Articles | Volume 15, issue 20
Research article
23 Oct 2015
Research article |  | 23 Oct 2015

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

S. A. Strode, B. N. Duncan, E. A. Yegorova, J. Kouatchou, J. R. Ziemke, and A. R. Douglass

Abstract. A low bias in carbon monoxide (CO) at northern high and mid-latitudes is a common feature of chemistry climate models (CCMs) that may indicate or contribute to a high bias in simulated OH and corresponding low bias in methane lifetime. We use simulations with CO tagged by source type to investigate the sensitivity of the CO bias to CO emissions, transport, global mean OH, and the hemispheric asymmetry of OH. We also investigate how each of these possible contributors to the CO bias affects the methane lifetime. We find that the use of specified meteorology alters the distribution of CO compared to a free-running CCM simulation, improving the comparison with surface observations in summer. Our results also show that reducing the hemispheric asymmetry of OH improves the agreement of simulated CO with observations. We use simulations with parameterized OH to quantify the impact of known model biases on simulated OH. Removing biases in ozone and water vapor as well as reducing Northern Hemisphere NOx does not remove the hemispheric asymmetry in OH, but it reduces global mean OH by 18 %, bringing the simulated methane lifetime into agreement with observation-based estimates.

Short summary
A low bias in carbon monoxide (CO) at northern latitudes is a common feature of chemistry climate models. We find that increasing Northern Hemisphere (NH) CO emissions or reducing NH OH concentrations improves the agreement with CO surface observations, but reducing NH OH leads to a better comparison with MOPITT. Removing model biases in ozone and water vapor increases the simulated methane lifetime, but it does not give the 20% reduction in NH OH suggested by our analysis of the CO bias.
Final-revised paper