Ozone is formed naturally when oxygen in the Earth's atmosphere is exposed to strong concentrations of ultraviolet light. It also forms artificially when nitrogen and oxygen emissions from internal combustion engines are exposed to sunlight.
Ozone is formed naturally in Earth's upper atmosphere. When the oxygen gas (O2) in the atmosphere is exposed to ultraviolet light, the chemical bond between the two oxygen atoms is broken. The extra oxygen atom bonds with an existing O2 molecule in the atmosphere, forming ozone (O3).
Ozone is also formed from car exhaust, which contains nitric oxide (2NO), an unstable nitrogen and oxygen molecule. Nitrogen molecules naturally break apart when exposed to sunlight, which means that when the nitric oxide is released into the atmosphere it breaks down and releases a free oxygen atom, which bonds with the existing O2 molecules and forms ozone gas.