About Ozone Ozone is a naturally occurring gas, a form of oxygen. Ozone is colourless and has a very harsh odour. Ozone can be found through the entire atmosphere, however the greatest concentration occurs at an altitude of about 25 km in the stratosphere. Ozone in the stratosphere is formed primarily through a reaction between oxygen and radiation from the sun. This band of ozone-rich air is called the ozone layer.
Ozone also occurs in very small amounts at ground level.
Even though both types of ozone are exactly the same molecule, their presence in different parts of the atmosphere has very different consequences. Ground level ozone is a component of urban smog, a serious air pollutant. Sunlight and reactions with other naturally occurring chemicals in the stratosphere also destroy ozone. In an unpolluted atmosphere, there is a balance between the amount of ozone produced and the amount of ozone destroyed. So the total concentration of ozone in the stratosphere remains relatively constant.