Which environmental factor contributes primarily to the incidence of smog formation?

The formation of smog is primarily influenced by high temperatures and sunlight. When the temperature rises, it can lead to the increased production of ground-level ozone, a key component of smog. Sunlight catalyzes chemical reactions that occur in the atmosphere, causing pollutants like nitrogen oxides and volatile organic compounds to react and form ozone. This process is more efficient in warm conditions where there is ample sunlight, leading to an exacerbation of smog formation.

High humidity, low temperatures, and heavy rainfall do not have the same direct impact on the chemical reactions that lead to smog. In fact, high humidity can sometimes help to reduce the concentration of pollutants in the air, while low temperatures can limit the photochemical reactions necessary for smog creation. Heavy rainfall typically clears particulates and other pollutants from the atmosphere, reducing smog levels rather than increasing them. Therefore, the combination of high temperatures and sunlight is key to the development of smog in urban environments.