Implications of RO2 Fate for α-Pinene SOA Volatility and Composition

ERIK HELSTROM, Lesly Franco Deloya, Hannah Kenagy, Manjula Canagaratna, Jesse Kroll, MIT

     Abstract Number: 396
     Working Group: Aerosol Chemistry

Abstract
The formation and composition of secondary organic aerosol (SOA) is governed by the atmospheric processing of volatile organic compounds (VOCs), with a key branch point being the fate of organic peroxy (RO₂) radicals. Laboratory chamber studies have been employed to interrogate the chemistry of SOA formation from the oxidation of many common VOCs under different environmental conditions. However, limits in both chamber conditions and instrumentation have historically pushed experimental design towards higher concentrations of both reactants and oxidants, inhibiting recently-identified RO₂ unimolecular pathways. The impacts of competition between RO₂ isomerization, RO₂+NO, and RO₂+HO₂ reactions on both particle composition and volatility have not been systematically explored. Using explicit chemical modeling to predict likely experimental RO₂ fates, we generate SOA from α-pinene in a series of chamber experiments where the relative abundances of OH, NO, and HO₂ radicals were manipulated to modify the prevailing RO₂ regime. The particles were sampled through a Thermal Denuder–Aerosol Mass Spectrometer (TD-AMS) in order to determine the amount, elemental ratios, and volatility distribution of aerosol as a function of RO₂ fate. Species level chemical information was collected using an Extractive Electrospray Ionization Time of Flight Mass Spectrometer (EESI-ToF-MS) to track SOA compositional variation. Accessing a more atmospherically relevant span of RO₂ fates in the laboratory invites the possibility of describing a range of SOA chemistry beyond the previous “high-NO_x vs. low NO_x” paradigm, to include environmental conditions in which RO₂ isomerization can become competitive with bimolecular RO₂ reactions.