In 1972, the chemical industry was producing about 700,000 metric tons of chlorofluorocarbons (CFCs) per year. A chlorofluorocarbon is a compound composed of just carbon, chlorine, and fluorine. Most of the CFCs produced in the early 70’s were either CFC-11 (CFCl3,) or CFC-12 (CF2Cl2.)
When these substances reach the stratosphere, they come into contact with radiant energy (ie. ultraviolet light) strong enough to break them down. For instance, radiant energy of wavelength less than 215 nm will break the covalent bond between one of the chlorine atoms and the carbon atom in CF2Cl2.
CF2Cl2(g) -> CF2Cl(g) + Cl(g)
The chlorine atoms released in this type of reaction can destroy ozone molecules, i.e:
Cl(g) + O3(g) -> ClO(g) + O2(g)
ClO(g) + O(g) -> Cl(g) + O2(g)
Each chlorine atom is thought to destroy an average of 1000 ozone molecules before it is converted into an inactive form. Two important inactive chlorine compounds are HCl and ClONO2. They are formed in reactions such as:
CH4(g) + Cl(g) -> CH3(g) + HCl(g)
ClO(g) + NO2(g) + M -> ClONO2(g) + M
(M = molecule to carry off excess energy)
In recent times, much research has been conducted into finding substitutes for CFCs; for example, since 1995 carbon dioxide has been used to blow polystyrene foam as opposed to CFCs. By reducing the output of CFCs, the depletion of the ozone layer will stop increasing in speed, and hopefully the original equilibrium between the formation of ozone and the natural destruction of ozone will be restored.