Long-Term Changes in Gas- and Particle-Phase Emissions From On-Road Diesel and Gasoline Vehicles

Gas- and particle-phase pollutants were measured separately for a) light-duty (LD) vehicles and b) medium- (MD) and heavy-duty (HD) diesel trucks. Measurements were made during summer 2006 at the Caldecott Tunnel in the San Francisco Bay area as part of a continuing campaign to track changes in vehicle emissions over time. When normalized to fuel consumption, NOx emission factors were found to be 3.0+-0.2 and 39+-3 g/kg for LD vehicles and MD/HD diesel trucks, respectively. Corresponding PM2.5 emission factors were 0.07+-0.02 and 1.4+-0.3 g/kg. Results from 2006 are compared to similar measurements made at the same site in 1997. NOx emission factors have decreased by 67+-3 and 25+-12% for LD vehicles and MD/HD diesel trucks, respectively. The ratio of HD to LD emission factor for NOx increased from 6+-1 to 12+-1 between 1997 and 2006, which indicates an increase in the relative importance of diesel trucks as a source of NOx emissions. This is compounded by the fact that in the U.S., diesel fuel sales have been increasing 3 times faster than gasoline sales. High time-resolution (1 Hz) measurements of NOx and CO2 were used to calculate NOx emission factors from individual HD truck exhaust plumes. MD/HD diesel trucks were also found to be a significant source of direct aldehyde emissions, which react in the atmosphere to form peroxyacyl nitrates.