Ozone is produced naturally in the stratosphere when highly energetic solar radiation strikes molecules of oxygen, O2, and cause the two oxygen atoms to split apart in a process called photolysis. If a freed atom collides with another O2, it joins up, forming ozone O3.
Most of the ozone in the stratosphere is formed over the
equatorial belt, where the level of solar radiation is greatest.
The circulation in the atmosphere then transports it towards the pole (see
So, the amount of stratospheric ozone above a location on the Earth varies
naturally with latitude, season, and from day-to-day.
||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. Over Canada is normally about 25% thicker in winter than summer. 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. At different temperatures and pressures (i.e. varying altitudes), there are different production and destruction reaction rates leading to a variation in concentration. 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.