Ozone or trioxygen, is an inorganic molecule with the chemical formula O3. It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O2, breaking down in the lower atmosphere to O2 (dioxygen). Ozone is formed from dioxygen by the action of ultraviolet (UV) light and electrical discharges within the Earth's atmosphere. It is present in very low concentrations throughout the latter, with its highest concentration high in the ozone layer of the stratosphere, which absorbs most of the Sun's ultraviolet (UV) radiation.
Ozone is mostly found in the stratosphere, a layer of the atmosphere between six and 30 miles (10-50 km) above the Earth's surface. This ozone layer forms an invisible protective shield over the planet, absorbing damaging UV radiation from the sun. Without it, life on Earth would not be possible.
In the stratosphere, ozone is created primarily by ultraviolet radiation. When high-energy ultraviolet rays strike ordinary oxygen molecules (O2), they split the molecule into two single oxygen atoms, known as atomic oxygen. A freed oxygen atom then combines with another oxygen molecule to form a molecule of ozone. There is so much oxygen in our atmosphere, that these high-energy ultraviolet rays are completely absorbed in the stratosphere.
In addition to the natural ozone balance, scientists have found that ozone levels change periodically as part of regular natural cycles such as the changing seasons, winds and long time scale sun variations. Moreover, volcanic eruptions may inject materials into the stratosphere that can lead to increased destruction of ozone.