Articles | Volume 14, issue 9
Atmos. Chem. Phys., 14, 4661–4678, 2014
Atmos. Chem. Phys., 14, 4661–4678, 2014

Research article 13 May 2014

Research article | 13 May 2014

Secondary organic aerosol formation exceeds primary particulate matter emissions for light-duty gasoline vehicles

T. D. Gordon1,2,*,**, A. A. Presto1, A. A. May1,***, N. T. Nguyen1, E. M. Lipsky3, N. M. Donahue1,2, A. Gutierrez4, M. Zhang4, C. Maddox5, P. Rieger5, S. Chattopadhyay6, H. Maldonado7, M. M. Maricq8, and A. L. Robinson1,2 T. D. Gordon et al.
  • 1Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA 15213, USA
  • 2Engineering and Public Policy, Carnegie Mellon University, Pittsburgh, PA 15213, USA
  • 3Department of Engineering, Penn State Greater Allegheny, McKeesport, PA 15131, USA
  • 4Mobile Source Operations, California Air Resources Board, El Monte, CA 91731, USA
  • 5Monitoring and Laboratory, California Air Resources Board, El Monte, CA 91731, USA
  • 6Planning and Technical Support, California Air Resources Board, El Monte, CA 91731, USA
  • 7Research Division, California Air Resources Board, Sacramento, CA 95814, USA
  • 8Research and Advanced Engineering, Ford Motor Company, Dearborn, MI 48120, USA
  • *now at: National Oceanic and Atmospheric Administration Earth System Research Laboratory, Chemical Sciences Division, 325 Broadway, Boulder, CO 80305, USA
  • **now at: Cooperative Institute for Research in Environmental Sciences, University of Colorado, 216 UCB, Boulder, CO 80309, USA
  • ***now at: Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA

Abstract. The effects of photochemical aging on emissions from 15 light-duty gasoline vehicles were investigated using a smog chamber to probe the critical link between the tailpipe and ambient atmosphere. The vehicles were recruited from the California in-use fleet; they represent a wide range of model years (1987 to 2011), vehicle types and emission control technologies. Each vehicle was tested on a chassis dynamometer using the unified cycle. Dilute emissions were sampled into a portable smog chamber and then photochemically aged under urban-like conditions. For every vehicle, substantial secondary organic aerosol (SOA) formation occurred during cold-start tests, with the emissions from some vehicles generating as much as 6 times the amount of SOA as primary particulate matter (PM) after 3 h of oxidation inside the chamber at typical atmospheric oxidant levels (and 5 times the amount of SOA as primary PM after 5 × 106 molecules cm−3 h of OH exposure). Therefore, the contribution of light-duty gasoline vehicle exhaust to ambient PM levels is likely dominated by secondary PM production (SOA and nitrate). Emissions from hot-start tests formed about a factor of 3–7 less SOA than cold-start tests. Therefore, catalyst warm-up appears to be an important factor in controlling SOA precursor emissions. The mass of SOA generated by photooxidizing exhaust from newer (LEV2) vehicles was a factor of 3 lower than that formed from exhaust emitted by older (pre-LEV) vehicles, despite much larger reductions (a factor of 11–15) in nonmethane organic gas emissions. These data suggest that a complex and nonlinear relationship exists between organic gas emissions and SOA formation, which is not surprising since SOA precursors are only one component of the exhaust. Except for the oldest (pre-LEV) vehicles, the SOA production could not be fully explained by the measured oxidation of speciated (traditional) SOA precursors. Over the timescale of these experiments, the mixture of organic vapors emitted by newer vehicles appears to be more efficient (higher yielding) in producing SOA than the emissions from older vehicles. About 30% of the nonmethane organic gas emissions from the newer (LEV1 and LEV2) vehicles could not be speciated, and the majority of the SOA formed from these vehicles appears to be associated with these unspeciated organics. By comparing this study with a companion study of diesel trucks, we conclude that both primary PM emissions and SOA production for light-duty gasoline vehicles are much greater than for late-model (2007 and later) on-road heavy-duty diesel trucks.

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