title: Formation and evolution of molecular products in α-pinene secondary organic aerosol.
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 α-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 α-pinene SOA mass, and are characterized as semivolatile/low-volatility monomers and extremely low volatility dimers, which exhibit comparable oxidation states yet different functionalities. Features of the α-pinene SOA formation process are revealed for the first time, to our knowledge, from the dynamics of individual particle-phase components. Although 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 α-pinene is consumed, which cannot be explained solely by gas-phase photochemical production. Additionally, distinct responses of monomers and dimers to α-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 α-pinene SOA.
authors.affiliation: Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125;
authors.forename: Xuan
authors.initials: X
authors.lastname: Zhang
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125;
authors.forename: Renee C
authors.initials: RC
authors.lastname: McVay
authors.affiliation: Department of Chemical and Biomolecular Engineering, Hong Kong University of Science and Technology, Hong Kong, China;
authors.forename: Dan D
authors.initials: DD
authors.lastname: Huang
authors.affiliation: Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125;
authors.forename: Nathan F
authors.initials: NF
authors.lastname: Dalleska
authors.affiliation: Laboratoire Interuniversitaire des Systèmes Atmosphériques, UMR CNRS 7583, Université Paris Est Créteil and Université Paris Diderot, 94010 Créteil, France.
authors.forename: Bernard
authors.initials: B
authors.lastname: Aumont
authors.affiliation: Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125; Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125;
authors.forename: Richard C
authors.initials: RC
authors.lastname: Flagan
authors.affiliation: Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125; Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125; seinfeld@caltech.edu.
authors.forename: John H
authors.initials: JH
authors.lastname: Seinfeld
keywords: air quality; climate; particulate matter; secondary organic aerosol