A global three-dimensional chemistry-transport model of the troposphere has been used to quantify the contribution of surface emissions, and of the oxidation of methane and non-methane hydrocarbons (NMHCs) to the distribution of carbon monoxide in the troposphere. A modeling technique has been developed, which allows the quantification of each component of the CO budget, without altering the distributions of other tropospheric species such as the hydroxyl radical OH. The calculated CO concentration are compared with long-term surface data, and regional as well as global budgets are quantified. Simulations results show that, if the impact of surface industrial emissions affect mostly the northern hemisphere, biomass burning emissions have a significant impact at all latitudes.