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Methane is the second most important long-lived greenhouse gas and plays a\udcentral role in the chemistry of the Earth's atmosphere. Nonetheless there\udare significant uncertainties in its source budget. Analysis of the isotopic\udcomposition of atmospheric methane, including the doubly substituted species\ud13CH3D, offers new insight into the methane budget as the sources\udand sinks have distinct isotopic signatures. The most important sink of\udatmospheric methane is oxidation by OH in the troposphere, which\udaccounts for around 84 % of all methane removal. Here we present\udexperimentally derived methane + OH kinetic isotope effects and\udtheir temperature dependence over the range of 278 to 313 K for CH3D\udand 13CH3D; the latter is reported here for the first time. We\udfind kCH4/kCH3D = 1.31 ± 0.01 and\udkCH4/k13CH3D = 1.34 ± 0.03 at room temperature,\udimplying that the methane + OH kinetic isotope effect is\udmultiplicative such that\ud(kCH4/k13CH4)(kCH4/kCH3D) = kCH4/k13CH3D,\udwithin the experimental uncertainty, given the literature value of\udkCH4/k13CH4 = 1.0039 ± 0.0002. In addition, the\udkinetic isotope effects were characterized using transition state theory with\udtunneling corrections. Good agreement between the experimental, quantum\udchemical, and available literature values was obtained. Based on the results we conclude that the OH reaction (the\udmain sink of methane) at steady state can produce an atmospheric clumped\udisotope signal\ud(Δ(13CH3D) = ln([CH4][13CH3D]/[13CH4][CH3D]))\udof 0.02 ± 0.02. This implies that the bulk tropospheric\udΔ(13CH3D) reflects the source signal with relatively small\udadjustment due to the sink signal (i.e., mainly OH oxidation)

atmospheric chemistry and physics

Kinetic isotope effects of &amp;lt;sup&amp;gt;12&amp;lt;/sup&amp;gt;CH&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;D + OH and &amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;CH&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;D + OH from 278 to 313 K