Ozone layer

The ozone layer is that part of the stratosphere which contains ozone. Ozone is notable for its ability to absorb certain frequencies of ultraviolet radiation. The ozone layer is not very dense; if it were compressed to the density of the troposphere, it would be only a few millimeters thick.

Ozone in the earth's atmosphere is generally created by ultraviolet light striking oxygen molecules containing twooxygen atoms (O₂), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O₂ to create ozone, O₃. The ozone molecule is also unstable and when ultraviolet light hits ozone it splits into a molecule of O₂ and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle[?], thus creating an ozone layer in the stratosphere.

The ozone layer can be destroyed by the presence of atomic chlorine, fluorine or bromine in the atmosphere leading to the so-called ozone hole in the polar stratosphere during winter months; these elements are found in certain stable compounds, especially chlorofluorocarbons (CFCs) which may find their way to the stratosphere and there be liberated by the action of ultraviolet light on them.

The concentration of atmospheric ozone in the ozone layer varies by a large factor worldwide, being thicker, near the equator, and thinner, at the poles. Ozone levels, over the northern hemisphere, are dropping by ~4% per year. Approximately ~4.6% of the Earth's surface is not covered by the ozone layer; these are the ozone holes.

On January 23, 1978 Sweden became the first nation to ban aerosol sprays that are thought to damage the ozone layer.

See:

• ozone hole
• ozone depletion