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

摘要

Unexpectedly large seasonal phase differences between CH4concentration and its 13C ∕ 12C isotopic ratio andtheir inter-annual variations observed in southern hemispheric time serieshave been attributed to the Cl + CH4 reaction, in which13CH4 is discriminated strongly compared to OH + CH4,and have provided the only (indirect) evidence of a hemispheric-scalepresence of oxidative cycle-relevant quantities of tropospheric atomic Cl.Our analysis of concurrent New Zealand and Antarctic time series ofCH4 and CO mixing and isotope ratios shows that a corresponding13C ∕ 12C variability is absent in CO. Usingthe AC-GCM EMAC model and isotopic mass balancing for comparing the periodsof presumably high and low Cl, it is shown that variations in extra-tropicalSouthern Hemisphere Cl cannot have exceeded0.9  ×  103 atoms cm−3. It is demonstrated that the13C ∕ 12C ratio of CO is a sensitive indicator forthe isotopic composition of reacted CH4 and therefore for itssources. Despite ambiguities about the yield of CO from CH4 oxidation(with this yield being an important factor in the budget of CO) anduncertainties about the isotopic composition of sources of CO (in particularbiomass burning), the contribution of Cl to the removal of CH4 in thetroposphere is probably much lower than currently assumed.