How is Ozone Formed?

Ultraviolet light (UV) from the sun and oxygen molecules (O₂) are responsible for most of ozone (O₃) production. Ozone is formed by photochemical reactions occurring in the stratosphere. These reactions primarily occur high in the atmosphere because the UV light that drives them is completely absorbed in the stratosphere. We call this type of UV light UV-C. The UV-C light carries enough energy to photolyze (or break apart) the oxygen molecule.

Where is Ozone Formed?

High amounts of ozone are produced above the tropics. Near the equator, the Sun is almost directly overhead during the day all year long. UV light at the equator is more intense because of the Sun's position here. The intense UV light drives photolysis to produce high amounts of ozone. However, the ozone produced in the tropics does not stay over that region. Stratospheric winds transport the ozone from the tropics towards the North Pole during the winter (the same is true during the Southern hemisphere's winter). The ozone builds up over the middle and high latitudes and reaches its highest abundance during the spring. The ozone builds up because the UV intensity is low in the winter, so the rate of ozone destruction is low. The amount of ozone then decreases during the summer and reaches its lowest abundance during the fall. Most of the ozone we find in the Northern hemisphere was produced over the tropics.