The ozone layer is a region of the stratosphere containing ozone, O3. It absorbs much of the Sun's harmful ultraviolet radiation, which can cause skin cancer and cataracts.Ozone is formed and broken down naturally: O2 → 2O and O2 + O → O3. This balance has been upset by chlorofluorocarbons (CFCs), which were used in refrigerators and aerosols.In the stratosphere, UV light breaks the C-Cl bond in CFCs to form chlorine radicals, for example CF2Cl2 → •CF2Cl + Cl•
The chlorine radicals then catalyse the breakdown of ozone: Cl• + O3 → ClO• + O2, then ClO• + O3 → 2O2 + Cl•. The overall reaction is 2O3 → 3O2. Because the Cl• is regenerated, one radical can destroy thousands of ozone molecules.Nitrogen monoxide radicals from aircraft and thunderstorms also break down ozone in a similar way. CFCs have been banned under the Montreal Protocol and replaced by alternatives such as HFCs, which do not contain chlorine.
The chlorine radicals then catalyse the breakdown of ozone: Cl• + O3 → ClO• + O2, then ClO• + O3 → 2O2 + Cl•. The overall reaction is 2O3 → 3O2. Because the Cl• is regenerated, one radical can destroy thousands of ozone molecules.Nitrogen monoxide radicals from aircraft and thunderstorms also break down ozone in a similar way. CFCs have been banned under the Montreal Protocol and replaced by alternatives such as HFCs, which do not contain chlorine.