Ozone: Ozone is found near the ground in the troposphere, and it is also a major component of smog. The ozone close to the ground should not be confused with the ozone in the upper layer of the atmosphere (the stratosphere), which screens out harmful ultraviolet rays. Ozone is formed when nitrogen oxides and volatile organic compounds mix in the sunlight, and therefore ozone levels are always higher in summer. Nitrogen oxides are formed by burning fossil fuels, and some sources of volatile organic compounds are factories and trees.