Which key pollutant is primarily linked to the process of photochemical smog formation?

The key pollutant primarily linked to the process of photochemical smog formation is nitric oxide (NO). Photochemical smog is created when sunlight interacts with pollutants such as nitrogen oxides and volatile organic compounds (VOCs) in the atmosphere. Nitric oxide is a precursor to nitrogen dioxide (NO2), which plays a significant role in this chemical reaction. When nitrogen oxides are emitted from sources like vehicle exhaust, they can react with sunlight to form Ozone (O3) and other secondary pollutants that contribute to the characteristic features of photochemical smog.

In contrast, other pollutants like lead, carbon monoxide, and sulfur dioxide do not play a central role in the formation of photochemical smog. Lead is primarily a heavy metal pollutant linked to neurological and developmental issues but not to smog formation. Carbon monoxide is more associated with incomplete combustion and its health effects rather than smog formation itself. Sulfur dioxide is a major contributor to industrial smog and acid rain but does not significantly contribute to the photochemical reactions that form photochemical smog. Thus, nitric oxide is correctly identified as the key pollutant in this context.