Ozone is continually being created and destroyed in a natural photochemical cycle.

In addition to this natural cycle, ozone is also being destroyed as a consequence of human activity.

Chlorofluorocarbons (such as dichlorodifluoromethane) have been used as aerosol propellants, as refrigerants, as solvents for cleaning circuit boards and as blowing agents in the manufacture of expanded platics. After release these long lived molecules diffuse through the troposphere (where uv levels are comparatively low) and reach the stratosphere where they undergo photolysis creating highly reactive chlorine atoms that catalyse the conversion of ozone molecules into oxygen molecules.

This is an example of homogeneous catalysis where the catalyst (Cl) and the other species involved (O3, O2 and O) are all in the same state, in this case the gaseous state.

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