Ozone Formation

Ozone is created when a single oxygen atom reacts with an oxygen molecule. Many chemical reactions in the atmosphere can produce a single, or free, oxygen atom. One reaction that people are able to detect occurs when lightning strikes water droplets. The electric jolt can sever the bonds between water's two atoms of hydrogen and one atom of oxygen, freeing the oxygen atom and allowing it to bond with a regular O₂ molecule to form ozone. If you have ever noticed a peculiar odor in the air after lightning has struck nearby, you were probably smelling ozone.

Most ground-level ozone that causes health concerns is formed when sunlight reacts with air pollution caused by human activities. The pollutants that lead to ozone formation can be divided into two categories. One category is molecules called nitrogen oxides (NOx).