| Overview | Energy Use | Agriculture | Industrial Processes | Nitrous Oxide Data Tables |
| U.S. Anthropogenic Nitrous Oxide Emissions,
1990-1995 | ||
| Nitrous Oxide | Carbon Equivalent | |
| Estimated 1995 Emissions (Million Metric Tons) | 0.47 | 39.1 |
| Change Compared to 1994 (Million Metric Tons) | -0.09 | -0.7 |
| Change from 1994 (Percent) | 1.8 | 1.8 |
| Change Compared to 1990 (Million Metric Tons) | 0.13 | 1.1 |
| Change from 1990 (Percent) | 3.0 | 3.0 |
| Principal Sources of U.S. Anthropogenic
Nitrous Oxide Emissions, 1990-1995 | ||||
| Source | Thousand Metric Tons Nitrous Oxide | Percent Change | ||
| 1990 | 1995 | 1990- 1995 | 1994- 1995 | |
| Energy Use | 188 | 184 | -2.2 | -0.6 |
| Agriculture | 164 | 172 | 4.7 | -4.3 |
| Industrial | 97 | 107 | 9.9 | 0.1 |
Compared to carbon dioxide or methane, nitrous oxide is released in small quantities from anthropogenic sources; however, nitrous oxide’s 100-year global warming potential (GWP) of 310 makes it a significant contributor to atmospheric warming [44]. As indicated in Table 26, the EIA estimates that anthropogenic nitrous oxide emissions in the United States totaled 462,000 metric tons in 1995. Sources for which data are available show slightly decreased emissions in 1995 associated with agricultural activities [45]. Although there are many known natural and anthropogenic sources, emissions of nitrous oxide have been difficult to quantify on a global scale, primarily because it has been one of the least-studied greenhouse gases to date.
Figure 9 shows U.S. emissions of nitrous oxide by source over the period 1990-1995. The largest source of anthropogenic nitrous oxide emissions is energy use, 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 a total of 184,000 metric tons of nitrous oxide in 1995, a decrease of 4,000 metric tons from the 1990 level.
The second-largest source of nitrous oxide emissions is agriculture, primarily fertilizer application and a small amount released from the burning of crop residues. Agricultural sources of nitrous oxide are estimated at 172,000 metric tons in 1995, an increase of 8,000 metric tons from 1990 and a decrease of 5,000 metric tons from 1994. However, there is substantial uncertainty and debate regarding the emissions implications of use of nitrogen-based fertilizers. Models used for estimation are based on limited sources of experimental data. For more information about methods and assumptions behind these estimates, see Appendix A.
Another important source of nitrous oxide emissions is industrial production of adipic acid and nitric acid, which together accounted for emissions of 107,000 metric tons of nitrous oxide in 1995, an increase of 10,000 metric tons from 1990 and virtually no change from 1994. Figure 10 shows changes in nitrous oxide emissions for the period 1990-1995. The largest increase in nitrous oxide emissions over the period is from industry. Emissions from energy use increased slightly from 1994 to 1995.
Overall, fertilizer use (which amplifies the natural flux of nitrous oxide from soil) and vehicular fuel combustion combine to account for approximately 70 percent of estimated emissions. Rapid growth in emissions from energy is largely responsible for the increase in anthropogenic nitrous oxide emissions observed since the early 1980s. In the 1990s, emissions from energy use have remained roughly stable, due primarily to changes in the composition of the motor vehicle fleet. Cars manufactured in the 1980s exhibited significantly higher nitrous oxide emissions rates than the majority of cars being retired from the fleet at that time. Beginning in 1990, however, the emissions rate for new cars declined below that of retiring vehicles [46]. Emissions of nitrous oxide in the United States will probably continue to rise slowly as the proportion of the motor vehicle fleet using catalytic converters increases.
| U.S. Nitrous Oxide Emissions from Energy Use, 1990-1995 | |
| Estimated 1995 Emissions (Thousand Metric Tons Nitrous Oxide) | 184 |
| Change Compared to 1994 (Thousand Metric Tons Nitrous Oxide) | -1 |
| Change from 1994 (Percent) | -0.6 |
| Change Compared to 1990 (Thousand Metric Tons Nitrous Oxide) | -4 |
| Change from 1990 (Percent) | -2.2 |
Nitrous oxide emissions are a byproduct of fuel combustion in both mobile and stationary sources. In 1995, energy-related emissions are estimated to total 184,000 metric tons, equal to 40 percent of estimated total U.S. anthropogenic nitrous oxide emissions (Table 26). The estimates indicate that emissions from energy use were increasing sharply during the 1980s but have been stable since 1990 (Figure 9).
Nitrous oxide emissions from motor vehicles are influenced by a variety of factors, including fleet size, vehicle miles traveled, and emission control technologies. In addition, vehicles equipped with catalytic converters to reduce emissions of nitrogen oxides, carbon monoxide, and nonmethane volatile organic compounds emit up to 20 times more nitrous oxide than comparable vehicles without such emission control devices [47]. Emissions from air, rail, and water transportation sources and from farm and construction equipment are included as “Other Mobile Sources” in Table 27.
Nitrous oxide emissions from mobile source combustion were estimated to be 145,000 metric tons in 1995 (Table 27). Approximately 88 percent of the emissions can be attributed to motor vehicles. Although passenger car fuel efficiency increased slightly in 1994, the fuel savings were roughly offset by increases in vehicle miles traveled, fleet size, fuel used per vehicle, and share of fleet with catalytic converters. Catalytic converters tend to increase emissions of nitrous oxide. This was particularly true of the early three-way catalyst systems used between 1983 and 1989; however, emissions from the advanced three-way catalyst systems introduced in 1990 are significantly lower. As a result, most of the 25-percent increase in emissions since 1987 occurred prior to 1991. Emissions have roughly stabilized in the 1990s, as vehicles with older higher-emitting catalytic converters have been retired and replaced with vehicles with advanced three-way catalyst systems.
During combustion, nitrous oxide is produced as a result of chemical interactions between nitric oxide and other combustion products. With most conventional combustion systems, high temperatures destroy almost all nitrous oxide, limiting the quantity that escapes; therefore, emissions from these systems are typically low.
In 1995, estimated nitrous oxide emissions from stationary combustion sources fell modestly, by 9,000 metric tons (Table 28). Combustion systems powered by coal clearly produce approximately 76 percent of total annual emissions. Electric utilities account for more than one-half of total emissions. Estimated nitrous oxide emissions from stationary combustion have increased by only 8 percent since 1987.
| U.S. Nitrous Oxide Emissions from
Agriculture, 1990-1995 | |
| Estimated 1995 Emissions (Thousand Metric Tons Nitrous Oxide) | 172 |
| Change Compared to 1994 (Thousand Metric Tons Nitrous Oxide) | -8 |
| Change from 1994 (Percent) | -4.3 |
| Change Compared to 1990 (Thousand Metric Tons Nitrous Oxide) | 8 |
| Change from 1990 (Percent) | 4.7 |
On a global scale, agricultural practices contribute approximately 70 percent of anthropogenic nitrous oxide emissions [48,49]. Their contribution was approximately 38 percent of U.S. emissions in 1994. Nitrogen flux to the atmosphere from the application of fertilizer is the principal source of agriculture-related emissions in the United States, and appears to be the single largest contributor of domestic anthropogenic nitrous oxide emissions, although estimates of agricultural nitrous oxide emissions are very uncertain (see Appendix A). The disposal of crop residues by burning also produces nitrous oxide that is released into the atmosphere; however, the amount is relatively minor.
Nitrous oxide uptake and emissions occur naturally as a result of nitrification and denitrification processes in soil. When nitrogen-based fertilizers are added to the soil, emissions generally increase, unless application precisely matches plant uptake and soil capture [50]. The EIA estimates that 167,000 metric tons of nitrous oxide were released into the atmosphere as a result of fertilization practices in 1995 (Table 29).
As described in Chapter 3, incomplete combustion of agricultural wastes produces various gases, including methane and nitrous oxide. In 1995, burning of crop residues produced estimated emissions of nearly 5,000 metric tons of nitrous oxide (Table 26). Large fluctuations in annual crop production result in negligible variations in emissions, because only a small fraction of crops become waste, only 10 percent of crop waste is assumed to be burned, and less than 1 percent of the burned waste is released as nitrous oxide. The amount of nitrous oxide emitted as a result of this agricultural practice has been roughly stable since 1987.
| U.S. Nitrous Oxide Emissions from
Industrial Processes, 1990-1995 | |
| Estimated 1995 Emissions (Thousand Metric Tons Nitrous Oxide) | 107 |
| Change Compared to 1994 (Thousand Metric Tons Nitrous Oxide) | 0 |
| Change from 1994 (Percent) | 0.1 |
| Change Compared to 1990 (Thousand Metric Tons Nitrous Oxide) | 10 |
| Change from 1990 (Percent) | 9.9 |
Nitrous oxide is also 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 increased by 9.9 percent since 1990, although they remained stable from 1994 to 1995.
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. Creation of 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 [51]. Currently, two plants (accounting for approximately 77 percent of total production) control emissions by thermally decomposing the nitrous oxide, and 98 percent of the potential emissions from those plants are eliminated by this technique [52].
In 1995, adipic acid production equaled 1994 production at 815,000 metric tons, resulting in stable levels of estimated nitrous oxide emissions from this source. However, this level of production represents a substantial increase from the 1993 level. Some of this growth may be attributed to increased demand for nylon fibers and polyvinyl chloride in 1994 and 1995.
Nitric acid is a primary ingredient in fertilizers. The process for manufacturing this acid involves 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 [53].
The 8.6 million tons of nitric acid manufactured in 1995 resulted in estimated emissions of 47,000 metric tons of nitrous oxide. 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 generalizable across the industry (see Appendix A).
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