Long-term changes in emissions of nitrogen oxides and particulate matter from on-road gasoline and diesel vehicles

Gas- and particle-phase pollutants were measured separately for (a) light-duty (LD) vehicles and (b) medium-duty (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 40±3 g kg−1 for LD vehicles and MD/HD diesel trucks, respectively. Corresponding particulate matter (PM2.5) emission factors were 0.07±0.02 and 1.4±0.3 g kg−1. The ratio of particulate black carbon to organic mass (BC/OM) for LD vehicles was 0.71±0.15. For diesel trucks, BC/OM was 2±1, indicating that PM2.5 was dominated by BC. Results from 2006 are compared to similar measurements made at the same site in 1997. For LD vehicles, NOx and PM2.5 emission factors decreased by 67±3% and 36±17%, respectively. Corresponding decreases for diesel trucks were 30±9% for NOx and 48±12% for PM2.5. The ratio of HD to LD emission factor for NOx increased from 6±1 to 13±1 between 1997 and 2006, which indicates an increase in the relative importance of diesel trucks as a source of NOx emissions. The absorption, scattering, and extinction cross-section emission factors parameters relevant to climate change and atmospheric visibility, were an order of magnitude higher for diesel trucks than LD vehicles. Single-scattering albedo, measured at λ=675 nm, was 0.31±0.06 and 0.20±0.05 for LD vehicle and diesel truck PM emissions, respectively.