In which areas has photochemical smog been reported?

Photochemical smog is primarily associated with areas that have high levels of motor vehicle traffic. This type of smog occurs when sunlight reacts with pollutants such as nitrogen oxides and volatile organic compounds, which are commonly emitted by motor vehicles. Urban areas with dense traffic generate these emissions in significant quantities, leading to the formation of photochemical smog, particularly on warm, sunny days.

The presence of large amounts of vehicles in a concentrated area creates a higher likelihood of these pollutants combining to form ground-level ozone, a key component of photochemical smog. This phenomenon is often observed in major metropolitan areas with heavy congestion, where the heat and sunlight enhance the chemical reactions necessary for smog formation.

In contrast, while large industries, chemical processing plants, and industries processing hazardous wastes may contribute to air pollution, they are more often associated with industrial smog rather than photochemical smog, particularly if they are not located in areas of intense sunlight and traffic patterns. Thus, high motor vehicle traffic is the most critical factor for the occurrence of photochemical smog.