What is the most likely cause of photochemical smog?

Photochemical smog is primarily caused by a complex reaction between sunlight and pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) emitted from various sources. Heavy motor vehicle traffic is a significant contributor to these pollutants, as vehicles emit both NOx and VOCs during fuel combustion. In urban areas with high traffic volumes, the combination of sunlight and these emissions leads to the formation of ground-level ozone and other photochemical oxidants that characterize photochemical smog.

While industrial emissions, residential heating, and agricultural activities can also contribute to air pollution, they do not play as direct and prominent a role in the formation of photochemical smog as heavy motor vehicle traffic does. Industrial emissions often consist of different types of pollutants that can lead to other forms of air pollution, but the specific conditions required for photochemical smog—including the reaction with sunlight—are primarily met in densely populated areas with extensive vehicle use.