Chemical Feedback From Decreasing Carbon Monoxide Emissions

Understanding changes in the burden and growth rate of atmospheric methane (CH₄) has been the focus of several recent studies but still lacks scientific consensus. Here we investigate the role of decreasing anthropogenic carbon monoxide (CO) emissions since 2002 on hydroxyl radical (OH) sinks and tropospheric CH₄ loss. We quantify this impact by contrasting two model simulations for 2002–2013: (1) a Measurement of the Pollution in the Troposphere (MOPITT) CO reanalysis and (2) a Control-Run without CO assimilation. These simulations are performed with the Community Atmosphere Model with Chemistry of the Community Earth System Model fully coupled chemistry climate model with prescribed CH₄ surface concentrations. The assimilation of MOPITT observations constrains the global CO burden, which significantly decreased over this period by ~20%. We find that this decrease results to (a) increase in CO chemical production, (b) higher CH₄ oxidation by OH, and (c) ~8% shorter CH₄ lifetime. We elucidate this coupling by a surrogate mechanism for CO-OH-CH₄ that is quantified from the full chemistry simulations.