title: Photolysis, OH reactivity and ozone reactivity of a proxy for isoprene-derived hydroperoxyenals (HPALDs).
abstract: The C(5)-hydroperoxyenals (C(5)-HPALDs) are a newly-recognized class of multi-functional hydrocarbons produced during the hydroxyl radical (OH)-initiated oxidation of isoprene. Recent theoretical calculations suggest that fast photolysis of these compounds may be an important OH source in high-isoprene, low-NO regions. We report experimental constraints for key parameters of photolysis, OH reaction and ozone reaction of these compounds as derived from a closely-related, custom-synthesized C(6)-HPALD. The photolysis quantum yield is 1.0 ± 0.4 over the range 300-400 nm, assuming an absorption cross section equal to the average of those measured for several analogous enals. The yield of OH from photolysis was determined as 1.0 ± 0.8. The OH reaction rate constant is (5.1 ± 1.8) × 10(-11) cm(3) molecule(-1) s(-1) at 296 K. The ozone reaction rate constant is (1.2 ± 0.2) × 10(-18) cm(3) molecule(-1) s(-1) at 296 K. These results are consistent with previous first-principles estimates, though the nature and fate of secondary oxidation products remains uncertain. Incorporation of C(5)-HPALD chemistry with the above parameters in a 0-D box model, along with experimentally-constrained rates for C(5)-HPALD production from isomerization of first-generation isoprene hydroxyperoxy radicals, is found to enhance modeled OH concentrations by 5-16% relative to the traditional isoprene oxidation mechanism for the chemical regimes of recent observational studies in rural and remote regions. This enhancement in OH will increase if C(5)-HPALD photo-oxidation products also photolyze to yield additional OH or if the C(5)-HPALD production rate is faster than has been observed.
authors.affiliation: Department of Chemistry, University of Wisconsin, Madison, WI, USA. gwolfe@chem.wisc.edu
authors.forename: Glenn M
authors.initials: GM
authors.lastname: Wolfe
authors.forename: John D
authors.initials: JD
authors.lastname: Crounse
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Parrish
authors.forename: Jason M
authors.initials: JM
authors.lastname: St Clair
authors.forename: Melinda R
authors.initials: MR
authors.lastname: Beaver
authors.forename: Fabien
authors.initials: F
authors.lastname: Paulot
authors.forename: Tehshik P
authors.initials: TP
authors.lastname: Yoon
authors.forename: Paul O
authors.initials: PO
authors.lastname: Wennberg
authors.forename: Frank N
authors.initials: FN
authors.lastname: Keutsch