title: Isomerization of Second-Generation Isoprene Peroxy Radicals: Epoxide Formation and Implications for Secondary Organic Aerosol Yields.
abstract: We report chamber measurements of secondary organic aerosol (SOA) formation from isoprene photochemical oxidation, in which radical concentrations were systematically varied and the molecular composition of semi- to low-volatility gases and SOA were measured online. Using a detailed chemical kinetics box model, we find that to explain the behavior of low-volatility products and SOA mass yields relative to input H2O2concentrations, the second-generation dihydroxy hydroperoxy peroxy radical (C5H11O6·) must undergo an intramolecular H-shift with a net forward rate constant of order 0.1 s-1or higher. This finding is consistent with quantum chemical calculations that suggest a net forward rate constant of 0.3-0.9 s-1. Furthermore, these calculations suggest that the dominant product of this isomerization is a dihydroxy hydroperoxy epoxide (C5H10O5), which is expected to have a saturation vapor pressure ∼2 orders of magnitude higher, as determined by group-contribution calculations, than the dihydroxy dihydroperoxide, ISOP(OOH)2(C5H12O6), a major product of the peroxy radical reacting with HO2. These results provide strong constraints on the likely volatility distribution of isoprene oxidation products under atmospheric conditions and, thus, on the importance of nonreactive gas-particle partitioning of isoprene oxidation products as an SOA source.
authors.forename: Emma L
authors.identifier: http://orcid.org/0000-0002-3041-9027
authors.initials: EL
authors.lastname: D'Ambro
authors.affiliation: Department of Chemistry, University of Copenhagen , Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.
authors.forename: Kristian H
authors.identifier: http://orcid.org/0000-0001-8070-8516
authors.initials: KH
authors.lastname: Møller
authors.forename: Felipe D
authors.initials: FD
authors.lastname: Lopez-Hilfiker
authors.forename: Siegfried
authors.initials: S
authors.lastname: Schobesberger
authors.forename: Jiumeng
authors.initials: J
authors.lastname: Liu
authors.forename: John E
authors.initials: JE
authors.lastname: Shilling
authors.forename: Ben Hwan
authors.initials: BH
authors.lastname: Lee
authors.affiliation: Department of Chemistry, University of Copenhagen , Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark.
authors.forename: Henrik G
authors.identifier: http://orcid.org/0000-0002-7275-8297
authors.initials: HG
authors.lastname: Kjaergaard
authors.forename: Joel A
authors.identifier: http://orcid.org/0000-0002-5098-4867
authors.initials: JA
authors.lastname: Thornton
mesh_terms: D000336:Aerosols / Q000737:chemistry*; D004852:Epoxy Compounds / Q000737:chemistry; D006861:Hydrogen Peroxide; D010084:Oxidation-Reduction; D014835:Volatilization
chemical_list: D000336:Aerosols; D004852:Epoxy Compounds; D006861:Hydrogen Peroxide