Grantee Research Project Results

Long-term trends in nitrogen oxide emissions from motor vehicles at national, state, and air basin scales.

Citation:

McDonald BC, Dallmann TR, Martin EW, Harley RA. Long-term trends in nitrogen oxide emissions from motor vehicles at national, state, and air basin scales. Journal of Geophysical Research:Atmospheres 2012;117(D21):D00V18 (11 pp.).

Abstract:

A fuel-based approach is used to estimate nitrogen oxides (NO_x = NO + NO₂) emissions from gasoline- and diesel-powered motor vehicles. Estimates are made at the national level for the period 1990–2010. Vehicle emissions are also estimated at the state level for California, and for the South Coast (Los Angeles) and San Joaquin Valley air basins. Fuel-based emission estimates are compared with predictions from widely used emission inventory models. Changes in diesel NO_x emissions vary over time: increasing between 1990 and 1997, stable between 1997 and 2007, and decreasing since 2007. In contrast, gasoline engine-related NO_x emissions have decreased steadily, by ∼65% overall between 1990 and 2010, except in the San Joaquin Valley, where reductions were not as large due to faster population growth. In the San Joaquin Valley, diesel engines were the dominant on-road NO_x source in all years considered (reaching ∼70% in 2010). In the urbanized South Coast air basin, gasoline engine emissions dominated in the past and have been comparable to on-road diesel sources since 2007 (down from ∼75% in 1990). Other major anthropogenic sources of NO_x are added to compare emission trends with trends in surface pollutant observations and satellite-derived data. When all major anthropogenic NO_x sources are included, the overall emission trend is downward in all cases (−45% to −60%). Future reductions in motor vehicle NO_x will depend on the effectiveness of new exhaust after-treatment controls on heavy-duty trucks, as well as further improvements to durability of emission control systems on light-duty vehicles.