Based on on-road measurements in their study, a team from the University of California Berkeley has estimated that, as of 2010, light-duty (LD) gasoline vehicles were responsible for 85% of CO; 18% of NOx; 18% of organic aerosol (OA); and 6% of black carbon (BC) emissions from on-road motor vehicles in the United States. Correspondingly, the study, reported in the ACS journal Environmental Science & Technology, also concluded that, as of 2010, diesel engines were the dominant on-road source of BC, OA, and NOx.
The researchers measured vehicle emissions of NOx, CO, fine particulate matter (PM2.5), OA and BC in bore 2 of the Caldecott tunnel in the San Francisco Bay Area. In bore 2, light-duty (LD) vehicles accounted for more than 99% of total traffic; heavy-duty trucks were not allowed.
Diesel vehicles (mostly medium-duty delivery trucks with two axles and six tires) accounted for less than 1% of all vehicles observed in the tunnel but were nevertheless responsible for (18 ± 3)%, (22 ± 6)%, and (45 ± 8)% of measured NOx, OA, and BC concentrations.
Gasoline- and diesel-engine contributions to total on-road vehicle emissions are functions of traffic volumes and vehicle exhaust emission rates, which are both subject to variations in space and time. An accurate understanding of the relative emissions contributions from these sources is needed for assessing their effects on air quality and human health. However, large uncertainties remain in current inventories of on-road vehicle emissions, particularly in the case of particulate black carbon (BC) and primary organic aerosol (OA).
… a broader reading of the relevant literature provides widely divergent findings about the relative importance of exhaust from gasoline versus diesel engines as sources of PM2.5, BC, and OA emissions … There are also major disagreements about the relative importance of contributions from gasoline and diesel vehicles to ambient fine-particle concentrations, as inferred from receptor modeling studies.
… The overall objective of this study was to determine current emission rates of gaseous and particulate pollutants from on-road light-duty vehicles, accounting for pollutant contributions due to diesel trucks that were present to a small degree in the on-road setting where vehicle emissions were measured.—Dallmann et al.
Among the findings of the study:
For CO2 and VOCs, for which fuel-specific emission factors for gasoline and diesel vehicles are similar, the contribution of exhaust emissions from diesel trucks is approximately proportional to fuel consumption. Because gasoline use tends to be much higher than corresponding diesel fuel sales in most parts of North America, gasoline engines tend to dominate these species.
Of the pollutants measured, gasoline vehicles showed higher emission factors compared to diesel for CO only. When combined with fuel sales data, the conclusion that gasoline engines are the dominant on-road source of CO is very clear—a finding that is consistent with other studies.
Emission factors of NOx and OA measured at the Caldecott tunnel were an order of magnitude higher for diesel trucks than for LD gasoline vehicles, and BC emission factors were approximately a factor of 50 higher for diesel engines. These differences in emission factors offset the relatively small (when compared to gasoline) amounts of diesel fuel consumed at regional, state, and national scales.
Diesel engines contribute greater than 50% of total on-road BC emissions, even at very low relative levels of diesel fuel consumption.
Future emissions from the on-road vehicle fleet will be strongly affected by the introduction of advanced emission control technologies for the diesel truck fleet. New control technologies such as diesel particle filters and selective catalytic reduction systems are now standard equipment for new HD diesel trucks. These systems are designed to reduce emissions of particulate matter and NOx. Newer trucks are often also equipped with an oxidation catalyst that is effective in reducing emissions of CO and VOC.
… As a result, the relative contributions of gasoline versus diesel engines to overall emissions of pollutants such as BC and OA might change rapidly in the coming years. Continued measurements of emissions from the on-road vehicle fleet are needed to track these changes and the resultant influences on the absolute and relative emission contributions from gasoline and diesel engines.—Dallmann et al.
- Timothy R. Dallmann, Thomas W. Kirchstetter, Steven J. DeMartini, and Robert A. Harley (2013) “Quantifying On-Road Emissions from Gasoline-Powered Motor Vehicles: Accounting for the Presence of Medium- and Heavy-Duty Diesel Trucks,” Environmental Science & Technology