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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/acp-2018-188
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
21 Feb 2018
Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).
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 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 13C/12C 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 and 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 13C/12C 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 can not have exceeded 0.9 × 103 atoms cm−3. It is demonstrated that the 13C/12C 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.
Citation: Gromov, S., Brenninkmeijer, C. A. M., and Jöckel, P.: A very limited role of tropospheric chlorine as a sink of the greenhouse gas methane, Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-188, in review, 2018.
Sergey Gromov et al.
Sergey Gromov et al.
Sergey Gromov et al.

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Short summary
Using the observational data on 13C(CO) and 13C(CH4) from extra-tropical Southern Hemisphere (ETSH) and the EMAC model we: (1) Provide an independent, observation-based evaluation of Cl atoms 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 extra-tropical Southern Hemisphere...
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