Isotopic source signatures of stratospheric CO inferred from in situ vertical profiles

The stratospheric CO budget is determined by CH₄ oxidation, OH-driven loss and atmospheric transport. These processes can be constrained using CO mole fractions and isotopic compositions, with the latter being largely unexplored. We present novel stratospheric observations of δ¹³C-CO and δ¹⁸O-CO vertical profiles, revealing distinct altitude-dependent trends. δ¹³C-CO decreases with altitude due to inverse ¹³C kinetic fractionation in the OH sink and ¹³C-depleted CO from CH₄ oxidation. In contrast, δ¹⁸O-CO increases with altitude, driven by ¹⁸O-rich oxygen from O(¹D) via O₃ photolysis and CO₂ photolysis. Our findings suggest that CO isotopes can act as valuable proxies for quantifying CO production from CO₂ photolysis. Incorporating CO mole fractions and isotopic data into global models enhances evaluations of the stratospheric CH₄ sink and OH abundance, improving our understanding of stratospheric water vapour and its radiative impacts.