Only about 10% of atmospheric ozone occurs in the troposphere, either naturally or as a pollutant. The remaining 90% occurs in the stratosphere, where ozone concentrations of up to several ppm serve a vital biological role in absorbing highenergy photons of ultraviolet radiation from the sun. In this book so far, we have been largely concerned with the troposphere because that is where the emissions, transformations, deposition and effects mainly occur. The stratosphere, though, is not some remote, disconnected and inert structure where nothing happens that has any influence. The stratosphere is an important reaction chamber for a number of processes that depend on either very cold conditions, very high solar UV fluxes, or both. In 1985, it was discovered that the ozone concentration above the continent of Antarctica was decreasing; this was found to be due to chemical reactions with chlorine derived from stable chemicals known as chlorofluorocarbons. The reduction in ozone concentration has now been observed globally, and has potentially severe implications for life on the surface.