Ozone has been cast in the roles of both villain and victim in the media of the past decade. Ozone is an everyday part of the atmosphere playing a number of complex roles and is constantly being created and destroyed.

We should have ozone around as long as we have an atmosphere with abundant oxygen. In its ordinary form as a free element, oxygen usually occurs as a diatomic molecule (consisting of two oxygen atoms) - O2. Ozone is a triatomic molecule consisting of three oxygen atoms - O3 - formed when certain kinds of energy, say an electric arc or very intense ultraviolet light, are applied to O2. Ozone is less stable than diatomic oxygen and most other gases in the atmosphere; every ozone molecule "wants to" give up its extra energy and its extra atom to become oxygen again.

Ozone makes the sweetish-pungent smell coming from the arc of electric motors. It is also emitted by laser printers, is created by lightning storms, and is one of the things that makes smog irritate our eyes and lungs.

Most of the ozone in the atmosphere is created when the high-energy ultraviolet rays of the Sun reach the rarefied gas molecules of the stratosphere. The ultraviolet energy excites the oxygen atoms, and they combine to form O3.

What is fascinating is that ultraviolet rays create ozone and destroy it - the high energy ultraviolet rays creating it, and the low energy ultraviolet rays breaking apart the O3 molecules. Most ultraviolet rays never reach the Earth's surface because O2 absorbs the high energy rays, and the low-energy ultraviolet rays are absorbed by ozone. So it is this ozone production and destruction process that absorbs excess ultraviolet and protects humans and other dwellers on Earth's surface from sunburn and skin cancer.

Ozone is not distributed evenly throughout Earth's atmosphere. Most of the naturally occurring ozone tends to be concentrated in the lower stratosphere, where many photochemical reactions involving ultraviolet rays are taking place. These high concentrations are not caused by local ozone production as there is insufficient ultraviolet to produce much ozone at lower altitudes. Ozone concentrations can be strongly influenced by heating and cooling (expansion and compression) and by winds that move it around from place to place.

Some ozone molecules do find their way down into the troposphere. On addition, humans create some ozone in the troposphere by various kinds of air pollution. When automobiles and other human sources emit carbon monoxide, methane, and non-methane hydrocarbons in the presence of nitrogen oxides, sunlight causes a reaction that produces ozone "smog" and raises important health issues in some of our traffic-choked cities.

Ozone in the upper troposphere and lower stratosphere functions as a greenhouse gas. Just how strong its greenhouse effect is, compared to carbon dioxide, is something scientists have yet to quantify, but it could be much weaker. Measurements of ozone concentrations, especially of historical trends, are few. Some suggest surface ozone may have doubled since pre-industrial times.

Related Articles:

- Gases Introduction
- Carbon Dioxide
- Halocarbons
- Methane
- Nitrous Oxide
- Ozone
- Water Vapour