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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
With a 100-year global warming potential (GWP) of 310, nitrous oxide is a significant contributor to atmospheric warming. Although there are many known natural and anthropogenic sources, emissions of nitrous oxide have been difficult to quantify, primarily because it has been one of the least studied greenhouse gases. Estimated U.S. anthropogenic nitrous oxide emissions totaled 1.0 million metric tons in 1997, down slightly from 1996 emissions levels but still 47,000 metric tons above 1990 levels (Table 25). Nearly all the increase from 1990 levels can be attributed to emissions from mobile combustion, which grew by more than 30 percent between 1990 and 1995. Emissions stabilized from 1995 through 1997, as growth in the light-duty truck fleet slowed and the next generation of lower-emitting catalytic converters was introduced. These trends can be expected to lead to lower emissions from mobile combustion over the next few years. Agricultural activities are the largest source of U.S. anthropogenic nitrous oxide emissions--primarily, the application of nitrogen to agricultural soils. Small quantities of nitrous oxide are also released from the burning of crop residues. Estimated emissions of nitrous oxide from agricultural sources were 642,000 metric tons in 1997, equal to 1990 levels and 7,000 metric tons (1.0 percent) higher than in 1996 (Figure 6). There are, however, large uncertainties connected with the emissions consequences of adding nitrogen to agricultural soils (see box on page 44).
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 residential, industrial, and electric utility energy use. Energy use was responsible for the release of 264,000 metric tons of nitrous oxide in 1997, an increase of 4,000 metric tons (1.4 percent) over the 1996 level.
Energy Use Nitrous oxide emissions from both mobile and stationary sources are byproducts of fuel combustion. Estimated 1997 energy-related emissions were 264,000 metric tons, approximately 26 percent of total U.S. anthropogenic nitrous oxide emissions (Table 25). Emissions from energy use are dominated by mobile combustion (1997 estimated emissions of 222,000 metric tons). Mobile Combustion Nitrous oxide emissions from motor vehicles are caused primarily by the conversion of pollutant nitrogen oxides (NO2) 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. Consequently, emissions also depend on the "mix" of vehicle age and type on the road. The rising popularity of light-duty trucks (with larger engines) tends to raise emissions, while the penetration of the latest model three-way catalytic converters (in 1996 and later model cars) tends to reduce emissions. In addition to emissions from passenger cars and light-duty trucks, emissions from air, rail, and water transportation sources and from farm and construction equipment are also included in the estimates. Overall nitrous oxide emissions from mobile source combustion in 1997 are estimated at 222,000 metric tons (Table 26). Approximately 92 percent of the emissions can be attributed to motor vehicles. The rapid growth in emissions between 1990 and 1995 is a function of 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 and 1997 model year cars has stabilized emissions from this source.
Stationary Combustion During combustion, nitrous oxide is produced as a result of chemical interactions between nitrogen oxides 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 1997, estimated nitrous oxide emissions from stationary combustion sources increased modestly, by 900 metric tons over 1996 and by a larger 9.8 percent since 1990 (Table 27). Nearly all the emissions increase of almost 4,000 metric tons between 1990 and 1997 can be attributed to coal-fired electricity generation, which grew in response to the growing demand for electricity. Coal-fired combustion systems produced some three-quarters of the 1997 emissions, and electric utilities accounted for approximately two-thirds of all stationary combustion emissions. Agriculture On a global scale, agricultural practices contribute approximately 70 percent of anthropogenic nitrous oxide emissions.(33) Similarly, in the United States, agricultural activities were responsible for 63 percent of 1997 nitrous oxide emissions. About three-quarters of agricultural emissions are associated with the application of commercial and animal-manure-based fertilizers. 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 1,850 metric tons or about 0.1 percent of total U.S. emissions of nitrous oxide from agricultural sources in 1997. Overall, emissions from agricultural activities were unchanged between 1990 and 1997. Nitrogen Fertilization of Agricultural Soils Nitrous oxide uptake and emissions occur naturally as a result of nitrification and denitrification processes in soil and crops. Nitrogen may be added to soil and crops through the application of commercial synthetic fertilizers or animal manure. When nitrogen-based fertilizers are added to the soil, emissions generally increase, unless application precisely matches plant uptake and soil capture.(34) Nitrous oxide may be emitted directly to the atmosphere from fertilizers or from crop residues. It may also be emitted directly from nitrogen-rich soils, called histosols. Nitrous oxide may also be released indirectly when nitrogen-enriched runoff reaches rivers and streams. Additional indirect emissions occur when a small portion of nitrogen from fertilizer application reaches the atmosphere in its elemental form and is converted to nitrous oxide. The EIA estimates that a total of 658,000 metric tons of nitrous oxide were released into the atmosphere as a result of direct and indirect emission associated with fertilization practices in 1997 (Table 28). This represents an increase of 5,000 metric tons (less than 1 percent) compared with the amount released in 1990 and a decrease of 7,000 metric tons (1.1 percent) compared with 1996.
Crop Residue Burning Incomplete combustion of agricultural wastes during burning of crop residues produces various gases, including nitrous oxide. Estimated emissions of nitrous oxide from this source were approximately 1,600 metric tons in 1997, up by 8.3 percent from 1996 (Table 25). As with methane emissions from crop residue burning, emissions of nitrous oxide grew as a result of increased corn, soybean, and pea production. Although emissions from this source remain very small, at about 0.2 percent of all U.S. nitrous oxide emissions, they have grown by 20 percent since 1990. The estimates of U.S. nitrous oxide emissions from the burning of crop residues presented in this report are roughly one-third of those published in the previous year's edition of Emissions of Greenhouse Gases in the United States. The downward revision is the result of a change in the assumed fraction of each crop burned, from 10 percent to 3 percent, as recommended by the U.S. Environmental Protection Agency (EPA) in its draft Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1996. (For more detail on the method used in this report, see Appendix A.) Solid Waste of Domesticated Animals Nitrous oxide is released as part of the microbial denitrification of animal manure. The 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. Estimated 1997 nitrous oxide emissions from animal waste were 235,000 metric tons, down by 1.7 percent from 1996 levels but still slightly higher than 1990 levels (Table 29). This makes animal waste the second-largest U.S. source of nitrous oxide emissions after the application of fertilizers. Nitrous oxide emissions from animal waste are dominated by emissions from cattle waste, which represent approximately 95 percent of emissions from the solid waste of domesticated animals. Thus, emissions levels have largely moved in sync with cattle populations, growing between 1990 and 1994, stabilizing in 1995, and declining in 1996 and 1997. Waste Management Nitrous oxide emissions from waste management account for almost 2 percent of all U.S. anthropogenic nitrous oxide emissions (Table 25). Emissions from human sewage in wastewater are responsible for more than 90 percent of the estimated emissions from this source, and the remainder are associated with waste combustion. Estimated emissions from waste management have grown slowly between 1990 and 1997 because estimates from human sewage in wastewater are partially scaled to population data. 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 somewhat lower than they otherwise would have been.
Waste Combustion In 1997, estimated nitrous oxide emissions from waste combustion were 834 metric tons, down by more than 6 percent from 1996 levels and 9 percent below 1990 levels. The total volume of waste generated in the United States increased by 16 percent between 1990 and 1997. In 1990, the share of waste burned hit a high of 11.5 percent. After dropping to 10 percent in 1991, the share of waste that was incinerated in the United States remained relatively stable through 1996--in the neighborhood of 10 percent of all waste generated. Thus, increasing levels of waste generation led to an increase in the total volume of waste incinerated and higher nitrous oxide emissions. The share of waste incinerated dropped from 10 percent in 1996 to 9 percent in 1997 as the share of waste recycled increased, with a corresponding drop in the estimate of nitrous oxide emissions. Human Sewage in Wastewater 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),(35) and nitrogen concentration. In 1997, nitrous oxide emissions from wastewater were 17,400 metric tons, a 0.9-percent increase from 1996 levels and a 11.2-percent increase from the 1990 level (Table 25). Estimates of nitrous oxide emissions from human waste correspond to population size and per capita protein intake. In addition to slow but steady population growth, the U.S. per capita protein intake has risen by 3.7 percent since 1990, accounting for the rise in emissions.(36) Industrial Processes Nitrous oxide is emitted as a byproduct of certain chemical production processes. Table 30 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 have decreased by 6,300 metric tons (6.7 percent) since 1990 and by 20,000 metric tons (18.6 percent) since 1996. All the decline can be traced to decreased emissions from adipic acid production, which dropped as emissions controls were added to the third of four manufacturing plants currently in operation.
Adipic Acid Production 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 plasticizers 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.(37) Between 1990 and 1994 emissions from adipic acid manufacture grew slowly until they reached 60,000 metric tons (Table 30). After remaining relatively stable in 1995 and 1996, emissions dropped sharply to just 42,000 metric tons in 1997. 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.(38) By mid-1997 a third plant had installed emissions controls, raising the share of production employing controls from 77 percent to 85.4 percent. With the controls in place for the full year in 1998, emissions from this source can be expected to fall further.(39)
Nitric Acid Production 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. Measurements at a DuPont plant indicate emissions factors of 2 to 9 grams of nitrous oxide per kilogram of nitric acid manufactured. The 9.1 million tons of nitric acid manufactured in 1997 resulted in estimated emissions of 45,000 metric tons of nitrous oxide (Table 30). There is, however, a considerable degree of uncertainty associated with this estimate, because the emissions factor for the DuPont plant may not in fact be representative of the entire industry (see Appendix A).
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