Impact of Emissions, Chemistry, and Climate on Atmospheric Carbon Monoxide: 100-year Predictions from a Global Chemistry-Climate Model

The possible trends for atmospheric carbon monoxide in the next 100 years have been illustrated using a coupled atmospheric chemistry and climate model driven by emissions predicted by a global economic development model. Various model runs with different assumptions regarding emissions or model parameters have been carried out to investigate the impacts of model and emission uncertainties on the predictions. We have found complicated interactions among emissions, atmospheric chemistry, and climate regarding the distributions and evolution of CO in the atmosphere. Based on the predicted emissions of methane and carbon monoxide, the model predicts an increasing trend of carbon monoxide in the next century with a tropospheric mole fraction of CO in 2100 double its present-day value. Methane emissions are found to have the most important effect on the future atmospheric CO budget. High methane emissions cause significant depletion of tropospheric OH, increase of CO concentrations, and lengthening of lifetimes of many chemical species including CO and CH4. The global average atmospheric lifetime of CO is predicted in our reference model run to be about 0.6 month longer than its present value (approximately 2 months). The predicted emissions of CO increase only slightly over the next century, so the impact of CO emissions on the predicted CO abundance appears to be less important than that of methane. Consequently, maintaining the emissions of CH4 at their current levels can prevent significant future changes in tropospheric chemistry, while similar controlling emissions of CO cannot achieve the same result. This study also indicates that climate variations, especially those causing changes in H2O concentrations, can influence atmospheric trends of carbon monoxide. A two-way interaction between chemistry and climate regarding CO is evident. Specifically, the budget of atmospheric CO affects the destruction of methane and the production of CO2, ozone, and sulfate aerosols and thus affects climate, while the resultant changes in climate modify the budget of CO-CH4 in turn through their effects on H2O and temperature.