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

## TL;DR

This study uses isotopic signatures of CO in the stratosphere to better understand processes like methane oxidation and OH loss.

## Contribution

The paper introduces novel in situ isotopic CO profiles to trace stratospheric CO sources and sinks.

## Key findings

- δ13C-CO decreases with altitude due to kinetic fractionation and CH4 oxidation.
- δ18O-CO increases with altitude from O(1D) and CO2 photolysis.
- CO isotopes can serve as proxies for CO2 photolysis and OH abundance.

## Abstract

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.