Artigo Updated Radiative Forcing Estimates of 65

8/12/2019 Artigo Updated Radiative Forcing Estimates of 65

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Updated radiat ive forcing estimates of 65halocarbons and nonmethane

hydrocarbons

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http://centaur.reading.ac.uk/1492/http://www.reading.ac.uk/centaurhttp://www.reading.ac.uk/centaurhttp://centaur.reading.ac.uk/1492/


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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D17, PAGES 20,493-20,505,SEPTEMBER 16, 2001

Updated radiative forcing estimates of 65 halocarbonsand nonmethane hydrocarbonsKamaljit Sihra, MichaelD. Hurley, Keith P. Shine,andTimothyJ. WallingtonAbstract. The direct radiative forcing of 65 chlorofiuorocarbons, ydrochlorofiuo-rocarbons,hydrofiuorocarbons, ydrofiuoroethers,halons, iodoalkanes,chloroalka-nes, bromoalkanes,perfiuorocarbonsand nonmethane hydrocarbons has beenevaluated using a consistentset of infrared absorption crosssections. For the ra-diative transfer models, both line-by-line and random band model approacheswereemployed or each gas. The line-by-line model was first validated against measure-ments aken by the AirborneResearchnterferometerEvaluationSystem ARIES)of the U.K. MeteorologicalOffice; the computed spectrally integrated radiance ofagreed o within 2% with experimentalmeasurements. hree model atmospheres,derived from a three-dimensionalclimatology,were used in the radiative forcingcalculations to more accurately represent hemispheric differences n water vapor,ozone concentrations,and cloud cover. Instantaneous,clear-sky radiative forcingvalues calculated by the line-by-line and band models were in close agreement.The band model valueswere subsequentlymodified to ensureexact agreementwiththe line-by-line model values. Calibrated band model radiative forcing values, foratmosphericprofiles with cloudsand using stratosphericadjustment, are reportedand compared with previous literature values. Fourteen of the 65 molecules haveforcings hat differ by more than 15% from those n the World MeteorologicalOrganization 1999]compilation.Elevenof the molecules ave not been reportedpreviously. The 65-molecule data set reported here is the most comprehensiveandconsistentdatabase yet available to evaluate the relative impact of halocarbons andhydrocarbonson climate change.1. Introduction

The Kyoto Protocol calls for international limits onthe emissionof greenhouse asessuch as CO2, N20,CH4, HFCs, and SF6. To compare the relative impactof different greenhousegas control strategies, t is nec-essary to place the climatic impacts of various green-house gaseson a common scale. For any meaningfulscale, knowledgeof the radiative forcing for each gas srequired. As a result of the Montreal Protocol a largenumber of different compoundsare under considerationas replacements or chlorofiuorocarbons.n comparingthe environmental impacts of these replacements, it isnecessary o consider their potential effect on globalclimate. Over the past 5-10 years, there have been

Departmentf Meteorology,niversityf Reading,eading,England,United Kingdom.2FordMotorCompany, earborn, ichigan.Copyright2001 by the AmericanGeophysical nion.Paper number 2000JD900716.0148-022701 2000JD 00716509.00

many studies which have reported cross-sectionalmea-surements nd radiative forcingcalculations or possibleCFC replacements e.g., Christidis et al., 1997; Uler-bauxet al., 1993; Goodet al., 1998; Hansen et al., 1997;Heathfield et al., 1998; Imasu et al., 1995; Ko et al.,1999; Myhre et al., 1998; Naik et al., 2000; Pinnocket al., 1995; Roehl et al., 1995]. Unfortunately, ypi-cally only a few gasesare studied n each nvestigation,and there are often significantdifferencesn the method-ologiesused n the differentstudies e.g., differentab-sorption spectra, radiative transfer models and differentatmospheric rofilesof the mostabundantgases).Com-parison of radiative forcing valuesreported from differ-ent laboratories s often not a straightforward process.A consistentand comprehensive et of well-documentedforcing calculations s needed to provide a clear relativeranking of the potential climatic impacts of the largenumber of potential CFC replacements. The most com-prehensivetudy o date s that of Jain et al. [2000],whoreport results for 39 gasesby combining distributionscalculated with a two-dimensional chemical transportmodel with detailed radiative transfer calculations.

In this study we have undertaken a systematic studyof the infrared spectra and radiative forcing of 65 halo-20,493


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20,494 SIHRA ET AL.: UPDATED HALOCARBON RADIATIVE FORCING ESTIMATEScarbons and hydrocarbons, 11 of which have not beenreported to our knowledge. To maximize the experi-mental precision, the infrared spectra of all of the com-pounds exceptCFC-11, HFC-245fa, perfiuorocyclobu-tane, trichloromethane, nd 1,2 dichloromethane) eremeasured using the same experimental techniquesandapparatus Pinnocket al., 1995]. Two different adia-tive transfer models were utilized for each of the gasesin order to quantify the impact of the model on theradiative forcing result.Radiative transfer calculations performed using theline-by-line approach were carried out using a modifiedversionof the Reference orwardModel (RFM) [Dud-hia, 1997],a fast forwardmodelbased n the GENLN2model Edwards, 987]. The RFM wascomparedwithother line-by-line models from the Intercomparison ofRadiation CodesUsed in Climate Models (ICRCCM)and validated against observations y the Airborne Re-search nterferometerEvaluation System (ARIES) ofthe U.K. MeteorologicalOffice. A discussion f the re-sults is given in section 2.Absorption spectra were measured over the 450 to2000cm range o includenot only the main spectralsignaturesypically oundbetween 00 and 1500cm ,but also he generallysmaller eatureswhich are presentin the 450 to 700 cm spectral ange. The 700 and1500cm- region s importantsince he principal asesfound in the atmosphere absorb most weakly withinthis range. The 450 to 700 cm region,encapsulat-ing strong surfaceemissions ut also strong absorptionbands of water vapor and carbon dioxide, s less mpor-tant [Pinnocket al., 1995]. However, or gases uchastoluene,with 7.1%of the integrated ross ectionallingin the 450 to 600 cm range hat contributes 6.5% othe radiative forcing, his frequency ange s certainlysignificant.The infraredspectra, he integrated bsorp-tion cross sections, and a discussion on contaminantsare given in section 4.The impact of thesegases n the atmospheres quan-tified by their "adjusted adiative orcing" Intergov-ernmentalPanel on Climate Change IPCC), 1995],

which is defined as the net change n irradiance at thetropopause rom the inclusionof a greenhouse as, afterthe stratospheric emperatures have been allowed to ad-just to this changeand return to a radiative equilibrium.To include clouds and stratospheric adjustment is com-putationally expensive or the line-by-linemodel. Hencethe clear-sky "instantaneous adiative forcing," whichdoes not include stratospheric adjustment, is also usedfor each gas to calibrate a narrowband model which isused or the calculationof the adjusted orcings.No pre-vious study has used line-by-line calculations or sucha large number of gases. A discussion f the models,radiative orcings, lobalwarmingpotentials GWPs),and a comparisonwith other values found in the litera-ture concludes his study.2. Validation of the Reference ForwardModel

Line-by-line model calculationswere performedusingthe Reference orwardModel (RFM) [Dudhia, 1997].The irradiance calculation was included using a four-point Gaussianquadrature that approximates he zenithangle ntegration o better than 0.1% [Cloughet al.,1992]. This flux form of the model, hereinafter e-ferred to as the FRFM, was run at a spectral reso-lution of 0.0025 cm , which is sufficient o resolveLorentz broadened lines from near the surface to theupper stratosphere. The ability to resolve lines abovethis height was found to produce a negligiblechange ndownward rradiance to the tropopause. The HITRAN1996 inecompilationRothman t al., 1998], CKD2.1"watervaporcontinuum Clough t al., 1992],and a linewing imit of 25 cm werealsoused or all modelcal-culations. Furthermore, for each vertical profile layer,the Planck function was permitted to vary linearly withoptical depth so that in the extremes, he radiance orirradiance) rom a layer boundary epresentedhe tem-perature of that boundary at opaque frequenciesandthe mean ayer temperature at optically thin frequencies[see,e.g., Wiscombe, 976; Clough t al., 1992]. This

Table 1. Comparison f Line-by-LineModels or Three AtmosphericCasesTropical Midlatitude Summer Subarctic WinterFsfc Ftrp Ftoa Fsfc Ftrp Ftoa Fc Ftrp Ftoa

ICRCCMMinimum 389.92 277.62 290.26 343.07 265.00 283.32 164.44 178.08 202.61Maximum 397.86 288.14 294.40 351.19 274.53 290.47 166.73 179.92 203.45FGENLN2 395.49 281.69 292.63 347.49 266.58 283.59 171.17 172.86 199.83LBLRTM 394.66 277.62 290.26 346.91 265.30 283.32FRFM 396.86 277.72 290.70 348.10 265.51 283.90 170.73 173.88 200.59airradiancesregivenn W m 2, where sfcs thedownwardrradiancet thesurface,trp s thenetupward rradiance t the tropopause, nd Ftoa is the upward rradiance t the top of the atmosphere.Abbreviationsre ICRCCM, Intercomparisonf RadiationCodesUsed n ClimateModels; GENLN2,Flux version f the GENLN2 ine-by-line odel; BLRTM,Atmosphericnvironmentesearchine-by-linemodel; and FRFM; flux Reference Forward Model.


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SIHRA ET AL.: UPDATED HALOCARBON RADIATIVE FORCING ESTIMATES 20,495

more realistic representationwas also used n all of themodel calculations. The model validation was carriedout in two parts: a comparisonwith other line-by-linemodels and a validation against observations.2.1. Model Comparison

The model comparisonwas performedusingarchiveddata from the four line-by-line modeling groups n theICRCCM comparisonEllingson t al., 1991].The datawere obtained from the Carbon Dioxide InformationAnalysis enterWebsite http://cdiac.esd.ornl.gov/ftp/ICRCCM-radiative_fluxes). he rangeof irradiancesfor these models is shown in Table 1 for three at-mosphericprofiles: tropical, midlatitude summer, andsubarcticwinter (ICRCCM cases25, 27, and 33, re-spectively). For each case,vertical concentrationpro-files of water vapor, ozone, and carbon dioxide werealso given (CO2 with a uniform concentration f 300ppmv). Irradiances rom the AtmosphericEnviron-mentResearchine-by-linemodel LBLRTM) are shownexplicitly, since this is closest n formulation to theFRFM, and those rom the flux versionof the GENLN2model (FGENLN2), previously t Reading [Pinno&and Shine,1998],are also ncluded or comparison.The range of ICRCCM values s much greater forthe tropical and midlatitude summer cases han forthe subarctic winter. The profile for the latter casehas a much lower surface temperature and a muchweaker vertical structure, which make differences,par-ticularly from the type of water vapor continuummodelused, less significant. Differing line wing limits, spec-tral databases, vertical resolution, and in one model,slightly different mixing ratios, all contribute to thevariability. For the tropical and midlatitude summercases the FRFM irradiances lie within the ICRCCMrange. For subarctic winter, the models run with agreater number of approximations end toward weakerabsorption emission) han the FRFM and FGENLN2.More important, for all three cases, he FRFM agreesto better than 1.5% with the LBLRTM and FGENLN2model irradiances. The instantaneous, clear-sky radia-tive forcings or a doublingof carbondioxideconcentra-tion from 300 to 600 ppmv for the midlatitude summercaseare 5.55 W m 2 for the FRFM, 5.59 Wm -2 forthe LBLRTM model,and 5.54 W m 2 for FGENLN2.These valuesare consistentwithin 1%, which is an ac-ceptable level of agreementgiven the differencesbe-tween the models. The averageof the ICRCCM line-by-linemodelsgivesa forcingof 5.63 W m 2, whichis somewhat higher than the FRFM value. However,the ICRCCM average s biasedby the modelscalculat-ing weakerabsorptions,or example,by using ine winglimits of 10 cm - instead of 25 cm -. For these modelsthe increasen CO2 concentrationrom 300 to 600 ppmvresults n the stronglyabsorbingCO2 band centeredat667 cm (15/m) to saturaten transmittance. incethis band is already partially saturatedat 300 ppmv in

the more comprehensivemodels such as the LBLRTMand FRFM, the change n irradiance,and hence orcing,is less pronounced.2.2. Comparison With Observations

A validation of the model, however, can necessarilybe achievedonly by a comparisonwith observations. nthis case, radiance measurementswere used that wererecordedby the ARIES instrument and provided by J.P. Taylor (personal ommunication,999). The ARIESinstrument is a thermal infrared interferometer capableof recording n either the zenith or nadir over an al-titude range of 30-9000 m. Validation was performedacrosshe 700 o 3000cm x spectral angeat 0.5 cm xresolution. The tropospheric temperature and concen-tration profilesof water vapor and ozonewere providedfrom in situ measurements on board the aircraft. Thestratosphericprofiles were taken from the global andmultiannual tmosphere f Christidis t al. [1997].Uni-form vertical profilesof carbon dioxide (358 ppmv),methane (1720 ppbv), nitrous oxide (312 ppbv), andcarbon monoxide 137 ppbv) were included o repre-sent the present-day atmosphere. Figure la showsanARIES zenith spectrumrecordedat midlatitude in Oc-tober 1998, Figure lb shows he calculated esult, andFigure lc shows he residuals ARIES minusRFM).The model showsexcellent agreementwith observa-tions across he entire spectral range, with frequency-integratedadiancesf 48.45W m 2 sr 1 for heARIESinstrumentand 47.65 W m 2 sr x for the RFM, a dif-ferenceof 1.7%, not including the negative radiancesbetween 2500 and 3000 cm -x and instrument noise be-tween1400and 1800cm x. A comparison f an ARIESspectrum measured n the nadir was also performed.The frequency-integrated adiances showeda smallerdifferencebetter than 0.5%), which s expected incenadir spectrashow esssensitivity o spectraldifferencesgiven the surfaceemission erm. The agreementbe-tween the measured and modeled data in Figure 1 pro-vides a validation of the RFM line-by-line model.3. The Radiative Transfer Models

The FRFM is not able to include clouds or strato-spheric djustment.The narrowbandmodel NBM) ofShine 1991] s therefore sed o calculatenstantaneouscloud-free orcings, or comparison gainst he FRFM,as well as the cloudy,adjusted orcings.The line-by-lineapproachemploys he most fundamental physicsandhighnumerical ccuracyo evaluate he radiative rans-fer equations. It is therefore thought to be the mostaccurate atmospheric adiative transfer model subjectto the limiting and significant aveats f imprecise pec-tral line parametersand line shape. Hence he forcingsfrom the FRFM are used to calibrate those from theNBM; the cloudy,adjusted orcingsare scaleddirectlyby a gas-dependent cale factor so that the instanta-


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20,496 SIHRA ET AL.' UPDATED HALOCARBON RADIATIVE FORCING ESTIMATES

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Figure 1. An AirborneResearchnterferometer valuationSystem ARIES) zenithspectrumrecorded t midlatituden October 998over he 700 o 3000cm spectral ange s shownnFigure a. The correspondingeference orwardModel (RFM) calculated pectrums shownnFigure b. The spectrally ntegrated adiances re 48.45W m sr 1 for the ARIES instrumentand 47.65 W m-2sr -1 for the RFM.

neous,clear-sky orcings rom the NBM agreeexactlywith those from the FRFM. The NBM has a spectralresolution f 10 cm , with 250 bandsacrosshe 0 to2500cm- range.The model sesheMalkmus tatisti-cal distribution f line strengthsMalkmus, 967] incethis "model"more closely epresentseality than anyof the other random band models. Transmittances arecalculated using either the HITRAN 1996 databaseorabsorption ross ectionsor the halocarbons,veragedinto 10 cm-1 intervals nd the CKD 2.1 watervaporcontinuummodel. rradiances re calculated gainus-ing four-point Gaussianquadrature. Both modelssub-layer the vertical profilesso hat variations n the Planckfunctioncan be neglected.Three cloud ayerscanbe in-cluded n the atmospheric rofileusinga basicparam-eterization of optical depth, with the fractional cover-age of each cloud layer taken from the International

SatelliteCloud ClimatologyProject (ISCCP) and withthe uppermostcloud tuned to give an agreementof ap-proximately1% with observed utgoingongwaveadi-ation at the top of the atmosphereFreckletont al.,1998]. Stratospheric djustmentwas ncludedby as-suming hat the combinationof constantsolar heatingrate and dynamicalheating balances he infrared cool-ing rate and using an iterative technique o adjust thestratospheric emperatures unti. the radiat,ive equilib-rium, upset by the inclusion of the new molecule, srestored Pinnock t al., 1995].Atmospheric rofilesof temperature,cloudcover,andwater vapor and ozoneconcentrationswere used o rep-resent the tropics and the extratropics of each hemi-sphere as described y Freckletont aL [1998]).Theprofileswere basedon multiyear averages f EuropeanCentre or Medium-RangeWeatherForcastsECMWF)


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SIHRA ET AL.: UPDATED HALOCARBON RADIATIVE FORCING ESTIMATES 20,497analysesand combinationsof different satellite instru-ment data (see Christidiset al. [1997] or a more com-plete description). Uniform profilesof carbon dioxide(358ppmv), methane 1720ppbv), and nitrous oxide(312ppbv) were used. The tropopause eight was de-finedas the "conventional"hermal ropopauseWorldMeteorological rganizationWMO), 1986].The back-ground concentrationof the trace gaseswas assumedto be zero, with forcings calculated for a 0.1 ppbv uni-form increase n concentration.The global mean forcingis hence calculated by area-weighting the three forc-ings and linearly scaling to 1.0 ppbv. This ensuresthat no sharp spectral features saturate and that ab-sorptions emain within the regime of the weak limit,one of the regimeswhere the NBM most accurately cal-culates ransmittanceseePinnocket al., 1995]. Thismethod is also more appropriate to calculate radiativeforcingsof trace gases or observedor anticipated atmo-spheric concentrationswhich are typically much lowerthan 1.0 ppbv. A significanterror arises rom the as-sumption of a uniform profile for short-lived gases.Forthis study we use correction factors from Jain et al.[2000] or gases ommon o our studies; or other gaseswe use a lifetime-dependent factor based on the resultsof Jain et al. [2000] o account imply or the impactof atmospheric lifetime. The validity of this methodis discussedn section5. Note that Jain et al. [2000]also calculate the forcing using the full latitudinal andseasonalvariations in background conditions; the fullprofilesgive a forcing ypically 2-3% higher than for aglobal mean profile, reaching7% for a few gases. Asdiscussed ater, such errors are no bigger than othersourcesof error, such as in absorption cross section orspecificationof tropopause position.4. Integrated Absorption Cross Sections

Absorption cross-sectionalspectra were measured atthe Ford Motor Company, with additional spectra ob-tained from the Spectroscopy nd Warming Potentialsof AtmosphericGreenhouseGases (SWAGG) project[Highwoodnd Shine,2000] and from Clerbaux t al.[1993]. The spectraof CFC-12 from HITRAN 1996[Varanasiand Nemtchinov,1994] and HFC-245fa (re-ported by Ko et al. [1999]) were also included; hegasesmeasuredby each of these sources,and their in-tegrated cross sections, are shown in Table 2. Wherepossible, he crosssectionswere recorded across he 450to 2000cm spectral ange;detailsof the Fordexperi-mentalsetuparegivenby Christidis t al. [1997].Forty-four of the trace gasesand their forcingscalculated us-ing this NBM have been reported by our group previ-ously Pinnock t al., 1995;Christidis t al., 1997;High-woodet al., 1999; Highwood nd Shine, 2000]. Elevenof the trace gaseshave not been previously reported toour knowledge. For completeness, n addition to thenew measurementsof infrared spectra for the hydroflu-oroethers, halons, and perfiuorocarbons listed in Ta-

ble 2, we also ran checksof the infrared spectra of the44 compoundshydrofiuorocarbons,ydrochlorofiuoro-carbons,and nonmethanehydrocarbons) hat we hadpreviouslymeasured Pinnocket al., 1995; Christidiset al., 1997;Highwood t al., 1999],someof whichhadbeenusedby Naik et al. [2000]and Jain et al. [2000].In the processof double checking he old spectra, wefound there was a small shift in the baseline of 13 of theold spectra. Accordingly, we remeasured he infraredspectra of all of the gases and report the new resultsgiven in Table 2. After remeasurement,we found thatseven of the 44 spectra had integrated cross sectionswhich were more than 10% different rom our publishedmeasurements. These compounds are indicated in Ta-ble 2. Smaller corrections are also made to some of theother gases. A small proportion of the infrared spec-tra were contaminated with water vapor lines between1400 and 2000 cm , which arisebecause f tracesofwater vapor in the infrared beam within the spectrom-eter. A Kalman filtering echnique Kalman, 1960]wasused effectively o removewater lines to within the levelof the noise for a completedescription f this methodsee$ihra [1998]).5. Radiative Forcings and GlobalWarming Potentials5.1. The Effect of Nonuniform Vertical Profiles

The adjusted, cloudy-sky,global, and annual meanradiative forcingsare shown n Table 3, for uniform1.0 ppbv vertical profiles and taking into account at-mosphericifetimes. Theseuniform 1.0 ppbv valuesareregarded s eferencealues, s hese llowa more eadycomparison etweendifferentstudies.The 1.0 ppbv val-ues will overestimate he forcingbecauseof the falloffin concentrationsabove the tropopause. For some racegases, otably CFC-11 and CFC-12, someobservationsexist with which to characterize the impact of nonuni-form distributionse.g.,Freckleton t al., 1998]. Forothers, 2-D chemical ransport modelscan be used odeduce erticalprofiles see, .g., Freckleton t al., 1998;Naik et al., 2000; Jain et al., 2000]. Suchan approachis clearly he most comprehensive,ut the lack of datawith which to evaluate such models, particularly forthe shorter-lived species,and the difficulties n repre-senting ransport, especiallyn 2-D models,must berecognized.n addition, assumptionsave o be madeabout the distributionsof emissions e.g., Freckletonet al. [1998]assumedmissionso be predominantlynthe NorthernHemisphere, hile Naik et al. [2000]andJain et al. [2000]specifya constant urfacemixingra-tio everywhere). Furthermore,an assumption as tobe made about whether the model is run to a steadystate. As an example of the problems n using model-derivedvertical profiles,Naik et al. [2000] eport thatthe CFC-11 model-derived orcing s 11% smaller thanfor a constant rofilecase;Freckleton t al. [1998]used


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20,498 SIHRA ET AL.: UPDATED HALOCARBONRADIATIVE FORCING ESTIMATESTable 2. IntegratedAbsorption ross ections ver he Range 50-2000 m Measuredn the PresentWorkat 296K (FordMotorCompany)ndPreviousesultsromClerbauxt al. 1993]and heSpec-troscopyndWarming otentialsf Atmosphericreenhouseases roject SWAGG)

Integrated ross ection10 17 cm ' molecule1 cm 1)Gas Chemicalormula Ford Clerbauxt al. [1993] SWAGGCFC-11 CClsF 9.31CFC-12 CCI.F. 13.59HCFC-123 CHCI.CFs 11.99cHCFC-124 CHC1FCFs 13.85cHCFC-141b CHsCCI.F 7.14cHCFC- 142b CHs C C1F. 9.58cHCFC-21 CHFCI. 7.48HCFC-225ca CHC1.CF.CFs 18.41cHCFC-225cb CC1F.CF.CHC1F 15.39cHCFC-22 CHC1F. 10.16cHFC-125 CHF.CFs 16.47cHFC-134 CHF.CHF2 10.57HFC-134a CFH.CFs 12.40cHFC-143 CHF.CH.F 7.14cHFC- 143a CFs CHs 12.71cHFC-152a CHsCHF. 6.87cHFC-161 CHsCH.F 2.46HFC-227ca CFs CF. CHF. 19.97HFC-227ea CFs CHFCFs 23.04cHFC-236cb CFs CF. CH.F 16.96HFC-236fa CFsCH.CFs 23.42cHFC-23 CHFs 11.61cHFC-245cb CFs CF.CHs 16.71HFC-245fa e CFs CHq.CHF. 19.57HFC-272ca CHsCF.CHs 5.69cHFC-32d CH.F2 5.65HFC-41 CHsF 1.57HFE-245fa2 CFs CHq.OCHF. 26.31HCFE-235da2 CFs CHC1OCHF. 27.44HFE-125 CFsOCHF. 31.55

HFE-143a CF30CHs 20.16HFE-356mff2 CFs CHq.OCH.CFs 27.33HFE-7100 C4F9OCHs 36.04HFE-7200 C4F90 C.H5 36.56i7100 i-C4F9OCH3 37.61n 7100 n-C 4F90CHs 33.71i7200 i-C4 F90C.H5 32.70H-Galden 1040X CHF. OCF.OC.F40CHF. 65.82Halon 1211 CF.BrC1 12.51Halon 2402 CF.BrCF.Br 15.40Halon 1301 CFsBr 16.84Trifluoromethyl sulphur CFsSF5 25.44cPentafluororidePerfluorometbane CF 4 21.93Perfluoroethane C2F6 21.68Perfluoropropane CsFs 27.39Perfluorocyclobutane C4FsPerfluorobut ane C4F lo 31.41Iodopentafluoroethane CFsCF.I 17.72Iodotrifluoromethane CFsI 15.47Dibromomet bane CHq.B r . 2.07Difluorobromomethane CHF.Br 10.04Methylbromide CHsBr 0.711,2 dichloroethane CH.C1CH.C1Trichloromethane CHClsAcetylene C.H. 3.90Benzene C6H 3.04Cyclohexane C H1 . 0.53Ethane C2H 0.38Ethene C2H4 1.67

12.17(14.43)6.8410.8317.4916.519.98(16.11)

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SIHRA ET AL.' UPDATED HALOCARBON RADIATIVE FORCING ESTIMATES 20,499Table 2. (continued)Gas Chemical Formula

Integratedross ection10 7 cm molecule cm)Ford Clerbauz t al. [1993] SWAGGn-butaned C4Ho 0.63n-pentane C5H2 0.72Propane CsHs 0.54Propylbenzene C6HsCH2CH2CHs 2.43Styr ne C6H5CHCH2 3.13Toluene CHsCHs 2.34

aWhere vailable,he 253K data rom he Clerbauzt al. [1993] ndSWAGG tudiesregiven; therwisehespectra ere ecordedt 287K Clerbauzt al., 1993] r 295K (SWAGG), s ndicatedy valuesn parenthesesandbrackets,espectively.n the case f HCFC-22,he SWAGG pectrum as ecordedt 273K. Data forHFC-245fa ere aken romKo et al. [1999], nd or CFC-12he 259-Kspectrumn HITRAN 1996wasused.bTaken rom HITRAN (1996).cWavenumberanges 700-2000m .dIntegratedbsorptionrossectionshich ifferrom urpreviouslyublishedesultsymorehan10%.eTaken rom Ko et al. [1999].Table 3. Global, Annual Mean, Cloudy-SkyRadiative Forcings or a 1.0-ppbv Change n TraceGas Mixing Ratio, for a Uniform Profile and Estimated for Lifetime-DependentProfiles Based onthe Resultsof Jain et al. [2000]

Radiative orcing,Wm - ppbv Gas ConstantProfile LifetimeAdjusted WMO [1999] Jain et al. [2000]CFC-11 0.269 0.242 0.25 0.240CFC-12 0.337 0.321 0.32 0.302HCFC-123 0.192 0.141 0.20 0.143HCFC-124 0.215 0.193 0.22 0.195HCFC-141b 0.150 0.130 0.14 0.131HCFC-142b 0.189 0.163 0.20 0.164HCFC-21 0.173 0.140 0.17HCFC-225ca 0.259 0.202 0.27 0.207HCFC-225cb 0.317 0.280 0.32 0.245HCFC-22 0.213 0.208 0.22 0.205HFC-125 0.236 0.223 0.23 0.249HFC-134 0.193 0.180 0.18 0.176HFC-134a 0.168 0.159 0.19 0.200HFC-143 0.134 0.118 0.13 0.115HFC-143a 0.154 0.148 0.16 0.160HFC-152a 0.119 0.095 0.13 0.097HFC-161 0.039 0.024 0.03 0.022HFC-227ca 0.268 0.249 0.31HFC-227ea 0.264 0.256 0.30 0.322HFC-236cb 0.239 0.217 0.23HFC-236fa 0.253 0.251 0.28 0.264HFC-23 0.177 0.171 0.20 0.248HFC-245cb 0.245 0.26HFC-245fa 0.256 0.241 0.25 0.261HFC-272ca 0.082 0.08HFC-32 0.114 0.105 0.13 0.155HFC-41 0.027 0.023 0.02HFE-245fa2 0.387 0.332 0.31HCFE-235da2 0.449 0.372 0.38HFE-125 0.424 0.407 0.44HFE-143a 0.198 0.172 0.27HFE-356mff2 0.362HFE-7100 0.402 0.347 0.31HFE-7200 0.412 0.303 0.30i7100 0.374n7100 0.465i7200 0.338


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20,500 SIHRAET AL.' UPDATED ALOCARBONADIATIVE ORCING STIMATESTable 3. (continued)

RadiativeForcing,Wm -' ppbv Gas Constantrofile Lifetimedjusted WMO1999] Jain tal. 2000]H-Galden 1040X 1.051Halon 1211 0.313Halon 2402 0.329Halon 1301 0.318CFaSF5 0.567CF4 0.117C.F6 0.268CaFs 0.283C4Fs 0.318C4Flo 0.375CFaCF.I 0.296CFI 0.275CH.Br. 0.016CHF.Br 0.166CHBr 0.007CH.C1CH.C1 0.021CHCI 0.136Acetylene 0.051Benzene 0.013Cyclohexane 0.004Ethane 0.003Ethene 0.037n-butane 0.004n-pentane 0.004Propane 0.003Propylbenzene 0.021Styrene 0.046Toluene 0.020

0.987 1.050.263 0.30 0.2510.289 0.32 0.2730.116 0.08 0.089b0.266 0.250.279 0.260.314 0.360.370 0.330.26 0.293b0.168 0.23 0.268b0.011 0.01 0.019b0.146 0.14 0.174b0.005 0.01 0.007b0.094 0.02

aWhere vailable,hese orrectionactors reapplied irectly; lsewhere,correctionactorbased n anempiricaleast quaresit to the esultsfJainetal. 2000]sapplied,singifetimesiven y WMO 1999].The orcingsromWMO 1999] nd he ifetime orrectedorcingsromJainet al. [2000] realsoncludedwhere ataareavailable.he eferenceorcingnd ifetimeorHFC-245fare romKo et al. [1999]. hechemical ormulae or the gases re given n Table 2.Radiative orcingor a constantracegasprofile.

latitudinally resolvedsatellite observations f CFC-11(whichwere ound o agreewith balloonsondebserva-tions)and calculated forcing nly6% smallerhan aconstant rofilecase. For a numberof gases ommonto the Naik et al. [2000] nd Jain et al. [2000] tudiesthe mpact fmodel-derivedrofilesiffersmarkedlyorsomegases e.g., the correctionsor HCFC-123 are 16%and 27%respectively). . Jain (personalommunica-tion,2000) eportshat resultsromJainet al. [2000]should be regardedas the most reliable becauseof themoredetailed pproachn that study.For he ivegasesthat Freckletont al. [1998] ndJainet al. [2000] avein common, two gaseshave derived correctionswhichdiffer by more than 30%.With thesewords f caution,he following pproachis adopted. or thosegases ommono thisstudyandthat of Jain et al. [2000]we adopt heir correctionac-tor for the impact of vertical profile. For nine of the39 gases f Jain et al. [2000],verticalprofileswerenotavailable, nd they assumedhat thesegases aveaconstantvertical profile,even or gaseswith short ife-times. We choose,nstead, o adopta simple orrection

factorbased n atmosphericifetime. t is recognizedthat sucha correctionactor s bestappliedusing hestratospheric ifetime, because t is this that determinesthe rate of falloffabove he tropopausesee reckletonet al., 1998;Naik et al., 2000]; ndeed, lotting he cor-rectionactors f Naik et al. [2000] gainst tratosphericlifetime ields tight relationshipsee igure ). How-ever, for most gases, he stratosphericifetime is notavailable.Althoughess obust, hereremains gen-eral relationship etweenhe lifetimeand the impactof the vertical rofile n the forcing Figure ). Usingthe Jain et al. [2000] esults,we havederivedan em-pirical curve fit such that the fractional correction tothe forcingor ifetimes reaterhan0.25yearss givenby I- 0.241 'aSs, here is the lifetimen years.For gases f shorter ifetime, we use he minimumvalue(0.61) n the Jain et al. [2000] tudy.There s clearlymuchuncertaintyn applying uch orrections,nd hiswill be a significantource f error;however,he errorsin using these correctionsshould be lower than the errorin applying no correctionat all.

After application f these orrections, ebelievehat


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SIHRAET AL- UPDATEDHALOCARBONADIATIVE ORCING STIMATES1.0

0.9

,- 0.8o.--.

oo 0.7o

.--.

u.. 0.6

0.5

I Jain ta -total_lifetime/ Naik tal. totalifetime/ [3Naiket al. - stratosphericifetime/ fit o aintal.:1-0.241'358)

20,501

0.4 0.1i i i i ,, , ,1 I i i i , ,1o. oo. ooo. iooo.

Lifetime years)Figure . The orrectionactorsequiredocalculateadiativeorcingshichccountorstratosphericecayromhosealculatedith uniformrofilereplottedsa functionflifetime.he lot howsnempiricaleastquaresit o he orrectionactorsrom ain tal.[2000]oestimateheactorsequiredor asesherehemodel-derivederticalrofilesre otavailable.

the uncertaintiesn the calculatedorcingsrisingromdifferent reatments f the verticalprofileof the gaseswill be comparableo the errorsn the spectroscopicmeasurements,he radiation cheme,nd he choice ftropopauseeach f the orderof 5-10%).5.2. Results

The globaland annualmean orcing alues, ver-agedwheremore hanonecrossection asused, reshown n Table 3. These assume hat radiative forcingvariesinearlywith cross ection, validassumptionftemperature-dependentand hangesreassumedo benegligiblesee hristidist al.,1997] nd hemagnitudeof errorsn the spectra, uch sbaseline rifts,are fre-quencyndependent.he orcingsf Jainet al. [2000]and WMO [1999] avealsobeen ncludedor compar-ison. HFC-245fa s not included y WMO [1999],andso he lifetimeandreferenceorcing f Ko et al. [1999]has been used instead. The 100-year global warmingpotentialsGWPs),computedsing ur orcingsndthe WMO [1999]ifetimes,regivenn Table4. Theradiativeorcingshow ood greementn general, ith51 of the65 gasesgreeingo within15%of the WMO[1999]aluesalthought shouldenotedhatsome fthe WMO 1999] alues ere ased nourownearlierwork).

For the instantaneousorcings not shown), he un-calibrated NBM valueswere found to vary between98and 104%of the correspondingRFM valuesor all butfivegases. hese emainingases,cetylene,enzene,toluene, erfluoromethane,ndmethyl romide, ll ex-hibit sharpspectraleatures nd showed eviations fup to 13%. This llustrateshat the moreapproximatemodel, he NBM, in general erforms erywell exceptwhen the infrared spectrumcontainssharply varyingspectraleatures. This "problem"s more ikely tobe observedwhen cross-sectionalpectra are used forlightermoleculese.g.,acetylene)inceor moleculartransitionshe line spacings inversely roportionalomolecularmomentsof inertia (henceheavymoleculesgenerally avestrongly verlappinginesand essde-tailed inestructure)see, .g., Griffiths nddellaseth,1986].To investigate hetherhis artifactof the NBM af-fects he adjusted,cloudy-skyorcingsn a consistentmanner,we nvestigaten alternativemethod f forcingagreementetweenheFRFM and heNBM.Thecrosssections f eachof the above ivegases nd CFC-11, rep-resentinghe "typical" ase, ere caledn frequencyythe differencen the global-meanorcing pectra f theNBM and FRFM for the instantaneous lear-sky orc-ing.These ffectiverossectionserehenusedn the


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20,502 SIHRA ET AL.: UPDATEDHALOCARBONRADIATIVE FORCINGESTIMATESTable 4. Global Warming Potentials GWPs) Relativeto CO2, CalculatedUsingLifetimes rom WMO [1999],the Lifetime-Corrected Forcingsof This Study, and a100-Year Time Horizon a

Forcing, Lifetime, 100-YearGas W m ' ppbv years GWPCFC-11 0.242 45 4,500CFC-12 0.321 100 10,600HCFC-123 0.141 1.4 80HCFC-124 0.193 6.1 540HCFC-141b 0.130 9.2 650HCFC-142b 0.163 18.5 1,900HCFC-21 0.140 2 170HCFC-225ca 0.202 2.1 130HCFC-225cb 0.280 6.2 540HCFC-22 0.208 11.8 1,800HFC-125 0.223 32.6 3,600HFC-134 0.180 10.6 1,200HFC-134a 0.159 13.6 1,300HFC-143 0.118 3.8 340HFC-143a 0.148 53.5 5,000HFC-152a 0.095 1.5 140HFC-161 0.024 0.25 10HFC-227ca 0.249 32.6 2,900HFC-227ea 0.256 36.5 3,200HFC- 236cb 0.217 14.6 1,300HFC-236fa 0.251 226 8,400HFC-23 0.171 243 13,000HFC-245fa 0.241 7.6 860HFC-32 0.105 5.6 710HFC-41 0.023 3.7 160HFE-245fa2 0.332 4.4 610HCFE-235da2 0.372 2.6 330HFE-125 0.407 165 14,000HFE-143a 0.172 5.7 620HFE-7100 0.347 5 440HFE-7200 0.303 0.77 60H-Galden 1040X 0.987 48 8,700Halon 1211 0.263 11 1,100Halon 1301 0.289 65 6,200CF4 0.116 50000 8,300C.F6 0.266 10000 12,100CFs 0.279 2600 9,200CaFs 0.314 3200 9,700C4Fo 0.370 2600 9,600CFaI 0.168 0.005


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SlHRA ET AL.' UPDATED HALOCARBON RADIATIVE FORCING ESTIMATES 20,503Table 5. Radiative ForcingsCalculated Using Measuredand Effective CrossSections or CFC-11and GasesWith Sharp Spectral SignaturesGas

Measured Cross Section Effective Cross SectionFRFM Adjusted Adjusted and FRFM Scaled Instantaneous Adjusted

CFC-11 0.3283 0.2710 0.2689 0.3283 0.2689Acetylene 0.0591 0.0466 0.0507 0.0591 0.0511Benzene 0.0166 0.0138 0.0134 0.0166 0.0130CF4 0.1350 0.1058 0.1168 0.1349 0.1168CHaBr 0.0078 0.0062 0.0065 0.0077 0.0065

aThe adjusted orcing refers to the narrowbandmodel (NBM) adjusted, cloudy-sky orcing. For themeasuredcrosssection, "scaled" epresents he final adjusted forcing after calibration to the FRFM values.The effectivecrosssection s calculatedby multiplying the measured rosssectionby the difference n clear-sky global forcing, as a function of frequency,between he RFM and NBM. In this case he instantaneousandadjusted NBM forcings herefore require no linear scalingcalibration, and the differencebetween columnsfourandsixhence escribeshe reliability f thismethod.Values re given n W m 2 ppbv-1.

Improvementsmade to the NBM, used n the studiesofPinnocket al. [1995],Christidiset al. [1997],and thisstudy, will also contributemarginally o the forcingdif-ferences.The forcing for HCFC-21 also showsa signif-icant differencerom that of WMO [1999] rom Chris-tidis et al. [1997]. n that studya cloudy-sky, djustedforcing f 0.185W m 2 ppbv wascalculatedor a uni-form profile comparedwith 0.173 n this study). A life-time corrected alueusing he WMO [1999]correctionwould hereforegive 0.148 W m 2 ppbv-(comparedwith 0.140here) and not 0.17 Wm -2 ppbv, as in-correctly abulatedby WMO [1999].PerfiuoromethaneCF4) is a strongergreenhouseasin this study han reported y WMO [1999] rom Myhreand$tordal 1997], ndby Jain et al. [2000].The cross-sectional pectrumof McDaniel et al. [1991]used orboth of these studies was recorded at a higher resolu-tion than the Ford spectrum, but bands and particu-larly strongines n the bandcenters, t 1283cm , for

example, are much weaker, giving an integrated crosssectionapproximately73% that of the Ford spectrum.This is due possibly to band saturation, as suggestedby Roehl et al. [1995]; he integratedcrosssectionofRoehlet al. [1995]agreeswith that used n this studyto within 7%, supporting he value derivedhere.The forcing or HFE-143a in this study s 36% lowerthan that reportedby WMO [1999],basedon the re-sultsof Good t al. [1998].The absorption ross ectionusedby Good t al. [1998]wassimulatedromcalculatedrotational/vibrational requenciesnd absolutentensi-ties as discussed y Goodand Francisco 1998]. Thesimulated spectrum showsdifferences n spectral fea-tures, which are attributed to convolvedor unresolvedsignaturesn the measured pectrum,although he Fordspectrum, t a resolution f 0.06 cm , showsdenti-cal features. The calculated aw forcingvaluesof Goodet al. [1998]usesimulated pectra,with values aryingfrom 6% (CFC-12) to 39% (HFC-134) stronger han

Table 6. Gases or Which the Global Mean Radiative ForcingCalculatedHere Differs From ThoseReportedby WMO [1999]by More Than 15%Forcing,W m- 2 ppbv-1

PercentageGas This Study WMO [1999] Difference ReferenceHCFC-21 0.140 0.17 -17.4HCFC-123 0.141 0.20 -29.7HCFC-142b 0.163 0.20 -18.4HCFC-225ca 0.202 0.27 -25.1HFC-32 0.105 0.13 -19.2HFC-134a 0.159 0.19 -16.6HFC-152a 0.095 0.13 -26.9HFC-161 0.024 0.03 -20.9HFC-227ca 0.249 0.31 -19.6CHCla 0.094 0.02 +370CHaBr 0.005 0.01 -52.8CFI 0.168 0.23 -27.0CF4 0.116 0.08 +46.3HFE-143a 0.172 0.27 -36.2

Christidiset al. [1997]Fisher et al. [1990]Fisher et al. [1990]Pinnocket al. [1995]Fisher et al. [1990]Uhristidiset al. [1997]Uhristidis t al. [1997]Uhristidiset al. [1997]Uhristidiset al. [1997]Myhre and Stordal 1997]Goodet al. [1998]

aThe sourceof the WMO [1999]data, where available, s indicated.


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20,504 SIHRA ET AL.: UPDATED HALOCARBON RADIATIVE FORCING ESTIMATES

those eportedby WMO [1999]. t is therefore ot in-consistentwith the overestimatedorcingof HFE-143a,as observed here.The remaining ases f WMO [1999] re based itheron the resultsof Fisher et al. [1990]or on "grey" it-erature where no comparison s possible. In particular,

the large difference n forcing or trichloromethane an-not be accountedor, since he WMO [1999] aluewasnot basedon a publishedestimate. The calculations yFisher et al. [1990]are basedon a radiative-convectivemodel with broader spectral bandscomparedwith thoseused here. Furthermore, a differing tropopauseheightdefinition can also affect the forcing value by up to9% [e.g.,Freckleton t al., 1998],whilediffering bsorp-tion cross sections have been shown to produce errorsof a similar magnitude. Different vertical profiles nthe referenceprofile and spectroscopic ata also com-bine as second-order effects to enhance this difference.It is impossible o quantify exactly the contribution ofthese effects to the differences observed between the twostudies given that this information was not published.A further nine trace gases HCFC-124, HCFC-225cb,HFC-23, HFC-227ea, HFC-236fa, C4Fs, C4F0, HFE-7100'andhalon-1211)show adiative orcingdifferencesfrom WMO [1999]of 10-15%. These orcings re basedmainly on the resultsof Fisher et al. [1990],on greyliterature, or on calculationsusing simple models. Er-rors of this magnitude are typically attributable to anyof the factors mentioned previously and are thereforeharder to assess iven the lack of information. The ra-diative forcings or these gases, and indeed all of thegases eported here, have been calculated in a consis-tent manner and hence are easier to compare relativeto a fixed forcing, such as CFC-11, if desired.

Our values can be comparedwith the comprehensivestudyof Jain et al. [2000],mostdirectlyby comparingthe constant profile cases n both studies. In general,the agreement s excellent. Of the 26 gases ommon oboth studies, 11 agree to better than 5%, while a fur-ther 6 agree o within 10%. However, n addition toCF4 whichwas discussedbove, he Jain et al. [2000]values or four gasesare much higher than ours. TheseareHFC-134a 23%higher),HFC-227ea 24%),HFC-23(34%), and HFC-32 (35%); by coincidence,his list in-cludes he two most abundant HFCs in the atmosphere:HFC-134a and HFC-23.

We are unable to fully explain these discrepancies.Differences n the absorption crosssection can explainabout half the discrepancy for HFC-23 and one thirdthat for HFC-32 but very little for the other two gases.The narrowband and line-by-line results agree well forthese gases, and our narrowband model is similar tothat usedby Jain et al. [2000]. It will probablybenecessaryo do a tight intercomparison f the two setsof work to resolve the cause of these differences.

6. ConclusionsCloudy-sky, djusted adiative orcings or 65 halo-carbonsand nonmethanehydrocarbons ave been cal-culated usinga narrowbandmodel and the FRFM line-by-linecode. The FRFM wascomparedwith other ine-by-line adiationcodes nd found o agree n irradianceto better than 1.5%and n CO forcing o betterthan1.0%. The FRFM was urthervalidated gainstob-servationsusing the ARIES instrument and found toagree o better than 1.8% n frequency-integratedadi-ance. Both models were used to calculate the radiativeforcings, sing bsorption ross ections easured rin-cipallyat the FordMotor Company. he radiative orc-ings or 11 of the gases,ncludingour somers, ave, othe author's nowledge,ot beenpublished reviously.The adjusted, loudy-skyadiative orcings re n goodagreement ith WMO [1999] aluesor 51 gases, hile14 gases howdifferencesreater han 15%. The agree-

mentbetweenur esults nd hose f Jainet al. [2000],for the 26 gasesn common,s generally xcellent, l-though four gaseswere found to differ by more than20%.The combination f the use of a validated ine-by-line code, he reassessmentf the infraredspectra, ndthe useof a singlemethodologyor all gases uggeststhat the forcings resented ereare a general mprove-ment over WMO [1999].The 65-moleculeata set re-ported here is the most comprehensive nd consistentdatabase et availableo evaluate he relative mpactofhalocarbons nd nonmethane ydrocarbons n climatechange.There remainsa need o compare bsorption

spectra, adiative orcingvalues, nd the impactof ver-tical profileson the forcing. The work presented erewill contribute o suchcomparisons.Acknowledgments. The work was funded by the De-partment of the Environment, Transport and the RegionsundercontractEPG/1/1/73. We thank A. Dudhia for pro-viding and supporting he RFM, J.P. Taylor for the provi-sion of ARIES data and E. J. Highwood for her help withthe SWAGG spectra and comments.We are grateful to tworeferees or many pertinent commentsand to A. Jain forclarifying some aspectsof his calculations.

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M.D. Hurley and T.J. Wallington, Mail Drop SRL 3083,Ford Motor Company,Dearborn, MI 48121-2053. (e-mail:[email protected])K.P. Shine and K. Sihra, Department of Meteorology,Uni-versity of Reading, Earley Gate, PO Box 243, Reading, RG66BB, England, UK. (e-mail: K.P. [email protected])(ReceivedApril 14, 2000; revisedAugust 16, 2000;acceptedOctober 5, 2000.)

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