Gas-Phase Oxidation Products and SOA Yields from a-Pinene Oxidation under a Range of RO2 Reaction Conditions

LESLY FRANCO DELOYA, Erik Helstrom, Hannah Kenagy, Yaowei Li, Frank Keutsch, Jesse Kroll, MIT

     Abstract Number: 428
     Working Group: Aerosol Chemistry

Abstract
Peroxy radicals (RO₂) are key intermediates in the oxidation of volatile organic compounds (VOCs) and the formation of secondary organic aerosol (SOA). RO₂ radicals can undergo a number of reactions, including reaction with hydroperoxy radicals (HO_x), reaction with nitric oxide (NO), or isomerization; given that each of these channels forms different products, this branching likely impacts the production of SOA. However, previous laboratory studies have typically examined SOA yields under either very high or very low concentrations of NO and have not systematically examined SOA yields at the full atmospheric range of RO₂ fates. In the present study, we examine gas-phase products and SOA yields under a wide range of RO₂ reaction conditions. We oxidized α-pinene in a 7.5 m³ environmental chamber and used varying concentrations of hydroxyl radical (OH) precursors, nitrous acid (HONO) and hydrogen peroxide (H₂O₂), to vary the relative branching among different RO₂ channels. A chemical ionization mass spectrometer (CIMS) was run in both water and ammonium mode to sample a wide range of gas-phase oxidation products and an aerosol mass spectrometer (AMS), Thermal Denuder-Aerosol Mass Spectrometer (TD-AMS), and scanning mobility particle sizer (SMPS) were used to collect particle-phase information. We identify gas-phase α-pinene organic oxidation products that serve as markers for different RO₂ channels; these provide experimental constraints on the RO₂ chemistry occurring in the chamber (e.g., RO₂+NO, RO₂+HO₂, RO₂ isomerization). This enables us to describe SOA yields as a function of RO₂ reaction conditions, which can be used to improve SOA parameterizations within chemical transport models.