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

Abstract

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 12C/ 13/C KIE values are reported as ɛ values, where ɛ = (KIE − 1) × 1000‰, and KIE = k 12/ k 13. The following average stable-carbon KIEs were obtained: (6.56 ± 0.12)‰ (isoprene), (6.47 ± 0.27)‰ (MACR), and (7.58 ± 0.47)‰ (MVK). The measured KIEs all agree within uncertainty to an inverse molecular mass (MM) dependence of OH ɛ(‰) = (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 ɛ values from this study, the inverse MM dependence changes only marginally to OH ɛ(‰) = (485 ± 14)MM −1. The addition of these isoprene OH ɛ values to a recently measured set of ɛ O 3 values in an analogous study [ Atmos. Environ. 2008, 42, 8728–8737] allows for estimates of the average change in the 12C/ 13C ratio due to processing in the troposphere.