Colloquium

Emissions from gasoline and diesel engines vary on diurnal, weekly, and decadal time scales. Rapid growth in diesel fuel use and decreasing NOx emission rates from gasoline engines have led to altered emission profiles. During the 1990s, on-road use of diesel fuel grew 3 times faster than gasoline. Over the same time period, the NOx emission rate from gasoline engines in California was reduced by a factor of ~2, whereas NOx emission rates from diesel engines were unchanged. Diesel engines therefore grew in both relative and absolute terms as a source of NOx, accounting for over half of all on-road NOx emissions as of 2000. Diesel truck emissions decrease by 60-80% on weekends. Counter-intuitive responses to these emission changes are seen in measured concentrations of ozone and fine particulate nitrate. In contrast, elemental carbon (EC) concentrations decrease on weekends as expected. Weekly and diurnal patterns in diesel truck activity contribute to variability in the ratio of organic carbon (OC) to EC in primary source emissions, and this could be a source of bias in assessments of the importance of secondary organic aerosol.