While ozone in the Earth's stratosphere is "good" because it protects us from harmful UV radiation from the sun, ozone located in the troposphere is "bad" - it is a form of pollution.

Tropospheric ozone is a major component of photochemical smog. You've probably seen photochemical smog before; on a sunny day in an urban area like Salt Lake City, you might notice a yellowish-brown haze hanging over the city. This haze is photochemical smog. It is created mainly from automobile exhaust interacting with sunlight. Car exhaust contains nitrogen oxide and carbon oxide compounds. When these compounds interact with sunlight, they are broken down, releasing the oxygen (O₂) atoms. The O₂ is free to combine with free oxygen atoms in the air to create O₃ (ozone). This ozone and other chemicals make up the hazy brown photochemical smog.

Photochemical smog is not only ugly, it is harmful to humans and other organisms. It irritates the respiratory system, eyes, nose, and throat of animals, causes difficulty breathing, and is harmful to plants.

The video on the next page describes the formation of photochemical smog, its major components, and how a temperature inversion in the troposphere can influence the amount of air pollution over an area.