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A low bias in carbon monoxide (CO) at northern high and mid-latitudes is acommon feature of chemistry climate models (CCMs) that may indicate orcontribute to a high bias in simulated OH and corresponding low bias inmethane lifetime. We use simulations with CO tagged by source type toinvestigate the sensitivity of the CO bias to CO emissions, transport,global mean OH, and the hemispheric asymmetry of OH. We also investigate howeach of these possible contributors to the CO bias affects the methanelifetime. We find that the use of specified meteorology alters thedistribution of CO compared to a free-running CCM simulation, improving thecomparison with surface observations in summer. Our results also show thatreducing the hemispheric asymmetry of OH improves the agreement of simulatedCO with observations. We use simulations with parameterized OH to quantifythe impact of known model biases on simulated OH. Removing biases in ozoneand water vapor as well as reducing Northern Hemisphere NO<sub><i>x</i></sub> does notremove the hemispheric asymmetry in OH, but it reduces global mean OH by18 %, bringing the simulated methane lifetime into agreement withobservation-based estimates

Implications of carbon monoxide bias for methane lifetime and atmospheric composition in chemistry climate models