title: Highly Oxidized Second-Generation Products from the Gas-Phase Reaction of OH Radicals with Isoprene.
abstract: The gas-phase reaction of OH radicals with isoprene has been investigated in an atmospheric pressure flow tube at 293 ± 0.5 K with special attention to the second-generation products. Reaction conditions were optimized to achieve a predominant reaction of RO2radicals with HO2radicals. Chemical ionization-atmospheric pressure interface-time-of-flight mass spectrometry served as the analytical technique in order to follow the formation of RO2radicals and closed-shell products containing at least four O atoms in the molecule. The reaction products were detected as adducts with the reagent ions using acetate, lactate, or nitrate in the ionization process. Observed signals were attributed to a series of C5-products with multiple hydroxy, hydroperoxy, and probably carbonyl groups. H/D exchange experiments supported the product identification. The generation of the detected second-generation products can be mechanistically explained starting from the OH radical reaction of hydroxy hydroperoxide isomers, HO-C5H8-OOH. These isomers represent the dominant products of the initial OH radical attack on isoprene. Dihydroxy dihydroperoxides, (HO)2-C5H8-(OOH)2, were analyzed as the main second-generation products beside the dihydroxy epoxides. A simple kinetic analysis revealed that the observed second-generation products in total (other than dihydroxy epoxides) were formed with an estimated molar yield of 10.0-1.5+2.1% with respect to converted hydroxy hydroperoxides. A formation yield of 5.8-0.9+1.3% has been deduced for the main product (HO)2-C5H8-(OOH)2. The detected, highly oxidized isoprene products represent potential secondary organic aerosol precursors. An annual, global (HO)2-C5H8-(OOH)2formation strength of (16-35) × 106metric tons is estimated based on product measurements of this study and literature data regarding the formation of the dihydroxy epoxide isomers for an annual isoprene emission of 454 × 106metric tons of carbon.
authors.affiliation: Leibniz Institute for Tropospheric Research, TROPOS , 04318 Leipzig, Germany.
authors.forename: Torsten
authors.identifier: http://orcid.org/0000-0003-2014-6018
authors.initials: T
authors.lastname: Berndt
authors.affiliation: Leibniz Institute for Tropospheric Research, TROPOS , 04318 Leipzig, Germany.
authors.forename: Hartmut
authors.initials: H
authors.lastname: Herrmann
authors.affiliation: Department of Physics, University of Helsinki , P.O. Box 64, Helsinki 00014, Finland.
authors.forename: Mikko
authors.initials: M
authors.lastname: Sipilä
authors.affiliation: Department of Physics, University of Helsinki , P.O. Box 64, Helsinki 00014, Finland.
authors.forename: Markku
authors.initials: M
authors.lastname: Kulmala