Final answer:
Chlorofluorocarbons (CFCs) break down in the upper atmosphere due to the action of ultraviolet (UV) radiation from the sun, which releases chlorine atoms that destroy ozone molecules.
Step-by-step explanation:
The cause of a CFC molecule breaking down once it reaches the upper atmosphere is ultraviolet (UV) radiation from the sun. The high energy UV radiation helps dissociate the CFC molecules, releasing highly reactive chlorine atoms. These chlorine atoms then catalyze the destruction of the ozone layer, partaking in a cycle that erodes the protective shield that absorbs harmful UV radiation. An example of this reaction is when CFC-13 interacts with UV light:
CFC-13 + hv → CFC-12 + Cl
The chlorine atom released is capable of destroying thousands of ozone molecules, which poses a significant threat to the ozone layer. As a result, chlorofluorocarbons have been replaced with more benign substances such as hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs).