AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
On-Road Gasoline and Diesel Vehicle Contributions to Fine Particulate Black Carbon and Primary Organic Aerosol Emissions
TIMOTHY DALLMANN, Thomas Kirchstetter, Robert Harley, University of California, Berkeley
Abstract Number: 390 Working Group: Urban Aerosols
Abstract Motor vehicles are an important source of fine particulate matter (PM2.5) emissions that include organic aerosol (OA) and black carbon (BC) as major components. Other recent on-road studies have highlighted the importance of gasoline engine-related contributions. However, separation of gasoline versus diesel contributions is a major source of uncertainty that has been discounted by other investigators. This issue arises even in settings where heavy-duty diesel truck traffic is not allowed or is present at very low levels. We measured emissions from thousands of in-use light-duty (LD) motor vehicles at the Caldecott tunnel in the San Francisco area. Measurements of PM2.5, OA, and BC, along with gaseous species, were made in tunnel lanes in which LD vehicles accounted for >99% of total traffic. Measured pollutant concentrations were apportioned between LD gasoline vehicles and diesel trucks. Fleet-average emission factors were then calculated for LD vehicles using a carbon balance method. Diesel trucks, while accounting for <1% of total vehicles, were responsible for 24 ± 4, 22 ± 5, and 45 ± 8% of the measured PM2.5, OA, and BC concentrations, respectively. These disproportionate emissions contributions from diesel trucks can result in significant bias in the on-road evaluation of LD vehicle emission factors unless analysts explicitly account for their presence. Fleet-average OA and BC emission factors for light-duty vehicles reported here are 10 and 50 times lower, respectively, than average heavy-duty diesel truck emission factors measured during the same field campaign. Emission factors and overall fuel consumption for gasoline and diesel engines were used to map out the relative importance of these sources to overall on-road vehicle emissions. In contrast to other recent studies, gasoline engines were found to be an insignificant source of BC, and a relatively minor source of POA emissions at urban, state, and national scales.