The oxidizing capacity of the Earth's atmosphere: Probable past and future changes

The principal oxidants in the lower atmosphere are ozone (O₃) and two by-products of O₃ photodissociation, the hydroxyl radical (OH) and hydrogen peroxide (H₂O₂). A number of critical atmospheric chemical problems depend on the earth's "oxidizing capacity," which is essentially the global burden of these oxidants. There is limited direct evidence for changes in the earth's oxidizing capacity since recent preindustrial times when, because of industrial and population growth, increasing amounts of O₃ precursor trace gases (carbon monoxide, nitrogen oxides, and hydrocarbons) have been released into the atmosphere. The concentrations of O₃ and possibly H₂O₂ have increased over large regions. Models predict that tropospheric O₃ will increase ∼0.3 to 1% per year over the next 50 years with both positive and negative trends possible for OH and H₂O₂. Models and the observational network for oxidants are improving, but validation of global models is still at an early stage.