Source Gases that Affect Stratospheric Ozone
This chapter on source gas trends describes past changes in atmospheric concentrations of ozone-depleting chemicals and discusses their implications for Earth's ozone layer. The observed atmospheric changes provide evidence of an atmosphere responding to international policy decisions given that natural contributions are relatively small and fairly constant. The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer has been successful at causing an initial decrease in the concentrations of ozone-depleting substances and the threat they pose to stratospheric ozone. Despite this success, full recovery of the ozone layer is not expected until mid- to late-century. Other non-halogenated gases, such as methane, nitrous oxide, hydrofluorocarbons, and sulfur-containing chemicals also influence stratospheric ozone and climate and are discussed. Observed changes in the atmospheric concentrations of both ozone-depleting substances and non-halogenated chemicals provide a basis for understanding trends in stratospheric ozone discussed in subsequent chapters.