What external factor can break down CFCs?

The correct answer is UV radiation because ultraviolet radiation from the sun is a significant external factor that can lead to the breakdown of chlorofluorocarbons (CFCs) in the atmosphere. When CFC molecules are exposed to UV radiation, they undergo photodissociation, a process in which the chemical bonds within the CFCs break down. This reaction releases chlorine atoms, which then participate in the depletion of ozone (O3) in the stratosphere, contributing to environmental concerns such as the thinning of the ozone layer.

Heat from appliances, while it can affect materials and lead to various chemical reactions, is not a primary factor in breaking down CFCs in the atmospheric context. Similarly, while water vapor and carbon dioxide are important components of the atmosphere, they do not cause the specific breakdown of CFCs like UV radiation does. Therefore, understanding the unique role of UV radiation in the breakdown of CFCs helps highlight the environmental impact of these substances in relation to ozone depletion.