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

• Main Authors: S. Gromov, C. A. M. Brenninkmeijer, P. Jöckel
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/18/9831/2018/acp-18-9831-2018.pdf

<p>Unexpectedly large seasonal phase differences between CH₄ concentration and its ¹³C&thinsp;∕&thinsp;¹²C isotopic ratio and their inter-annual variations observed in southern hemispheric time series have been attributed to the Cl&thinsp;+&thinsp;CH₄ reaction, in which ¹³CH₄ is discriminated strongly compared to OH&thinsp;+&thinsp;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 <span style="" class="text">¹³C&thinsp;∕&thinsp;¹²C</span> 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&thinsp; × &thinsp;10³&thinsp;atoms&thinsp;cm⁻³. It is demonstrated that the ¹³C&thinsp;∕&thinsp;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.</p>