Changes

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, [[lightning]].  It is also created by [[man-made]] sparks, such as in an electrical motor.   

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, [[lightning]].  It is also created by [[man-made]] sparks, such as in an electrical motor.   

In the upper atmosphere 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.   

Some activists claimed that this ozone layer has been threatened in recent decades by the release of [[chlorofluorocarbons]], on the theory that these act as [[catalyst]]s to break down the O₃ molecules.

Some activists claimed that this ozone layer has been threatened in recent decades by the release of [[chlorofluorocarbons]], on the theory that these act as [[catalyst]]s to break down the O₃ molecules.