Reactive Nitrogen Species Emission Trends in Three Light-/Medium-Duty United States Fleets.

Repeated, fuel-specific, emission measurements in Denver (2005/2013), Los Angeles (LA) (2008/2013), and Tulsa (2005/2013) provide long-term trends in on-road reactive nitrogen emissions from three light-/medium-duty U.S. fleets. Reductions in oxides of nitrogen (NOx) emissions ranged from 21% in Denver (from 5.6 ± 1.3 to 4.4 ± 0.2 g of NOx/kg of fuel) to 43% in Tulsa (from 4.4 ± 0.3 to 2.5 ± 0.1 g of NOx/kg of fuel) since 2005, while decreases in fleet ammonia (NH3) emissions ranged from no change in Denver (from 0.45 ± 0.09 to 0.44 ± 0.02 g of NH3/kg of fuel) since 2005 to a 28% decrease in LA (from 0.80 ± 0.02 to 0.58 ± 0.02 g of NH3/kg of fuel) since 2008. The majority of the reduction in gasoline vehicle NOx emissions occurred prior to the full implementation of the Tier II emission standards in 2009. High in-use NOx emissions from small-engine diesel passenger vehicles produced a significant contribution to the fleet means despite their small numbers. NH3 emissions decreased at a slower rate than NOx emissions as a result of modest NH3 emission reduction among the newest vehicles and increased emissions from a growing number of older vehicles with active catalytic converters. In addition, the reactive nitrogen emissions from many new model year vehicles are now dominated by NH3.