Long-term trends of carbon monoxide inferred using a two-dimensional model

A global two-dimensional chemistry model is used to study the long-term trends of CH₄, CO, and OH from pre-industrial times to 2020 with given emission scenarios according to the increase of world population. The calculated global-averaged concentration of CO is 27 ppbv before 1840, the concentration of CO is 76 ppbv in 1991, and is estimated to be 105 ppbv in 2020. From 1840 to 1991, the concentration of OH changed from 7.17×10⁵ to 5.79×10⁵ molecules/cm³, i.e., decreased by 19%. The long-term trends of CH₄ derived from the model are in good agreement with observation results. The annual increase of CH₄ during 1983–1991 is 12.1–13.3 ppbv by this model and 11.1–11.6 ppbv by observation. The calculated growth rate of CO in 1980s is 1.03–1.06%/yr i.e., 6.9–7.9 ppbv/yr. The model is used to investigate why the CO concentration decreased at the beginning of 1990s. We find that the decrease of CO emissions and depletion of stratospheric ozone are the best explanation, which account for 70% and 30% of the decrease of CO concentration, respectively. The model results also show that possible reduction of CH₄ emission has little influence on the change of CO concentrations though the reduction of CO emission can counteract the growth of CH₄ significantly.