Ozone

Ozone is a chemical compound consisting of three oxygen atoms. It is represented by the symbol O₃.

It is unstable at standard temperatures and pressures, and a powerful oxidizer. At the earth's surface it occurs naturally as a result of electrical discharges, most prominently in the form of lightning. It is also created by man-made sparks, such as in an electrical motor.

In the stratosphere it serves a very important function, by acting to filter out most of the sun's ultraviolet radiation.

Scientists have measured an annual, temporary thinning layer of ozone over the north and south poles, called an "ozone hole".

Much of this is a result of the breakdown of chlorofluorocarbons and the release of elemental chlorine, which acts as a catalyst to break down the O₃ molecules.

(Above) The mechanism by which ozone is broken down in the stratosphere by chlorine atoms. Note that the propagation steps repeat thousands of times, such that one molecule of a CFC can destroy thousands of molecules of O₃]]

• In April 1991, the Environmental Protection Agency announced that ozone levels in the Northern Hemisphere had decreased by as much as 5 percent over the United States in the past 10 years.
• A minority opinion holds that ozone depletion has little to do with CFCs and that the ozone hole may be more natural than man-made. S. Fred Singer, an environmental scientist at the University of Virginia, has argued that a rise in methane or changes in the sun's cycle may affect ozone. He also believes the lack of historical data about ozone depletion makes it difficult to determine if similar changes occurred centuries before hairspray cans were invented. [1]