Impacts of Peroxy Radical Fates on α-Pinene Secondary Aerosol Composition
ERIK HELSTROM, Lesly Franco Deloya, Hannah Kenagy, Anita Avery, Manjula Canagaratna, Jesse Kroll, MIT
Abstract Number: 573
Working Group: Aerosol Chemistry
Abstract
Biogenic volatile organic compounds such as α-pinene are major (and often-studied) precursors of secondary organic aerosol (SOA). A vital branching point in the chemistry of these species in the atmosphere is the fate of peroxy (RO2) radicals, formed upon attack by an oxidant (e.g., OH). Though previous laboratory investigations have assessed the effects of RO2 chemistry on SOA yields, the impact of RO2 fate on molecular composition of the aerosol has not been explored in a systematic way. Here, we use explicit chemical modeling to inform a series of α-pinene oxidation experiments that span various atmospheric RO2 chemical regimes, modifying the relative competition between the RO2 + NO, RO2 + HO2, RO2 + RO2, and isomerization pathways. An Aerosol Mass Spectrometer (AMS) and Extractive Electrospray Ionization Time of Flight Mass Spectrometer (EESI-ToF-MS) are deployed to characterize SOA composition in each of these experiments. We demonstrate direct measurements of the dynamic evolution of SOA composition during its formation and comprehensive changes in aerosol composition between experiments. We evaluate these changes in the signal response of the EESI to the SOA as a function of RO2 regime, identifying major species dependent on the importance of RO2 reactions with both NO and HO2, as well as on RO2+RO2 reactions. We comprehensively evaluate these changes by exploring shifts in the average elemental ratios, double bond equivalency, and the relative abundance of nitrate functionalization and dimerization. We then compare these speciated measurements with the bulk measurements made in the AMS. Further, we evaluate potential molecular tracers of RO2 chemistry, possibly enabling the field characterization of the RO2 chemical conditions during ambient SOA formation.