Kinetic isotope effects of ¹²CH₃D + OH and ¹³CH₃D + OH from 278 to 313K

Authors

L M. T Joelsson, University of Copenhagen
J A. Schmidt, University of Copenhagen
E J. K Nilsson, Lund University
T Blunier, University of Copenhagen
David W. T Griffith, University of WollongongFollow
S Ono, Massachusetts Institute of Technology
Matthew S. Johnson, University of Copenhagen

RIS ID

107186

Publication Details

Joelsson, L. M. T., Schmidt, J. A., Nilsson, E. J. K., Blunier, T., Griffith, D. W. T., Ono, S. & Johnson, M. S. (2016). Kinetic isotope effects of ¹²CH₃D + OH and ¹³CH₃D + OH from 278 to 313K. Atmospheric Chemistry and Physics, 16 (7), 4439-4449.

Abstract

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 13CH3D, 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 84% of all methane removal. Here we present experimentally derived methane+OH kinetic isotope effects and their temperature dependence over the range of 278 to 313K for CH3D and 13CH3D; the latter is reported here for the first time. We find kCH4/kCH3D Combining double low line 1.31 ± 0.01 and kCH4/k13CH3D Combining double low line 1.34 ± 0.03 at room temperature, implying that the methane+OH kinetic isotope effect is multiplicative such that (kCH4/k13CH4)(kCH4/kCH3D) Combining double low line kCH4/k13CH3D, within the experimental uncertainty, given the literature value of kCH4/k13CH4 Combining double low line 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 (δ(13CH3D) Combining double low line ln([CH4][13CH3D]/[13CH4][CH3D])) of 0.02 ± 0.02. This implies that the bulk tropospheric δ(13CH3D) reflects the source signal with relatively small adjustment due to the sink signal (i.e., mainly OH oxidation).