Three-Dimensional Modeling of HCFC-123 in the Atmosphere

Mar 31, 2009 - luorocarbons, production and consumption of HCFC-123 is limited under the Montreal Protocol on Substances that Deplete the Ozone Layer...
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Environ. Sci. Technol. 2009, 43, 3208–3213

Three-Dimensional Modeling of HCFC-123 in the Atmosphere: Assessing Its Potential Environmental Impacts and Rationale for Continued Use DONALD J. WUEBBLES* AND KENNETH O. PATTEN Department of Atmospheric Sciences, University of Illinois, Urbana, Illinois 61801

Received August 18, 2008. Revised manuscript received February 14, 2009. Accepted March 8, 2009.

HCFC-123 (C2HCl2F3) is used in large refrigeration systems and as a fire suppression agent blend. Like other hydrochlorofluorocarbons, production and consumption of HCFC-123 is limited under the Montreal Protocol on Substances that Deplete the Ozone Layer. The purpose of this study is to update the understanding of the current and projected impacts of HCFC123 on stratospheric ozone and on climate and to discuss the potential environmental effects from continued use of this chemical for specific applications. For the first time, the Ozone Depletion Potential (ODP) of a HCFC is determined using a threedimensional model (MOZART-3) of atmospheric physics and chemistry. All previous studies have relied on results from twodimensional models. The derived HCFC-123 ODP of 0.0098 is smaller than previous values. Analysis of the projected uses and emissions of HCFC-123, assuming reasonable levels of projected growth and use in centrifugal chiller and fire suppressant applications, suggests an extremely small impact on the environment due to its short atmospheric lifetime, low ODP, low Global Warming Potential (GWP), and the small production and emission of its limited applications. The current contribution of HCFC-123 to stratospheric reactive chlorine is too small to be measurable.

Introduction 1,1-Dichloro-2,2,2-trifluoroethane, C2HCl2F3, or as it is commonly called, HCFC-123, a halogen-substituted ethane of exclusively industrial origin, is a nonflammable, volatile, colorless liquid at standard pressure and temperature and has a faint odor of ether. HCFC-123 has long been used in large refrigeration systems and, more recently, in a fire suppression agent blend that was approved as a Halon 1211 replacement agent in 1994 under the U.S. Environmental Protection Agency’s (EPA’s) Significant New Alternatives Program. The EPA approved this blend under the generic name HCFC Blend B. It is sold, for example, under the trade name ‘Halotron I’ by American Pacific Corporation. HCFC123 accounts for more than 95% of the content of the blend. Like other hydrochlorofluorocarbons (HCFCs), consumption (defined as production plus imports minus exports) of HCFC-123 in the developed (non-Article 5) countries is being * Corresponding author phone: (217)244-1568; fax: (217)244-4393; e-mail: [email protected]. 3208

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controlled under the Montreal Protocol on Substances that Deplete the Ozone Layer, with elimination of 99.5% of its maximum allowable consumption by the year 2020. However, in two earlier journal articles, Wuebbles and Calm (1) and Calm et al. (2) examined the potential impacts of HCFC-123 on stratospheric ozone and came to the conclusion that continued use of this compound for those limited uses would lead to insignificant effects on stratospheric ozone and that there are advantages in continued use of this compound for specific applications in comparison to the potential climate effects of switching to alternatives. As stated by the title of the Wuebbles and Calm paper (1), there is an environmental rationale for retention of some endangered chemicals. Ten years later, there is renewed discussion internationally about allowing continued use of some of the HCFCs with very low Ozone Depletion Potentials (ODPs). The purpose of this study is to update the understanding of the current and projected impacts of one of those chemicals, HCFC-123, on stratospheric ozone and on climate as well as discuss the potential environmental effects from continued use of this chemical beyond 2020 for specific niche applications so that decision makers have the best information possible. For the first time, the Ozone Depletion Potential for HCFC-123 is determined using a three-dimensional model of atmospheric physics and chemistry. All previous studies have relied on results from two-dimensional models that are generally no longer considered to be state-of-the-art tools for study of the complex atmospheric processes required for evaluating ODPs. Historically, the primary use of HCFC-123 has been as a replacement for CFC-11 in very large low-pressure air conditioning systems, called chillers. Such chillers are the core of central systems to air condition large buildings. The typical lifetime of a chiller is at least 25 years. Note that HCFC123 is used only in chillers with centrifugal compressors and not in other air conditioning technology. HCFC-123 was first commercialized in 1989 as a replacement for CFC-11. The majority of the new centrifugal chillers being produced are still using either HCFC-123 or HFC-134a. The amount of HCFC-123 banked in chillers throughout the world is 9130 tonnes in the U.S., 205 in Canada, 1800 in China, and 4290 in Japan (but this is the sum of HCFC-123 and HFC-134a for Japan) (3). UNEP (3) also implies that larger centrifugal chillers are primarily built for U.S. and China markets (40% each), with only 10% each going to Japan and Europe. If we assume 50% to perhaps a high end of 80% of the chillers in Japan are HCFC-123 (conversion rate to HFC-134a is unclear), then the total amount of HCFC-123 banked in chillers worldwide is 13285 to 14565 tonnes (or 29.3 million to 32.1 million lbs). UNEP (3) also estimates that chillers only have leakage emissions of about 0.5% per year (newer chillers likely have smaller emission rates), so unless there are significant releases other than leakage, the total emissions per year from chillers should be on the order of 66 to 73 tonnes (146,000 to 160,000 lbs). It is not discussed whether all of the HCFC-123 is emitted or reclaimed when the chiller is no longer used (as discussed later, the resulting effect on ozone for existing chillers is still small even if it is all released). In any case, as more companies reduce their use of HCFC-123 in chillers in the coming years because of concerns about the Montreal Protocol, the total emissions will likely decline. Although a relatively small use compared to refrigerant applications, the other major use of HCFC-123 that can lead to atmospheric release is in firefighting blends. HCFC-123 has been used as an effective streaming fire suppression agent both directly and in mixtures like Halotron I (4). The use of HCFC-123 in localized situations for fire suppression has 10.1021/es802308m CCC: $40.75

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advantages over other alternatives to Halon-1211 (e.g., see comparison of capabilities of different alternatives in Table 11-11 in UNEP (3)). EPA (5, 6) estimates that 210 tonnes (463,000 lbs) of Halotron I was sold in the United States for the year 2006. Since more than 95% of the raw material used to manufacture the Halotron I blend is HCFC-123, we will assume these amounts are equivalent to HCFC-123. The amount of production and use of HCFC-123 as a fire suppression agent outside the U.S. is believed to add no more than an additional 20%. This would lead to a total worldwide production of HCFC-123 for this use in 2006 of less than 250 tonnes. According to EPA (5), use of streaming agents like Halotron I are estimated to grow at a rate of roughly 3% per year over the next few decades. If we assume this holds for Halotron I, then by 2015, annual production of Halotron I would be 274 tonnes (326 tonnes assuming similar growth in the worldwide production). By 2025, production of Halotron I would be 379 tonnes (452 tonnes worldwide). The worldwide installed base for Halotron I currently (as of May 2008) is 1.8 million kg or 1800 tonnes (29). EPA (2006) estimates that about 3.5% of this class of agent is emitted each year (based on manufacturers information) or roughly 63 tonnes for the year 2006. Assuming an annual growth of 3% in production and 3.5% in emissions out to 2050 based on estimated sales of Halotron I would mean a rough estimate of an installed base of approximately 3000 to 3500 tonnes worldwide by 2015 and emissions of approximately 108 to 122 tonnes per year. For 2025, the installed base would increase to approximately 4700 to 5600 tonnes, with emissions of approximately 170 to 200 tonnes per year. HCFC-123 is used as an intermediate in some methodologies in the production of HFC-125 (CF3CHF2), of trifluoroacetylchloride, and of various agricultural chemicals. This use is of relevance only if the chemical handling and production process results in emissions of HCFC-123. While there are no available estimates of these emissions, the general opinion is that these emissions are very small. HCFC-123 also has had limited use in foam blowing processes and as a solvent. However, these uses have been very limited and the U.S. companies, i.e., DuPont and Honeywell, have moved away from HCFC-123 for these applications. There are no accurate estimates of current production or emissions for the use of HCFC-123 as a solvent, but all evidence suggests that the amount is very small. The current total annual worldwide market for HCFC123 is estimated to be on the order of 2500 tonnes (5.5 million lbs.) (29). Of this total, it is estimated that annual demand as a refrigerant is approximately 1140-1230 tonnes (2.5-2.7 million lbs.). Annual demand as an intermediate chemical is about 1140 tonnes -- very little of which is emitted to the atmosphere. Demand for fire protection use accounts for the rest, approximately 136 to 227 tonnes (300,000 to 500,000 lbs.). Several factors make it more difficult to estimate the total production and emissions occurring now. First, since very few companies now produce HCFC-123, the Alternative Fluorocarbons Environmental Acceptability Study (AFEAS) no longer estimates HCFC-123 based on compiling an aggregation of total production and emissions (doing so would reveal individual manufacturer market data). Second, chiller refrigerant losses have been significantly reduced over the past decade. Third, because of concerns about coming regulations and corresponding falling demand, some companies no longer develop chillers using HCFC-123. The banning or restrictions on use of HCFC-123 in parts of Europe for noncritical uses is further enhancing the overall decrease in use of this compound. With the assumption that refrigeration and fire suppression are the only uses of relevance, the total emissions per

year of HCFC-123 are currently approximately 130 to 135 tonnes. This can be contrasted with the use of other HCFCsfor example, for 2004 the emissions of HCFC-22 were approximately 310,000 tonnes (7). The vast majority of the HCFC-123 use and emissions is in the Northern Hemisphere, primarily at midlatitudes. McCulloch (7), for example, estimates that approximately 96% of the emissions of HCFC123 from chiller use occur in the Northern Hemisphere.

Methodology Once emission has occurred, the primary removal for HCFC123 in the atmosphere occurs through its reaction with the hydroxyl radical, OH. The bimolecular second-order reaction with hydroxyl (OH) is the dominant reaction, but HCFC-123 also has much less important reactions with chlorine atoms or excited oxygen atoms, O(1D). The reaction rate for HCFC123 with OH used in this study is k ) 6.3 × 10-13 exp(-850/ T), where T is temperature in K (12). Photolysis is not important except for those few molecules that reach the mesosphere. CF3C(O)Cl is the major reaction product of HCFC-123 with OH and subsequent reactions removing the first chlorine atom (13, 14). While much of the CF3C(O)Cl will be removed from the atmosphere through wet deposition (forming hydrochloric and trifluoroacetic acid), it is expected that the remainder will photolyze or react to quickly release the other chlorine atom. In this study, the state-of-the-art three-dimensional model of global atmospheric chemistry and physics developed by the National Center for Atmospheric Research (NCAR) called the Model for OZone And Related Tracers version 3 (MOZART-3) has been used to explicitly calculate the impact of halocarbon emissions added at the Earth surface into the atmosphere on ozone depletion. The MOZART-3 chemistrytransport model (CTM) includes a complete representation of tropospheric, stratospheric, and upper atmospheric processes (15). It incorporates a full stratosphere, including the chemistry of chlorine species (Cly) and bromine species (Bry) important in stratospheric ozone calculation as well as updated hydrogen, nitrogen, and hydrocarbon oxidation chemistry relevant to stratospheric and tropospheric chemistry included in the lower atmospheric version, MOZART version 2 (16). State-of-the-art representations of relevant heterogeneous and physical processes for winter/spring polar vortex related to ozone destruction are also fully included in the model. This model has been evaluated extensively via comparisons with measurements of atmospheric trace gases from satellite data and a large number of aircraft field campaigns (e.g., refs 15-18). Chemical reaction-rate constants and photochemical data for reactions other than those of HCFC-123 follow the recommendations of Sander et al. (19). The MOZART-3 CTM is driven by meteorology fields derived from the Whole Atmosphere Community Climate Model (WACCM) version 1b (20). The MOZART-3 thus has a 2.8° resolution in latitude and longitude and a hybrid sigma-pressure vertical coordinate including 66 layers from the surface to 5.1 × 10-6 mbar (approximately 140 km). The MOZART-3 model was used here to evaluate the atmospheric lifetime and Ozone Depletion Potential (ODP) for HCFC-123. A steady-state background atmosphere corresponding to the year 2000 was derived assuming fixed surface mixing ratios for long-lived gases (such as nitrous oxide, methane, chlorofluorocarbons, and other halogenated source gases) and fixed emissions for short-lived gases. Separate studies were then completed with added emissions of HCFC-123 and CFC-11 (for derivation of the ODP) to give about a one percent decrease in globally averaged ozone. Emissions of HCFC-123 were assumed to occur entirely on land at midlatitudes in the Northern Hemisphere, from 30 VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Derived annually- and zonally averaged distribution for HCFC-123 (in ppt) in the ODP study using MOZART-3 based on the assumptions described in the text. The figure is intended to show the relative changes in altitude and latitude of the derived distribution for this otherwise unrealistic scenario. The white dotted line in this and the following figures indicates the annually and zonally averaged average tropopause.

FIGURE 3. Derived annually- and zonally averaged change in ozone (in percent) based on the derived HCFC-123 using MOZART-3 based on the assumptions described in the text.

FIGURE 4. Derived annually- and zonally averaged change in ozone (in ppb) based on the derived HCFC-123 using MOZART-3 based on the assumptions described in the text. FIGURE 2. Derived annually- and zonally averaged change in reactive chlorine (Cly) (in ppt) based on the derived HCFC-123 using MOZART-3 based on the assumptions described in the text. to 60 °N (which corresponds to where the vast majority of current emissions also occur). For HCFC-123, the assumed surface flux was 6.34 × 106 tonnes per year (or 1.78 × 109 molec. cm-2 s-1 or) - this is much larger than the current flux but necessary to get a one percent globally averaged ozone destruction in the model (the standard perturbation for ODP studies historically to ensure statistically significant ozone perturbation). Each of the MOZART-3 perturbations was run to a steady state; this took ten model years for HCFC-123 due to stratospheric adjustment to the added chlorine. The resulting annually- and zonally averaged distribution of HCFC-123 (in ppt) is given in Figure 1, while Figure 2 gives the corresponding change in total reactive chlorine (Cly, in ppt). The resulting changes in annually- and zonally averaged ozone are given in Figures 3 (in percent change) and 4 (in ppb change). As expected, the derived changes in ozone occur primarily in the stratosphere, with changes in the upper stratosphere corresponding to the effects of chlorine gasphase catalytic cycles, and the changes in the lower stratosphere related to the effects of heterogeneous chemistry (11). Available atmospheric observations of HCFC-123 are very limited. Monthly mean baseline mixing ratios for HCFC-123 measured at Cape Grim in Tasmania from 1998 to 2004 (8) indicate that the mixing ratio in 2004 was approximately 0.064 ppt. A strong annual cycle is evident in their data because of the annual cycle in OH and the short atmospheric lifetime (∼1.3 years (11)) of HCFC-123. Overall the atmospheric mixing ratios have been increasing over the seven 3210

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year period at approximately 6 ( 1%/yr. A gas chromatograph-mass spectrometer measurement operated by the Norwegian Institute for Air Research at the Zeppelin station on Svalbard observed monthly mean HCFC-123 mixing ratios of 0.89 to 1.27 ppt during 2003 with indications of a slow decline since 2001 (9). These are the only reported measurements of HCFC-123 since a value of 0.03 ppt for 1996 reported for Cape Grim by Oram (10), which is consistent within uncertainty with the more recent Cape Grim measurements (8). The existing measurements are troubling because the differences between the Northern Hemisphere and Southern Hemisphere concentrations are larger than would be expected based on the atmospheric lifetime of HCFC-123. With approximately 96% of the emissions occurring in the Northern Hemisphere, we would expect the Northern Hemisphere concentrations of HCFC-123 to be about 2-3 times those measured in the Southern Hemisphere, not the 16 times found in the limited available observations. While we can speculate that the Northern Hemisphere measurements were perhaps affected by local emissions, the reason for the differences is unknown at this time. Nonetheless, the Zeppelin station measurement is still a very small concentration compared to the atmospheric concentration of other HCFCs such as HCFC-22, about 164 ppt in 2004 (11). We tried to use the model calculations used in the ODP study to estimate the current flux of HCFC-123 by comparing the mixing ratios of HCFC-123 measured at Cape Grim (8) and at Ny-Ålesund (9) during 2003 to the mixing ratios calculated in MOZART-3 in the surface layer at corresponding model grid points and to zonally averaged concentrations from the model. When we multiply the HCFC-123 flux used

in the MOZART ODP study by the fraction (mixing ratio in measurement)/(mixing ratio in MOZART-3) for either the Cape Grim or the Norwegian measurements, the flux required to sustain the measured HCFC-123 is greater than 2000 metric tonnes per year in both cases. Uncertainties in this analysis include the distinction between point measurements at a fixed location and model output on a 2.8° resolution grid in latitude and longitude; differences between the meteorology generated by WACCM 1b (20) and the actual Earth meteorology of 2003 (and prior years); and differences produced by our assumed emissions distribution versus the actual emissions of HCFC-123. The estimated flux based on MOZART-3 comparison to measurements is still more than a factor of 10 larger than the HCFC-123 flux both recommended by UNEP (3) of 66 to 73 tonnes per year and the larger estimates of 130 to 135 tonnes made above. WMO (2007) (11) estimates that the atmospheric lifetime of HCFC-123 is 1.3 years. The MOZART-3 model analyses give 1.27 years, in excellent agreement with the past estimates. Since it takes on average approximately three years for surface emissions at midlatitudes to reach the stratosphere, only a fraction of the HCFC-123 emitted at the ground would be expected to reach the stratosphere. Also, since it takes roughly a year for transport across the equatorial region into the Southern Hemisphere, the majority of the HCFC-123 emitted should be found in the troposphere of the Northern Hemisphere (as seen in Figure 1).

Results and Discussion Ozone Depletion Potential for HCFC-123. The concept of Ozone Depletion Potentials (21, 22) was developed in response to the growing concern of ozone depletion as a metric by which the significance of an ozone depleting substance would be calculated. An ODP is a relative measure of the expected cumulative effect on stratospheric ozone per unit mass emission of a gas compared with the expected effect from the same mass emission of CFC-11. Therefore, it is defined as the change in total ozone per unit mass emission of the gas, relative to the change in total ozone per unit mass emission of CFC-11 (22, 11). ODP by itself does not, however, indicate the actual emissions occurring or the amount of ozone depletion that has occurred or might occur in the future. ODPs are a single-value scientific index except for very short-lived compounds with lifetimes of less than one year. ODPs have proved valuable to policy considerations and are an integral part of national and international deliberations on ozone-protection policy, including the Montreal Protocol and the U.S. Clean Air Act. The ODP of CFC-11 by definition is 1.0 and, according to the U.S. Clean Air Act, 0.2 is the upper bound of ODP values for any permissible chemical. Additionally, the U.S. Environmental Protection Agency (EPA) has generally been concerned with chemicals possessing ODPs of 0.05 or larger, but this value is subjective and lower ODPs may still be of concern to some policymakers. Traditionally, ODPs have been evaluated on the basis that most chemicals evaluated have long enough lifetimes (greater than 1 year) to spread throughout the troposphere globally. The ODP concept was modified (23, 24) to better accommodate short-lived compounds with atmospheric lifetimes less than one year. Previously, the University of Illinois two-dimensional model calculated an ODP for HCFC-123 of 0.013 (25). The most recent WMO ozone assessment (11) gives a value of 0.02, but this is not based on new results but is a rounding up of earlier two-dimensional modeling studies (previous assessments had given an ODP of 0.014). From the results of this study using the MOZART-3 three-dimensional model, we derive an ODP for HCFC-123 of 0.0098, somewhat smaller but similar to previous estimates. The derived ODP for HCFC-

123 is among the smallest derived for the HCFC family of compounds or for the broader range of other ozone depleting substances (ODSs) (11). Overall Effects on Ozone. The concept of Effective Equivalent Stratospheric Chlorine (EESC) is often used to gauge the net effect of tropospheric halocarbon emissions and trends on the loading of ozone-depleting halogen in the stratosphere (e.g., refs 11 and 26). Given that human-related emissions of halocarbons containing chlorine and bromine are considered responsible for most of the decrease in stratospheric ozone over the last few decades, EESC is generally assumed to provide a rough estimate of the ozone depletion and the time scale for ozone recovery due to controls on human related halocarbon emissions in an otherwise unchanging atmosphere (which is not really quite the case because of, for example, changing emissions and concentrations of methane and nitrous oxide). Nonetheless, midlatitude stratospheric halogen levels and corresponding effects on ozone are often estimated using EESC based on ground based measurements of halocarbons and estimated future atmospheric concentrations based on assumed future emissions (usually following the Montreal Protocol) (11, 26). HCFC-123 emissions and atmospheric concentrations are usually not included in EESC derivations because the emissions and concentrations of HCFC-123 are too small to have a material effect on stratospheric chlorine. The total contribution of HCFCs to EESC is estimated by WMO (2007) not to exceed 100 ppt out of roughly 3000 ppt total, with most of the HCFC contribution coming from HCFC-22 and lesser contributions from HCFC-141b and HCFC-142b. HCFC-22, HCFC-141b, and HCFC-142b are scheduled for earlier production bans due to higher ODPs and are therefore not being considered for future exemptions to allow continued use. HCFC-123 has a negligible contribution in these analyses. Based on the Northern Hemisphere concentration estimated above, we estimate that the current contribution of HCFC-123 to EESC is less than 0.001 ppt (i.e., emissions would have to be 1000 times larger to have a 1 ppt contribution to EESC). If one tries to estimate the contribution of Halotron I to this, it comes out to be less than 0.0005 ppt. Note that this value is much smaller than earlier estimates made by EPA in the 1996 Federal Register; we think they meant to say LESS THAN 0.001 ppt but instead stated a value a thousand times higher, 0.001 ppb. Even if use of HCFC-123 were to expand 10-fold worldwide, the contribution of HCFC123 to EESC would still be less than 0.005 ppt. This is negligible compared to the roughly 3000 ppt of chlorine currently found in the lower stratosphere. The analyses in Wuebbles and Calm (1) and Calm et al. (2) came to the conclusion that even “worst case” scenarios for continuing future use of HCFC-123, including its use as a refrigerant, would only increase the equivalent chorine loading (e.g., EESC) by at most 0.002 to 0.007%, an indiscernible impact. The recent reductions in the estimated use of HCFC-123 would put the projections at the lower end of this effect on EESC. If Halotron I use were to grow at 3% per year over the next few decades, its contribution to EESC would still only be less than or equal to 0.0009 ppt in 2015 and less than or equal to 0.0014 ppt in 2025 assuming 3.5% loss per year (as emission rate). These estimates assume all of the chlorine reaches the stratosphere, so given the short lifetime of HCFC-123 and its designed use where the majority of it is converted to other chemicals in the removal of heat from the fire combustion zone, the actual contribution would be even less. Based on the previous discussion, the 2006 worldwide installed bank for Halotron I and HCFC-123 for fire extinguishing is estimated as 1800 tonnes (4.0 million lb.). Given the short atmospheric lifetime of HCFC-123, even a sudden release of all materials banked worldwide would not have a VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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significant impact (likely less than a few ppt) on the chlorine loading in the atmosphere. Also, the resulting very small effects on stratospheric ozone worldwide would likely not be measurable and would likely end after a few years. In a realistic situation, the emissions are expected to remain small enough that the equivalent chlorine would never get anywhere close to a level of concern relative to other forcings on ozone. Global Warming/Climate Impacts. The current atmospheric concentration of HCFC-123 is too small to have a measurable effect on climate compared to radiative forcing from increased carbon dioxide and a number of other radiatively important gases. Major absorption of infrared radiation (IR) by many relevant greenhouse gases occurs in the window region (8-12 µm). Information on where and how strongly a molecule absorbs infrared radiation is obtained from a determination of its vibrational spectroscopy. Essentially, any compound with a C-F, C-Cl, or C-H bond will likely absorb in the window region. The radiative efficiency (radiative forcing per ppb) of HCFC-123 is 0.14 W m-2 ppb-1 (11, 27). This is derived using the infrared absorption cross sections from the National Institute of Standards and Technology (28). The radiative forcing from wave numbers 1750 to 2000 cm-1 for HCFC-123 is then estimated to be less than 0.00002 W m-2. This value is negligible compared to the 2.5 W m-2 that has occurred from increasing concentrations of carbon dioxide and other greenhouse gas over the same time period (27). HCFC-123 has a 100-year integrated Global Warming Potential (GWP) of 77 (compared to a value of 1 for CO2) (11, 27). This value is comparable to or lower than many commonly used hydrocarbons and generally much smaller than the values for many other halogenated compounds. As an example, the 100-year GWP for HCFC-123 is much less than the value of 9810 for HFC-236fa, a comparable but less widely used alternative streaming agent in fire suppression applications. In terms of potential replacements as a fire suppression agent, no recognized HFC (hydrofluorocarbon) would have a lower GWP. Existing GWP studies (e.g., 11, 27) suggest that alternative molecules with very low GWPs would likely have too many hydrogens to be useful as a fire suppression agent (29). The current Halotron I blend formulation contains a small percentage of CF4, a high GWP gas. Accounting for the CF4 content, however, one could emit over 40 times the amount of Halotron I before one would have the same impact on climate as using HFC-236fa. However, CF4 lasts in the atmosphere for many thousands of years, so its effects on climate would continue for a very long time. The amount of CF4 emissions worldwide from Halotron I are currently estimated to be less than 1 tonne annually (29), but its long atmospheric lifetime and strength as a greenhouse gas make it a concern. As analyzed in this study, HCFC-123 has a very low impact on the environment (i.e., the current contribution of HCFC123 to stratospheric reactive chlorine is so small as to not be measurable) due to its short atmospheric lifetime, low ODP, low GWP, and the small production and emission of its limited applications. In addition, its atmospheric lifetime is long enough that HCFC-123 is not a volatile organic compound and should not have an effect on local and regional air quality. Given these small impacts on ozone and climate, limited use of HCFC-123 could potentially be permitted. However, these conclusions are based on analyses of only this compound, while decision makers consider contributions from a number of gases affecting ozone and climate change in developing appropriate policy. 3212

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Acknowledgments This research was supported in part from a contract with the American Pacific Corporation and from a grant with the U.S. Environmental Protection Agency. The views expressed are those of the authors and do not necessarily reflect those of the sponsoring organizations.

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