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Discussion papers
https://doi.org/10.5194/acp-2019-1047
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1047
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 22 Nov 2019

Submitted as: research article | 22 Nov 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Strong Sensitivity of the Isotopic Composition of Methane to the Plausible Range of Tropospheric Chlorine

Sarah A. Strode1,2, James S. Wang1,2,a, Michael Manyin2,3, Bryan Duncan2, Ryan Hossaini4, Christoph A. Keller1,2, Sylvia E. Michel5, and James W. C. White5 Sarah A. Strode et al.
  • 1Universities Space Research Association, Columbia, MD, USA
  • 2NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 3SSAI, Lanham, MD, USA
  • 4Lancaster Environment Centre, Lancaster University, Lancaster, UK
  • 5Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, USA
  • anow at: the Institute for Advanced Sustainability Studies, Potsdam, Germany

Abstract. The 13C isotopic ratio of methane, δ13C of CH4, provides additional constraints on the CH4 budget to complement the constraints from CH4 observations. The interpretation of δ13C observations is complicated, however, by uncertainties in the methane sink. The reaction of CH4 with Cl is highly fractionating, increasing the relative abundance of 13CH4, but there is currently no consensus on the strength of the tropospheric Cl sink. We use a set of GEOS global model simulations with different predicted Cl fields to test the sensitivity of the δ13C of CH4 to the diversity of Cl output from chemical transport models. We find that δ13C is highly sensitive to both the amount and geographic distribution of Cl. Simulatlons with Cl providing 0.28 % or 0.66 % of the total CH4 loss bracket the δ13C observations for a fixed set of emissions. Thus, even when Cl provides only a small fraction of the total CH4 loss and has a small impact on total CH4, it provides a strong lever on δ13C. The geographic distribution and seasonal cycle of Cl also impacts the hemispheric gradient and seasonal cycle of δ13C. The large effect of Cl on δ13C compared to total CH4 broadens the range of CH4 source mixtures that can be reconciled with δ13C observations. Stronger constraints on tropospheric Cl are necessary to improve estimates of CH4 sources from δ13C observations.

Sarah A. Strode et al.
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Sarah A. Strode et al.
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Short summary
The 13C : 12C isotopic ratio in methane (CH4) provides information about CH4 sources, but loss of CH4 by reaction with OH and chlorine (Cl) also affects this ratio. Tropospheric Cl provides a small and uncertain sink for CH4, but has a large effect on its isotopic ratio. We use the GEOS model with several different Cl fields to test the sensitivity of methane's isotopic composition to tropospheric Cl. Cl affects the global mean, hemispheric gradient, and seasonal cycle of the isotopic ratio.
The 13C : 12C isotopic ratio in methane (CH4) provides information about CH4 sources, but loss...
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