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

Joelsson, L. M T; Schmidt, J. A.; Nilsson, E. J K LU ; Blunier, T.; Griffith, D. W T; Ono, S. and Johnson, M. S. (2016) In Atmospheric Chemistry and Physics 16(7). p.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;... (More)

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).

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author
organization
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type
Contribution to journal
publication status
published
subject
in
Atmospheric Chemistry and Physics
volume
16
issue
7
pages
11 pages
publisher
Copernicus Gesellschaft Mbh
external identifiers
  • scopus:84964670741
  • wos:000374703000013
ISSN
1680-7316
DOI
10.5194/acp-16-4439-2016
language
English
LU publication?
yes
id
d918d9c4-3cf6-4281-8421-cf815b208153
date added to LUP
2016-10-03 13:08:24
date last changed
2017-10-22 05:19:58
@article{d918d9c4-3cf6-4281-8421-cf815b208153,
  abstract     = {<p>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 CH<sub>3</sub>D and <sup>13</sup>CH<sub>3</sub>D; the latter is reported here for the first time. We find kCH<sub>4</sub>/kCH<sub>3</sub>D Combining double low line 1.31 ± 0.01 and kCH<sub>4</sub>/k<sup>13</sup>CH<sub>3</sub>D Combining double low line 1.34 ± 0.03 at room temperature, implying that the methane+OH kinetic isotope effect is multiplicative such that (kCH<sub>4</sub>/k<sup>13</sup>CH<sub>4</sub>)(kCH<sub>4</sub>/kCH<sub>3</sub>D) Combining double low line kCH<sub>4</sub>/k<sup>13</sup>CH<sub>3</sub>D, within the experimental uncertainty, given the literature value of kCH<sub>4</sub>/k<sup>13</sup>CH<sub>4</sub> 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(<sup>13</sup>CH<sub>3</sub>D) Combining double low line ln([CH<sub>4</sub>][<sup>13</sup>CH<sub>3</sub>D]/[<sup>13</sup>CH<sub>4</sub>][CH<sub>3</sub>D])) 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).</p>},
  author       = {Joelsson, L. M T and Schmidt, J. A. and Nilsson, E. J K and Blunier, T. and Griffith, D. W T and Ono, S. and Johnson, M. S.},
  issn         = {1680-7316},
  language     = {eng},
  month        = {04},
  number       = {7},
  pages        = {4439--4449},
  publisher    = {Copernicus Gesellschaft Mbh},
  series       = {Atmospheric Chemistry and Physics},
  title        = {Kinetic isotope effects of <sup>12</sup>CH<sub>3</sub>D+OH and <sup>13</sup>CH<sub>3</sub>D+OH from 278 to 313K},
  url          = {http://dx.doi.org/10.5194/acp-16-4439-2016},
  volume       = {16},
  year         = {2016},
}