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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: S. Gromov, C. A. M. Brenninkmeijer, P. Jöckel
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2018
• Subjects: Physics; QC1-999; Chemistry; QD1-999; Antarctic; New Zealand; Antarc*
• Online Access: https://doi.org/10.5194/acp-18-9831-2018; https://doaj.org/article/52cd5205ea4e405bbda899479e651213

Unexpectedly large seasonal phase differences between CH 4 concentration and its 13 C ∕ 12 C isotopic ratio and their inter-annual variations observed in southern hemispheric time series have been attributed to the Cl + CH 4 reaction, in which 13 CH 4 is discriminated strongly compared to OH + CH 4 , 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 CH 4 and CO mixing and isotope ratios shows that a corresponding 13 C ∕ 12 C 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 × 10 3 atoms cm −3 . It is demonstrated that the 13 C ∕ 12 C ratio of CO is a sensitive indicator for the isotopic composition of reacted CH 4 and therefore for its sources. Despite ambiguities about the yield of CO from CH 4 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 CH 4 in the troposphere is probably much lower than currently assumed.