Changes

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 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>

Scientists have measured an annual cyclical thinning and thickening in the layer of ozone over the north and south poles. (Activists refer to this as 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.<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 O₃ 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>