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Volume 18, issue 13 | Copyright

Special issue: The Modular Earth Submodel System (MESSy) (ACP/GMD inter-journal...

Atmos. Chem. Phys., 18, 9831-9843, 2018
https://doi.org/10.5194/acp-18-9831-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Jul 2018

Research article | 12 Jul 2018

A very limited role of tropospheric chlorine as a sink of the greenhouse gas methane

Sergey Gromov1,3, Carl A. M. Brenninkmeijer1, and Patrick Jöckel2 Sergey Gromov et al.
  • 1Max Planck Institute for Chemistry, Atmospheric Chemistry Department, Mainz, Germany
  • 2Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Weßling, Germany
  • 3Institute of Global Climate and Ecology Roshydromet & RAS (IGCE), Moscow, Russia

Abstract. Unexpectedly large seasonal phase differences between CH4 concentration and its 13C12C isotopic ratio and their inter-annual variations observed in southern hemispheric time series have been attributed to the Cl+CH4 reaction, in which 13CH4 is discriminated strongly compared to OH+CH4, and have provided the only (indirect) evidence of a hemispheric-scale presence of oxidative cycle-relevant quantities of tropospheric atomic Cl. Our analysis of concurrent New Zealand and Antarctic time series of CH4 and CO mixing and isotope ratios shows that a corresponding 13C12C variability is absent in CO. Using the AC-GCM EMAC model and isotopic mass balancing for comparing the periods of presumably high and low Cl, it is shown that variations in extra-tropical Southern Hemisphere Cl cannot have exceeded 0.9 × 103atomscm−3. It is demonstrated that the 13C12C ratio of CO is a sensitive indicator for the isotopic composition of reacted CH4 and therefore for its sources. Despite ambiguities about the yield of CO from CH4 oxidation (with this yield being an important factor in the budget of CO) and uncertainties about the isotopic composition of sources of CO (in particular biomass burning), the contribution of Cl to the removal of CH4 in the troposphere is probably much lower than currently assumed.

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Using the observational data on 13C (CO) and 13C (CH4) from the extra-tropical Southern Hemisphere (ETSH) and EMAC model we (1) provide an independent, observation-based evaluation of Cl atom concentration variations in the ETSH throughout 1994–2000, (2) show that the role of tropospheric Cl as a sink of CH4 is seriously overestimated in the literature, (3) demonstrate that the 13C/12C ratio of CO is a sensitive indicator for the isotopic composition of reacted CH4 and therefore for its sources.
Using the observational data on 13C (CO) and 13C (CH4) from the extra-tropical Southern...
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