One of the least-studied greenhouse gases to date, nitrous oxide is an important contributor to atmospheric warming because of its 100-year global warming potential of 320.(Note 1) Although there are many known natural and anthropogenic sources, emissions of nitrous oxide have been difficult to quantify on a global scale.
This year, emissions estimates in this report have been expanded to include burning of crop residues and stationary source combustion from residential, industrial, and electric utility energy use. As indicated in Table 28, the Energy Information Administration (EIA) estimates that anthropogenic nitrous oxide emissions in the United States totaled 459,000 metric tons in 1993. Sources for which data are available show increased emissions in 1994 (estimates for mobile source combustion are not yet available). Most anthropogenic nitrous oxide emissions in the United States can be attributed to agricultural and energy-related sources. In particular, 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. The dramatic growth in emissions from energy are largely responsible for the increase in anthropogenic nitrous oxide emissions observed since the early 1980s (Figure 6). In recent years, emissions from energy use have risen at a much slower rate, due 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.(Note 2) Emissions of nitrous oxide in the United States will probably continue to rise as consumption of fertilizer and the proportion of the motor vehicle fleet using catalytic converters increase.
Table 28. Estimated U.S. Emissions of Nitrous Oxide, 1987-1994
(Thousand Metric Tons of Nitrous Oxide)
NA = not available.
Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Totals may not equal sum of components due to independent rounding.
Sources: Estimates presented in this chapter. Crop residue burning - U.S. Department of Agriculture, National Agricultural Statistics Service, Crop Production annual reports. Emissions calculations based on Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), pp. 4.69-4.73.
Figure 6. U.S. Emissions of Nitrous Oxide by Source, 1980-1994
Source: Estimates presented in this chapter.
On a global scale, agricultural practices contribute approximately 70 percent of anthropogenic nitrous oxide emissions.(Note 3) Application of fertilizer is the principal source of agriculture-related emissions in the United States. The disposal of crop residues by burning also produces nitrous oxide that is released into the atmosphere; however, the amount is relatively minor.
For this report, nitrous oxide emissions from fertilizer use were estimated using a simplified calculation suggested by A.R. Mosier.(Note 6) Emissions factors ranging in order of magnitude from 0.001 to 0.1 were applied to the nitrogen content of fertilizer consumed annually in the United States, producing low, median, and high estimates. In 1994, the median estimate (which assumes that 1 percent of the nitrogen in fertilizer is emitted as nitrous oxide) indicates that 180,000 metric tons of nitrous oxide was released into the atmosphere as a result of fertilization practices (Table 29). Estimates for prior years have been modified from those in last year's report to represent the nitrogen content of annual fertilizer consumption for the calendar year.
Table 29. U.S. Nitrous Oxide Emissions from Nitrogen Fertilizer Use,
1987-1994
(Thousand Metric Tons of Nitrous Oxide)
Note: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994).
Sources: Emissions coefficients from Intergovernmental Panel on Climate Change, Methane and Nitrous Oxide Methods in National Emissions Inventories and Options for Control, Proceedings, Research for Man and Environment (Netherlands, February 1993), p. 274. Total nitrogen content of U.S. fertilizer consumption from Tennessee Valley Authority.
For this reason, the EIA has adopted the Intergovernmental Panel on Climate Change (IPCC) estimate that, in developed countries, 10 percent of crop wastes are burned.(Note 7) This estimate is uncertain because burning of crop residues in the United States is uncommon and is actually prohibited in many States for air quality reasons. Therefore, this value probably represents a maximum quantity.
In 1994, burning of crop residues produced emissions of 6,000 metric tons of nitrous oxide (Table 28). Large fluctuations in annual crop production result in negligible variations in emissions. Since 1987, the amount of nitrous oxide emitted as a result of this agricultural practice has been roughly stable.
Nitrous oxide emissions are also produced as a byproduct of fuel combustion in both mobile and stationary sources. In 1993, energy-related emissions totaled 184,000 metric tons, or 40 percent of total U.S. anthropogenic nitrous oxide emissions (Table 28). The estimates indicate that emissions from energy use have been increasing over time, nearly doubling since 1983.
Emissions from air, rail, and water transportation sources and from farm and construction equipment are included as "Other Mobile Sources" in Table 30. Estimates for each of these sources were derived by applying specific emissions factors to energy consumption data as reported by the EIA.
Table 30. U.S. Nitrous Oxide Emissions from Mobile Sources, 1987-1994
(Thousand Metric Tons of Nitrous Oxide)
*Less than 500 metric tons of nitrous oxide.
NA = not available.
Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Totals may not equal sum of components due to independent rounding.
Sources: Calculations based on vehicle miles traveled from U.S. Department of Transportation, Federal Highway Statistics, various years, Table VM-1. Vehicle emissions coefficients from Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), pp. 1.64-1.68.
Nitrous oxide emissions from mobile source combustion were estimated to be 146,000 metric tons in 1993 (Table 30). Approximately 89 percent of the emissions can be attributed to motor vehicles. Driven by increases in VMT, fleet size, and share of fleet with catalytic converters, emissions from mobile source combustion have increased by 25 percent since 1987.
Emissions were estimated by applying the emissions factors for coal, oil, and natural gas to EIA's consumption data for each of those fuels in the commercial, residential, industrial, and electric utility sectors. In 1994, nitrous oxide emissions from stationary combustion sources rose slightly to 39,000 metric tons (Table 31). Combustion systems powered by coal clearly produce the most nitrous oxide, approximately 76 percent of annual emissions. As a sector, electric utilities consistently account for more than one-half of total emissions. Estimated nitrous oxide emissions from stationary combustion have increased by only 8 percent since 1987.
Table 31. U.S. Nitrous Oxide Emissions from Stationary Combustion Sources,
1987-1994
(Thousand Metric Tons of Nitrous Oxide)
*Less than 500 metric tons of nitrous oxide.
NA = not available.
Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Totals may not equal sum of components due to independent rounding.
Sources: Emissions coefficients from Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), p. 1.50. Energy consumption data from Energy Information Administration, State Energy Data Report 1993, DOE/EIA-0214(93) (Washington, DC, May 1995); and Monthly Energy Review, DOE/EIA-0035(95/07) (Washington, DC, July 1995).
Nitrous oxide is also emitted as a byproduct of certain chemical production processes. Table 32 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 20 percent since 1987.
Table 32. U.S. Nitrous Oxide Emissions from Industrial Processes,
1987-1994
(Thousand Metric Tons of Nitrous Oxide)
Notes: Data in this table are revised from the data contained in the previous EIA report, Emissions of Greenhouse Gases in the United States 1987-1992, DOE/EIA-0573 (Washington, DC, November 1994). Totals may not equal sum of components due to independent rounding.
Sources: Adipic acid production from Chemical and Engineering News, annual report on "Top 50 Industrial Chemicals" (April issue, various years). Nitric acid production from Chemical Manufacturer's Association, Chemical Industry Statistical Handbook (Washington, DC, 1994). Adipic acid emissions coefficient from M. Thiemens and W. Trogler, "Nylon Production: An Unknown Source of Atmospheric Nitrous Oxide," Science, Vol. 251 (February 22, 1991), p. 932. Nitric acid emissions coefficient from Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), p. 2.9.
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.(Note 12) 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.(Note 13)
In 1994, adipic acid production increased to 815,000 metric tons, resulting in increased levels of estimated nitrous oxide emissions from this source. Some of this growth may be attributed to increased demand for nylon fibers and polyvinyl chloride in 1994.
The emissions estimates presented in this report were calculated by multiplying the annual quantity of nitric acid produced by the midpoint of the emissions range determined at the DuPont plant. The 8.8 million tons of nitric acid manufactured in 1994 resulted in emissions of 48,000 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.