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. Emissions of nitrous oxide in the United States will probably continue to rise slowly as the proportion of the motor vehicle fleet using catalytic converters increases. Mobile Combustion Nitrous oxide emissions from motor vehicles are influenced by a variety of factors, including fleet size, vehicle miles traveled, and emission control technologies. Approximately 88 percent of the emissions can be attributed to motor vehicles. 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.