Secondary Organic Aerosol Formation of Tailpipe Emissions from On-road Gasoline Vehicles

On-road gasoline vehicles are a major source of secondary organic aerosol (SOA) in urban areas. We tested a fleet of on-road gasoline vehicles using a cold-start unified cycle on the dynamometer to investigate SOA formation from the OH radical oxidation of gasoline vehicle tailpipe emissions using a smog chamber and a Potential Aerosol Mass (PAM) oxidation flow reactor. These vehicles were recruited from California in-use on-road vehicles and covered a wide range of emission standards, including Super Ultra-Low Emission vehicles (SULEVs) that meet the most stringent emission standard. The PAM reactor complements the smog chamber by enabling us to characterize SOA production from the oxidation of gasoline vehicular exhaust over longer OH exposure times. Comprehensive chemical analysis of non-methane hydrocarbons (NMHCs) in tailpipe emissions from gasoline vehicles has been carried out to determine SOA precursors, including intermediate volatility and semi-volatile organic compounds. We observed less SOA production from newer, lower NMHC emitting vehicles compared to older, higher-emitting vehicles. No SOA production was observed for SULEV vehicles during smog chamber experiments, but SOA production for SULEV vehicles was about a factor of 4 greater than primary organic aerosol in the PAM reactor. In addition, we have investigated the SOA formation potential and the composition of SOA under a range of conditions, including organic aerosol concentrations, SOA precursor concentrations and OH exposure, by comparing the SOA formation in the smog chamber to the PAM reactor. Our measurements of SOA formation and characterization of NMHCs identify the major classes of SOA precursors and determine the effectiveness of the tightening of emission standards to reduce SOA. Our results will significantly improve our understanding of SOA formation in the atmosphere.