title: Isoprene Peroxy Radical Dynamics.
abstract: Approximately 500 Tg of 2-methyl-1,3-butadiene (isoprene) is emitted by deciduous trees each year. Isoprene oxidation in the atmosphere is initiated primarily by addition of hydroxyl radicals (OH) to C4or C1in a ratio 0.57 ± 0.03 (1σ) to produce two sets of distinct allylic radicals. Oxygen (O2) adds to these allylic radicals either δ (Z or E depending on whether the allylic radical is cis or trans) or β to the OH group forming six distinct peroxy radical isomers. Due to the enhanced stability of the allylic radical, however, these peroxy radicals lose O2in competition with bimolecular reactions. In addition, the Z-δ hydroxy peroxy radical isomers undergo unimolecular 1,6 H-shift isomerization. Here, we use isomer-resolved measurements of the reaction products of the peroxy radicals to diagnose this complex chemistry. We find that the ratio of δ to β hydroxy peroxy radicals depends on their bimolecular lifetime (τbimolecular). At τbimolecular≈ 0.1 s, a transition occurs from a kinetically to a largely thermodynamically controlled distribution at 297 K. Thus, in nature, where τbimolecular> 10 s, the distribution of isoprene hydroxy peroxy radicals will be controlled primarily by the difference in the relative stability of the peroxy radical isomers. In this regime, β hydroxy peroxy radical isomers comprise ∼95% of the radical pool, a much higher fraction than in the nascent (kinetic) distribution. Intramolecular 1,6 H-shift isomerization of the Z-δ hydroxy peroxy radical isomers produced from OH addition to C4is estimated to be ∼4 s-1at 297 K. While the Z-δ isomer is initially produced in low yield, it is continually reformed via decomposition of the β hydroxy peroxy radicals. As a result, unimolecular chemistry from this isomer contributes about half of the atmospheric fate of the entire pool of peroxy radicals formed via addition of OH at C4for typical atmospheric conditions (τbimolecular= 100 s and T = 25 C). In contrast, unimolecular chemistry following OH addition at C1is slower and less important.
authors.affiliation: Division of Geological and Planetary Sciences and ‡Division of Engineering and Applied Science, California Institute of Technology , Pasadena, California 91125, United States.
authors.forename: Alexander P
authors.initials: AP
authors.lastname: Teng
authors.affiliation: Division of Geological and Planetary Sciences and ‡Division of Engineering and Applied Science, California Institute of Technology , Pasadena, California 91125, United States.
authors.forename: John D
authors.initials: JD
authors.lastname: Crounse
authors.affiliation: Division of Geological and Planetary Sciences and ‡Division of Engineering and Applied Science, California Institute of Technology , Pasadena, California 91125, United States.
authors.forename: Paul O
authors.identifier: 0000-0002-6126-3854
authors.initials: PO
authors.lastname: Wennberg