NITROUS-OXIDE EMISSIONS FROM VEHICLES

Nitrous oxide is one of the trace gases that contribute to greenhouse warming as well as stratospheric ozone depletion. The role of vehicular emissions in the global N2O budget is currently in doubt due to two recent findings: (1) catalytic converters increase the levels of N2O emissions from vehicles, and (2) past measurements are questionable due to the discovery of artifact N2O formation during collection. This study combined new tailpipe emission measurements with literature values to arrive at average vehicular N2O emissions. Nitrous oxide measurements were made on nine vehicles. The measured emissions were combined with literature values from an additional 32 vehicles to determine typical N2O emission levels. Emissions averaged 3.6 mg/mile from noncatalyst cars, but increased to an average of 29 mg/mile from vehicles with oxidation catalysts, 61 mg/mile with dual-bed catalysts, and 45 mg/mile with three-way catalysts. No difference was seen between the late-model cars tested and earlier literature values. The artifact problem observed during other emission measurements was not a problem during vehicle testing. Vehicular emissions of N2O were estimated to be 125 Gg in the United States and about 200 Gg in the world. Based on this value and the rate of N2O increase in the stratosphere, vehicles in the United States emit about two percent of anthropogenic N2O emissions and contribute only 0.1 percent of the calculated temperature increase from greenhouse gases.