CFCs and the Ozone Layer

Ozone, O₃, forms a layer in the upper atmosphere where it absorbs much of the ultraviolet radiation arriving from the Sun. That absorption protects living things from the skin cancers, eye damage and crop damage that high-energy UV causes, so the ozone layer is beneficial. Ozone near the ground is different: there it is a pollutant formed in photochemical smog.

Chlorofluorocarbons, CFCs such as CF₂Cl₂ and CFCl₃, were used as refrigerants, aerosol propellants, solvents and foam-blowing agents because they are unreactive, non-flammable and non-toxic. That same unreactivity lets them survive long enough to drift up into the stratosphere.

Scientific evidence from different research groups, gathered by satellite and by balloon measurements, showed that CFCs were responsible for the fall in ozone. The evidence led to legislation, the Montreal Protocol of 1987, which banned the manufacture and use of CFCs. Chemists then developed chlorine-free alternatives: hydrofluorocarbons (HFCs) such as CH₂FCF₃ are now used in refrigerators and air conditioning. They contain no C–Cl bond, and the C–F bond is too strong to be broken by UV, so they do not produce ozone-destroying radicals. Hydrocarbons such as butane are used as propellants.

The ozone layer is recovering slowly, because CFCs already released stay in the atmosphere for decades. The story is used in exams as an example of how scientific evidence, gathered and checked by different groups, leads to legislation and to new chemistry.