climatic changeis quantitativelydescribedby the global warmingpotential(GWP) whichincorporatesboth the molecule'slifetime and the abilityto absorbradiation(its radiative forcing).A molecule'sradiativeforcing is a measureof the changein net irradiance(W m-2) at the tropopausaefter allowingfor stratospherictemperaturesto readjustto stratosphericequilibrium.The time-integratedradiativeforcingresultingfrom the instantaneousreleaseof a kilogram of trace gasexpressedrelative to that of a kilogram of a referencegas, stronglya moleculeabsorbsis obtainedthrougha determination of the molecule'svibrational profile, i.e., band position and band strength.This work usesprior evaluation of the vibrationalprofile of dimethyland fluorinatedethers.By combiningthiswith evaluationsof their lifetimes,the environmental significanceof thesespeciesis determinedrelative to their ability to affect global climate. Presented in this work are lifetimes,relative radiative forcings,and global warming potentials calculated for dimethyl ether and the fluorinated ethers CH,OCF 3 (E143a), CHF2OCHF: (E134), and CHF2OCF3 (E125). usually CO2, defines a molecule'sGWP [Intergovernmental Panelon ClimateChange(IPCC), 1990,1996].GWPs,however, 2. Lifetime Copyright1998by the American GeophysicalUnion. A molecule's lifetime is defined as the ratio of its atmo- Paper number 98JD01880. 0148-0227/98/98JD-01880509.00 sphericburden to that of its combinedlossmechanismsand is the time it takesfor a substancteo decreaseby (l/e) 36.8% of 28,181 28,182 GOOD ET AL.: LIFETIMES AND GLOBAL WARMING POTENTIALS FOR ETHERS Table 1. Lifetime of Ether SpeciesDue to OH LossOnly ScalingRelative to Prinn et al. [1995] (4.9 years) Species Rate Expression Calculated Model CH3OCH3 CH3OCF3 CHF2OCHF2 CHF2OCHF2 a6.7x 10-2e-3/r bl.9 X 10-2e-Sss/r bl.9 X 10-2e-26/r b4.7X 10-3e-295/r 0.014 4.7 24.1 134.0 0.012 4.7 24.6 137.0 All valuesexpressedin years. aWallingtonet al. [1988]. bHsuandDemote[1995]. ScalingRelative to WMO [1995] (5.9 years) Calculated Model 0.017 5.7 29.0 162.0 0.014 5.7 29.7 165.0 its original concentration[e.g., WMO, 1995; Wuebbles1, 995]. For the moleculesevaluated,photolysisshouldbe relatively unimportant,implyingreactionwith hydroxyrl adical(OH) is the principal removal mechanism.The UV-vis absorptionof the fluorinated ethers has been neither measured nor calcu- lated. The first excitedstate of dimethyl ether is found to be about 185 nm [Bremneret al., 1991].Ethers releasedinto the atmospherewill not encounterradiation of this energyuntil they reachpast the stratosphere.For this reasonthe lifetimes of fluorinated ethers are expectedto be dominatedby their reactionwith tropospherichydroxylradical.The expressionfor a molecule'slifetime is shownin (1) wherek is the rate constantfor reactionwith hydroxylradicalat a specifiedtemperature takenasthe globallyaveragedatmospherictemperature, 277 K [Pratherand Spivakovsky1,990]. r = 1/k[OH] (1) Rate constantsare thoseof Hsu et al. [1995]and Wallingtonet al. [1988].The concentrationof hydroxylradicalis takenfrom thework of Prinnetal. [1995]who determineda globalweighted-average lower-atmosphericOH concentration of 9.7 _+ 0.6 x l0sradicalscm-3 basedontheirobservationosf methyl chloroform concentrationand evaluationof its budget.The derivedatmosphericlifetime of methylchloroform(4.9 years due to troposphericOH lossand 4.8 yearsdue to total atmosphericOH loss)alongwith its atmosphericconcentrationare usedto estimatethe globallyaveragedOH concentration. Another approachto lifetime determinationscan be made through the use of global atmospheric chemical-transport modelsthat representall of the knownrelevantprocessesT.he zonally averaged two-dimensionalchemical-radiative-transport model used in this studyto determine the atmospheric lifetimesof the ethersdeterminesthe distributionsof important trace constituentsin the troposphere,stratosphere,and mesosphereT. he photochemicalmechanismtypicallyrepresentsthe chemicalandphysicalinteractionsof about50 chemical speciesand stratosphericaerosolsand accountsfor about 150 chemicaland photochemicalreactionsin the atmosphere [Wuebbleset al., 1995, 1997;Kinnisonet al., 1994]. A commonpracticefor more accuratelyevaluatingatmosphericlifetimesof gasesreactingprimarilywithOH isto scale the lifetimeof a particularspeciesx,, relativeto methylchlo- roform such that Tx/TCHCC1k,c--H,cc,/kx (2) The methyl chloroform-hydroxylradical rate expressionis 1.8 x 10-12e-lss/r [DeMoteetal., 1994].In Table1, col- umns3 and5 listthe atmosphericlifetimeof eachspeciesusing (2). Column3 usesa methylchloroformlifetimeof 4.9yearsas determinedby Prinn et al. [1995] for troposphericOH loss, while column5 usesa 5.9 year methyl chloroformlifetime as recommendedby WMO [1995]. Columns4 and 6 list lifetimes of the ethersascomputedfrom the chemicaltransportmodel. Column 4 is scaledwith the 4.9 year methyl chloroformlifetime, while column 6 usesthe 5.9 year chloroform lifetime. Plate 1 illustratesthe resultsof the chemicaltransportmodel. The concentration of each ether is shown as a function of altitude. Dimethyl ether with its fast rate of reaction with hydroxylradicaldegradesveryquicklywithin the troposphere. From model calculationsthe lifetime of dimethyl ether is found to be -0.015 years(-5.1 days).Little dimethylether would be transportedinto the stratosphereand would thus havelittle impacton the stratosphere'sozonelayerif reactions from its degradationproductscould affect ozone. Slowerreactionratesand longerlifetimesaccompanyincreasingfluorination. CHF2OCHF2, (E134) and CHF2OCF3, (E125) are shown in Table 1 to have substantial lifetimes, with E125 having a lifetime of 134-165 years.Theseethersare predictedto reachwell beyondthe troposphere;seePlate 1. Previousevaluationsimplied shorterlifetimes for fluorinated ethers;however,our analysiscontradictstheseconclusions[Cooperet al., 1992;Zhanget al., 1992].In the kineticinvestigationbyHsu et al. [1995]a relativerate techniquewasusedwhichnegatesthe effectsof unwantedsidereactionscausedby impurities.As a result,the rate data from Hsu et al.'s [1995] investigationare substantiallyslowerthan thoseof Zhang et al. [1992]. Slower reaction rates lead to longer atmosphericlifetimes and may impart significantglobalwarmingpropertiesto theseethers dependingon the molecule'sspecificinfrared absorptionfea- tures. 3. Radiative Forcing Bandpositionsandbandstrengthsfor eachether havebeen determinedfrom ab initio methodologyin a previousstudy [Goodetal., 1998].Optimizationswereperformedwith Becke nonlocalthreeparameterexchangeand correlationfunctional with the Lee-Yang-Parr correctional functional method (B3LYP). The B3LYP calculationswere performedwith the large6-311++G(3df,3pd) basisset.Thismethodwasfoundto have arms error of 2.7% in determiningband positionsof theseethers[Goodetal., 1998].Absorptioncrosssectionswere integratedovera spectrawl idthof 100cm-. Integratedband strengthspredictedby ab initio calculationslie between 10 to GOOD ET AL.' LIFETIMES AND GLOBAL WARMING POTENTIALS FOR ETHERS Ethe(rVolumme ixin,rgatio) 28,183 15- I 1.53e-10 ,, 1.38e-10 1.23e-10 1,07e-10 9.20e- 11 7,67e-ll 6.14e-ll 4.60e-ll - 3.07e-ll 1.53e-ll O.00e+00 -50 0 50 Latitude (degree) 50".'.-,._.._.._. C_(.V.o_l3u_m.eiCxFin[3agt!o), ( E 3O 20 lO 1.53e-10 1.40e-10 1.26e-10 1.13e-10 9.92e-ll 8.56e- 11 7.21e-ll 5.85e-11 4.49e-ll 3.14e-ll 1.78e-ll -5O 0 50 Latitude (degree) Plate1. Atmospherciconcentratiodnistributioonf dimethyel ther,CH3OC3F (E143a),CHF2OCH2F (E134)a, ndCHF2OCF(3E125)f,rom2-Dchemical-transmpoordtelT.hemodeilsfirstruntosteadsytate withcurrenst urfacemolefractionspecifiefdor majorsourcegasesS. incethecurrentatmospheraicbundanceosftheethersstudiedhereareexpectetdobequitesmallt,heirglobadl istributioninsthebackground atmosphearreeassumetodbezeroT. hesurfacaebundancoefsdimethyeltherE, 143aE, 134,andE125,are thenperturbedwithl ppboneat a time,andthemodelisrunto steadystate. 28,184 GOOD ET AL.: LIFETIMES AND GLOBAL WARMING POTENTIALS FOR ETHERS CHF20CHF2(Vol,ume,,m!xi.ngratio) 4O E 30 v :< 20 lO 1.63e-10 1.53e-10 1.48e-10 1.43e-10 1.37e-10 1.32e-10 1.26e-10 1.21e-10 1.16e-10 1.10e-10 1.05e-10 9.95e-11 -5O 0 5O Latitude (degree) CHF:2;O,CFV3ol me mixing ratio 50 / 4O 30 20 10 I 1.54e-10 1.53e-10 1.51e-10 1.50e-10 1.49e-10 1.48e-10 1.46e- 10 1.45e-10 1.44e-10 1.43e-10 1.42e-10 -5O 0 5O Latitude (degree) Plate 1. (continued) 15% aboveexperimentallyderivedresultsThiserroriswithin havebeenusedintheclimatecommunitmy odel(CCM) ofthe thedeviationof 10-25%betweenexperimentaldlyetermined NationalCenterfor AtmospheriRcesearch(NCAR) to comcross-sectiodnata [Pinnocket al. 1995].Bandstrengthsare putelongwaveandshortwaveradiation[Briegleb1,992].This thenusedwitha radiativetransfermodelfor theglobalatmo- narrowbandmodelincorporatesintegratedcrosssectionsinto sphereto determineradiationbudgetsasa functionof altitude. 30, 100cm-1 bandsextendingfrom0.0 to 3000cm-. The The longwaveand shortwaveradiation schemesof this model radiativetransfermodelcontainvserticalprofilesof pressure GOOD ET AL.: LIFETIMES AND GLOBAL WARMING POTENTIALS FOR ETHERS 28,185 Table2a. RadiativeForcing(sW m-2 ppb-) Using ExperimentalVibrational Data Calculated Forcing McDaniel Clerbaux Varanasi Agent et al. et al. et al. Average (IPCC) Relative to CFC- 11 per Unit Molecule CFC-11 0.24 CFC-12 0.29 HCFC-22 0.20 ... 0.25 0.245 (0.22) 1.00 0.32 0.32 0.31 (0.28) 1.27(1.27) 0.23 0.22 0.216(0.19) 0.88(0.86) Table 3. Global Warming PotentialsCalibration Relative to IPCC [1996] Values Greenhouse Gas 20 year Integration 100 year Integration 500 year Integrationa CFC-11 CFC-12 HCFC-22 4700(4900) 7520 (7800) 3860 (4000) 3680(3800) 7890 (8100) 1460(1500) 1320(1400) 3900 (4200) 450 (520) IPCC values are in parentheses.All values expressedrelative to CO2. Instantaneousinjectionof 1 kg of each gas. afrom Wuebbles[1995]. and temperature. Vertical mixing ratios of H20, 03, CO2, CH4, andN20 over35 levelsaswell ascloudcoverageat three levelsare incorporated.Calculatedradiativeforcingsfor CFC11, CFC-12, and HCFC-22 usingexperimentallyderivedband strengthscomparewell with IPCC [1996] estimates.Table 2a comparesthe model-estimatedradiativeforcingvaluesaswell as their ratio to CFC-11 with recent IPCC [1996] estimates. The band strengthsusedwere thoseof McDaniel et al. [1991], Varanasietal. [1988],andClerbauxetal. [1993].The averageof the three data setsdiffers from IPCC estimatesby 11.4, 10.7, and 13.7% for CFC-11, CFC-12, and HCFC-22, respectively. Column 6 in Table 2a shows that the relative ratio to CFC-11 comparesextremelywell with IPCC [1996] estimates. Table 2b alsolistsradiativeforcingvaluesfor speciesusing absorbance cross sections as determined from ab initio meth- odology.Structurallysimilar HFC analogsto the fluorinated ethers were investigatedfor comparativepurposes.For all speciesthe raw radiativeforcingvaluesare overestimated(Table 2b, column 2). Their relative values, however, compare reasonablywell with literature values.Column 3 of Table 2b listsforcingvaluesrelativeto CFC-11 and scaledby the IPCC [1996f]orcingvalueforCFC-11(0.22W m-2 ppb-).Column 4 of Table 2b listsforcingvaluesrelative to CFC-12 and scaled by the IPCC [1996] forcingvalue for CFC-12 (0.28 W m--2 ppb-). Dimethyl ether, asexpected,hasa very low radiativeforcing as predictedfrom its vibrationalspectra.Dimethyl ether'sin- tenseCH stretchinbgandsresonatearound3000cm- well outsidethe 7-12 /zm window region. Each of the fiuorinated ethersin Table 2 has a forcingvalue considerablylarger than its HFC analog.E143a hasa forcingvalue slightlylower than that of CFC-12, while the remaining two ethers, E134 and E125, have larger radiative forcingsper unit amount in the atmosphere. The C-F and C-O stretching modes of these ethers resonate within the 7-12 /zm band; thus increasesin fluorinationare accompaniedby an increasein the molecule's radiativeforcing[Goodet al., 1998]. 4. Global Warming Potentials GWP calculationsintegratethe molecule'satmosphericlifetime and its radiativeforcinginto one expressionto evaluate the cumulative effect of emissionsof a greenhousegas on climate. The time-integrated radiative forcingsfor CO2 are based on the approach of IPCC [1996], using a CO2 decay responsecalculated by Siegenthaler and Joos carbon cycle model [IPCC, 1996;Siegenthalearnd Joos,1992]. Table 3 tabulates estimated global warming potentials for CFC-11, CFC-12, and HFC-22 relative to CO2 compared to IPCC [1996]estimatesshownin parenthesesA. n averageerror of 3.3% existsbetweenthis work and the IPCC [1996] estimates.Table 4 listsglobalwarmingpotentialsfor the ethers. As expected,the longlifetimesandintenseabsorptionfeatures within the 7-12/zm window of E134 and E125 result in substantial global warming potentials. Table 4 also compares GWP valuesfor HFCs structurallysimilar to eachether. Lifetimesand globalwarmingpotentialsfor eachHFC were taken from IPCC [1996]. HFC-125 has a lifetime of 32.6 years,thus its GWP decreasessubstantiallyover longer integrationperiods. Conversely,the GWP of E125 increasesfrom 20 to 100 year integrationsand then declinesdue to its relatively large lifetime of 165 years. E134 has a lifetime longer than both HFC-134 and HFC-134a, thus leadingto higher globalwarming potentialsacrossall integrationperiods.Over short integrationperiodsthe GW? of E134 is similar in magnitudeto that of E125 due to similaritiesin their radiativeforcings(0.49 and0.47W m-2 ppb forE125andE134,respectivelyO).ver longerintegrationperiodshowever,the differencesin lifetimes beginsto dominatethe GWP. Over a 100year integrationthe Table2b. RadiativeForcing(sW m-2 ppb-) UsingabInitioVibrationaDl ata Forcing Agent Raw Forcing Value Forcing Scaled Relative to CFC-11 (0.22W m-2 ppb-) Forcing Scaled Relative to CFC-12 (0.28W m-2 ppb-1) Literature Value CFC- 11 CFC-12 CF3CH3 (HCF143a) CHF2CHF2(HFC134) CF3CHF2 (HFC125) CH3OCH 3 CH3OCF3 (E143a) CHF2OCHF2 (E134) CHF2OCF3 (E125) 0.29 0.34 0.18 0.25 0.36 0.02 0.31 0.57 0.60 0.22 0.26 0.14 0.19 0.27 0.02 0.24 0.43 0.46 0.24 0.28 0.15 0.21 0.29 0.02 0.26 0.47 0.49 0.22" 0.28 0.168b 0.18b 0.25b ... ... ... IPCC [1996]. bFromPinnocketal. [1995]. 28,186 GOOD ET AL.: LIFETIMES AND GLOBAL WARMING POTENTIALS FOR ETHERS Table 4. Comparisonof Alternative Hydrocarbons Species Chemical Formula Lifetime, years Global Warming Potential, 20 years Global Global Warming Warming Potential, Potential, 100 years 500 years HFC-125 E125 HFC-134 E134 HFC-134a HFC-143a E143a HFC-143 DME CHF2CF 3 CHF2OCF 3 CHF2CHF2 CHF2OCHF 2 CH2FCF3 CH3CF3 CH3OCF3 CH2FCHF 2 CH3OCH 3 32.6 165.2 10.6 29.7 14.6 48.3 5.7 3.8 0.015 4600 11800 2900 9760 3400 5000 2200 1000 1.2 2800 1400 1000 5720 1300 3800 656 300 0.3 920 9120 310 1830 420 1400 202 94 0.1 HFC valuesfrom IPCC [1996]. GWP for E134 (5720) is 41% that of the E125 GWP (14,000), while over a 500 year integrationthe GWP of E134 (1830) is only20% that of the E125 GWP (9120). E143a hasa lifetime and thus GWP intermediate to that of HFC-143 and HFC- 143a over all time integrations. 5. Conclusions Dimethyl ether appearsto be atmosphericallybenignwith respectto its ability to affect global climate.A short atmo- sphericlifetime and a lackof significantabsorptionfeaturesin thewindowregioncombineto yield a smallGWP. Unlessthere are unrealisticallylarge emissions,dimethyl ether shouldnot contributesignificantlyto globalwarming. E143a appearsto be an improvementover analogoushalo- carbons,while E134 and E125 are long-livedspecieswith sig- nificantglobalwarmingproperties.Plate 1 indicatesthat sub- stantial concentrations of these ethers will reach the stratosphereF. or theseethers,photolysismaybecomean important removalmechanismcapableof competingwith reaction with hydroxylradical.The importanceof photolysisas a removalmechanismand the atmosphericfate of formed degradationproductsare neededto fully characterizethe environmental significanceof theseethers. Acknowledgments. Work at the University of Illinois was supported in part by the NASA ACMAP and by the U.S. EPA. 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(ReceivedMay 4, 1998;revisedMay 21, 1998; acceptedMay 29, 1998.)