Most of the ozone in the stratosphere is formed over the equatorial belt, where the level of solar radiation is greatest. Under normal circumstances highest ozone values are found over places such as Canada and Siberia, whilst the lowest values are found around the equator. 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.