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Nitrous Oxide Emissions
Estimated U.S. anthropogenic nitrous oxide emissions totaled 1,224.2 thousand metric tons in 1999, 0.1 percent more than in 1998 and 5 percent above 1990 levels (Table 24). Nearly all the increase from 1990 levels can be attributed to emissions from the nitrogen fertilization of agricultural soils and emissions from mobile combustion, which grew by 23.3 and 61.5 thousand metric tons, respectively, between 1990 and 1999. Table 24. Estimated U.S. Emissions of Nitrous Oxide, 1990-1999 The largest component of U.S. anthropogenic nitrous oxide emissions is emissions from agricultural activities, almost three-quarters of which result from nitrogen fertilization of agricultural soils. 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 870.1 thousand metric tons in 1999, less than 1 percent below 1998 levels but 3 percent above 1990 levels (Figure 6). Figure 6. U.S. Emissions of NItrous Oxide by Source, 1990-1999 (Thousand Metric Tons Nitrous Oxide) 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.46 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 utility energy use. Energy use was responsible for the release of 278.9 thousand metric tons of nitrous oxide in 1999, 3 percent higher than in 1998 and 32 percent higher than in 1990. Industrial production of adipic acid and nitric acid, which releases nitrous oxide as a byproduct, accounted for emissions of about 56.6 thousand metric tons of nitrous oxide in 1999, a 41-percent decrease from 1990 levels and a 3-percent decline from 1998 levels (Table 24). The large decline in emissions from this source is a result of the implementation of emissions control technology at three of the four adipic acid plants operating in the United States.
Nitrous oxide emissions from both mobile and stationary sources are byproducts of fuel combustion. Estimated 1999 energy-related emissions were 278.9 thousand metric tons, 23 percent of total U.S. anthropogenic nitrous oxide emissions (Table 24). Emissions from energy use are dominated by mobile combustion (82 percent of 1999 totals). Nitrous oxide emissions from mobile source combustion in 1999 were 227.7 thousand metric tons, 2 percent above 1998 levels (Table 25). 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. Ninety-four percent of the emissions can be attributed to motor vehicles (Table 25). Emissions grew rapidly between 1990 and 1995 due to increasing motor vehicle use, the shifting composition of the light-duty vehicle fleet toward light trucks, and the gradual replacement of low emitting pre-1983 vehicles in the fleet with higher emitting post-1983 vehicles. The shift to advanced three-way catalytic converters in 1996 through 1999 model year cars has slowed but not abated emissions growth from this source. Table 25. U.S. Nitrous Oxide Emissions from Mobile Sources, 1990-1999 Nitrous oxide emissions from motor vehicles are caused primarily by the conversion of pollutant 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. 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 are typically low. In 1999, estimated nitrous oxide emissions from stationary combustion sources were 51.1 thousand metric tons, 4 percent higher than in 1998 and 15 percent higher than in 1990 (Table 26). Nearly two-thirds (63 percent) of the emissions increase from this source between 1990 and 1999 can be attributed 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 59 percent of the 1999 emissions of nitrous oxide from stationary combustion, and electric utilities accounted for 55 percent of all nitrous oxide emissions from stationary combustion. Table 26. U.S. Nitrous Oxide Emissions from Stationary Combustion Sources, 1990-1999 On a global scale, agricultural practices contribute approximately 70 percent of anthropogenic nitrous oxide emissions.47 Similarly, in the United States, agricultural activities were responsible for 71 percent of 1999 nitrous oxide emissions. Seventy- three percent of agricultural emissions are associated with nitrogen fertilization of agricultural soils (Table 24). 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 1.6 thousand metric tons or 0.2 percent of total U.S. emissions of nitrous oxide from agricultural sources in 1999. Nitrous oxide emissions from agricultural activities grew by 3 percent between 1990 and 1999.
Nitrogen Fertilization of Agricultural Soils 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.48 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. EIA estimates that a total of 636.8 thousand metric tons of nitrous oxide was released into the atmosphere as a result of direct and indirect emissions associated with fertilization practices in 1999 (Table 27). Estimated emissions increased by 4 percent compared with 1990 and decreased by less than 1 percent compared with 1998. Nitrous oxide emissions from the application of nitrogen-based fertilizers and biological fixation in crops accounted for 61.5 percent of total nitrous oxide emissions from this source during 1999. Table 27. U.S. Nitrous Oxide Emissions from Nitrogen Fertilization of Agricultural Soils, 1990-1999 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). In 1999, estimated emissions of nitrous oxide from crop residue burning were 1.6 thousand metric tons, down by 3 percent from 1998 levels (Table 24). The small decrease is mainly attributable to reduced corn, soybean, and wheat production. Emissions from this source remain very small, at 0.1 percent of all U.S. nitrous oxide emissions. Solid Waste of Domesticated Animals Estimated 1999 nitrous oxide emissions from animal waste management were about 231.7 thousand metric tons, down by 1 percent from 1998 levels and 1 percent higher than 1990 levels (Table 28), 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 94 percent of emissions from the solid waste of domesticated animals. Thus, changes in estimated emissions result primarily from changes in cattle populations. Cattle populations grew during the first half of the decade, leading to higher emissions through 1995, but have since declined slowly, bringing emissions close to 1990 levels. Table 28. U.S. Nitrous Oxide Emissions from Solid Waste of Domesticated Animals, 1990-1999 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 the 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. Nitrous oxide emissions from waste management are estimated at about 18.5 thousand metric tons for 1999, 2 percent of all U.S. anthropogenic nitrous oxide emissions (Table 24). During 1999, emissions from human sewage in wastewater were responsible for 96 percent of the estimated emissions from this source, and the remainder was associated with waste combustion. Estimated emissions from waste management grew by 12 percent between 1990 and 1999 and by 1 percent between 1998 and 1999. 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.
In 1999, estimated nitrous oxide emissions from waste combustion were 0.8 thousand metric tons, up by 4 percent from 1998 levels but 13 percent below 1990 levels. While the share of waste burned is estimated to be unchanged between 1998 and 1999, total waste generated increased by 4 percent. The total volume of waste generated in the United States increased by 33 percent between 1990 and 1999; however, the share of waste burned in 1999 was just 7.5 percent, compared with 11. 5 percent in 1990.49 Nitrous oxide is emitted from wastewater that contains nitrogen-based organic materials, such as those found in human or animal waste. It is produced by two natural processes: nitrification and denitrification. 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),50 and nitrogen concentration. In 1999, nitrous oxide emissions from wastewater were about 17.8 thousand metric tons, a 1-percent increase from 1998 levels and a 13-percent increase from the 1990 level (Table 24). Estimates of nitrous oxide emissions from human waste are scaled to population size and per capita protein intake. U.S. population has grown by 9.3 percent since 1990. U.S. per capita protein intake rose steadily between 1990 and 1999, with a brief respite in 1995 and 1996. Today, U.S. per capita protein intake is 7.3 percent above 1990 levels. Data on protein intake are taken from the United Nations Food and Agriculture Organization (FAO).51
Nitrous oxide is emitted as a byproduct of certain chemical production processes. Table 29 provides estimates of emissions from the production of adipic acid and nitric acid, the two principal known sources. Emissions from the combination of these two processes were 56.6 thousand metric tons in 1999, a decrease of 39.9 thousand metric tons (41 percent) since 1990 and 1.5 thousand metric tons (3 percent) since 1998. All of the decline can be traced to decreased emissions from adipic acid production. Table 29. U.S. Nitrous Oxide Emissions from Industrial Processes, 1990-1999 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.52 Between 1990 and 1994, emissions from adipic acid manufacture grew by 18 percent, reaching 67.0 thousand metric tons (Table 29). After remaining relatively stable in 1995 and 1996, emissions dropped sharply to 12.1 thousand metric tons in 1998, and they remained at that level in 1999. Through 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 emissions from the plants.53 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 percent in 1996 to 92 percent in 1997. With emissions controls in place for the full year, 97 percent of emissions from U.S. adipic acid production were controlled in 1998.54 Estimated emissions of nitrous oxide from uncontrolled adipic acid production decreased from 21.8 thousand metric tons in 1997 to 6.9 thousand metric tons in 1999, and 1999 emissions of nitrous oxide from controlled plants remained relatively constant at 5.2 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, 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. The 8,093.9 thousand metric tons of nitric acid manufactured in 1999 resulted in estimated emissions of 44.5 thousand metric tons of nitrous oxide (Table 29). This estimate was 4 percent lower than 1998 levels but 12 percent higher 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.55 If you would like to receive any information relating to any of our greenhouse gas reports via e-mail, click here and subscribe by entering your e-mail address.
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