The most likely cause of photochemical smog is due to:

Photochemical smog is primarily formed as a result of chemical reactions between sunlight and pollutants, particularly nitrogen oxides (NOx) and volatile organic compounds (VOCs) that are predominantly emitted from motor vehicles. Heavy motor vehicle traffic is a significant source of these pollutants, as cars, trucks, and buses release NOx and VOCs into the atmosphere. When these compounds react under the influence of sunlight, they create secondary pollutants, including ozone at ground level, which is a major component of photochemical smog.

In urban areas with high traffic volumes, the conditions are often favorable for the formation of this type of smog, especially during sunny and warm weather. This relationship between traffic emissions and photochemical reactions highlights why heavy motor vehicle traffic is identified as the most likely cause of photochemical smog. Understanding the sources of pollution is crucial for environmental health specialists in developing strategies to mitigate air quality issues.