Which of the following is a major component of photochemical smog?

Photochemical smog is primarily characterized by the presence of ozone, which is formed when sunlight reacts with pollutants such as volatile organic compounds (VOCs) and nitrogen oxides. These reactions typically occur in urban areas with heavy traffic and significant sunlight, leading to the production of ground-level ozone, a key component of photochemical smog. Ozone at this level is harmful to human health and the environment, but it differs from the ozone found in the stratosphere, which protects us from the sun's ultraviolet rays.

While carbon monoxide, aldehydes, and methane are pollutants that can contribute to air quality issues, they are not defining components of photochemical smog. Carbon monoxide is primarily a product of incomplete combustion, aldehydes are often products of industrial processes and vehicle emissions, and methane is a potent greenhouse gas but not a significant player in the formation of photochemical smog. The ambient conditions and the photochemical reactions that produce ozone are what specifically characterize photochemical smog.