Why are ozone-depleting substances so effective at destroying ozone?
Ozone-depleting substances are effective ozone-depleters for two reasons. The first is that they are not reactive (chemically speaking), which means they survive long enough in the atmosphere to drift up into the stratosphere. The second is that they help the natural reactions that destroy ozone. Unlike most chemicals released into the atmosphere at the Earth’s surface, ozone-depleting substances are not “washed” back to Earth by rain or destroyed in reactions with other chemicals. They simply do not break down in the lower atmosphere and they can remain in the atmosphere from 20 to 120 years or more. Once they reach the stratosphere, UV-C radiation breaks up these molecules into chlorine (from CFCs, methyl chloroform, carbon tetrachloride) or bromine (from halons, methyl bromide) which, in turn, break up ozone (O3). Both chlorine and bromine activate and speed up the ozone destruction reactions without being altered or destroyed themselves. Thus, a single chlorine atom can destroy up
