Chapter 7
Evaluating the Potential Effects of Halon Replacements on the Global Environment 1
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Donald J. Wuebbles , Peter S. Connell , and Kenneth O. Patten 1
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Department of Atmospheric Sciences, University of Illinois, 105 South Gregory Avenue, Urbana, IL 61801-3070 Global Climate Research Division, Lawrence Livermore National Laboratory, Livermore, CA 94550
The variety of compounds being considered as replacements for halons needs to be evaluated to ensure that their use will not significantly impact the environment. This chapter examines the possibility of any concern about effects on global atmospheric ozone and climate from halons and their replacements, describes the approaches being used to evaluate these effects, and reviews existing evaluations of potential effects of these compounds on ozone and climate. These studies suggest that use of extremely long-lived gases such as perfluorocarbons and sulfur hexafluoride should be discouraged because of their potential significant effects on the radiative forcing affecting climate. Other compounds, including several HCFCs and HFCs, appear to have much less effect on ozone and climate than the compounds they would replace. Although initial studies of CF I indicate little concern, there remain uncertainties in its potential effects on ozone. 3
Under the Clean Air Act, the United States has essentially eliminated the production and import of the halons, C F C l B r (or H-1211), CF3Br (H-1301), and C 2 F B r (H2402), beginning January 1, 1994. This has been done in compliance with the international Copenhagen Agreement modification of the Montreal Protocol on Substances That Deplete the Ozone Layer (1). The primary environmental concern from use of halons and other bromine-containing compounds has been the potential for the bromine released from these compounds to destroy significant numbers of ozone molecules in the stratosphere. The halons being banned are part of the group of chemicals, including chlorofluorocarbons such as CFCI3 (CFC-11) and CF2CI2 (CFC12) and other chlorocarbons, that appear to be largely responsible for the significant decrease in stratospheric ozone observed over the last few decades (2-4). In addition to the concerns about their effects on ozone, the halons and other halocarbons are also greenhouse gases, with strong absorption features in the infrared. Growing atmospheric concentrations of these gases can lead to changes in the radiative forcing on climate, and thus to potential changes in the Earth's climate (5-7). Although national and international policy has not yet been developed and there 2
0097-6156/95/0611-0059$12.00/0 © 1995 American Chemical Society In Halon Replacements; Miziolek, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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remain significant uncertainties about the effects of human activities on climate, the concern about potential effects on climate are real and need to be considered. A variety of industrial and government groups are currently trying to find appropriate replacements for the many important uses of halons, particularly in fire fighting applications. As part of this search, the potential effects on ozone and climate from these replacements need to be evaluated. The purpose of this chapter is to examine compounds being considered as halon replacements and the approaches being used to evaluate the potential effects of these replacements on ozone and climate. Concentrations and Lifetimes The tropospheric concentration of a halon or replacement compound is dependent on the rate of emission into the atmosphere and the atmospheric lifetime of the constituent. Measurements of halons H-1211 and H-1301 show that their current global mixing ratios (the ratio of its volume density or concentration to the volume density of air) are about 2.5 and 2.0 pptv (parts per trillion by volume), respectively, and are currently increasing at about 3 and 8 % per year, respectively (8, 7, 3). These rates of increase have slowed appreciably in recent years, consistent with the reduction in production and emissions of these compounds (9,7). Despite such small concentrations, production of these compounds is being halted because of the capability of the bromine contained in these compounds to destroy ozone (see later discussion on ozone). Numerical models indicate that H-1211 and H-1301 are essentially nonreactive in the troposphere and are destroyed through photolysis in the stratosphere, resulting in atmospheric lifetimes of about 20 and 65 years, respectively (10). Because of their long atmospheric lifetimes, the destruction of these halons generally releases their bromine into the stratosphere where the bromine is most effective in affecting ozone. The long atmospheric lifetimes also imply that halons already emitted will be releasing bromine into the stratosphere for several more decades after production is stopped. A number of compounds are being evaluated as replacements for the current uses of halons. Compounds being considered include: perfluorocarbons, such as C2F6 (also referred to as Fluorocarbon 116, FC-116), C F (FC-218), C4F10 (FC-31-10), and C6F14 (FC-51-14); hydrofluorocarbons, such as C3HF7 (HFC-227ea), C2H2F4 (HFC134a), and C H F (HFC-23); hydrochlorofluorocarbons, such as CHF C1 (HCFC-22), C2HF3CI2 (HCFC-123) and C H F C 1 (HCFC-124); and several other compounds, such as CF3I and SF6. A list of these compounds is given in Table I. (based on information concerning halon replacements under consideration provided by the U.S. Environmental Protection Agency). In some applications, mixtures of such compounds are being considered; only the individual compounds are examined here although the effects of a mixture can be evaluated in terms of the proper ratios of the individual effects. The atmospheric lifetime of such compounds is important to determining their potential effects on ozone and climate including calculating the indices Ozone Depletion Potentials and Global Warming Potentials. 3
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Atmospheric Lifetime. After emission into the atmosphere, the time scale for removal of a gas, its atmospheric lifetime, is generally defined as the ratio of total atmospheric burden to integrated global loss rate. The lifetime is the time it takes for the global amount of the gas to decay to 1/e or 36.8% of its original concentration after initial emission into the atmosphere. The atmospheric lifetime integrates over spatial and temporal variations in the local atmospheric chemical loss frequencies for the compound. The lifetime must take into account all of the processes determining the removal of a gas from the atmosphere, including photochemical losses within the atmosphere (typically due to photodissociation or reaction with OH), heterogeneous removal processes (e.g., loss into clouds or into raindrops), and permanent removal
In Halon Replacements; Miziolek, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
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7. WUEBBLES ET AL.
uptake by the land or ocean. Atmospheric lifetimes of a number of gases have been determined based on current knowledge of these loss processes; these lifetimes have recently been updated for the IPCC (7) and W M O (3) assessments. As shown in Table I, atmospheric lifetimes of greenhouse gases of interest range from a few days (e.g., for CF3I) to thousands of years (e.g., for SF6 and several perfluorocarbons). In Table I, most of the values for atmospheric lifetimes are based on the recent international assessments and references therein. The lifetime for H 2402 has not been reevaluated and is based on an earlier ozone assessment (2), while lifetimes for perfluorocarbons have been evaluated by Ravishankara et al. (11). The lifetime for C3F8 is not available, but, using other perfluorocarbons as a guide, is likely to be on the order of 5000 years. The atmospheric lifetimes of the perfluorocarbons and SF6 are extremely long, implying that any emissions of these gases will remain in the atmosphere for well over a thousand years. Lifetimes of HCFCs and HFCs range from very short lifetimes for gases such as HCFC-123 that react rapidly with hydroxyl (OH) to lifetimes of comparable size to the halons. The atmospheric lifetime of CF3I is extremely short, on the order of a few days, due to its photolysis at near-ultraviolet wavelengths (similar results found both with our model and in ref. 13). Table I. Atmospheric lifetimes and calculated Ozone Depletion Potentials for halons and potential replacements
Species
Chemical Formula
Atmospheric lifetime Ozone Depletion (years) Potentials (ODPs)
Halons H-1211
CF ClBr CF Br C2F4Br2 2
H-1301 H-2402 HCFCS. HCFC-22 HCFC-123 HCFC-124 HFCs HFC-23 HFC-32 HFC-125 HFC-134a HFC-227ea PFCs FC-116 FC-218 FC-31-10 FC-51-14 Other Sulfur hexafluoride Trifluoroiodomethane
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CF HC1 C2F HC1 C F HC1 2
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2
2
4
CHF3
CH F C HF CH FCF 2
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2
5
2
3
C3HF7
C F C F 2
6
3
8
C4F10 C6F14
SF
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CF3I
20 65 22
5 13 7
13.3 1.4 5.9
0.05 0.02 0.03
250 6.0 36 14 41
