Characterization of gas- and particle-phase emissions from on-road motor vehicles

Motor vehicles are a significant source of air pollutants that may cause adverse health effects, degrade visibility, soil buildings and materials, damage crops, and alter the earth's radiative energy balance leading to global climate change. Vehicle emissions are affected by changes in emission control technologies, fuel properties, and growth in vehicle population and amount of driving. Measurement of on-road motor vehicle emissions is useful both in the determination of emissions trends over time and in analysis of the relative importance of different sources.;Gas- and particle-phase pollutants were quantified separately for light-duty (LD) vehicles, and medium- (MD) and heavy-duty (HD) diesel trucks during summer 2006 at the Caldecott tunnel in the San Francisco Bay area. HD diesel truck emission factors (pollutant mass emitted per unit fuel burned) were found to be at least an order of magnitude higher than those of LD vehicles for nitrogen oxides (NOx), fine particulate mass (PM2.5), total particle number, and a variety of carbonyls. Size-resolved particulate number and volume emission factors were also at least an order of magnitude higher from diesel trucks for all particle diameters (Dp) measured (10 nm < Dp < 290 nm). The optical properties of PM emissions were directly measured at lambda = 675 nm; 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. The single-scattering albedo of fresh PM emissions was subsequently calculated.;Results from 2006 were 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, between 1997 and 2006 as measured at the Caldecott tunnel. Corresponding decreases for diesel trucks were 30+/-9% for NOx and 48 +/- 12% for PM2.5. Particle number emission factors for LD vehicles and diesel trucks also decreased between 1997 and 2006. The HD/LD emission factor ratio 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.;Diesel trucks are the dominant on-road source of not only NOx, but also aldehydes, which are toxic, photochemically reactive, and malodorous. Formaldehyde emissions from LD vehicles decreased by 61 +/- 7% between 2001 and 2006, likely due to fleet turnover and the removal of methyl tent-butyl ether (MTBE) from gasoline. Increased use of ethanol in gasoline after 2003 appears to have slowed the decrease in acetaldehyde emissions that would have otherwise occurred due to fleet turnover.;Emissions measurements for 226 individual HD trucks driving through the tunnel showed that particulate black carbon (BC) emissions were log-normally distributed. The highest-emitting 10% of trucks were responsible for ∼40% of total BC and particle number emissions from all HD trucks sampled. There was minimal overlap among high-emitters of these two pollutants: only 1 truck was found to be among the highest-emitting 10% for both BC and particle number. This result is consistent with the hypothesis that vapor condensation or adsorption onto existing BC particle surfaces competes effectively with nucleation processes that form new ultrafine particles.