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In the stratosphere it serves a very important function, by acting to filter out most of the [[sun]]'s ultraviolet radiation.
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".<ref>http://ozonewatch.gsfc.nasa.gov/facts/hole.html</ref>
Scientists have measured an annual cyclical thinning in the layer of ozone over the north and south poles. When the thickness of the layer is exceptionally depleted, it is called an "ozone hole".<ref>http://ozonewatch.gsfc.nasa.gov/facts/hole.html</ref>
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 O<sub>3</sub> molecules.<ref>Chlorofluorocarbons (CFCs) themselves are not involved in the catalytic process; upon reaching the stratosphere, they are subject to higher levels of ultraviolet radiation that decompose the CFC and release atomic chlorine. [http://www.ciesin.columbia.edu/TG/OZ/ozndplt.html Catalytic destruction of ozone] </ref>
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 O<sub>3</sub> molecules.<ref>Chlorofluorocarbons (CFCs) themselves are not involved in the catalytic process; upon reaching the stratosphere, they are subject to higher levels of ultraviolet radiation that decompose the CFC and release atomic chlorine. [http://www.ciesin.columbia.edu/TG/OZ/ozndplt.html Catalytic destruction of ozone] </ref>
<center>[[Image:Ozonemech.gif]]</center>
<center>[[Image:Ozonemech.gif]]</center>
(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 O<sub>3</sub>]]
(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 O<sub>3</sub>
*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.
*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.