Impact of Increased Nitrogen Oxide Emissions from Diesel Engines on Regional Atmospheric Chemistry

Over the last 30 years, major efforts have been made to control emissions from light-duty passenger vehicles (in North America, these vehicles are almost all powered by spark-ignition gasoline engines). Progress in reducing these emissions has been more rapid for carbon monoxide and volatile organic compounds than for nitrogen oxides. Emission reductions have occurred despite increases in the number of vehicles on the road, the amount they are driven, and the amount of gasoline burned. Efforts to control heavy-duty compression-ignition (diesel) engine emissions began more recently, and to date have resulted in only minor reductions in the emission rate of nitrogen oxides. Furthermore, since use of diesel fuel has been growing at a rate that is about three times faster than gasoline, nitrogen oxide emissions from diesel engines have been increasing in both relative and absolute terms. The overall result is that mobile source (mainly gasoline engine) emissions of CO and VOC are declining, while total (gasoline+diesel) mobile source NOx emissions are approximately unchanged. Weekly cycles are observed on regional to continental scales in the measured atmospheric concentrations of nitrogen oxides. In California, an observed effect of higher ozone on weekends has become much more widespread between 1980 and 2000. The spreading of this weekend ozone effect is consistent with the increasingly important role of diesel engines as a source nitrogen oxides, and the strong weekly cycle in diesel engine activity and emissions.