Stratospheric ozone depletion by chlorofluorocarbons

Man-made chlorofluorocarbons (CFCs) such as${\rm CCl}_{2}{\rm F}_{2}$and${\rm CCl}_{3}{\rm F}$are inert in the lower atmosphere and can survive for a hundred years or more without reaction. The only important destruction process for CFCs is ultraviolet photolysis in the stratosphere, with the release of atomic chlorine. Chlorine atoms attack stratospheric ozone with the formation of the free radical CIO which reacts further to regenerate atomic chlorine. This chain reaction can cause the removal of 100 000 molecules of ozone per Cl atom, and coupled with the emission to the atmosphere of one million tons of CFCs per year, produces ozone depletion on a significant global scale. Under the special meteorological conditions of the Antarctic winter stratosphere, chlorine and nitrogen chemistry occur which permit massive ozone depletion in the lower stratosphere when sunlight returns in the spring. Similar chemistry has also been found in experiments carried out in the Arctic stratosphere. Analysis of long-term records from ground stations has confirmed the loss of 2-3% ozone since 1970 in the Northern Hemisphere between 30°N and 64°N, with the heaviest losses in the winter. The Montreal Protocol of 1987 provides a framework for international control of emissions of CFCs, and its 1990 modification calls for elimination of further production within the next decade. Substitues for the CFCs are now being developed rapidly, with special attention to HCFCs (e.g.${\rm CHCIF}_{2}$) and HFCs (e.g.${\rm CH}_{2}{\rm FCF}_{3}$) whose primary removal occurs through oxidation in the lower atmosphere.