Inverse modeling of the global CO cycle: 2. Inversion of ¹³C/¹²C and ¹⁸O/¹⁶O isotope ratios

Following a three‐dimensional inverse modeling study on atmospheric CO mixing ratios [ Bergamaschi et al ., this issue], we present the expansion of our model for the treatment of the stable isotope ratios 13 C/ 12 C and 18 O/ 16 O in carbon monoxide. The individual isotopomers were included in the model as independent tracers, and the inversion scheme was extended, allowing the simultaneous optimization (with respect to observational data) of modeled atmospheric mixing and isotope ratios. Observational data of 13 C/ 12 C and 18 O/ 16 O ratios were taken from a set of five globally distributed sites, all of which exhibit pronounced seasonal cycles and which as a whole clearly define large latitudinal gradients. Incorporation of 13 C/ 12 C ratios in the inversion resulted in a clear constraining of the total amount of CO arising from CH 4 oxidation. The present study suggests an average CO yield of 86% (80–88%), provided that (1) the reaction of CH 4 +Cl has no significant impact on the δ 13 C of the resulting CO and (2) CO from the ocean helps to balance the δ 13 C in the Southern Hemisphere. Otherwise, a further reduced CO yield would be necessary, as low as 71% in case of an average kinetic isotope effect of 1.013 [ Lowe et al ., 1999] or 69% in case of a δ 13 C value of −25.0‰ for the oceanic source. Despite a lack of experimental investigations on 18 O/ 16 O in CO arising from CH 4 or nonmethanehydrocarbons (NMHC) oxidation, inclusion of 18 O/ 16 O into the inversion gives further significant constraints on the global CO cycle. It is shown that apart from the large technological CO source, additional sources with dominant emissions in the Northern Hemisphere are required, such as biogenic emissions (either direct or via oxidation of biogenic NMHCs) or anthropogenic NMHCs, in order to reproduce observed atmospheric 18 O/ 16 O ratios. The extended inversion scheme allows CO budgets to be derived, which reproduce, within two standard deviations of observational data, simultaneously CO mixing ratios (from the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory network), 13 C/ 12 C, and 18 O/ 16 O ratios. Incorporation of stable isotopes renders inversion results much more robust compared to inversions of CO mixing ratios only.