Ozone occurs in two layers of the atmosphere.
The layer surrounding the earth's surface is the troposphere. It is a key ingredient of urban smog. The troposphere extends to a level about 10 miles up, where it meets the second layer, the stratosphere. Ozone occurs naturally in the stratosphere and is produced and destroyed at a constant rate. These ozone depleting substances degrade slowly and can remain intact for many years as they move through the troposphere until they reach the stratosphere.