Multi-Generational Autoxidation Chemistry in the α-Pinene+Oh System and Dual Roles for NO in SOA Formation

MASAYUKI TAKEUCHI, Jean Rivera-Rios, Nga Lee Ng, Ivan Piletic, Emma D'Ambro, Joel A. Thornton, Benjamin Murphy, Havala Pye, Georgia Institute of Technology

     Abstract Number: 429
     Working Group: Aerosol Chemistry

Abstract
Evidence for autoxidation, rapid gas-phase oxidation of organic vapors, has recently emerged, though its implication for the formation of secondary organic aerosol (SOA) in varying environmental conditions (e.g., NO level) is not well understood. The presence of a high NO level suppresses first-generation autoxidation reactions by shortening the lifetime of peroxy radicals (RO₂). However, for cyclic precursors, a high NO level also promotes further ring-opening processes of first-generation products via alkoxy radical decomposition followed by RO₂+NO. The higher-generation RO₂ can potentially undergo similar or faster autoxidation than first-generation RO₂ due to increased conformational flexibility, though studies on autoxidation of higher-generation RO₂ are limited. Moreover, higher-generation RO₂ are likely more functionalized and contain more labile sites to abstract H atoms to propagate autoxidation. In this study, we expanded an explicit α-pinene+OH mechanistic model by including multi-generational autoxidation chemistry. SOA formation and properties from the model results were compared with our chamber experiment data to evaluate the model responses and to understand the dual roles of NO in autoxidation processes. For ambient relevant levels of NO (0.01−10 ppb) and HO₂ (1−100 ppt) in a monoterpene-rich environment (i.e., southeastern U.S.), our model revealed that the majority of SOA is of low volatility (log₁₀(C^*, µg m^-3) <0) rather than semi-volatile and that SOA formation potential (mass yield) from α-pinene+OH oxidation is very similar among urban (Atlanta, GA) and rural sites (Centreville, AL), consistent with prior ambient measurements. This occurs because the competing effects of NO in suppressing α-pinene RO₂ autoxidation and facilitating multi-generational autoxidation offset each other for the range of NO level tested. Since ambient NO_x emissions also contribute to atmospheric oxidative capacity, future reductions of NO_x are likely to lead to benefits of decreasing PM level via reduced SOA from α-pinene+OH oxidation.