12C/13C kinetic isotope effects of the gas-phase reactions of isoprene, methacrolein, and methyl vinyl ketone with OH radicals

The stable-carbon kinetic isotope effects (KIEs) for the gas-phase reactions of isoprene, methacrolein (MACR), and methyl vinyl ketone (MVK) with OH radicals were studied in a 25 L reaction chamber at (298 +/- 2) K and ambient pressure. The time dependence of both the stable-carbon isotope ratios and the concentrations was determined using a gas-chromatography combustion isotope ratio mass spectrometry (GCC-IRMS) system. The volatile organic compounds (VOCs) used in the KIE experiments had natural-abundance isotopic composition thus KIE data obtained from these experiments can be directly applied to atmospheric studies of isoprene chemistry. All C-12/(13)/C KIE values are reported as epsilon values, where epsilon = (KIE - 1) x 1000 parts per thousand, and KIE = k(12)/k(13). The following average stable-carbon KIEs were obtained: (6.56 +/- 0.12)parts per thousand (isoprene), (6.47 +/- 0.27)parts per thousand (MACR), and (7.58 +/- 0.47)parts per thousand (MVK). The measured KIEs all agree within uncertainty to an inverse molecular mass (MM) dependence of (OH)epsilon(parts per thousand) = (487 +/- 18)MM-1, which was derived from two previous studies [J. Geophys. Res. 2000, 105, 29329-29346: J. Phys. Chem. A 2004, 108, 11537-11544]. Upon adding the isoprene, MACR, and MVK (OH)epsilon values from this study, the inverse MM dependence changes only marginally to (OH)epsilon(parts per thousand) = (485 +/- 14)MM-1. The addition of these isoprene (OH)epsilon values to a recently measured set of (O3)epsilon values in an analogous study [Atmos. Environ. 2008, 42, 8728-8737] allows for estimates of the average change in the C-12/C-13 ratio due to processing in the troposphere. (C) 2009 Elsevier Ltd. All rights reserved.