4. Nitrous Oxide
4. Nitrous OxideNitrous oxide (N2O) is a potent greenhouse gas , with a direct global warming potential 170 to 290 times that of carbon dioxide (see Chapter 1, Table 3). It is a stable compound that does not decay readily in the atmosphere, with a long atmospheric lifetime of over 121 years. Nitrous oxide is emitted from both anthropogenic and natural sources. Neither the mechanisms by which nitrous oxide enters the atmosphere nor the pathways by which it leaves the atmosphere are fully understood, but it is generally believed that the principal natural source of nitrous oxide is microbial action in soils, particularly in damp tropical forests.
Nitrous oxide concentrations in the atmosphere are rising slowly. Samples taken from ice cores in the Arctic and Antarctic regions suggest that pre-industrial nitrous oxide concentrations ranged from 270 to 290 parts per billion. Systematic modern measurements suggest that concentrations rose from about 300 parts per billion to 307 parts per billion between 1979 and 1988, an annual growth rate of rate of about 0.3 percent. (99)
Estimates of U.S. anthropogenic emissions of nitrous oxide are of varying reliability. The most important source of anthropogenic nitrous oxide emissions is probably the enhancement of natural emissions caused by the application of nitrogen fertilizer to agricultural soils. However, the extent to which this enhancement occurs is site-specific, varying with temperature, soil conditions, fertilizer type, and crop type. Without exceptionally detailed information, it is generally not possible to estimate nitrous oxide emissions from agricultural sources to any degree of reliability higher than an order of magnitude.
Estimates of emissions from energy-related sources are incomplete, because previously derived emissions coefficients from many combustion sources are now thought to have been based on unreliable sampling methods. Thus, the energy-related emissions reported here are known to underestimate actual energy-related emissions to some degree. Emissions from industrial processes are also subject to considerable uncertainty with respect to the effectiveness of emissions control measures and the actual amount of nitrous oxide released.
Table 27 illustrates recent trends for U.S. sources of nitrous oxide emissions, based on available information and estimation methods. Emissions from mobile sources grew rapidly in the late 1980s, due mostly to the increasing penetration of vehicles equipped with catalytic converters, which contribute to nitrous oxide formation. In aggregate, however, there has been no strong trend in estimated emissions during the 1990s.
Nitrous oxide emissions from the biosphere to the atmosphere originate mainly from soils. (100) Nitrous oxide is produced naturally in soils by the microbial processes of nitrification-the oxidation of ammonia to nitrate (NO3)-and denitrification- the reduction of nitrates or nitrites (NO2) to gaseous nitrogen. The application of commercial nitrogen fertilizers provides an additional source of nitrogen that can enhance natural nitrous oxide emissions from the soil. In well-aerated conditions, where soil moisture content does not limit aeration, nitrous oxide emissions from the nitrification of ammonium-based fertilizers can be substantial. (101) (102)
Many individual factors control nitrification and denitrification, including:
Adding nitrogen to the soil generally increases nitrous oxide emissions. (104) The temporal pattern of emissions following fertilization is generally that of a large efflux of nitrous oxide occurring for a short time. Emission rates decline sharply after about 6 weeks and thereafter fluctuate around a low baseline, independent of the amount of fertilizer applied. (105)
While there are many known factors that regulate nitrous oxide production, it is difficult to determine how these factors will interact under field conditions to produce measured fluxes. (106) Nitrification and denitrification, as well as the regulators of nitrous oxide/ nitrogen ratios from denitrification, all have their own sets of optimum environmental conditions. As a result, one process may be the primary nitrous oxide producer in one set of field conditions, but as soil conditions change, another process may predominate. (107)
The method first recommended by the IPCC for calculating nitrous oxide emissions from nitrogen fertilizers was based on the amount of each type of commercial fertilizer nitrogen consumed (in mass units of nitrogen) and an emissions coefficient for the fraction of applied nitrogen that is released as nitrous oxide for each fertilizer type. More recent work suggests that the method of using different emissions coefficients for different types of fertilizer is not adequately supported by field research. Because the findings were not based on a consistent set of controlled experiments, there was no basis for determining whether the observed variations in emissions rates were caused by differences in fertilizer type or by some other uncontrolled variable, such as soil type, climate, or crop type.
Until the uncertainty regarding nitrous oxide emissions is resolved, the IPCC suggests that countries assume that 1 percent of the nitrogen (both mineral and organic) applied annually as fertilizer is released to the atmosphere. This method implies total U.S. emissions of 162,000 metric tons of nitrous oxide in 1993 (Table 28); however, the estimates may vary by an order of magnitude. (108)
Combustion-related nitrous oxide emissions account for approximately 40 percent of total U.S. nitrous oxide emissions (Table 27). Despite declining emissions from stationary combustion, total energy-related nitrous oxide emissions have risen by 17 percent since 1987, due to a 28- percent increase in emissions from the transportation sector (Table 27).
Nitrous oxide is produced directly from the combustion of fossil fuels . Several factors influence nitrous oxide emissions, including the air-fuel mix, combustion temperature, and pollution control equipment. Until a few years ago, fuel combustion was thought to be a major source of nitrous oxide emissions. However, the discovery of a sampling error, which resulted in erroneously high emissions factors, revealed that combustion is actually a minor anthropogenic source.
To estimate nitrous oxide emissions from stationary sources, IPCC coefficients were applied to energy data published in EIA's State Energy Data Report and Monthly Energy Review. Reliable emissions factors for many potential sources of nitrous oxide emissions are not available, however. In 1993, stationary combustion sources are estimated to have released approximately 44,000 metric tons of nitrous oxide (Table 29). Coal-burning electric utilities and fuel-oil-fired commercial boilers account for most of these emissions.
Nitrous oxide is produced in motor vehicle engines through reactions with atmospheric nitrogen. Research indicates that catalytic converters-installed to control emissions of carbon monoxide, nonmethane volatile organic compounds, and nitrogen oxides-actually promote the formation of nitrous oxide. Nitrous oxide is produced during the reaction of nitrogen oxide (NO) and ammonia (NH3) over the platinum in the catalytic converter. (109) As the share of the U.S. motor vehicle fleet equipped with catalytic converters has increased over the years, so have emissions of nitrous oxide from this source.
To develop estimates of nitrous oxide emissions from mobile sources, emissions factors recommended by the IPCC, specific for various vehicle types and model years, were used. To use these factors, data on equipment use (i.e., miles traveled by vehicles of different types and model years) are required. While this information is not collected directly, the U.S. Department of Transportation's Federal Highway Administration (FHWA) reports annual estimates of vehicle miles traveled (VMT) for different types of vehicles (passenger cars, trucks, buses, and motorcycles).
The EIA also collects transportation data from vehicle- owning households through the Residential Transportation Energy Consumption Survey (RTECS). Surveys were conducted in 1983, 1985, 1988, and 1991. A custom database sort was used to compute VMT for household-sector passenger cars and trucks by model year for 1983, 1985, 1988, and 1991. A weighted average emissions coefficient was then calculated and applied to non-household-sector passenger cars and light trucks (business-owned vehicles, fleets, rental cars, etc.). Emissions estimates for other years were interpolated from the weighted average estimates for survey years. Estimates for 1992 were extrapolated from 1991 survey data and fleet age data reported by the American Automobile Manufacturers Association. (110) Results of those computations are shown in Table 30.
Nitrous oxide emissions from other mobile sources (ships, boats, trains, farm and construction equipment, and gasoline- powered aircraft) represent only 4 percent of total transportation- related emissions. To develop these estimates, IPCC coefficients were applied to energy consumption data reported in EIA's State Energy Data Report, Monthly Energy Review, and Fuel Oil and Kerosene Sales, and in Oak Ridge National Laboratory's Transportation Energy Data Book. Total emissions from "other transportation" are shown in Table 30.
Moderate amounts of nitrous oxide are also emitted from nitrogen-using chemical processes. The manufacture of nitric acid and adipic acid are two known sources, and other processes are thought to contribute nitrous oxide emissions as well. Table 31 summarizes estimates of emissions from nitric acid and adipic acid production.
Adipic acid is the main constituent of nylon (nylon 6,6). It is also used in the production of plasticizers, lubricants , and polyurethanes, and as a flavor additive in food products. The compound is formed by the oxidation of nitric acid with ketone-alcohol (cyclohexanol). Approximately 0.3 metric tons of nitrous oxide are released per metric ton of adipic acid formed during the production process. (111)
There are only three companies (Du Pont, Monsanto, and Allied Chemical) that manufacture adipic acid in the United States. These companies do not report adipic acid production individually. Aggregate production values were obtained from Chemical and Engineering News, which reports that 708,000 metric tons of adipic acid were produced in the United States in 1993. (112) Some U.S. plants control nitrous oxide emissions as a byproduct of waste control procedures designed to reduce nitrogen oxide (NOx) emissions. For this report, adipic acid production was allocated between plants with controls and those without controls. Controls were assumed to be 99 percent effective. This implies emissions of 50,000 metric tons of nitrous oxide.
Nitric acid is a raw material used primarily as a feedstock in fertilizer production, but also in the production of adipic acid and explosives. (113) Reimer et al. report that an estimated 50 to 65 millions metric tons of nitric acid is produced globally on an annual basis, of which about 1.6 million metric tons is used by the adipic acid industry. (114) Offgas measurements at a single Du Pont factory indicate emissions ranging from 2 to 9 grams of nitrous oxide per kilogram of nitric acid produced, implying U.S. emissions of 15,000 to 70,000 metric tons annually from nitric acid production. (115) This report uses the midpoint of that range. Currently, no abatement technique specifically directed at removing nitrous oxide is used in nitric acid production facilities; however, the emissions factor used includes any effect of other abatement systems that may be applied. It is not yet clear whether the emissions from the single Du Pont plant tested are representative of the U.S. nitric acid industry as a whole.
