A "hole" in the ozone layer was detected over Antarctica in 1985. Since this startling discovery, scientists have investigated the processes involved in ozone depletion and have tried to create models to predict future trends.
The Ozone Layer and CFCs
The ozone layer is a region in the lower stratosphere (about 25 km up). Ozone, which has 3 oxygen atoms per molecule rather than the pair of atoms in molecular oxygen, is produced when ultraviolet light interacts with diatomic oxygen. The presence of ozone in the upper atmosphere is vital to life on the surface, since it screens out ultraviolet light that could damage animal and plant life. Ozone is destroyed by chlorofluorocarbons (CFCs) and other chlorine and bromine compounds (click here for details on the chemical reactions). CFCs, which are used as coolants, aerosol propellants, and cleaning fluid by the electronics industry, are stable gases with lifetimes of a few decades to more than a century. As CFCs find their way into the stratosphere, chlorine atoms are stripped off by ultraviolet light, leading to destruction of ozone. Interactions between ozone and chlorine compounds are now fairly well understood. Much of the chlorine from the breakdown of CFCs eventually reacts with nitrogen oxides to form a reservoir of stable compounds that do not react with ozone (details).
Polar Stratospheric Clouds Increase Ozone Depletion
For years we were told that global warming and ozone depletion were unrelated. Now it turns out that global warming contributes to ozone destruction. During the local winter (at different times in the Arctic and Antarctic) a circular river of air forms, powered by the warm temperatures in the temperate zone. This vortex blocks the warm air flowing from the equator and isolates the region around the pole, making it colder. This is especially true at higher altitudes, as greenhouse gases reflect heat away from the stratosphere. The cooling of the stratosphere increases the efficiency of reactions responsible for ozone loss.
When the temperature in the stratosphere falls below 190 K (-83oC), Polar Stratospheric Clouds (PSCs) form. These clouds consists of icy nitric acid-containing particles. The surface of these particles provides a location for a special class of rapid chemical reactions that that convert nonreactive chlorine compounds into ozone-eating forms (details)..
When sunlight returns at the end of the polar winter, the chlorine compounds formed in the PSCs are photo-dissociated into chlorine radicals that rapidly destroy ozone through the chlorine catalytic cycle.
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