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4. Nitrous Dioxide Emissions
Overview Estimated U.S. anthropogenic nitrous oxide emissions totaled 1,125 thousand metric tons in 2002, 1.1 percent less than in 2001 and 0.2 percent below 1990 levels (Table 23). Almost all of the decrease from 2001 can be attributed to nitrogen fertilization of agricultural soils. Emissions from this source decreased by 16 thousand metric tons of nitrous oxide compared with 2001 levels. Emissions of nitrous oxide from industrial sources in 2002 increased by 3 thousand metric tons from their 2001 level. The decrease in emissions of nitrous oxide from 1990 can be attributed to emissions from nitrogen fertilization of agricultural soils and industrial sources (adipic acid and nitric acid production), which fell by a combined 67 thousand metric tons between 1990 and 2002, more than offsetting the increase of 59 thousand metric tons in emissions from mobile combustion sources since 1990. Weighted by global warming potential, total nitrous oxide emissions in 2002 were equivalent to 333.1 million metric tons carbon dioxide, or 4.9 percent of total U.S. greenhouse gas emissions. In 2001, total nitrous oxide emissions were equivalent to 336.8 million metric tons of carbon dioxide, or 4.9 percent of total U.S. greenhouse gas emissions. Sources of U.S. nitrous oxide emissions include energy use, agriculture, waste management, and industrial processes. The largest component of U.S. anthropogenic nitrous oxide emissions is emissions from agricultural activities, representing 70.3 percent of the total. Nitrogen fertilization of agricultural soils represents 73.7 percent of emissions from agricultural activities. Most of the remainder is from the handling of animal waste in managed systems. Small quantities of nitrous oxide are also released from the burning of crop residues. Estimated emissions of nitrous oxide from agricultural sources were 791 thousand metric tons (or 234.2 million metric tons carbon dioxide equivalent) in 2002, 2.0 percent below 2001 levels and 2.8 percent below 1990 levels (Figure 4). There are large uncertainties connected with the emissions consequences of adding nitrogen to agricultural soils. Models used for estimation are based on limited sources of experimental data.65 The uncertainty increases when moving from emissions associated with animal manure to soil mineralization and atmospheric deposition, where both estimating emissions and partitioning emissions between anthropogenic and biogenic sources become increasingly difficult. The second-largest source of anthropogenic nitrous oxide emissions is energy consumption, which includes mobile source combustion from passenger cars, buses, motorcycles, and trucks and stationary source combustion from commercial, residential, industrial, and electric power sector energy use. Energy use was responsible for the release of 263 thousand metric tons of nitrous oxide or 78.0 million metric tons carbon dioxide equivalent in 2002 (23.4 percent of total U.S. nitrous oxide emissions), only slightly (less than 0.05 percent) lower than in 2001 but 31.5 percent higher than in 1990. Industrial production of adipic and nitric acid, which releases nitrous oxide as a byproduct, accounted for emissions of 50 thousand metric tons of nitrous oxide or 14.9 million metric tons carbon dioxide equivalent in 2002 (4.5 percent of total U.S. nitrous oxide emissions), a 6.5-percent increase from 2001 levels and a 47.8-percent decrease from 1990 levels. The increase in emissions from this source in 2002 is a result of increased production of both adipic acid and nitric acid by 86 and 336 thousand metric tons, respectively, compared with compared with 2001 (10.3 and 4.7 percent, respectively). The large decline in emissions of nitrous oxide from adipic acid production since 1990 is a result of the implementation of emissions control technology at three of the four adipic acid plants operating in the United States.
Energy Use The energy use category includes nitrous oxide emissions from both mobile and stationary sources as byproducts of fuel combustion. Estimated 2002 energy-related emissions were 263 thousand metric tons, or 23.4 percent of total U.S. anthropogenic nitrous oxide emissions (Table 23). Emissions from energy use are dominated by mobile combustion (81.4 percent of nitrous oxide emissions from energy use in 2002). Mobile Combustion Nitrous oxide emissions from mobile source combustion in 2002 were 214 thousand metric tons or 63.5 million metric tons carbon dioxide equivalent, a decrease of 0.2 thousand metric tons nitrous oxide or 0.1 million metric tons carbon dioxide equivalent (0.1 percent) from 2001 levels (Table 24). In addition to emissions from passenger cars and light-duty trucks, emissions from air, rail, and marine transportation and from farm and construction equipment are also included in the estimates. Motor vehicles are the source of 94.3 percent of nitrous oxide emissions from mobile combustion (Table 24). Nitrous oxide emissions from motor vehicles are caused primarily by the conversion of nitrogen oxides (NOx) into nitrous oxide (N2O) by vehicle catalytic converters. The normal operating temperature of catalytic converters is high enough to cause the thermal decomposition of nitrous oxide. Consequently, it is probable that nitrous oxide emissions result primarily from “cold starts” of motor vehicles and from catalytic converters that are defective or operating under abnormal conditions. This implies that the primary determinant of the level of emissions is motor vehicle operating conditions; however, different types of catalytic converters appear to differ systematically in their emissions, and emissions probably vary with engine size. Thus, emissions also depend on the “mix” of vehicle age and type on the road. Nitrous oxide emissions from mobile sources grew rapidly between 1990 and 1995 due to increasing motor vehicle use, the shifting composition of the light-duty vehicle fleet toward light trucks that have lower fuel economy and higher per-mile emission factors, and the gradual replacement of low emitting pre-1983 vehicles that did not use catalytic converters with higher emitting post-1983 vehicles that do use catalytic converters. This growth moderated between 1995 and 1999 due to the introduction of more advanced, lower-emitting catalytic converters. After peaking in 1999, emissions have declined slowly as vehicle turnover has led to a fleet dominated by the more advanced catalytic converters. Stationary Combustion In 2002, estimated nitrous oxide emissions from stationary combustion sources were 49 thousand metric tons or 14.5 million metric tons carbon dioxide equivalent, 0.3 percent higher than in 2001 and 9.8 percent higher than in 1990 (Table 25). The emissions increase from this source between 1990 and 2002 can be attributed principally to coal-fired electricity generation, which grew in response to the growing demand for electricity and lower costs and improved availability at coal-fired power plants. Coal-fired combustion systems produced 63.6 percent of the 2002 emissions of nitrous oxide from stationary combustion. During combustion, nitrous oxide is produced as a result of chemical interactions between nitrogen oxides (mostly NO2) and other combustion products. With most conventional stationary combustion systems, high temperatures destroy almost all nitrous oxide, limiting the quantity that escapes; therefore, emissions from these systems typically are low. Agriculture
Nitrous oxide emissions from agricultural activities fell by 2.8 percent between 1990 and 2002. Agricultural activities were responsible for 70.3 percent of U.S. nitrous oxide emissions in 2002, roughly the same percentage that agricultural practices contribute to nitrous oxide emissions globally.66 Nitrogen fertilization of agricultural soils accounted for 73.7 percent of U.S. agricultural emissions of nitrous oxide (Table 23). Nearly all the remaining agricultural emissions can be traced to the management of the solid waste of domesticated animals. The disposal of crop residues by burning also produces nitrous oxide that is released into the atmosphere; however, the amount is relatively minor, at 2 thousand metric tons or 0.2 percent of total U.S. emissions of nitrous oxide from agricultural sources in 2002. Nitrogen Fertilization of Agricultural Soils EIA estimates that a total of 583 thousand metric tons of nitrous oxide (or 172.5 million metric tons carbon dioxide equivalent) was released into the atmosphere as a result of direct and indirect emissions associated with fertilization practices in 2002 (Table 26). Estimated emissions decreased by 2.5 percent compared with 2001 levels and were 3.4 percent lower than in 1990. Nitrous oxide emissions from the application of nitrogen-based fertilizers and biological fixation in crops accounted for 57.3 percent of total nitrous oxide emissions from this source during 2002. Nitrogen uptake and nitrous oxide emissions occur naturally as a result of nitrification and denitrification processes in soil and crops, generally through bacterial action. When nitrogen compounds are added to the soil, bacterial action is stimulated, and emissions generally increase, unless the application precisely matches plant uptake and soil capture.67 Nitrogen may be added to the soil by synthetic or organic fertilizers, nitrogen-fixing crops, and crop residues. Nitrogen-rich soils, called “histosols,” may also stimulate emissions. Adding excess nitrogen to the soil also enriches ground and surface waters, such as rivers and streams, which generate indirect emissions of nitrous oxide. Additional indirect emissions occur from “atmospheric deposition,” in which soils emit other nitrogen compounds that react to form nitrous oxide in the atmosphere. Crop Residue Burning In 2002, estimated emissions of nitrous oxide from crop residue burning were 2 thousand metric tons (or 0.5 million metric tons carbon dioxide equivalent), down by less than 0.5 thousand metric tons nitrous oxide (4.8 percent) from 2001 levels (Table 23). The small decrease is mainly attributable to decreased corn and soybean production. Emissions from this source remain very small, at 0.2 percent of all U.S. nitrous oxide emissions. When crop residues are burned, the incomplete combustion of agricultural waste results in the production of nitrous oxide, as well as methane (discussed in Chapter 3). Solid Waste of Domesticated Animals Estimated 2002 nitrous oxide emissions from animal waste management were 207 thousand metric tons (or 61.2 million metric tons carbon dioxide equivalent), down by 0.5 percent from 2001 levels and 1.3 percent lower than 1990 levels (Table 27), making animal waste the second-largest U.S. agricultural source of nitrous oxide emissions, after nitrogen fertilization of soils. Nitrous oxide emissions from animal waste are dominated by emissions from cattle waste, which account for 92.6 percent of emissions from the solid waste of domesticated animals. Nitrous oxide is released as part of the microbial denitrification of animal manure. The total volume of nitrous oxide emissions is a function of animal size and manure production, the amount of nitrogen in the animal waste, and the method of managing the animal waste. Waste managed by a solid storage or pasture range method may emit 20 times more nitrous oxide per unit of nitrogen content than does waste managed in anaerobic lagoon and liquid systems. Generally, solid waste from feedlot beef cattle is managed with the solid storage or pasture range method, accounting for the majority of nitrous oxide emissions. Solid waste from swine is generally managed in anaerobic lagoons and other liquid systems. Anaerobic digestion yields methane emissions but only negligible amounts of nitrous oxide. Thus, changes in estimated emissions result primarily from changes in cattle populations. For example, beef cattle populations grew during the first half of the 1990s, leading to higher emissions through 1995, but have since declined slowly, lowering emissions nearly to 1991 levels. Waste Management
Nitrous oxide emissions from waste management are estimated at 20 thousand metric tons (or 6.0 million metric tons carbon dioxide equivalent) for 2002, 1.8 percent of all U.S. anthropogenic nitrous oxide emissions (Table 23). During 2002, emissions from human sewage in wastewater were responsible for 95.7 percent of the estimated emissions from this source, and the remainder was associated with waste combustion. Estimated emissions from waste management increased by 1.4 percent between 2001 and 2002 and by 22.1 percent between 1990 and 2002. Because of the lack of reliable data and an effective estimation method, no estimate of emissions from industrial wastewater was calculated, leaving estimated emissions from waste management lower than they otherwise would be had a viable estimation method been available. Waste Combustion In 2002, estimated nitrous oxide emissions from waste combustion were 1 thousand metric tons, down 5.6 percent from 2001 levels and 3.6 percent above 1990 levels. Data on the amount of waste generated in the United States in 2002 were not available in time for this report; therefore, EIA scaled the 2002 estimates for waste combustion to the growth in U.S. gross domestic product. The share of waste burned is estimated to have been unchanged from 2001 to 2002, and the total volume of waste generated is estimated to have increased by 9.0 percent. The total volume of waste generated in the United States increased by 58.2 percent between 1990 and 2002; however, the share of waste burned in 2002 was just 7.5 percent, compared with 11.5 percent in 1990.68 Human Sewage in Wastewater In 2002, nitrous oxide emissions from wastewater were 19 thousand metric tons (or 5.7 million metric tons carbon dioxide equivalent), a 1.3-percent increase from 2001 levels and a 23.7-percent increase from the 1990 level (Table 23). Estimates of nitrous oxide emissions from human waste are scaled to population size and per capita protein intake. U.S. population has grown by 13.7 percent since 1990. U.S. per capita protein intake rose steadily between 1990 and 1999, before declining slightly in 2000, 2001, and 2002. Today, U.S. per capita protein intake is 6.5 percent above 1990 levels. Data on protein intake are taken from the United Nations Food and Agriculture Organization (FAO).69 Nitrous oxide is emitted from wastewater that contains nitrogen-based organic materials, such as those found in human or animal waste. Two natural processes—nitrification and denitrification—combine to produce nitrous oxide. Nitrification, an aerobic process, converts ammonia into nitrate; denitrification, an anaerobic process, converts nitrate to nitrous oxide. Factors that influence the amount of nitrous oxide generated from wastewater include temperature, acidity, biochemical oxygen demand (BOD),70 and nitrogen concentration. Industrial Sources
Emissions of nitrous oxide from industrial sources were 50 thousand metric tons (or 14.9 million metric tons carbon dioxide equivalent) in 2002, an increase of 3 thousand metric tons or 0.9 million metric tons carbon dioxide equivalent (6.5 percent) from 2001 and a decrease of 46 thousand metric tons or 13.7 million metric tons carbon dioxide equivalent (47.8 percent) since 1990. Nitrous oxide is emitted as a byproduct of certain chemical production processes. Table 28 provides estimates of emissions from the production of adipic acid and nitric acid, the two principal known sources. Adipic Acid Production Emissions from adipic acid production rose from 12 thousand metric tons of nitrous oxide (or 3.5 million metric tons carbon dioxide equivalent) in 2001 to 13 thousand metric tons (or 3.9 million metric tons carbon dioxide equivalent) in 2002—an increase of 10.3 percent. As discussed below, emissions from this source have been in the range of 12 to 14 thousand metric tons of nitrous oxide per year since 1998. Adipic acid is a fine white powder that is used primarily in the manufacture of nylon fibers and plastics, such as carpet yarn, clothing, and tire cord. Other uses of adipic acid include production of plasticizer for polyvinyl chloride and polyurethane resins, lubricants, insecticides, and dyes. In the United States, three companies, which operate a total of four plants, manufacture adipic acid by oxidizing a ketone-alcohol mixture with nitric acid. Nitrous oxide is an intrinsic byproduct of this chemical reaction. For every metric ton of adipic acid produced, 0.3 metric ton of nitrous oxide is created.71 Between 1990 and 1996, emissions from adipic acid manufacture grew by 23.2 percent, reaching 70 thousand metric tons of nitrous oxide (or 20.7 million metric tons carbon dioxide equivalent) before dropping sharply to 27 thousand metric tons of nitrous oxide (or 7.8 million metric tons carbon dioxide equivalent) in 1997 (Table 28). Beginning in 1996, two of the four plants that manufacture adipic acid controlled emissions by thermally decomposing the nitrous oxide. This technique eliminates 98 percent of potential nitrous oxide emissions from the process.72 During the first quarter of 1997, a third plant installed emissions controls, increasing the share of adipic acid production employing emissions abatement controls from 74.1 percent in 1996 to 91.6 percent in 1997. In 1998, with emissions controls in place for the full year, 97.4 percent of emissions from U.S. adipic acid production were controlled.73 Estimated emissions of nitrous oxide from uncontrolled adipic acid production decreased from 22 thousand metric tons in 1997 to 8 thousand metric tons in 2002, and 2002 emissions of nitrous oxide from controlled plants remained relatively constant at 5 thousand metric tons. With the share of adipic acid production employing abatement controls now at nearly 100 percent, future changes in nitrous oxide emissions from this source are expected to result primarily from changes in plant production levels in response to market demand. Nitric Acid Production The 6.8 million metric tons of nitric acid manufactured in 2002 resulted in estimated emissions of 37 thousand metric tons of nitrous oxide, equivalent to 11.0 million metric tons of carbon dioxide (Table 28). This estimate was 5.2 percent higher than 2001 levels and 6.1 percent lower than 1990 levels. The emissions factor used to estimate nitrous oxide emissions from the production of nitric acid was based on measurements at a single DuPont plant, which indicated an emissions factor of 2 to 9 grams of nitrous oxide emitted per kilogram of nitric acid manufactured, suggesting an uncertainty of plus or minus 75 percent in the emissions estimate.74 Nitric acid, a primary ingredient in fertilizers, usually is manufactured by oxidizing ammonia (NH3) with a platinum catalyst. Nitrous oxide emissions are a direct result of the oxidation. Nitrous Oxide Emissions Tables
Released: October 2003 |
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