Atmospheric carbon monoxide budget of the southern hemisphere: Implications of ¹³C/¹²C measurements

A large seasonal cycle has been observed in the 13 C/ 12 C isotopic ratio of CO in clean air in the extratropical southern hemisphere. This ratio is determined by the mixture of CO from isotopically distinct sources and is strongly influenced by the relative contributions of surface sources and the oxidation of CH 4 . We use a zonally averaged atmospheric model to relate atmospheric CO mixing ratios and 13 C/ 12 C isotopic ratios to the magnitude and distribution of CO sources and to explain the average seasonal cycles observed. The average 13 C/ 12 C ratio of CO emitted by surface sources in the southern hemisphere is larger than in the northern hemisphere and the southward flux of CO into the extratropical southern hemisphere is additionally enriched in 13 C as a result of oxidation during transport. These effects partially offset the effect of highly depleted 13 C/ 12 C ratios in CO produced by CH 4 oxidation. In the extratropical southern hemisphere, seasonal variation in the fraction of atmospheric CO derived from CH 4 oxidation produces large changes in 13 C/ 12 C which are partially offset by seasonal variations of surface sources. A good fit to observed average cycles can be obtained using surface source strengths consistent with previous estimates. However, the southern hemisphere data place strong constraints on the CH 4 oxidation source and imply that either the CO yield per molecule of CH 4 is about 0.7, compared with previous estimates of around 0.8, or that unidentified processes associated with CH 4 oxidation cause 13 C enrichment of about 4‰ in the CO produced.