| Overview | Energy Use | Agriculture | Industrial Processes | Nitrous Oxide Data Tables |
Figure 9 shows estimated U.S. emissions of nitrous oxide by source over the period 1990-1996.
The second-largest source of nitrous oxide emissions is agriculture, primarily fertilizer application and a small amount
released from the burning of crop residues. Emissions of nitrous oxide from agricultural sources in 1996 are estimated
at 146,000 metric tons, a decrease of 18,000 metric tons (11.0 percent) from 1990 and a decrease of 13,000 metric tons
(7.9 percent) from 1995. These estimates are derived from data on nitrogen-based fertilizer use, which declined in 1996;
however, there is substantial uncertainty and debate regarding the emissions implications of the use of nitrogen-based
fertilizers. Models used for estimation are based on limited sources of experimental data. For more information about
the methods and assumptions behind the 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 111,000 metric tons of nitrous oxide in 1996, an increase of 14,000 metric tons (14.1
percent) from 1990 levels and an increase of 3,000 thousand metric tons (2.6 percent) from 1995 levels. Table 26 shows
trends in nitrous oxide emissions for the period 1989-1996. The largest increase in nitrous oxide emissions over the
period was from industry.
Overall, fertilizer use (which amplifies the natural flux of nitrous oxide from soil) and motor vehicle fuel combustion
combine to account for approximately 65 percent of estimated emissions. Rapid growth in emissions from energy use
was largely responsible for the increase in anthropogenic nitrous oxide emissions observed during the 1980s. In the
1990s, emissions from energy use have remained roughly stable (Figure 9).
Nitrous oxide emissions from motor vehicles are influenced by a variety of factors, including fleet size, vehicle miles
traveled, and emissions control technologies. In addition, vehicles equipped with catalytic converters to reduce
emissions of nitrogen oxides, carbon monoxide, and nonmethane volatile organiccompounds emit up to 20 times
more nitrous oxide than comparable vehicles without catalytic converters.(52) 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 in 1996 are estimated at 148,000 metric tons (Table 27).
Approximately 88 percent of the emissions can be attributed to motor vehicles. Although passenger car fuel efficiency
increased slightly in 1996, 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 1996, estimated nitrous oxide emissions from stationary combustion sources increased modestly, by 2,000 metric tons from 1995 (Table 28). Estimated nitrous oxide emissions from stationary combustion sources have increased by only 7.4 percent since 1989. Coal-fired combustion systems produced approximately three-quarters of the 1996 emissions, and electric utilities accounted for approximately two-thirds of the total.
On a global scale, agricultural practices contribute approximately 70 percent of anthropogenic nitrous oxide emissions.(53),(54) Their contribution was approximately 33 percent of U.S. emissions in 1996. 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 C). The disposal of crop residues by burning also produces nitrous oxide that is released into the atmosphere; however, the amount is relatively minor, at 5,000 metric tons or 3.4 percent of total emissions of nitrous oxide from agricultural sources in 1996.
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.(55) The EIA estimates that 141,000 metric tons of nitrous oxide were released into the atmosphere as a result of fertilization practices in 1996 (Table 29). This represents a decrease of 19,000 metric tons (11.7 percent) compared with the amount released in 1990 and a decrease of 13,000 metric tons (8.6 percent) compared with the amount released in 1995.
As described in Chapter 3, incomplete combustion of agricultural wastes produces various gases, including methane
and nitrous oxide. In 1996, the burning of crop residues produced estimated emissions of approximately 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 1980.
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 14,000 metric tons (14.1 percent) since 1990 and by 3,000 metric
tons (2.6 percent) since 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. Nitrous oxide is an intrinsic byproduct of this chemical reaction. For every
metric ton of adipic acid produced, 0.3 metric tons of nitrous oxide is created.(56) Currently, two plants (accounting for
approximately 77 percent of total production) control emissions by thermallydecomposing the nitrous oxide, and
98 percent of the potential emissions from those plants are eliminated by this technique.(57)
Since 1994, adipic acid production has totaled about 800,000 metric tons, resulting in stable estimated nitrous oxide
emissions from this source; however, this level of production represents an increase of 70,000 metric tons (9.2 percent)
from the 1993 level.(58) Some of the growth can be attributed to increased demand for nylon fibers and polyvinyl
chloride, beginning in 1994.
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.(59)
The 9.1 million tons of nitric acid manufactured in 1996 resulted in estimated emissions of 50,000 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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