Other Definitions
ozone layer (dict)
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Ozone LayerThe ozone layer is that part of the Earth's stratosphere which contains relatively high concentrations of ozone. The total quantity of ozone in the ozone layer is not very large; if just the ozone were compressed to the pressure of the air at sea level, it would be only a few millimeters thick. The ozone layer was discovered in 1913 by the french physicists Charles Fabry and Henri Buisson. Ozone is notable for its ability to absorb certain wavelengths of ultraviolet radiation: - "The stratospheric ozone layer is important because, among other things, it reduces the amount of solar ultraviolet-B radiation reaching the Earth's surface. UV-B is the main cause of basal and squamous cell skin cancers, but not of malignant melanoma; the latter is primarily caused by UV-A-- which is not absorbed by ozone." http://www.sepp.org/ozone/natsci_ozone.html
Ozone in the earth's atmosphere is generally created by ultraviolet light striking oxygen molecules containing two oxygen 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 "thickness" of the ozone layer - that is, the total amount of ozone in a column overhead - varies by a large factor worldwide, being in general smaller near the equator and larger as one moves towards the poles. It also varies with season, being in general thicker during the spring and thinner during the fall. The reasons for this latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar intensity. Most of the ozone is created over the tropics, and carried to other latitudes by the stratospheric circulation. As a result, the highest ozone concentrations are found in the spring, rather than in the summer, and at high northern and southern latitudes, rather than near the equator. The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), hydroxyl (OH), and atomic chlorine and bromine (see ozone depletion). While there are natural sources for all of these species, the concentrations of Cl and Br have increased markedly in recent years due to the release of large quantities of manmade organohalogen compounds, especially chlorofluorocarbons (CFCs). These highly stable compounds find their way to the stratosphere, where Cl and Br atoms are liberated by the action of ultraviolet light on them. Each chlorine atom is capable of breaking down approximately one hundred thousand ozone molecules during the time that it resides in the stratosphere, and bromine is even more efficient. Ozone levels, over the northern hemisphere, have been dropping by ~4% per decade. Over approximately 5% of the Earth's surface, around the north and south poles, much larger (but seasonal) declines have been seen; these are the ozone holes. On January 23, 1978 Sweden became the first nation to ban CFC-containing aerosol sprays that are thought to damage the ozone layer. The United States and other developed nations followed suit soon after. Chlorofluorocarbons continued to be used in other applications, such as refrigeration and industrial cleaning, until after the discovery of the antarctic ozone hole in 1985. After negotiation of an international treaty (the Montreal Protocol), CFC production was sharply limited beginning in 1987 and phased out completely by 1996. On August 2, 2003, scientists announced that the depletion of the ozone layer may be slowing down due to the international ban on chlorofluorocarbons. http://news.independent.co.uk/world/environment/story.jsp?story=429802 Three satellites and three ground stations confirmed that the upper atmosphere ozone depletion rate has slowed down significantly during the past decade. The study was organized by the American Geophysical Union. Some breakdown can be expected to continue due to CFCs used by nations which have not banned them, and due to gases which are already in the stratosphere. External links - http://www.unep.org/ozone/Public_Information/4Aii_PublicInfo_Facts_OzoneLayer.asp
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See also
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