The ozone layer varies naturally with season. Weather conditions can also cause considerable daily variations. Ozone is also naturally broken down in the stratosphere. In an unpolluted atmosphere there is a balance between the amount of ozone being produced and destroyed and so the total concentration remains relatively constant. The highest ozone concentrations are in the lower stratosphere, between about 18 and 26 km.
Ozone also occurs in very small amounts in the troposphere.
It is produced at ground level through a reaction between sunlight and, e.g., gases emitted from cars. As a pollutant it should not be confused with the separate problem of stratospheric ozone depletion.