The carbon kinetic isotope effects (KIEs) of the room-temperature reactions of several light alkanes and ethene with OH radicals were measured in a 30 L PTFE reaction chamber at ambient pressure using gas chromatography coupled with online combustion and isotope ratio mass spectrometry (GCC-IRMS). For simplicity, KIEs are reported in per mil according to (‰) ) (k 12 /k 13 -1) × 1000. The following average KIEs were obtained, (all in ‰) : ethane, 8.57 ( 1.95; propane, 5.46 ( 0.35; n-butane, 5.16 ( 0.67; methylpropane, 8.45 ( 1.49; n-pentane, 2.85 ( 0.79; methylbutane, 2.91 ( 0.43; n-hexane, 2.20 ( 0.07; n-heptane, 1.96 ( 0.26; n-octane, 2.13 ( 0.39; cyclopentane, 1.84 ( 0.13; cyclohexane, 4.46 ( 0.51; methylcyclopentane, 1.77 ( 0.53; ethene, 18.6 ( 2.9. As well, the room-temperature rate constant for the reaction of methylcyclopentane + OH, not previously reported in the literature, was determined using relative rates: (8.6 ( 2.2) × 10 -12 cm 3 molecule -1 s -1 , including the estimated 25% uncertainty in the rate constant for cyclopentane + OH. KIE values for propane, n-butane and n-hexane have been reported previously [J. Geophys. Res. [Atmos.] 2000, 105, 29329]. Our KIE for n-hexane is in agreement with the previous measurement, but our values for propane and n-butane are both higher. The dependence between the KIE and chemical structure is discussed, and a method for estimating unknown carbon KIEs for the reactions of light alkanes with OH radicals is presented. With only one exception, predictions using this method agree within a factor of 2 of the experimental KIE results.

The carbon kinetic isotope effects (KIEs) of the room temperature reactions of benzene and several light alkyl benzenes with OH radicals were studied in a reaction chamber at ambient pressure using gas chromatography coupled with online combustion and isotope ratio mass spectrometry (GCC‐IRMS). The KIEs are reported in per mil according to ɛ (‰) = (KIE − 1) × 1000, where KIE = k12/k13. The following average KIEs were obtained, (all in ‰): benzene 7.53 ± 0.50; toluene 5.95 ± 0.28; ethylbenzene 4.34 ± 0.28; o‐xylene 4.27 ± 0.05, p‐xylene 4.83 ± 0.81; o‐ethyltoluene 4.71 ± 0.12 and 1,2,4‐trimethylbenzene 3.18 ± 0.09. Our KIE value for benzene + OH agrees with the only reported value known to us [Rudolph et al., 2000]. It is shown that measurements of the stable carbon isotope ratios of light aromatic compounds should be extremely useful to study atmospheric processing by the OH radical.

A technique for the accurate measurement of the carbon kinetic isotope effects (KIEs) of the reactions of light non-methane hydrocarbons (NMHC) with OH radicals in the gas phase is presented. The method uses a variant of established relative rate techniques, which has been optimized for measurements of KIEs. The instrumentation consists of three basic components: (1) a gas-phase reaction chamber, (2) a gas chromatograph (GC) for separating the components of the reaction mixture, and (3) an isotope ratio mass spectrometer coupled on-line to the GC. Using this technique, measurements of hydroxyl radical reaction KIEs with C 2 -C 9 saturated and unsaturated hydrocarbons having rate constants between 10 -11 and 10 -13 cm 3 molecule -1 s -1 are achievable. This method has been used successfully to measure the carbon KIE for the reactions of OH with a wide range of hydrocarbons in the gas phase.

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