Ozone

Ozone (O₃) plays an extremely important role in the upper atmosphere (stratosphere) and lower atmosphere (troposphere). In the stratosphere (10–50 km above sea level), UV radiation breaks O₂ molecules into atomic oxygen (O); O₂ and O then recombine to form O₃. This first step absorbs almost all UV-C radiation (200–280 nm) from the sun before it passes below ~40 km above sea level; this is the rate-limiting step in O₃ formation.

O₃ is photolyzed by UV-B radiation (280–320 nm) back to O and O₂. This process absorbs a large amount of UV-B radiation, but some still reaches Earth’s surface where it is the primary cause of sunburn. With depleted amounts of O₃ in the atmosphere, more UV-B radiation reaches sea level.

A significant amount of Earth’s O₃ (~10%) exists in the troposphere (~0–10 km). Here it is not produced by the photolysis of O₂ because the required UV-C radiation has been absorbed in the stratosphere. Instead, O₃ is produced primarily during the oxidation of CH₄ and NO. Anthropogenic generation of these compounds has greatly increased over the past century and led to an increase in tropospheric O₃, which is linked to health problems such as asthma.