Isomer-Selective Study of the OH-Initiated Oxidation of Isoprene in the Presence of O 2 and NO: 2. The Major OH Addition Channel Buddhadeb Ghosh, Alejandro Bugarin, Brian T. Connell, and Simon W. North* Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012, College Station, Texas 77842 ReceiVed: September 18, 2009; ReVised Manuscript ReceiVed: January 6, 2010 We report the first isomeric-selective study of the dominant isomeric pathway in the OH-initiated oxidation of isoprene in the presence of O 2 and NO using the laser photolysis-laser induced fluorescence (LP-LIF) technique. The photolysis of monodeuterated/nondeuterated 2-iodo-2-methylbut-3-en-1-ol results exclusively in the dominant OH-isoprene addition product, providing important insight into the oxidation mechanism. On the basis of kinetic analysis of OH cycling experiments, we have determined the rate constant for O 2 addition to the hydroxyalkyl radical to be 1.0 -0.5 +1.7 × 10 -12 cm 3 s -1 , and we find a value of 8.1 -2.3 +3.4 × 10 -12 cm 3 s -1 for the overall reaction rate constant of the resulting hydroxyperoxy radical with NO. We also report the first clear experimental evidence of the (E) form of the δ-hydroxyalkoxy channel through isotopic labeling experiments and quantify its branching ratio to be (10 ( 3)%. This puts a rigorous upper limit on the branching of the (E)-δ-hydroxyalkoxy radical channel. Since our measured isomeric-selective rate constants for the dominant outer channel in OH-initiated isoprene chemistry are similar to the overall rate constants derived from nonisomeric kinetics, we predict that the remaining outer addition channel will have similar reactivity. I. Introduction Isoprene (2-methyl-1,3-butadiene) is the dominant non- methane organic compound (NMOC) emitted into the atmo- sphere by vegetation with an annual global emission rate of ∼500 Tg. 1 It represents almost 50% of all biogenic non-methane hydrocarbons on a global scale, 2 with an atmospheric concentra- tion between 1-30 ppb. 3,4 In the atmosphere, isoprene reacts with the OH radical, NO 3 radical, Cl radical, and O 3, 5 although a strong diurnal emission rate and high reactivity toward the OH radical results in chemistry dominated by OH-initiated oxidation (90%) with an atmospheric lifetime with respect to the OH radical of approximately 1.5 h. 6 The OH-initiated oxidation of isoprene is a central issue in atmospheric chemistry and is responsible for 50-100% of ozone production attributed to VOC oxidation in the continental U.S., 4 which has adverse effect on vegetation 7,8 and human health. 7 Isoprene oxidation also leads to the formation of organic nitrates, which are responsible for removal of as much as 7% of the NO emitted in North America in the summer time, 6 accounting for 4% of nitrogen oxide transported from North America. 9 Isoprene is responsible for a 10% increase in the half-life of methane. 10 Finally, the oxidation of isoprene leads to US and global biogenic SOA formation of 50% and 58%, respectively. 11,12,13 The addition of OH to isoprene results in four distinct hydroxyl alkyl isomers (Figure 1), each of which ultimately leads to different first-generation end products. Although there has been no direct experimental determination of the branching between channels, theoretical work has predicted branching of 0.67, 0.02, 0.02, and 0.29, respectively, for isomers I, II, III, and IV, 14 with an overall rate of (1.0 ( 0.1) × 10 -10 molecule -1 cm 3 s -1 . 14–19 End product analysis studies also suggest that OH addition occurs preferentially at the terminal carbons. 20,21 The radicals formed from OH addition to the terminal carbons react with molecular oxygen under atmospheric conditions to form peroxy radicals, which subsequently react with NO to form alkoxy radicals. This contrasts with the dominant pathway for the radicals formed from OH addition to the inner carbons. For these radicals, recent studies have demonstrated that prompt isomerization to form R-hydroxyl alkyl radicals occurs followed by reaction with oxygen to form C 5 carbonyl compounds and HO 2 . 22–24 Theoretical studies suggest that decomposition is the sole fate for the -hydroxyalkoxy radicals 25–27 leading to the formation of methylvinyl ketone, methacrolein, and formaldehyde as first- generation end products. 28–34 The δ-hydroxyalkoxy radicals however, undergo prompt 1,5 hydrogen shift 35,36 followed by hydrogen abstraction or reaction with O 2 to form C 5 or C 4 hydroxycarbonyl compounds. 37 Hydrogen abstraction by O 2 during the oxidation process generates HO 2 , which reacts with NO to regenerate OH radicals. Thus, the time-dependent kinetics of OH in the presence of NO and O 2 is a sensitive probe of the detailed mechanism of the oxidation process. Until recently, studies of isoprene chemistry have been non- isomer-specific, that is, they reflect the reactivity of the combined pathways and are often insensitive to specific details of the isomeric pathways. In a previous study, we demonstrated that isomer-selective kinetic experiments permit the investigation of minor important channels in isoprene oxidation that are * To whom correspondence should be addressed. Figure 1. Initial branching of hydroxyalkyl radicals followed by the reaction of OH radical with isoprene. J. Phys. Chem. A 2010, 114, 2553–2560 2553 10.1021/jp909052t  2010 American Chemical Society Published on Web 02/01/2010