Kinetic isotope effects of (CH3D)-C-12 + OH and (CH3D)-C-13+OH from 278 to 313 K

摘要

Methane is the second most important long-lived greenhouse gas and plays a central role in the chemistry of the Earth's atmosphere. Nonetheless there are significant uncertainties in its source budget. Analysis of the isotopic composition of atmospheric methane, including the doubly substituted species (CH3D)-C-13, offers new insight into the methane budget as the sources and sinks have distinct isotopic signatures. The most important sink of atmospheric methane is oxidation by OH in the troposphere, which accounts for around 84aEuro-% of all methane removal. Here we present experimentally derived methaneaEuro-+aEuro-OH kinetic isotope effects and their temperature dependence over the range of 278 to 313aEuro-K for CH3D and (CH3D)-C-13; the latter is reported here for the first time. We find k(CH4)/k(CH3)D = 1.31 +/- 0.01 and k(CH4)/k(13)CH(3)D = 1.34 +/- 0.03 at room temperature, implying that the methaneaEuro-+aEuro-OH kinetic isotope effect is multiplicative such that (k(CH4)/k(13)CH(4))(k(CH4)/k(CH3)D) = k(CH4)/k(13)CH(3)D, within the experimental uncertainty, given the literature value of k(CH4)/k(13)CH(4) = 1.0039 +/- 0.0002. In addition, the kinetic isotope effects were characterized using transition state theory with tunneling corrections. Good agreement between the experimental, quantum chemical, and available literature values was obtained. Based on the results we conclude that the OH reaction (the main sink of methane) at steady state can produce an atmospheric clumped isotope signal (Delta((CH3D)-C-13) = ln([CH4][(CH3D)-C-13]/[(CH4)-C-13][CH3D])) of 0.02 +/- 0.02. This implies that the bulk tropospheric Delta((CH3D)-C-13) reflects the source signal with relatively small adjustment due to the sink signal (i.e., mainly OH oxidation).