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U.S. Emissions of Greenhouse Gases in Perspective Appendix A: Estimation Methods Appendix B: Carbon Coefficients Used in this Report Appendix C: Uncertainty in Emissions Estimates Appendix D: Emissions Sources Excluded Appendix E: Emissions of Energy-Related Carbon Dioxide in the United States, 1949-1997 Appendix F: Common Conversion Factors
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Overview
Beyond the three principal gases that account for some 98 percent of GWP-weighted U.S. emissions (carbon dioxide, methane, and nitrous oxide), there are an array of gases that affect climate in diverse ways. Most are engineered chemicals that do not occur in nature. The consequences of their emissions for the climate are considerably less than those of the other 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 rapidly. In the case of HFCs, the rapid 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 rushed 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 for use in various applications within 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 global warming potentials (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 approximate range of 7,000 to 9,000. Among HFCs, HFC-23 is the most potent greenhouse gas, with a GWP of 11,700. Table 31 summarizes U.S. emissions of halocarbons and other gases from 1990 to 1997, and Table 32 shows U.S. emissions of HFCs, PFCs, and sulfur hexafluoride in million metric tons carbon equivalent. As Table 32 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. The emissions estimates presented in Tables 31 and 32 are taken primarily from a draft EPA report, Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-1996, Review Draft (Washington, DC, March 1998). This is a departure from the emissions estimates presented in past years, which for 1990, 1994, and 1995 were drawn from EPA and other sources and for other years were estimated by EIA. The new estimates presented this year differ from past estimates as a result of differences between the methodologies and data sources used in the new EPA report and those used by the EIA and by the EPA in earlier sources. For HFCs, PFCs, and sulfur hexafluoride, the total new emissions estimates (expressed on a GWP-weighted basis) range from 12 percent to 21 percent lower than the earlier estimates for the 1990-1995 period. More information on methodologies and data sources for emissions of halocarbons and related gases is presented in Appendix A. Hydrofluorocarbons (HFCs) 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. Previously, HFC-23 emissions were estimated as being 2 to 4 percent of HCFC-22 production; however, 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 1996 HFC-23 emissions at 2,660 metric tons.(40) Annual emissions dropped by 22 percent between 1992 and 1995 before rebounding by 14 percent in 1996. Currently, demand for HCFC-22 as a chemical feedstock is growing at an annual rate of 7 percent, and HCFC-22 is continuing to make inroads in refrigeration applications as a replacement for CFCs.(41) 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 impact of rising HCFC-22 demand in the short term. HCFC-22 production (except for use as a feedstock) is scheduled to be eliminated by 2020 under the Copenhagen Amendments.
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 only 500 metric tons, but they are growing rapidly. In 1993, Ford Motor Company sold nearly 40,000 vehicles, each of which used approximately 2 pounds of HFC-134a in its air conditioner.(42) 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 out (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 nearly 1,500 metric tons from GM-made vehicles on the road in 1996.(43) GM based this 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, the GM emissions estimate implies that total U.S. HFC-134a emissions from mobile air conditioners were equal to about 4,500 metric tons in 1996. 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, a significant 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. A spokesman for Elf Atochem North America Inc. estimates that this number will grow to 10 million in 1998, as supplies of CFC-12 become tighter.(44) 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 significant than that 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 a significant 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 before leveling off at 10,000 metric tons, or 12 percent of 1996 sales for all uses.(45) HFC-134a has the distinct competitive advantage of being the only nonflammable liquefied gas propellant available on the market. 1,1-Difluoroethane (HFC-152a) As a non-ozone-depleting substance with a GWP of only 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 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.(46) DuPont scientists believe that HFC-152a will capture the primary share of the propellant market, because it is less expensive than HFC-134a (the primary alternative), has a much lower GWP, and is a better solvent (an important characteristic if ingredients are to remain in solution).(47) 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 1995, however, DuPont's reported emissions dropped to only 18 metric tons. The EPA estimated 1990 emissions of HFC-152a at 2,620 metric tons, dropping to only 310 metric tons in 1992, before climbing back to 1,080 metric tons in 1996. 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. For 1996, the EPA estimates emissions at 3,170 metric tons for HFC-125, 230 metric tons for HFC-143a, 2,060 metric tons for HFC-227ea, and 80 metric tons for HFC-236fa. Perfluorocarbons (PFCs) Perfluorocarbons are compounds composed of carbon and fluorine. PFC emissions are not regulated or reported, although their high GWPs (6,900 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 that 0.6 kilogram of perfluoromethane (CF4, also known as carbon tetrafluoride) and 0.06 kilogram of perfluoroethane (C2F6) are emitted as a result of each metric ton of aluminum smelted.(48) These coefficients, in conjunction with aluminum production figures, suggest U.S. emissions of 2,160 metric tons of perfluoromethane and 216 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.(49) 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.(50) The EPA's Atmospheric Pollution Prevention Division believes 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.(51) This is equivalent to emissions of about 70 metric tons of perfluoroethane and smaller amounts of the other gases. For 1996, the EPA estimates total emissions of all greenhouse gases from semiconductor manufacturing at 1.4 million metric tons carbon equivalent.(52) 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 at least partially. Since 1992, DuPont--the sole manufacturer of perfluoroethane--has been asking its customers to limit PFC use.(53) A number of semiconductor manufacturing firms have joined an EPA program to reduce PFC emissions voluntarily.(54) In addition, a number of PFC distributors are developing PFC emissions control equipment.(55) Recycling, abatement, and other control options remain in the early stages of development, however, and PFC substitutes are not yet available.(56) 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 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.(57) The EPA estimates 1996 emissions of sulfur hexafluoride at 1,430 metric tons, equivalent to emissions of more than 9 million metric tons of carbon. The EPA's estimates indicate a slight decrease in emissions between 1995 and 1996, from 1,460 metric tons to 1,430 metric tons.(58)
Ozone-Depleting Substances 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. Chlorofluorocarbons (CFCs) 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 phasing out CFCs by 1996.(59) 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.(60) In 1994, production and sales declined by nearly 80 percent, to only 7,000 metric tons,(61) 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.(62) AFEAS data suggest that use of CFC-12 as a blowing agent dropped by more than 90 percent between 1988 and 1996.(63) The use of CFC-12 in refrigeration, however, declined more slowly until 1994. During 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.(64) 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 rapidly, from negligible levels in the early 1990s. Bromofluorocarbons (Halons) 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.(65) Emissions of halons are low, although the exact figure is uncertain. Other Chemicals Several other chemicals combine high GWPs and significant emissions levels to produce potential effects on global climate: carbon tetrachloride, methyl chloroform, chloroform, and methylene chloride. 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. 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 4,000 metric tons in 1996. For 1998, the EPA has 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.(66) 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 4,400 metric tons in 1996.(67) 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 1996 emissions of 24,600 metric tons, down significantly from 46,000 metric tons in 1990.(68) Criteria Pollutants That Affect Climate Overview Certain criteria pollutants also affect climate: carbon monoxide (CO), nitrogen oxides (NOx), and nonmethane volatile organic compounds (NMVOCs).(69) 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 these 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 600 U.S. testing sites has declined by 15 percent since 1987.(70) 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, 1900-1996.(71) 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 significantly 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.
Carbon Monoxide 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 79 percent of 1996 emissions. Total emissions in 1996 amounted to about 80.4 million metric tons, an amount considerably below the 128.8 million metric tons seen in 1970 (Table 33). Between 1990 and 1996, carbon monoxide emissions decreased by 8.0 percent, due largely to decreases in emissions of approximately 4 million metric tons each from highway vehicles (down by 8.5 percent) and forest fires (down by 75.2 percent). Otherwise, 1996 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 0.8 million metric tons (5.7 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 Oxides 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. Since 1990, total U.S. emissions of nitrogen oxides have hovered around 22 million metric tons per year (Table 34). 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). Emissions are split between transportation and stationary sources. Total emissions of nitrogen oxide during 1996 (21.3 million metric tons) were about 1.4 percent lower than their 1990 level of 21.6 million metric tons. 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 (47.6 percent) of the 17.2 million metric tons of NMVOC emissions during 1996 came from industrial processes (Table 35), of which solvent use was the largest source. Most (80.0 percent) of the remaining 9.0 million metric tons of emissions were from combustion of transportation fuels. Emissions of NMVOCs declined by some 11.7 million metric tons (38.1 percent) from 1970 to 1996, while fuel consumption in the transportation sector increased and activity in the industrial sector expanded.(72) 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 17.2 million metric tons in 1996, down 7.3 percent from their level in 1995, and down by 9.0 percent (1.7 million metric tons) from 1990 levels. During 1996, energy-related activities (transportation and stationary source fuel combustion) accounted for approximately 43.7 percent (0.7 million metric tons) of the total decrease of 1.7 million metric tons in emissions of NMVOCs, compared with 1990 levels. Solid waste disposal sources accounted for 29.6 percent (0.5 million metric tons) of the decrease in emissions from 1990.
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