A new three-dimensional chemical transport model of the troposphere, named intermediate model of global evolution of species, has been developed to study the global distributions, budgets, and trends of 41 chemical compounds, including the most important species that determine the oxidation capacity of the atmosphere. The modeled global distributions of species such as methane, carbon monoxide, nitrogen oxides, and ozone are generally in good agreement with observations. The lifetime of methane, which can be regarded as a measure of the oxidizing capacity of the atmosphere, is found to be equal to 11 yr, in agreement with recent estimates. The model also shows that the deposition of ozone at the Earth's surface (1100 Tg/yr) balances the sum of the net phyotochemical production (550 Tg/yr) and the flux from the stratosphere (550 Tg/yr).