Which pollutant is associated with the phenomenon known as "photochemical smog"?

The correct answer is ozone, as it plays a crucial role in the development of photochemical smog. Photochemical smog occurs when sunlight interacts with pollutants such as nitrogen oxides and volatile organic compounds. During this interaction, ozone is formed through a series of chemical reactions, which are particularly prevalent on sunny days with stagnant air.

Ozone at ground level is a key component of photochemical smog and can have significant health effects, including respiratory issues and eye irritation. It is important to note that while nitrogen dioxide and other precursors contribute to the formation of ozone, the actual manifestation of smog is primarily characterized by the presence of ground-level ozone itself.

Other listed pollutants, such as carbon monoxide and sulfur dioxide, are associated with different types of air pollution problems, such as industrial emissions or vehicle exhaust, but they do not directly contribute to the phenomenon of photochemical smog in the same way that ozone does.