AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
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Aqueous-phase Photooxidation of Dimeric Compounds Arising from alpha-pinene
RAN ZHAO, Dana Aljawhary, Alex K. Y. Lee, Jonathan Abbatt, University of Toronto
Abstract Number: 145 Working Group: Aerosol Chemistry
Abstract Our current understanding of the chemistry occurring in the alpha-pinene ozonolysis reaction system is still incomplete. In particular, the chemistry of dimers and higher molecular weight organic compounds remains elusive. Kinetic data for the photochemistry of dimeric compounds is limited due to the fact that the dimeric compounds are comprised of a large number of trace compounds with highly variable structures and functionalities, making the separation and identification of each individual compound highly challenging. In this study, we have investigated the aqueous-phase photooxidation of dimeric compounds arising from the alpha-pinene ozonolysis system. SOA was produced in a 1 m$^3 chamber, and the water-soluble fraction of particles collected on a filter was photo-oxidized in the bulk aqueous phase. Employing Aerosol Chemical Ionization Mass Spectrometry (Aerosol-CIMS), we have observed rapid decay of dimeric compounds upon oxidation by the OH radical. Using pinonic acid as a reference compound, we obtained the effective 2nd order rate constant of total dimer signal reacting with the OH radical (k$^(II)$_(OH)). The k$^(II)$_(OH) values monitored by three different reagent ions (iodide, acetate and protonated water clusters) showed agreement within a factor of two. Given that the three reagent ions are detecting three overlapping yet diverging populations of compounds, we have arrived at a generalized k$^(II)$_(OH) value for OH oxidation of dimeric compounds: 1.5 ± 0.8 × 10$^9 M$^(-1) s$^(-1). Results from this study demonstrate that dimeric compounds from alpha-pinene ozonolysis react with the OH radical essentially at the collision-limited rate. The k$^(II)$_(OH) derived from this work can be incorporated into existing cloudwater chemistry models. We further performed positive matrix factorization analysis to the Aerosol-CIMS data, and the results are discussed.