HR: 1340h
AN: A43A-0884    [Abstracts]
TI: Tracking the Chemical Evolution of Oxidized Organic Aerosol: Results from Aerosol Mass Spectrometer Analyses of Aged Diesel Emissions
AU: * Sage, A M
EM: asage@andrew.cmu.edu
AF: Center for Atmospheric Particle Studies; Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213, United States
AU: Weitkamp, E A
EM: eweitkam@andrew.cmu.edu
AF: Center for Atmospheric Particle Studies; Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213, United States
AU: Robinson, A L
EM: alr@andrew.cmu.edu
AF: Center for Atmospheric Particle Studies; Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213, United States
AU: Donahue, N M
EM: nmd@andrew.cmu.edu
AF: Center for Atmospheric Particle Studies; Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213, United States
AU: Jimenez, J L
EM: jose.jimenez@colorado.edu
AF: CIRES and Dept. of Chemistry, University of Colorado at Boulder, Boulder, CO 80309, United States
AB: Regional chemical transport models predicated on laboratory yield curves significantly underpredict the secondary organic aerosol (SOA) production in an aging urban air mass. The high-flux, volatile organic compounds included in these models cannot account for the large quantities of organic material that condense downwind of anthropogenic sources. Furthermore, the mass spectra of laboratory-generated SOA from these traditional high-volatility precursors do not agree with those observed in aged ambient air masses. From these observations, it is clear that atmospheric abundance is not the sole criterion for identifying SOA precursors. We have proposed that precursor vapor pressure also plays an important role, hypothesizing that SOA can form from the atmospheric oxidation of a large suite of compounds of varying vapor pressures. Here, we support this hypothesis by using an Aerosol Mass Spectrometer to track the chemical evolution of diesel exhaust as it is photochemically oxidized in an environmental chamber. Upon exposure to atmospheric oxidants, gas-phase compounds emitted from a diesel generator react to form substantial amounts of SOA. This chemistry is corroborated by changes in the mass spectrum of the organic aerosol with increasing oxidant exposure time. Explicit knowledge of the condensed-phase mass spectrum of the primary emissions from our generator allows us to decompose each mass spectrum recorded throughout the experiment into primary and residual, secondary spectra. Our analysis reveals that the chemical composition of the residual spectrum, and thus the SOA formed in these experiments, is not constant in time, but that the condensing organic material becomes increasingly oxidized over the course of an experiment. This conclusion is supported by mathematical deconvolutions of the laboratory spectra using both principal component analysis and positive matrix factorization. After several hours of aging, the total mass spectrum of the chamber aerosol closely resembles that of ambient aged organic particulate matter. This observation is consistent with our hypothesis that the species in an air mass that are responsible for SOA formation change with time. The precursors of early-forming SOA give rise to condensable products that are considerably less oxidized than those which contribute to the later SOA that forms after several generations of gas-phase oxidation. We attribute this SOA formation pattern to the presence of relatively reduced, but low vapor- pressure species that, upon oxidation, produce SOA with high yields. These species can efficiently contribute to rapid, early SOA formation, suggesting that large, saturated semi-volatile species with low vapor pressures may contribute significantly to ambient SOA concentrations.
DE: 0317 Chemical kinetic and photochemical properties
DE: 0345 Pollution: urban and regional (0305, 0478, 4251)
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting