5. Halocarbons and Minor Gases
5. Halocarbons and Minor GasesIn addition to carbon dioxide, methane , and nitrous oxide , all of which are emitted in large volumes, there is a group of human-made compounds that are emitted in smaller volumes. These gases include chlorofluorocarbons (CFCs), bromofluorocarbons (halons ), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and other compounds that defy easy categorization.
These compounds are human-made gases that rarely or never appear in nature. Thus, their pre-industrial atmospheric concentrations can generally be assumed to be zero, and in their absence, the atmosphere is virtually transparent to radiation at the wavelengths they absorb. Consequently, adding even small amounts of these gases to the atmosphere causes the absorption of disproportionate amounts of infrared radiation. A kilogram of any of the gases listed in this chapter may have a direct warming potential hundreds or thousands of times that of a kilogram of carbon dioxide. However, emissions of all of the gases listed in this chapter combined total less than 1 million metric tons, a small fraction of emissions of carbon dioxide or methane. Table 32 lists the major gases covered in this chapter and, for each gas, its global warming potential , 1990 U.S. sales, estimated emissions, and principal uses.
The production and use of CFCs, HCFCs, and several other chemicals are regulated as ozone-depleting substances pursuant to U.S. adherence to the Montreal Protocol and under the Clean Air Act Amendments of 1990. U.S. production and use of these chemicals are being phased out. Several other minor greenhouse gases are also regulated as "hazardous airborne pollutants" under the Clean Air Act because they are known or suspected carcinogens.
The requirement to phase out CFC use has stimulated the production and, ultimately, emissions of a range of substitute chemicals. These new chemicals, typically HCFCs and HFCs, are much less damaging to stratospheric ozone, but they may have stronger effects on global climate. It is difficult to obtain information about CFC substitutes. Production and sales data are fragmentary. Collectors of statistics, such as industry groups, the U.S. International Trade Commission, and the EPA, are struggling to update their data collection efforts to track these new products. Many of these gases are currently produced and emitted in amounts less than 5,000 metric tons per year.
Emissions information is scarcer. The EPA recently added CFCs, halons, and several other chemicals to its listing of hazardous airborne pollutants as "ozone-depleting substances," and since 1991 has required establishments with "releases and transfers" of ozone-depleting substances in excess of 10,000 or 25,000 pounds to report emissions and recycling. These data are published annually in the EPA's Toxics Release Inventory (TRI). (116) Unfortunately, the data cover only a fraction of total emissions. Separately, the EPA has provided estimates of 1990 national-level emissions for most of the principal halocarbons. (117) There is rarely a straightforward linkage between production and emissions of these chemicals. CFCs, HCFCs, and HFCs are used primarily in three classes of applications:
Estimating the emissions of other gases presents diverse problems. Halons are used in fire extinguishing systems and generally are not released at all unless there is a fire or firefighting exercise, or the equipment fails or needs refurbishing. Other gases are used as feedstocks and generate only incidental fugitive emissions. Thus, estimating emissions for these gases is exceptionally difficult.
Table 33 summarizes available information about U.S. emissions of these gases. The emissions estimates are probably within 25 percent of the actual (unobservable) figure for the gases with larger emissions, and for the period before 1992. After 1991, the phasing-in of non-CFC substitutes and the expansion of recycling programs make estimation of emissions much more uncertain. Better data will probably be available in future years, as the International Trade Commission and the EPA upgrade their data collections.
While U.S. data for these gases are not very satisfactory, major chemical companies collaborate to report production and sales of CFCs and certain HCFCs through the Alternative Fluorocarbons Environmental Acceptability Study (AFEAS). (118) The world figures set a "ceiling" on estimated U.S. sales.
The following sections describe each type of gas in more detail.
CFCs are chemical siblings to hydrocarbons , particularly methane . Where a methane molecule contains a single carbon atom and four hydrogen atoms, in CFCs the hydrogen atoms are replaced with varying numbers of fluorine and chlorine atoms.
Ironically, it is the ozone-depleting potential of CFCs that renders their status as greenhouse gases uncertain. While CFCs absorb thousands of times more solar radiation, molecule for molecule, than does carbon dioxide, CFCs also destroy ozone . Ozone (O3) is an unstable form of oxygen (normally found as O2) whose capacity to absorb harmful ultraviolet radiation also makes it a potent greenhouse gas. Thus, while CFCs contribute to global warming by absorbing solar radiation directly, they contribute to global cooling by destroying ozone. Currently, scientists working with the IPCC are uncertain as to whether CFCs have a net warming or cooling effect.
For this reason, and because CFCs are already regulated under the Montreal Protocol , the Framework Convention on Climate Change (the Rio Treaty) specifically excluded CFCs from coverage as greenhouse gases. However, in this report, they are included for completeness.
Production and sales of CFCs are declining. The Montreal Protocol (signed in 1987) bound the United States and other signatories to phase out production of CFCs by the year 2000. The United States implemented the Montreal Protocol in the Form of Title VI (Stratospheric Ozone Protection) of the Clean Air Act Amendments of 1990. In regulations published pursuant to the Clean Air Act Amendments, the United States bound itself to phase out production and sales of ozone-depleting substances, primarily CFCs, by 1996. The regulations lay out a phaseout schedule that requires domestic producers of CFCs to reduce production to a specified fraction of 1986 production levels each year, declining to zero by January 1996.
CFC-11 is used primarily as a blowing agent for foam insulation, packing materials, and food packaging materials. CFC-11 is also used as a refrigerant in large commercial chillers. Production, sales, and estimated emissions of CFC-11 were all about 60,000 metric tons in 1990. (119) Based on the number of CFC-using chillers in the United States, the chiller market is probably no larger than about 10,000 metric tons, implying that the blowing agent market in 1990 was about 50,000 metric tons, down from about 70,000 metric tons in the mid-1980s. International Trade Commission data show that CFC-11 sales declined to 53,000 metric tons in 1992, implying a further shrinkage in the blowing agent market and consequent reductions in CFC-11 emissions.
Another source of emissions reductions will be recycling programs, which will both reduce emissions directly and extend the usefulness of CFC-using equipment. The Toxics Release Inventory reports "transfers to recycling" and disposals by large establishments. Reported recycling from this source was only 96 metric tons in 1992, and disposals (which presumably would prevent emissions) were 175 metric tons. (120)
CFC-12, commonly known by the trade name "freon," is the most ubiquitous CFC. In the late 1980s, CFC-12 usage was divided between automobile air conditioners, as a blowing agent for insulation and foam packaging, as a refrigerant in refrigerators and freezers, and as a refrigerant in commercial refrigeration and air conditioning systems. Production in 1990 was about 95,000 metric tons, with sales of about 82,000 metric tons. (121) Figure 7 illustrates estimated trends in CFC-12 production, sales, and estimated emissions. The trends shown in Figure 7 illustrate some of the difficulties in estimating emissions of halocarbons. After the signing of the Montreal Protocol , production and sales of CFC-12 rose rapidly in 1988 and 1989. However, there is no evidence that actual end-use consumption of CFC-12 rose. There was no boom in the sales of refrigerators, automobiles, or commercial chillers that would tend to account for this growth. The most likely explanation is stockpiling of CFC-12 by end users in anticipation of the end of CFC production in 1995. Similarly, sales of CFC-12 dropped sharply in 1990 and 1991, well in advance of any serious effort to phase out CFC-12 from its principal uses in automobiles and home refrigeration.
The EPA estimated 1990 U.S. emissions of CFC-12 at 112,000 metric tons. AFEAS data suggest that worldwide use of CFC-12 as a blowing agent shrank by 80 percent between 1988 and 1992. (122) It is probable that very little CFC-12 is now used as a blowing agent in the United States. On the other hand, replacement refrigerants for automobiles, refrigerators, and commercial air conditioning systems are only now becoming widely available, and emissions from the existing capital stock have probably changed little through the early 1990s. Therefore, it likely that the decline in post-1990 CFC-12 emissions has been very slow so far, with the disappearance of the blowing agent market being offset by increasing emissions from the growing stock of automobiles and refrigerators.
At present, domestic CFC-12 sales run tens of thousands of tons higher than domestic production, implying that manufacturers are selling previously stockpiled CFC-12.
Figure 7. Production, Sales, End Use and Estimated Emissions of CFC-12, 1980-1992
CFC-113, sometimes called "freon 113," is used almost entirely for solvent applications, particularly for cleaning electronic circuit boards. This application, and AFEAS information on CFC-113 end use, implies that virtually all CFC-113 is released to the atmosphere as soon as it is produced. No U.S. sales data have been available since 1986, when sales were reported at 78,000 metric tons, equivalent to about 40 percent of world sales of 200,000 metric tons. (123) The EPA estimated emissions of CFC-113 at 50,000 metric tons in 1990, implying U.S. 1990 sales of about 50,000 metric tons.
AFEAS data show northern hemisphere sales rising to 249,000 metric tons in 1989 and then declining to only 106,000 metric tons in 1992. (124) The emissions estimate shown in Table 33, which was calculated by scaling AFEAS data by the ratio of U.S. to AFEAS production in 1986 and 1990, shows rapid declines in emissions.
This estimate is supported by data from the Toxics Release Inventory, which reported emissions in 1991 at about 20,000 metric tons, declining to only 11,000 metric tons in 1992. (125) The Toxics Release Inventory indicates substantial CFC-113 recycling activity: firms reported recycling 2,555 metric tons of CFC-113 in 1992, while treating a further 570 metric tons. (126) TRI recycling and disposals have been deducted from the emissions estimate in Table 33.
CFC-114 is a specialty chemical used as a solvent. The U.S. Department of Energy (DOE) uses (and emits to the atmosphere) CFC-114 while enriching uranium for nuclear power plants and military purposes. The DOE's enrichment plant in Paducah, Kentucky (one of two in the United States) emitted 213 metric tons of CFC-114 in 1989. (127) Estimated 1986 production was 4,000 metric tons, 21 percent of 1986 world sales of 19,000 metric tons. However, according to AFEAS data, northern hemisphere CFC-114 sales declined to only 4,600 metric tons in 1991. The EPA estimated 1990 national-level emissions at 5,000 metric tons. Emissions by large users, as reported in the Toxics Release Inventory, were about 489 metric tons in 1992, down from 860 metric tons in 1991. (128) Declining TRI emissions and declining world sales imply a sharp reduction in emissions through 1992.
CFC-115 is used primarily as a blending agent for some specialty refrigerants. U.S. sales in 1986 were reported to be 10,000 metric tons, 87 percent of 1986 northern hemisphere sales reported by AFEAS. Since most CFC-115 production is in the United States, AFEAS data can be used to estimate post-1986 U.S. sales and production. AFEAS northern hemisphere CFC-115 sales rose to 14,000 metric tons in 1988 and 1989 and remained at 10,000 metric tons in 1992. All AFEAS CFC-115 sales were reported to be used for "long lifetime" applications, defined as having a retention period of 1 to 12 years.
The EPA estimated 1990 emissions at 3,000 metric tons. Emissions reported in the Toxics Release Inventory were about 160 metric tons in 1991, rising to 190 metric tons in 1992. (129) There was no recycling, and disposals were 0.05 metric tons in 1992. Based on the rising emissions in the TRI, and the absence of any decline in worldwide CFC-115 sales in the AFEAS data, CFC-115 emissions are unlikely to have declined since 1990.
HCFCs are chemical cousins of CFCs that contain at least one hydrogen molecule. HCFCs are less stable than CFCs, and consequently do not have the enormously long atmospheric lifetimes of CFCs. This makes HCFCs much less potent as ozone -depleting chemicals, compared to CFCs, and they are thus a common near-term choice to replace CFCs. HCFCs, however, are considered "Class II" ozone-depleting substances under the Montreal Protocol and the Clean Air Act Amendments, and their use is to be phased out by 2015. In regard to climate change, HCFCs combine higher global warming potentials with less important indirect cooling effects than CFCs. The EPA has added 11 HCFCs, including HCFC-22, HCFC-123, HCFC-124, HCFC-141b, and HCFC-142b, to the Toxics Release Inventory. Emissions of these HCFCs will be reported for the first time for the year 1994 and will be published in 1996. (130)
HCFC-22, with a global warming potential of 1,600, is used primarily in residential air conditioning systems. It is also the most widely available and least expensive potential substitute for CFCs in several applications. Sales in 1990 were 108,000 metric tons. The EPA estimated 1990 emissions at 82,000 metric tons. HCFC-22 sales in 1991 and 1992 continued at roughly 1990 levels, although U.S. production considerably exceeded sales. (131)
AFEAS data suggest that almost all HCFC-22 usage is for "medium lifetime" applications, although there has been a sudden increase in worldwide HCFC-22 usage for "short lifetime" applications since 1990. (132) The emissions estimate in Table 33 was calculated by prorating U.S. HCFC-22 sales into short-, medium-, and long-lifetime applications according to the shares shown for worldwide AFEAS data, assuming that short-lifetime applications emitted HCFC-22 immediately, and that medium- and long-lived applications emitted HCFC-22 over the subsequent 10 and 12 years, respectively. This calculation indicates rising HCFC-22 emissions, partly from the growing use of HCFC-22 as a replacement blowing agent, and partly as a delayed response to increasing HCFC-22 usage during the 1980s.
HCFC-142b, with a GWP of 4,000, is a CFC replacement that has been produced in small quantities since 1981. There are no available estimates of U.S. production, sales, or emissions. (133) However, the AFEAS collected information on northern hemisphere production and sales of HCFC-142b. This information puts a ceiling on possible sales or emissions in the United States. AFEAS reported 1990 world sales of about 19,000 metric tons, of which about 2,400 metric tons was for short-lived uses. (134) AFEAS data indicate that world sales rose to 31,000 metric tons by 1992, indicating increased use of HCFC-142b as a CFC substitute.
HCFC-123 is a replacement refrigerant for CFC-11 in commercial chillers. There was no known commercial production and no known emissions in 1990. However, this chemical may be of importance in the future. HCFC-124, with a GWP of 470, is an HCFC used as a CFC-12 replacement in sterilizers, with estimated 1990 emissions of about 3,000 metric tons. HCFC-141b, with a GWP of 620, has two uses: as a solvent and in a blend as a replacement blowing agent for CFC-11. The EPA estimated 1990 emissions of CFC-11 at about 300 metric tons.
Bromofluorocarbons are also similar to CFCs except that they include at least one bromine atom in their chemical makeup. Halon is a trade name for a family of these chemicals that are used as fire suppressants. Because halons are inert, nontoxic, and evaporate without leaving a residue, they are widely used in fire-extinguishing systems that protect high-value equipment, such as computer centers, military communications systems, and aircraft. Halons are being phased out pursuant to the Montreal Protocol , and production will end in 1996.
The U.S. military, which is one of the largest consumers of halons, is organizing a program to reduce emissions of halons through training with substitute chemicals, recycling of existing halons, and housekeeping steps to reduce leakage.
Emissions are uncertain, but are clearly very low. Sales in the mid-1980s totaled about 8,000 metric tons (3,000 tons of halon1211 and 5,000 tons of halon1301). The International Trade Commission reported 1992 sales of 4,500 metric tons of halon-1301. (135) The EPA estimated 1990 emissions of about 1,000 metric tons for halon-1211 and 1,000 metric tons for halon-1301.
HFCs are hydrochlorofluorocarbons (HCFCs) without the chlorine, composed of one or two carbon atoms and varying numbers of fluorine and hydrogen atoms. Several HFCs may be used as CFC replacements. Since they have no chlorine, HFCs have no effect on the ozone layer. Like HCFCs, however, they may have high direct radiative forcing effects. Thus, HFCs are unambiguously greenhouse gases , with substantial global warming effects.
The HFC with the largest current emissions is HFC-23. HFC-23 has no current uses, but is it emitted as a byproduct in the manufacture of HCFC-22. (136) HFC-23 emissions are estimated at 2 to 4 percent of HCFC-22 production, which implies emissions of about 3,000 to 5,000 metric tons per year. HFC23 has an exceptionally high global warming potential -12,600-with no known offsetting indirect effects.
Other HFCs include HFC-134a (used in automobile air conditioners as a CFC replacement, beginning in the early 1990s), with estimated 1990 emissions of 500 metric tons, and HFC-152a (a chemical used as a blending agent in the refrigerant R-500, which is typically used in dehumidifiers), with 1990 national emissions estimated by the EPA at 300 metric tons. HFC-134a is a strong greenhouse gas, with a GWP of 4,300, while HFC-152a is relatively weak greenhouse gas, with a GWP of 130. The President's Climate Change Action Plan proposed to reduce emissions of HFC-23 through a voluntary program with HCFC-22 producers.
AFEAS is collecting information on worldwide emissions of HFC-134a, to be published in late 1994. When available, the AFEAS data should give insight into the extent to which HFC-134a has replaced CFC-12 in automotive markets. There are a number of other HFCs that may be used as CFC replacements. However, no information is available about production, sales, or emissions of those chemicals, and it is likely that their emissions were negligible in the early 1990s. (137)
Perfluorocarbons are HFCs without the hydrogen. They are composed of one or two carbon atoms and 4 to 6 fluorine atoms. Like HFCs, PFCs contain no chlorine, so they do not affect the ozone layer and do not indirectly cause global cooling. PFCs are also unambiguously greenhouse gases . They have no commercial uses and are emitted as a byproduct of aluminum smelting. The EPA estimates that 0.6 kilogram of perfluoromethane , also called carbon tetrafluoride (CF4), and 0.06 kilogram of perfluoroethane (C2F6) are emitted for each metric ton of aluminum smelted, implying 1990 emissions of about 2,400 metric tons of CF4 and 240 metric tons of C2F6. (138) CF4 is also reportedly emitted by U.S. uranium enrichment plants, although estimates of the volumes are not available. (139)
Emissions of PFCs are unregulated and unreported. They do, however, have large direct global warming potentials (4,000 for CF4 and 9,000 for C2F6). The President's Climate Change Action Plan proposes to reduce emissions of PFCs through a voluntary program with aluminum producers.
Two other PFCs-perfluorocyclobutane (C4F8) and perfluorohexane (C6F14)-are known to exist, but there is no information on any sources of emissions for these chemicals.
Several other industrial chemicals have significant potential effects as greenhouse gases and are emitted to the atmosphere in substantial quantities. These include methyl chloride, carbon tetrachloride, sulfur hexafluoride , chloroform, and methylene chloride . Several of these chemicals are regulated as "hazardous airborne pollutants" under the Clean Air Act Amendments of 1990.
Carbon tetrachloride was a common household chemical and dry-cleaning solvent until it was discovered to be a carcinogen. At present, the principal use of carbon tetrachloride is as a feedstock in the manufacture of CFC-11 and CFC-12. Carbon tetrachloride is regulated as an ozone -depleting chemical under the Montreal Protocol and as a carcinogen under the Clean Air Act Amendments, and production must be phased out by 1996. Like CFCs, carbon tetrachloride has a high direct global warming potential (1,400) offset to an unknown degree by indirect global cooling effects. Sales of carbon tetrachloride peaked in 1988, along with the peak in CFC production, at about 385,000 metric tons. By 1991, sales had declined to only 173,000 metric tons. (140) Carbon tetrachloride is emitted to the atmosphere either as an incidental byproduct of CFC manufacture or as a result of its continued use in solvent applications. The EPA's Toxics Release Inventory estimated carbon tetrachloride emissions (from large emitters) at only 720 metric tons in 1990. (141) However, the EPA separately estimated 1990 national-level emissions at about 30,000 metric tons, almost all of which must be coming from solvent use of carbon tetrachloride. It is possible to generate a crude estimate of solvent use of carbon tetrachloride by "backing out" the quantities necessary for the manufacture of CFC-11 and CFC-12. This method does not produce a reliable time series, but it is useful in a general way. This method implies solvent usage of carbon tetrachloride of 30,000 to 40,000 metric tons annually, down from 50,000 to 70,000 metric tons in the early 1980s.
Methyl chloroform is a greenhouse gas with about the same global warming potential as nitrous oxide (its 100-year GWP is 110). In the United States, it is primarily used as a solvent. Like carbon tetrachloride, methyl chloroform is regulated as an ozone -depleting chemical under the Montreal Protocol and the Clean Air Act Amendments, and it is required to be phased out by 1996. Like CFCs, methyl chloroform has indirect cooling effects that may partially or completely offset its global warming effects.
U.S. sales were about 308,000 metric tons in 1990 and declined to 249,000 metric tons in 1992. (142) The EPA has also estimated national-level emissions of 316,000 metric tons in 1990. The EPA's national-level figure suggests that substantially all of the methyl chloroform sold in the United States is emitted to the atmosphere in short order, so this approach was used to estimate emissions for other years. Estimated recycling and disposals of 14,000 metric tons in 1992 by firms reporting to the Toxics Release Inventory were deducted from estimated emissions. (143) Without accounting for indirect cooling effects, the impact of U.S. methyl chloroform emissions on global warming would be equivalent to emissions of 7 to 9 million metric tons of carbon equivalent.
Chloroform is a relatively weak greenhouse gas with a GWP of 5. Its principal use is as a feedstock for the manufacture of HCFC-22, and secondarily as a solvent. There are no national-level emissions estimates currently available. Sales in 1990 were 196,000 metric tons. (144) Since most chloroform is not emitted to the atmosphere, but is converted to HCFC-22 or HFC-23, it is likely that national emissions are no greater than a few thousand metric tons annually. Chloroform is regulated as a carcinogen. The EPA's Toxics Release Inventory reported chloroform emissions of about 9,000 metric tons in 1992, down from 13,000 metric tons in 1988. (145) With its low GWP and low level of emissions, the warming effect of U.S. chloroform emissions is less than the equivalent of 0.02 million metric tons of carbon, even without considering its indirect effects.
Methylene chloride is also a weak greenhouse gas , with a global warming potential of 10. Like other chlorine-containing compounds, methylene chloride also has indirect effects on the ozone layer, but given its short atmospheric lifetime (less than 1 year) its indirect effects are probably not large. U.S. sales in 1992 were 136,000 metric tons. (146) Methylene chloride emissions are regulated as a potentially carcinogenic substance. Reported methylene chloride emissions under the Toxics Release Inventory were a substantial portion of U.S. sales: 59,000 metric tons in 1988, declining to 34,000 metric tons in 1992. (147) TRI reporting firms were also active in recycling and disposing of methylene chloride: these activities accounted for some 20,000 metric tons of methylene chloride in 1991 and 1992. The range of possible 1992 methylene chloride emissions is from 34,000 metric tons (the TRI number) to 82,000 metric tons (assuming that all unaccounted-for sales of methylene chloride are emissions). Assuming emissions at the top level, the impact of U.S. methylene chloride emissions on global warming is equivalent to emissions of 0.3 million metric tons of carbon, without considering indirect effects.
Sulfur hexafluoride (SF6) is a compound with a very high global warming potential (13,600). It is used in small amounts as an insulator for transformers, switchgear, and other electrical equipment. One recent article estimated global production and emissions at 5,000 metric tons in 1989, which sets an upper limit for possible U.S. emissions. (148) No further information is available on production, consumption, sales, or emissions of sulfur hexafluoride.
