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Ix. Halocarbon Global Warming Potentials

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Ix. Halocarbon Global Warming Potentials IX. HALOCARBON GLOBAL WARMING POTENTIALS Relative Effects on Global Warming of Halogenated Methanes and Ethanes of Social and Industrial Interest D. A. Fisher and Charles H. Hales E. I. du Pont de Nemours & Company Wilmington, DE Wie-Chyung Wang, Malcolm K. W. Ko and N. Dak Sze Atmospheric and Environmental Research, Inc. Cambridge, MA _" / / EDING PAGE BLAr,_K NOT FILMED GLOBALWARMING EXECUTIVE SUMMARY Halocarbon Global Warming Potentials have been defined and calculated in order to allow estimates of the relative environmental effects of halocarbons to be made. The results presented here indicate that the HGWPs of the hydrohalocarbons depend primarily on the atmospheric lifetime of the compounds and to a lesser degree on the molecular IR absorption characteristics. The reduction in HGWP that might be expected due to use replacement of a CFC by a hydrohalocarbon can be estimated by taking the ratio of the HGWP of the hydrohalocarbon to the HGWP of the CFC it would replace. For example, the reduction in HGWP in replacing uses of CFC-12 by HCFC-134a is (0.26 ± .010)/(3.05) = 0.085 ± 0.003. Of course, the relative quantities of the compound required in the use application must also be taken into account. Although the HGWP values reported here agree between models reasonably well once accounting is made for the differences in lifetimes, uncertainties in the values still exist due to the uncertainties in mo- deled chemistry and dynamics and their direct effect on the chemical lifetimes of these compounds. We expect that these values will be updated once better data is available for the ultraviolet reactions and the hydroxyl radical reactions with the respective compounds. The HGWP values appear to be reasonably robust parameters since their calculated values are nearly insensitive to assumed values of other radiative gases. The minor shifting of the HGWP values in primarily influenced by the changes in calculated lifetimes and therefore the abundance in the atmosphere. Calculated time-dependent relative global warmings for halocarbons are initially on order unity but decrease or increase depending on whether their lifetimes are shorter or longer than that of the reference gas. At longer times, the Relative Global Warmings asymptotically approach the HGWP values. 381 PRECEDING PAGE BLA_'K NGT FILMED N92"15447 RELATIVE EFFECTS ON GLOBAL WARMING OF HALOGENATED METHANES AND ETHANES OF SOCIAL AND INDUSTRIAL INTEREST Donald A. Fisher and Charles H. Hales E.I. du Pont de Nemours & Company Wei-Chyung Wang, Malcolm K. W. Ko and N. Dak Sze Atmospheric and Environmental Research, Inc. _ING PAGE BLI'.',_K NOT FILMED GLOBALWARMING ABSTRACT The relative potential global warming effects for several halocarbons (CFCs -11, 12, 113, 114, and 115; HCFCs 22, 123, 124, 141b, and 142b; and HFCs 125,134a, 143a, and 152a; carbon tetrachloride; and methyl chloroform) have been calculated by two atmospheric modeling groups. These calculations were based on atmospheric chemistry and radiative convective models to determine the chemical profiles and the radiative processes. The resulting relative greenhouse warming when normalized to the effect of CFC-11 agree reasonably well as long as we account for differences between modeled lifetimes. Differ- ences among results are discussed. Sensitivity of relative warming values is determined with respect to trace gas levels assumed. Transient relative global warming effects are analyzed. 1. INTRODUCTION A systematic environmental evaluation of replacements for fully halogenated chlorofluorocarbons (CFCs) includes the potential effects of each replacement chemical on global warming. While the major focus for rising environmental concerns centers on the potential effects of long-lived CFCs on stratospheric ozone, the role of these gases as contributors to an enhanced greenhouse or global warming also needs examina- tion. This concern is based on the ability of these gases to absorb infrared radiation in the atmospheric 'window' between 8 and 12/am. First, a brief background to establish the role of these gases in the 'greenhouse warming' phenomena. The radiative and thermal balance of planet Earth is established primarily by balancing the incoming (UV and visible) solar energy to the Earth's surface and atmosphere with the outgoing (infrared) radiation from the Earth's surface and atmosphere eventually being lost to space. Infrared energy is partially blocked at many wavelengths by naturally occurring gases such as carbon dioxide, methane, and stratospheric water vapor. These gases absorb energy at fundamental frequencies characteristic of their structure. This energy contributes to local warming and is eventually re-radiated. Changing gas concentrations change the radia- tive balance of the infra-red cooling process. Hence, the concern that increasing CO2 concentration will significantly impede infrared cooling of the Earth with a decrease in energy loss to space and a corresponding increase in the Earth's surface temperature. The atmosphere 'window' between 8 and 12/am is virtually transparent, i.e. since few gases are present that absorb energy at these wavelengths, outgoing infrared radiation passes through the atmosphere essen- tially unimpeded. However, both C-CI and C-F bonds have natural vibrational, bending, and rotational excitation frequencies in this infrared frequency range such that CFCs absorb the infrared energy to be- come very effective greenhouse gases. Their effectiveness as greenhouse gases is accentuated by their long lifetimes. Since functional replacements may also absorb infrared energy in the window region, it is important to estimate their potential impact on global climate as part of this evaluation. As will be demon- strated below, the effectiveness of these gases is determined in large part by their lifetimes. Longer lived gases build up higher tropospheric concentrations. Of fundamental importance in determining the effec- tiveness as greenhouse gases are the infra-red band strengths, i.e. the wave length intervals and the amount of energy absorbed within these intervals by each molecule. Besides the influence on the infrared radiative fluxes at the top of the atmosphere and at the surface, there is yet another aspect of the radiative perturbations from halogenated compounds -- namely the addi- 385 PRECEDING PAGE B' _'_'" "-',,,a NOT F:LMED GLOBALWARMING tional heating induced in the tropical upper troposphere and lower stratosphere which has the potential to increase temperatures in this region. Any change in the temperatures near the tropopause region is of considerable significance for the tropospheric-stratospheric exchange water vapor. Candidate alternatives are composed of either carbon, hydrogen and fluorine (hydrofluorocarbons or HFCs) or carbon, hydrogen, chlorine and fluorine (hydrochlorofluorocarbons or HCFCs). For simplici- ty, both classes of compounds are referred to as hydrohalocarbons. Because they contain hydrogen, the hydrohalocarbons are subject to destruction in the atmosphere through reaction with hydroxyl radicals. This destruction mechanism leads to much shorter atmospheric lifetimes for the hydrohalocarbon com- pounds (see [Fisher et al. (1989a)] for a discussion of the chemistry affecting the atmospheric lifetimes of these gases). By contrast, the only known destruction process of CFCs is through photolysis in the upper stratosphere. As will be shown below, the shorter atmospheric lifetime of HCFCs and HFCs is a primary factor in reducing their potential to affect global warming. This paper examines the calculated greenhouse effects of several one and two carbon halocarbons. Esti- mates of these effects will be quantified in terms of a relative potential to enhance global warming (halocarbons global warming potential or HGWP). Sensitivity to assumed levels of trace gases will be examined. Tran- sient relative warming effects will be analyzed. Table 1 Compounds Examined in this Study HALOCARBON FORMULA IUPAC NAME CFC-| | CC13F METHANE, TRICHLOROFLUORO- CFC-12 CC12F2 METHANE, DICHLORODIFLUORO- CFC- 113 CCI2FCCIF2 ETHANE, 1,1,2-TRICHLORO- 1,2,2-TRIFLUORO- CFC- 114 CCC1F2CC1F2 ETHANE, 1,2-DICHLORO- 1,1,2,2-TETRAFLUORO- CFC-115 CCIF2CF3 ETHANE, CHLOROPENTAFLUORO- HCFC-22 CHCIF2 METHANE, CHLORODIFLUORO- HCFC-123 CF3CHC12 ETHANE, 2,2-DICHLORO-1,1, I-TRIFLUORO- HCFC- 124 CF3CHCIF ETHANE, 2-CHLORO-1,1,1,2-TETRAFLUORO- HFC- 125 CF3CHF2 ETHANE, PENTAFLUORO- HFC- 134a CF3CH2F ETHANE, 1,1,1,2-TETRAFLUORO- HCFC- 14 lb CC12FCH3 ETHANE, 1,1-DICHLORO-I-FLUORO- HCFC- 142b CCIFECH3 ETHANE, 1-CHLORO- 1,1-DIFLUORO- HFC- 143a CF3CH3 ETHANE, 1,1, I-TRIFLUORO- HFC- 152a CHF2CH3 ETHANE, 1, I-DIFLUORO- CARBON- TETRACHLORIDE CC14 METHANE, TETRACHLORO- METHYL CHLOROFORM CCI3CH3 ETHANE, I,I,I-TRICHLORO- 386 GLOBALWARMING Halocarbon Global Warming Potential is based on a concept similar to Ozone Depletion Potential and is used to describe the relative potential of each halocarbon as a greenhouse gas. No attempt is made to calculate HGWPs for non-halocarbon gases such as carbon dioxide and methane. Because of the current atmospheric concentrations and spectral locations of the infrared absorption bands of these other gases, calculated global warming is not a linear function with increases in their atmospheric concentrations. In contrast, a calculated warming is linearly proportional to concentrations of halocarbons. Thus, Green- house Warming Potentials for carbon dioxide and methane would not be meaningful. Two atmospheric modeling groups, Atmospheric and Environmental Research, Inc.
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