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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.

Abstract. With the emergence of wide-spread application of new optical techniques to monitor δ13C in atmospheric CO2 there is a growing need to ensure well-calibrated measurements. We characterized one commonly available instrument, a cavity ring-down spectrometer (CRDS) system used for continuous in situ monitoring of atmospheric 13CO2. We found no dependency of δ13C on the CO2 concentration in the range of 303–437 ppm. We designed a calibration scheme according to the diagnosed instrumental drifts and established a quality assurance protocol. We find that the repeatability (1-σ) of measurements is 0.25‰ for 10 min and 0.15‰ for 20 min integrated averages, respectively. Due to a spectral overlap, our instrument displays a cross-sensitivity to CH4 of 0.42 ± 0.024‰ ppm−1. Our ongoing target measurements yield standard deviations of δ13C from 0.22‰ to 0.28‰ for 10 min averages. We furthermore estimate the reproducibility of our system for ambient air samples from weekly measurements of a long-term target gas to be 0.18‰. We find only a minuscule offset of 0.002 ± 0.025‰ between the CRDS and Environment Canada's isotope ratio mass spectrometer (IRMS) results for four target gases used over the course of one year.

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