|
Report Contents Report#:EIA/DOE-0573(98)
Related Links |
Criteria Pollutants That Affect Climate
Beyond the three principal gases (carbon dioxide, methane, and nitrous oxide) that account for some 98 percent of U.S. greenhouse gas emissions weighted by global warming potential (GWP), there are an array of gases that affect climate in diverse ways. Some are engineered chemicals that do not occur in nature. The consequences of their emissions for the climate are considerably less than those of the principal greenhouse gases, for various reasons. In some cases, the quantities emitted are small; in other cases, the impact of a particular gas on the climate may be difficult to quantify or measure. The Kyoto Protocol has crystallized these ambiguities by defining three classes of gases that "count" for emissions estimation: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). This chapter describes emissions sources and estimates emissions for HFCs, PFCs, and sulfur hexafluoride. Emissions are also estimated for two other categories of gases that do not "count" under the Kyoto Protocol but do have indirect and difficult-to-measure effects on climate:
As a group, emissions of HFCs, PFCs, and sulfur hexafluoride are rising. In the case of HFCs, the rise in emissions reflects the introduction of HFCs specifically as replacements for CFCs, whose use is being phased out under the Montreal Protocol because they damage the Earth's ozone layer. CFCs have been widely used as refrigerants, aerosol propellants, and foam blowing agents for many years, but with CFC production virtually ceasing by 1996, HFCs have been introduced into the market to fill the void in many key applications. Emissions of PFCs and perfluoropolyethers (PFPEs) have also been rising in the 1990s (although not as rapidly as HFC emissions), mainly because of the recent commercial introduction of new PFCs and PFPEs both as CFC substitutes and for use in various applications in the semiconductor manufacturing industry. HFCs, PFCs, and sulfur hexafluoride are emitted in small quantities, but they have disproportionate effects because their long atmospheric lifetimes and extreme scarcity in the atmosphere give them extremely large GWPs. Sulfur hexafluoride is the most potent of the greenhouse gases, with a GWP of 23,900. PFCs also tend to have particularly high GWPs, falling in the range of 7,000 to 9,000. Among HFCs, HFC-23 is the most potent greenhouse gas, with a GWP of 11,700. Table 28 summarizes U.S. emissions of halocarbons and other gases from 1990 to 1998, and Table 29 shows U.S. emissions of HFCs, PFCs, and sulfur hexafluoride in million metric tons carbon equivalent. As Table 29 indicates, throughout the 1990s HFC emissions have accounted for roughly one-half of the total carbon-equivalent emissions of HFCs, PFCs, and sulfur hexafluoride combined. Table 28. U.S. Emissions of Halocarbons and Miscellaneous Greenhouse Gases, 1990-1998 The emissions estimates presented in Tables 28 and 29 are taken primarily from the U.S. Environmental Protection Agency (EPA) report, Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-1997.(48) The 1998 preliminary estimates for HFC-23, PFCs, and sulfur hexafluoride represent advance estimates developed by EPA and provided to EIA. The 1998 preliminary estimates for the remaining HFCs were developed primarily by extrapolating the trends shown in the EPA estimates through 1997. More information on methodologies and data sources for emissions of halocarbons and related gases is presented in Appendix A.
HFCs are compounds containing carbon, hydrogen, and fluorine. They do not destroy ozone. The market for HFCs is expanding as CFCs are being phased out. It is difficult to keep pace with the variety of HFCs that are being developed and the quantities being produced. Consequently, accurate data are difficult to obtain. HFC-23 Although emissions of HFC-23 are relatively small, its high GWP gives it a substantial direct effect. HFC-23 is created as a byproduct in the production of HCFC-22. Small amounts are also used in semiconductor manufacture and as a fire-extinguishing agent. The EPA has developed its most recent emissions estimates on the basis of actual measurements of feed components and HFC-23 process stream concentrations at HCFC-22 production plants. Using this approach, the EPA estimates 1997 HFC-23 emissions at 2,570 metric tons.(49) Annual emissions dropped by 22 percent between 1992 and 1995 but increased by 11 percent in 1997. Demand for HCFC-22 as a chemical feedstock is growing at a7-percent annual rate, and HCFC-22 is continuing to make inroads in refrigeration applications as a replacement for CFCs.(50) The Climate Change Action Plan (CCAP) includes a voluntary program with HCFC-22 producers to reduce HFC-23 emissions, which may help to offset the rising demand for HCFC-22 in the short term. HCFC-22 production (except for use as a feedstock) is scheduled to be eliminated by 2020 under the Copenhagen Amendments.(51) 1,2,2,2-Tetrafluoroethane (HFC-134a) HFC-134a, with a GWP of 1,300, is gaining importance as a replacement for CFCs, especially in automotive air conditioners. Emissions in 1990 were estimated at 560 metric tons, but they are growing rapidly (by 3,400 percent since 1990) (Table 28). In 1993, Ford Motor Company sold nearly 40,000 vehicles, each of which used approximately 2 pounds of HFC-134a in its air conditioner.(52) Previous models used about 2.5 pounds of CFC-12. Nearly all 1994 and subsequent model year automobiles use HFC-134a as their air conditioner refrigerant. In addition, HFC-134a conversion packages are now available for older cars. Automobile air conditioners are subject to leakage, with sufficient refrigerant leaking (15 to 30 percent of the charge) over a 5-year period to require servicing. On its Form EIA-1605, General Motors (GM) reported total HFC-134a emissions of about 2,150 metric tons from GM-made vehicles on the road in 1998.(53) GM based its estimate on an assumed annual leakage rate of 10 percent per year. With GM vehicles accounting for about one-third of the U.S. light-duty fleet,(54) the GM emissions estimate implies that total U.S. HFC-134a emissions from mobile air conditioners were equal to about 6,500 metric tons in 1998. Emissions from this source are expected to continue to increase in the near future, as the replacement of vehicles using CFCs proceeds at a rapid pace.In addition to its use in all new automobiles, an automotive aftermarket for HFC-134a has been developing. Spurred by rising prices for CFC-12, 5 million cars were retrofitted for HFC-134a use in 1997.(55) Furthermore, many of the air conditioners in mid-1990s models (which were among the first automobiles to use HFC-134a) are now due to be serviced. A spokesperson for Elf Atochem North America estimates the U.S. aftermarket for HFC-134a at 45 to 50 million pounds, or roughly 35 percent of total annual demand. He believes that, as the market for HFC-134a matures, the aftermarket will eventually be about twice the size of the original equipment market.(56) HFC-134a is also used as a refrigerant in most new refrigerators built in the United States and in commercial chillers, but leakage from these sources is much less than from automotive air conditioners. Leakage occurs primarily during servicing of the units rather than during normal operation. Short-term uses of HFC-134a, on the other hand, are becoming an important source of emissions. Such uses include aerosols and open-cell foam blowing, which are denoted as short term because most of the HFC-134a used will be emitted to the atmosphere within a short period of time. According to the Alternative Fluorocarbons Environmental Acceptability Study (AFEAS), worldwide sales of HFC-134a for short-term applications jumped almost fourfold between 1994 and 1995. However, sales for short-term uses leveled off at 10,000 metric tons in 1996 and then dropped to 7,400 metric tons (or 7 percent of sales for all uses) in 1997.(57) New developments in the U.S. market may, however, reverse this recent downward trend. Beginning in September 1998, U.S. regulations require aerosol manufacturers to use propellants with a maximum volatile organic compound (VOC) level of 45 to 55 percent. HFC-134a is one of only two chemicals that meet this requirement (the other being HFC-152a). Also, in January 1999, the major marketers of tire inflaters began requiring the use of nonflammable material, creating additional demand for HFC-134a. With global consumption of HFC-134a rising at an annual rate of 10 to 15 percent, there is some concern that demand will surpass supply in the near future. For many years, the HFC-134a market was characterized by excess capacity and low prices, but since November 1998 there have been three price increases, indicating a tightening of the market.(58) Elf Atochem estimates that in 1999 global demand for HFCs will be roughly equal to the total world production capacity of 120,000 metric tons. Yet neither Elf Atochem nor the global market leader, ICI, is planning to build new capacity, in part because of concerns about the possibility of new regulations stemming from the Kyoto Protocol. Demand is thus expected to exceed production starting in 2000.(59) It is possible that capacity constraints will act as a brake on HFC-134a consumption and emissions in the future. The recent tightening of supply in the United States has already caused producers to divert HFC-134a from the export to the domestic market.(60) 1,1-Difluoroethane (HFC-152a)As a non-ozone-depleting substance with a GWP of 140, HFC-152a is an attractive potential replacement for CFCs. It can be used as a blowing agent, an ingredient in refrigerant blends (e.g., in R-500), and in fluoropolymer manufacturing applications. It is also compatible with the components used in aerosol products. Unlike CFCs, however, HFC-152a is flammable. Only one U.S. company (DuPont) produces HFC-152a, using the trade name Dymel-152a. In 1995 the company reported having doubled its production capacity from 1992 levels, to 35 million pounds.(61) DuPont probably was producing HFC-152a at nearly full capacity in 1994, corresponding to production of about 8,000 metric tons. The company reported 1994 HFC-152a emissions of 180 metric tons on its Form EIA-1605. In 1997, however, DuPont's reported emissions dropped to only 36 metric tons. The EPA estimated 1990 emissions of HFC-152a at 1,500 metric tons, and they are believed to have been somewhat lower in 1998. Other HFCs Other hydrofluorocarbons with considerable radiative forcing potential include HFC-125 (C2HF5), HFC-143a (C2H3F3), HFC-227ea (C3HF7), and HFC-236fa (C3H2F6), with 100-year GWPs of 2,800, 3,800, 2,900, and 6,300, respectively. Emissions of these HFCs are small but growing rapidly, as they continue to find applications as substitutes for CFCs. The EPA estimated 1997 emissions at 3,570 metric tons for HFC-125, 430 metric tons for HFC-143a, and 170 metric tons for HFC-236fa.
Perfluorocarbons are compounds composed of carbon and fluorine. PFC emissions are not regulated or reported, although their high GWPs (6,500 for perfluoromethane and 9,200 for perfluoroethane) have drawn the attention of the CCAP. PFCs are also characterized by long atmospheric lifetimes (up to 50,000 years); hence, unlike HFCs, they are essentially permanent additions to the atmosphere. As byproducts of aluminum production, they arise during discrete periods of process inefficiency. Emissions can be reduced by improving process efficiency. The Voluntary Aluminum Industrial Partnership, aimed at reducing PFC emissions from the aluminum industry, is a CCAP initiative. The principal quantifiable source of PFCs is as a byproduct of aluminum smelting. The EPA estimates U.S. emissions at 1,430 metric tons of perfluoromethane and 140 metric tons of perfluoroethane in 1997. U.S. primary aluminum production has been increasing since 1994, and the trend is expected to continue as the automobile industry expands its use of aluminum.(62) Another source of PFC emissions is semiconductor manufacturing. Perfluoromethane and perfluoroethane are used as etchants and cleaning agents in semiconductor manufacturing. Although anywhere from 5 to 95 percent of the CF4 and C2F6 is destroyed, the process produces fugitive emissions of perfluoroethane, perfluoromethane, and sulfur hexafluoride. The United States consumed an estimated 800 tons of perfluoroethane and perfluoromethane in 1995.(63) The EPA's Climate Protection Division estimates that emissions of PFCs, HFC-23, and sulfur hexafluoride from the semiconductor industry totaled about 1 million metric tons carbon equivalent in 1994, with about 60 to 70 percent of GWP-weighted emissions consisting of perfluoroethane. (64) For 1996, the EPA estimates total emissions of all greenhouse gases from semiconductor manufacturing at 1.3 million metric tons carbon equivalent.(65) It is difficult to assess trends in PFC emissions from the semiconductor industry. On the one hand, the continued rapid expansion of the worldwide semiconductor market may lead to increased PFC use and emissions. On the other hand, industry efforts to curb emissions may help to offset these market forces to some extent. Since 1992, DuPont--the sole manufacturer of perfluoroethane--has been asking its customers to limit PFC use.(66) A number of semiconductor manufacturing firms have joined an EPA program to reduce PFC emissions voluntarily.(67) In 1999, the World Semiconductor Council, comprising manufacturers from Europe, the United States, Japan, and Korea, has voluntarily committed to reduce emissions of PFCs by 10 percent from 1990 levels. In addition, a number of PFC distributors are developing PFC emissions control equipment.(68) Recycling, abatement, and other control options remain in the early stages of development, however, and PFC substitutes are not yet available.(69) A variety of other perfluorinated compounds are beginning to be used in the semiconductor industry, including C3F8 (manufactured by 3M), C4F10 (with a GWP of 7,000), C6F14 (with a GWP of 7,400), NF3 (manufactured by Air Products), and CHF3.
Sulfur hexafluoride (SF6) is used as an insulator for circuit breakers, switch gear, and other electrical equipment. In addition, its extremely low atmospheric concentration makes it a useful atmospheric tracer gas for a variety of experimental purposes. It is also a fugitive emission from certain semiconductor manufacturing processes, and it is used as a cover gas during magnesium production and processing, to prevent the violent oxidation of molten magnesium in the presence of air. Sulfur hexafluoride has a high GWP of 23,900, but it is not produced or used in large quantities. In 1989, global production and emissions were estimated at 5,000 metric tons.(70) The EPA's estimates indicate a gradual increase in U.S. emissions between 1990 and 1995, from 1,120 metric tons to 1,530 metric tons and holding steady thereafter.(71) The impact of ozone-depleting substances on global climate is ambiguous, and they are not included among the greenhouse gases to be controlled under the Kyoto Protocol. Emissions of CFCs, HCFCs, halons, and other chlorine-containing gases are therefore considered separately in this report. CFCs are derivatives of hydrocarbons. Hydrocarbons are composed of hydrogen and carbon atoms. In CFCs, the hydrogen atoms are replaced with chlorine and fluorine atoms, yielding an array of usually nontoxic, nonflammable gases useful in a wide variety of applications. CFCs have no natural source, and their high molecular stability allows them to migrate to the stratosphere, where they destroy ozone. Although molecule for molecule they absorb thousands of times more infrared radiation than does carbon dioxide, their net warming effect is reduced because of their effect on ozone. Ozone (O3), beneficial in the stratosphere for its ability to absorb harmful ultraviolet radiation, is also a potent greenhouse gas. Thus, while the direct warming potential of CFCs is far greater than that of carbon dioxide, their indirect effect on ozone reduces their net radiative forcing effects by half (see discussion in Chapter 1). The Copenhagen Amendments of the Montreal Protocol requires the phaseout of CFC production by 1996.(72) The United States is implementing these provisions through the Clean Air Act Amendments of 1990 and subsequent EPA regulations, which specify allowable production quotas and taxes on inventories and stocks. All production ceased in January 1996, with the exception of small amounts used in metered dose inhalers for asthma patients, for which no substitutes are available. Emissions of CFCs contained in mobile air conditioners, chillers, and other equipment built prior to the regulations will continue at least into the next decade. CFC-11 is used principally as a blowing agent for foams and packaging materials and as a refrigerant in large commercial chillers. Sales have been declining steadily since 1989, with production following roughly the same trend, except for a spike in 1992.(73) In 1994, production and sales declined by nearly 80 percent, to only 7,000 metric tons,(74) implying that CFC-11 has been phased out of the blowing agent market completely, with residual CFC-11 probably used only to recharge existing chillers. CFC-12 is often known by its trade name, "freon-12." Exceedingly versatile, its end uses include air conditioning (both automotive and commercial); refrigeration (refrigerators and freezers of varying scales); and as a blowing agent for foams, insulations, and packaging. Pursuant to the Montreal Protocol, production and sales dropped dramatically in 1990 and 1991, falling below estimates of end-use applications and emissions. In recent years, end use has gradually declined with the ongoing phaseout of CFCs.(75) AFEAS data suggest that use of CFC-12 as a blowing agent dropped by more than 90 percent between 1988 and 1996.(76) The use of CFC-12 in refrigeration, however, declined more slowly until 1994. In 1994, automobile, refrigerator, and commercial chiller manufacturers essentially ceased using CFC-12 in their products. At present, emissions are being sustained by the existing stock of CFC-using equipment. Other CFCs include CFC-113, CFC-114, and CFC-115. CFC-113 and CFC-114 are used principally as solvents. EPA-estimated emissions of both of these CFCs have been declining rapidly since 1989, although small CFC-114 emissions from metered dose inhalers are likely to continue for a few more years. The EPA granted the International Pharmaceutical Aerosol Consortium 338 metric tons of CFC-114 essential use allowances for 1998.(77) CFC-115 is used primarily as a blending agent for some specialty refrigerants. CFC-115 emissions have also declined during the 1990s, although not as rapidly as CFC-113 and CFC-114 emissions. Hydrochlorofluorocarbons (HCFCs) HCFCs are essentially CFCs that include one or more hydrogen atoms. The presence of hydrogen makes the resulting compounds less stable, and as a result they are more susceptible to photodecomposition, have much shorter atmospheric lifetimes than CFCs, and are less likely to migrate to the stratosphere where they would destroy ozone. They are therefore popular interim substitutes for CFCs. The Copenhagen Amendments placed HCFCs under control, with HCFC-22 slated for elimination by 2020 and all others by 2030. HCFC-22 is the most commonly used refrigerant for home air conditioning systems. It is the most widely available and least expensive potential substitute for CFCs in a variety of applications; however, the available evidence suggests that HCFC-22 gained most of its market share at the expense of CFCs in the late 1980s. Nonetheless, use of HCFC-22 for long- and medium-lifetime applications has created a "banked" inventory of the compound that is now being emitted at a rate of 70,000 to 80,000 metric tons per year. A number of other HCFCs are gaining importance as CFCs are phased out. HCFC-141b is used primarily as a solvent and as a blowing agent for closed-cell foams, and HCFC-142b is used mainly for long-lifetime applications--particularly as a blowing agent for closed-cell foams. HCFC-123 is a replacement for CFC-11 in refrigeration applications, and HCFC-124 is a potential replacement for CFC-12 in sterilizers. EPA-estimated emissions of all these HCFCs have risen from zero in the early 1990s to 60,000 metric tons total in 1998. Bromofluorocarbons are similar to CFCs except that they contain at least one bromine atom. They are inert, nontoxic, and evaporate without leaving any residue, making them popular for use as fire suppressants for high-value equipment, such as computer centers and aircraft. The trade name "halon" is applied to several of these chemicals, which are used as fire suppressants. Halons are particularly destructive to stratospheric ozone; consequently, production ceased in 1996 pursuant to the Montreal Protocol. However, attempts to smuggle halon-1301 into the United States have been reported.(78) Emissions of halons are low, although the exact figure is uncertain. Several other chemicals, including carbon tetrachloride, methyl chloroform, chloroform, and methylene chloride, combine high GWPs and sufficiently high emissions levels to produce potential effects on global climate. Several of these chemicals are regulated under the Clean Air Act Amendments of 1990. Most carbon tetrachloride is used as a feedstock in the production of CFC-11 and CFC-12. Carbon tetrachloride is regulated by the Clean Air Act Amendments as a known carcinogen and under the Montreal Protocol as an ozone-depleting chemical. Production ceased in January 1996. Emissions declined rapidly in the 1990s and, according to the EPA, reached negligible levels in 1996. Emissions remained negligible in 1997. Like carbon tetrachloride, methyl chloroform is regulated under the Clean Air Act Amendments as an ozone-depleting chemical covered by the Montreal Protocol. Used primarily as a solvent, it was required to be phased out by 1996. Emissions have declined rapidly, from about 160,000 metric tons in 1990 to negligible levels in 1996 and 1997. For 1998, the EPA granted the National Aeronautics and Space Administration and the Air Force a total of 60 metric tons of essential use allowances for the use of methyl chloroform in cleaning, bonding, and surface activation applications on the space shuttle and the Titan rocket.(79) Chloroform is used primarily as a feedstock for HCFC-22, with secondary use as a solvent. It is a weak greenhouse gas with a GWP of 5. Total emissions should be low, because most chloroform is incorporated into HCFC-22 during its production. As a carcinogen, chloroform is reported to the EPA's Toxics Release Inventory (TRI). The TRI indicates that emissions have been decreasing and were only 3,365 metric tons in 1997.(80) Like chloroform, methylene chloride is a weak greenhouse gas (GWP of 9). Its short atmospheric lifetime of less than 1 year probably prevents it from reaching the stratosphere where it would be damaging to ozone. As a result, its indirect cooling effects are likely to be small. A potential carcinogen, methylene chloride emissions are regulated and included in the TRI, with 1997 emissions of almost 22,000 metric tons, down from 46,000 metric tons in 1990.(81) Criteria Pollutants That Effect Climate
Overview Certain criteria pollutants also affect climate: carbon monoxide (CO), nitrogen oxides (NOx), and nonmethane volatile organic compounds (NMVOCs).(82) The Clean Air Act of 1970 required that air quality standards be established for pollutants with adverse effects on public health or welfare. They are termed "criteria pollutants" because the EPA based each National Ambient Air Quality Standard (NAAQS) on health-based criteria from scientific studies. Although these gases are not considered to be greenhouse gases themselves, estimates of their emissions are presented here because of their indirect effects on atmospheric concentrations of greenhouse gases, including carbon dioxide, methane, and ozone. Ozone is produced largely from atmospheric chemical reactions involving the criteria pollutants. Ozone is highly reactive with other atmospheric gases, and its concentration is influenced by meteorological conditions. As a result, it remains in the troposphere for only hours or days. Hence, concentrations of tropospheric ozone tend to be centered around cities where high levels of criteria pollutants are found. Ozone concentrations are measured at individual urban sites throughout the United States. The EPA reported that the composite average ozone concentration for its 660 U.S. testing sites has declined by 19 percent since 1988.(83) The EPA Office of Air Quality Planning and Standards has compiled emissions data for the various criteria pollutants in the document National Air Pollutant Emission Trends Update, 1900-1997.(84) The emissions estimates in this report are taken from that document. The EPA continues to modify emissions data with improved estimation methods and updated information. Since the passage of the Clean Air Act of 1970 and subsequent amendments, implementation of pollution control measures and replacement of older, less fuel-efficient vehicles have restrained potential growth in criteria pollutant emissions that otherwise would have been expected from growth in the economy, increased driving, and expansion of industrial output. Current emissions of both carbon monoxide and NMVOCs are well below peak levels seen in the early 1970s, despite year-to-year fluctuations. Although emissions of nitrogen oxides are now higher than in 1970, the level of emissions has been relatively stable in the 1990s. Most emissions of carbon monoxide result from incomplete oxidation during combustion of fuels used for transportation. Transportation emissions, primarily from highway vehicles, accounted for about 77 percent of 1997 emissions. Total emissions of carbon monoxide in 1997 amounted to 79.2 million metric tons (Table 30). Between 1990 and 1997, carbon monoxide emissions decreased by 7.6 million metric tons (9 percent), due largely to decreases in emissions of approximately 6.9 million metric tons from highway vehicles (down by 13 percent). Table 30. U.S. Carbon Monoxide Emissions, 1990-1998 Carbon monoxide emissions from residential fuel use and forest fires also declined in 1997, by 0.9 million (25 percent) and 1.9 million (35 percent) metric tons, respectively, compared with 1990. Otherwise, 1997 emissions were generally the same as or slightly above 1990 levels. One exception was emissions from "other off-highway vehicles," which were not as well controlled as other sources. Emissions from this source during 1996 were 1.2 million metric tons (9 percent) higher than in 1990. Carbon monoxide emissions are expected to decrease through the year 2000 as a result of more stringent tailpipe standards and other factors. Nitrogen oxide emissions are related to air-fuel mixes and combustion temperatures during the burning of fuels. Emissions are reduced by the use of pollution control equipment, such as catalytic converters. Total U.S. emissions of nitrogen oxides between 1990 and 1997, have ranged between 21 and 22 million metric tons per year (Table 31). Although this does not represent a decline (as seen with the other criteria pollutants), it is much lower than the rate of growth in fuel consumption (such as consumption of gasoline by motorists and coal by electric utilities). Table 31. U.S. Nitrogen Oxide Emissions, 1990-1998 Total U.S. emissions of nitrogen oxide during 1997 (21.4 million metric tons) were about 1 percent higher than their 1990 level of 21.2 million metric tons. In the United States, the majority of nitrogen oxide emissions are from transportation and stationary fuel combustion sources. During 1997, nitrogen oxide emissions from transportation sources accounted for 10.5 million metric tons (49 percent) of total U.S. emissions of this gas, followed by stationary fuel combustion sources, which accounted for 9.7 million metric tons (45 percent) of total U.S. emissions of nitrogen oxide. Emissions are expected to decline with implementation of various additional emissions control measures.Nonmethane Volatile Organic Compounds NMVOCs are a principal component in the chemical and physical atmospheric reactions that form ozone and other photochemical oxidants. Nearly half (49 percent) of the 17.3 million metric tons of NMVOC emissions during 1997 came from industrial processes (Table 32), of which solvent use was the largest source. Most (79 percent) of the remaining 8.8 million metric tons of emissions were from combustion of transportation fuels. Table 32. U.S. Emissions of Nonmethane Volatile Organic Compounds, 1990-1998 Emissions of NMVOCs declined by some 10.5 million metric tons (38 percent) from 1970 to 1997, while fuel consumption in the transportation sector increased and activity in the industrial sector expanded.(85) This improvement was accomplished by some reformulation of petroleum products, implementation of pollution abatement measures, and changes in industrial processes. Emissions from solvent utilization declined as a result of the substitution of water-based emulsified asphalt for asphalt liquefied with petroleum distillates. Emissions of NMVOCs were 17.3 million metric tons in 1997, down by 0.1 million metric tons (0.4 percent) from their level in 1996, and down by 1.6 million metric tons (8.2 percent) from 1990 levels. Energy-related emissions have declined by 1.1 million metric tons (13 percent) since 1990, while emissions associated with solid waste disposal have declined by 0.5 million metric tons (55 percent) since 1990. |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
