AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
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Formation and Evolution of Molecular Products in alpha-Pinene Secondary Organic Aerosol
XUAN ZHANG, Renee McVay, Dandan Huang, Nathan Dalleska, Bernard Aumont, Richard Flagan, John Seinfeld, California Institute of Technology
Abstract Number: 8 Working Group: Aerosol Chemistry
Abstract Much of our understanding of atmospheric secondary organic aerosol (SOA) formation from volatile organic compounds derives from laboratory chamber measurements, including mass yield and elemental composition. These measurements alone are insufficient to identify the chemical mechanisms of SOA production. We present here a comprehensive dataset on the molecular identity, abundance, and kinetics of alpha-pinene SOA, a canonical system that has received much attention owing to its importance as an organic aerosol source in the pristine atmosphere. Identified organic species account for 58 ~ 72% of the alpha-pinene SOA mass, and are characterized as semi/low-volatility monomers and extremely low volatility dimers, which exhibit comparable oxidation state yet different functionalities. Features of the alpha-pinene SOA formation process are revealed for the first time from the dynamics of individual particle-phase components. While monomeric products dominate the overall aerosol mass, rapid production of dimers plays a key role in initiating particle growth. Continuous production of monomers is observed after the parent alpha-pinene is consumed, which cannot be explained solely by gas-phase photochemical production. Additionally, distinct responses of monomers and dimers to alpha-pinene oxidation by ozone vs. hydroxyl radicals, temperature, and relative humidity are observed. Gas-phase radical combination reactions together with condensed-phase rearrangement of labile molecules potentially explain the newly characterized SOA features, thereby opening up further avenues for understanding formation and evolution mechanisms of alpha-pinene SOA.