Photochemical smog is formed primarily when primary pollutants interact with what?

Photochemical smog is primarily formed when primary pollutants, such as nitrogen oxides and volatile organic compounds (VOCs), interact with sunlight. This interaction leads to a series of chemical reactions that produce secondary pollutants like ozone and other photochemical oxidants, which characterize photochemical smog. The presence of sunlight is crucial because it provides the energy needed for these reactions to occur, leading to the formation of harmful compounds that can affect air quality and human health.

In contrast, while water vapor, heavy metals, and cold temperatures may play roles in the environment, they do not directly contribute to the formation of photochemical smog. Water vapor can influence other types of smog, heavy metals are more associated with particulate pollution, and cold temperatures may actually inhibit the conditions conducive to the formation of photochemical smog, which typically occurs in warmer, sunnier climates.