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Emissions of Greenhouse Gases Report
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Nitrous Oxide Emissions
Total Emissions | Agriculture | Energy Use | Industrial Sources | Waste Management |
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Total Emissions |
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Nitrous oxide emissions represent aout 5 percent of all U.S. greenhouse gas emissions. |
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The major sources of U.S. nitrous oxide emissions are agriculture and energy use, with industrial processes and waste management making smaller contributions (see Table 19 below). The largest source is agricultural activities (see Figure 17 on right), including nitrogen fertilization of soils and disposal of animal wastes. |
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Annual U.S. nitrous oxide emissions rose from 1990 to 1994, then fell from 1994 to 2003. They began rising sharply from 2003 to 2006, largely as a result of increases in the application of synthetic fertilizers. |
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| Agriculture |
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Agricultural soils—both direct and indirect emissions—account for more than three-quarters of all U.S. emissions of nitrous oxide from agriculture (see Figure 18 on right and Table 20 below). |
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When nitrogen compounds are added to the soil, bacterial action is stimulated, leading to increased emissions of nitrous oxide unless the application precisely matches plant uptake and soil capture. |
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Nitrous oxide is also released as part of the microbial denitrification of animal manure. Emissions are a function of animal size and manure production, the amount of nitrogen in the animal waste, and the method of managing the waste. Waste managed by solid storage or pasture range methods may emit 20 times as much nitrous oxide per unit of nitrogen content as waste managed in anaerobic lagoon and liquid systems. |
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| Energy Use |
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Nitrous oxide is a byproduct of fuel combustion in mobile and stationary sources (see Figure 19 on right). |
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More than three-quarters of U.S. nitrous oxide emissions from energy use can be traced to motor vehicles, primarily passenger cars and light trucks (see Table 21 below). Emissions from passenger cars did not increase from 2005 to 2006, emissions from light trucks showed the largest increase, and the rest of the increase in vehicle emissions of nitrous oxide is attributed to off-road vehicles (included in “Other Mobile Sources”). |
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Nitrous oxide emissions from motor vehicles are caused by the conversion of nitrogen oxides into nitrous oxide by catalytic converters. Because the normal operating temperature of catalytic converters is high enough to cause the thermal decomposition of nitrous oxide, emissions in this category result primarily from “cold starts” and defective catalytic converters. |
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Nitrous oxide emissions from stationary combustion sources are dominated by coal-fired generation at electric power plants. |
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| Industrial Sources |
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Production of adipic acid and nitric acid releases nitrous oxide as a byproduct (see Table 22 below). |
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Adipic acid is a fine white powder used primarily in the manufacture of nylon fibers and plastics. Three companies operate four U.S. plants that manufacture adipic acid by oxidizing a ketone-alcohol mixture with nitric acid. This chemical reaction causes nitrous oxide emissions. |
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A large decline in nitrous oxide emissions from industrial processes since 1996 (see Figure 20 on right) is the result of emissions control technology installed at three of the four adipic acid plants in the United States. |
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Nitric acid, a primary ingredient in fertilizers, usually is manufactured by oxidizing ammonia with a platinum catalyst. This oxidation causes nitrous oxide emissions. |
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| Waste Management |
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Emissions from human sewage account for nearly all U.S. nitrous oxide emissions from waste management. The remainder is associated with waste combustion (see Figure 21 on right and Table 23 below). |
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Estimates of nitrous oxide emissions from human waste are scaled to population size and per-capita intake of protein. |
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Nitrous oxide is emitted from wastewater that contains nitrogen-based organic materials, such as those found in human or animal waste. Factors that influence the amount of nitrous oxide generated from wastewater include temperature, acidity, biochemical oxygen demand, and nitrogen concentration. |
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