Common components of photochemical smog may not contain :

The photochemical reaction is brought about by the energy of the light being absorbed. Smoke and fog combine to generate the word "smog." The main source of air pollution, photochemical smog, is made up of both primary and secondary pollutants. Nitrogen oxides and volatile organic compounds are the main ones. Hydrocarbons that have two or three double or triple covalent bonds between nearby carbon atoms are said to be unsaturated. The main sources of unsaturated hydrocarbons include, for example, gasoline, natural gas, and ethylene. Some hydrocarbons, such as butadiene and benzene, are dangerous to human health and have the potential to cause cancer. When sulfur is released into the atmosphere, it is oxidation in the troposphere, creating sulfuric acid, which when combined with water creates acid rain.
The right response is, therefore "${\mathrm{SO}}_2$".