Final answer:
Photochemical smog is formed when primary pollutants such as nitrogen oxides and volatile organic compounds interact with sunlight. The reaction between these pollutants in the presence of sunlight leads to the formation of smog.
Step-by-step explanation:
Photochemical smog is formed when primary pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) interact with sunlight. These primary pollutants, which can come from sources like vehicle emissions and industrial processes, undergo chemical reactions in the presence of sunlight to produce secondary pollutants, including ozone. The reaction between NOx and VOCs in the presence of sunlight leads to the formation of photochemical smog, which appears as a brown haze above cities.