Which layer above the troposphere contains most atmospheric ozone?

The layer above the troposphere that contains most atmospheric ozone is the stratosphere.

The stratosphere begins roughly 10–15 kilometers above Earth and extends to about 50 kilometers. About 90% of atmospheric ozone is concentrated there, forming the ozone layer. Ozone is still a trace gas: peak concentrations are only about 2–8 parts per million.

Ozone forms when ultraviolet light splits oxygen molecules, O₂, and the resulting oxygen atoms combine with other O₂ molecules to make O₃. Ozone then absorbs much of the Sun’s harmful ultraviolet radiation, heating the stratosphere and producing its characteristic temperature increase with altitude.

A common mix-up is confusing the ozone layer with the troposphere, where ozone is an air pollutant and greenhouse gas near the ground. The stratospheric ozone layer was identified in the early 20th century, and its depletion by chlorofluorocarbons led to the 1987 Montreal Protocol.