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. As a result, most of the 25-percent increase in emissions since 1987 occurred prior to 1991. During combustion, nitrous oxide is produced as a result of chemical interactions between nitric oxide and other combustion products. 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. Their contribution was approximately 38 percent of U.S. emissions in 1994. Fertilizer Use Nitrous oxide uptake and emissions occur naturally as a result of nitrification and denitrification processes in soil. Crop Residue Burning As described in Chapter 3, incomplete combustion of agricultural wastes produces various gases, including methane and nitrous oxide. 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.