Ethane is the second most abundant hydrocarbon in the atmosphere, after methane, impacting on air quality, human health and climate. A quantification of its effects requires accurate knowledge of sources, processes along transport, and sinks. Carbon stable isotopic ratio investigations, complementarily to concentration measurements, were demonstrated to give more insight in source apportionment and atmospheric processing of organic compounds.
Yet, apportionment of atmospheric ethane sources, processing and sinks by using the 13C isotopic composition (δ13C) requires accurate knowledge of the stable carbon kinetic isotope effect (KIE) of its atmospheric degradation through oxidation by hydroxyl (OH) radicals. Moreover, the interpretation of tropospheric ambient data should account for the temperature dependence of KIE, since the tropospheric temperatures can vary extremely, over the range of 180-320 K.
In this work, the KIE temperature dependence for the oxidation of ethane by OH radicals in the tropospherically relevant temperature range was comprehensively investigated by experimental measurements and theoretical calculations. A framework to apply the observed KIE for interpreting ambient observations is presented. Experiments to determine the KIE temperature dependence of ethane oxidation by OH radicals were carried out with natural isotope abundances in the reactant, at ambient pressure, and in a temperature range of 243 to 303 K. Propane was used as a reference compound to verify the ethane chemistry. Chemical reactions were carried out in a ~12 L FEP reaction chamber, suspended in a newer developed temperature controlled oven. Using Thermal Desorption – Gas Chromatography – Combustion – Isotopic Ratio Mass Spectrometry (TD-GC-C-IRMS), the KIE of ethane with OH radical was derived from the temporal evolution of the ethane δ13C and concentration. At 303 ± 0.1 and 288.0 ± 0.3 K, the KIE values for the ethane oxidation by OH were found to be 7.45 ± 0.48 and 7.16 ± 0.54 ‰, respectively, showing a remarkable improvement of the measurement precision compared to the only available KIE value of 8.57 ± 1.95 ‰ at 296 ± 4 K reported by Anderson et al. (2004). Over the investigated temperature range, a slight increasing tendency of KIE toward lower temperatures was found, being approximately 0.4 ± 0.1 ‰ per 10 K.
Quantum mechanical calculations together with semi-Classical Transition State Theory (SCTST) were employed to theoretically determine the temperature dependence of KIE for reaction of ethane with OH in the temperature range of 150 – 400 K. The goal of this study was to better understand the reaction kinetics, as well as to investigate KIE at temperatures where experimental work has strong limitations. The quantum mechanical calculations were performed employing the M06-2X density functional theory method with an aug-cc-pVTZ basis set. Energy refinements were carried out at high-level theory CCSD(T) using aug-cc-pVxZ (x = D,T,Q) basis set, with extrapolation to the complete basis set limit (CBS). To this end, the 3-parameter expression aug-Schwartz6(DTQ) was employed in CBS. Tunneling effects and internal rotation treatments were also included in the KIE computation. For the first time, the anharmonicity of molecular vibration, and internal rotation was considered for the ethane + OH system. The theoretically computed KIE values are overestimated by approximately a factor of six, compared to the experimental results. However, a slightly negative temperature dependence of the theoretical KIE supports the experimental findings.
Implications of using the refined KIE values in investigating the isotopic fractioning of ethane during its chemical degradation in the atmosphere are discussed.