To solve this problem, we need to understand the components of photochemical smog.$\\$Photochemical smog is a type of air pollution that results from the reaction of sunlight with pollutants such as hydrocarbons and nitrogen oxides.$\\$It typically contains the following components:$\\$• Ozone (O$_3$): Formed by the reaction of sunlight with nitrogen oxides and volatile organic compounds.$\\$• Acrolein: An aldehyde that is a byproduct of the photochemical reactions.$\\$• Peroxyacetyl nitrate (PAN): A secondary pollutant formed from the reaction of nitrogen oxides and hydrocarbons.$\\$Chlorofluorocarbons (CFCs), on the other hand, are not typically associated with photochemical smog.$\\$CFCs are stable compounds that are primarily involved in stratospheric ozone depletion rather than tropospheric photochemical smog.$\\$Therefore, the component that is not a common part of photochemical smog is:$\\$Chlorofluorocarbons (CFCs).$\\$This corresponds to Option 4.