Chlorofluorocarbons (CFCs) are synthetic chemicals that are odourless, non-toxic, non-flammable and chemically inert. However, researchers found that the release of CFCs into the atmosphere caused ozone depletion. The term 'Ozone Depletion Potential' (ODP) is used to express the efficiency of a gas in depleting stratospheric ozone.
ODP = (amount of ozone removed by a unit mass of a particular CFC) / (amount of ozone removed by a unit mass of CFCl3)
By definition ODP (CFCl3) = 1
CFCs are not broken down by solar radiation present in the troposphere but instead accumulate here and are eventually transported into the stratosphere. At approximately 40km CFCs begin to be photolysed leading to the release of Cl atoms e.g. CFCl3 + hu ®CFCl2 + Cl
This causes an increase in chlorine reservoir compounds (HCl + ClONO2). During the winter, heterogeneous reactions between HCl and ClONO2 produce Cl2. This is readily photolysed when the sun returns in spring, producing active chlorine which is a main cause in ozone depletion.
Better alternatives to CFCs are Hydrofluorocarbons (HFCs) and Hydrochlorofluorocarbons (HCFCs). These both contain C-H bonds which can be broken down in the troposphere; therefore, there is little or no ozone depletion. HFCs is more preferable since it does not contain chlorine.
In June 1990 representatives of 93 nations, including the United Kingdom, agreed to phase out CFC production by the end of the twentieth century. The ozone layer will recover if all ozone depleting substances are stopped and normal levels could be reached by 2050.