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The '''ozone layer''' is a layer in [[Earth's atmosphere]] which contains relatively high concentrations of [[ozone]] |
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The '''ozone layer''' is a layer in [[Earth's atmosphere]] which contains relatively high concentrations of [[ozone]] (O<sub>3</sub>). This layer absorbs 97–99% of the [[Sun]]'s high frequency [[ultraviolet light]], which is potentially damaging to the life forms on Earth.<ref name="NASA">{{cite web|url=http://www.nas.nasa.gov/About/Education/Ozone/ozonelayer.html|title=Ozone layer|accessdate=2007-09-23}}</ref> It is mainly located in the lower portion of the [[stratosphere]] from approximately {{convert|30|to|4 hello0|km}} above Earth, though the thickness varies seasonally and geographically.<ref>{{cite web|url=http://www.ozonelayer.noaa.gov/science/basics.htm|title=Science: Ozone Basics.|accessdate=2007-01-29}}</ref> The ozone layer was discovered in 1913 by the French physicists [[Charles Fabry]] and [[Henri Buisson]]. Its properties were explored in detail by the British meteorologist [[G. M. B. Dobson]], who developed a simple [[Spectrophotometry|spectrophotometer]] (the [[Dobson spectrometer|Dobsonmeter]]) that could be used to measure stratospheric ozone from the ground. Between 1928 and 1958 Dobson established a worldwide network of ozone monitoring stations, which continue to operate to this day. The "[[Dobson unit]]", a convenient measure of the [[area density|columnar density]] of ozone overhead, is named in his honor. |
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==Origin of ozone== |
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[[Image:Ozone cycle.svg|thumb|350px|[[Ozone-oxygen cycle]] in the ozone layer.]] |
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The photochemical mechanisms that give rise to the ozone layer were discovered by the British physicist [[Sydney Chapman (mathematician)|Sidney Chapman]] in 1930. Ozone in the Earth's stratosphere is created by [[ultraviolet light]] striking [[oxygen]] [[molecule]]s containing two oxygen [[atom]]s (O<sub>2</sub>), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O<sub>2</sub> to create ozone, O<sub>3</sub>. The ozone molecule is also unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O<sub>2</sub> and an atom of atomic oxygen, a continuing process called the [[ozone-oxygen cycle]], thus creating an ozone layer in the [[stratosphere]], the region from about {{convert|10|to|50|km|ft}} above Earth's surface. About 90% of the ozone in our atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about {{convert|20|and|40|km}}, where they range from about 2 to 8 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only 3 millimeters thick.<ref>{{cite web|url=http://www.nasa.gov/facts/Earth/earth_facts_archives.html|title=NASA Facts Archive|accessdate=2011-06-09}}</ref> |
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==Ultraviolet light and ozone == |
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[[Image:Ozone altitude UV graph.svg|thumb|280px|left|Levels of ozone at various altitudes and blocking of [[ultraviolet radiation]].]] |
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[[Image:Ozone solar UV absorb DNA action.jpg|thumb|300px|UV-B energy levels at several altitudes. Blue line shows DNA sensitivity. Red line shows surface energy level with 10% decrease in ozone]] |
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Although the concentration of the ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet (UV) radiation coming from the sun. UV radiation is divided into three categories, based on its [[wavelength]]; these are referred to as UV-A (400–315 [[nano-metre|nm]]), UV-B (315–280 nm), and UV-C (280–100 nm). UV-C, which would be very harmful to all living things, is entirely screened out by ozone at around {{convert|35|km|ft}} altitude. UV-B radiation can be harmful to the skin and is the main cause of [[sunburn]]; excessive exposure can also cause genetic damage, resulting in problems such as [[skin cancer]]. The ozone layer is very effective at screening out UV-B; for radiation with a wavelength of 290 nm, the intensity at the top of the atmosphere is 350 million times stronger than at the Earth's surface. Nevertheless, some UV-B reaches the surface. Most UV-A reaches the surface; this radiation is significantly less harmful, although it can potentially cause genetic damages. |
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==Distribution of ozone in the stratosphere== |
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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 towards the poles. It also varies with season, being in general thicker during the spring and thinner during the autumn in the northern hemisphere. The reasons for this latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar intensity. |
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Since stratospheric ozone is produced by solar UV radiation, one might expect to find the highest ozone levels over the tropics and the lowest over polar regions. The same argument would lead one to expect the highest ozone levels in the summer and the lowest in the winter. The observed behavior is very different: most of the ozone is found in the mid-to-high latitudes of the northern and southern hemispheres, and the highest levels are found in the spring, not summer, and the lowest in the autumn, not winter in the northern hemisphere. During winter, the ozone layer actually increases in depth. This puzzle is explained by the prevailing stratospheric wind patterns, known as the [[Brewer-Dobson circulation]]. While most of the ozone is indeed created over the tropics, the stratospheric circulation then transports it poleward and downward to the lower stratosphere of the high latitudes. However in the southern hemisphere, owing to the [[ozone hole]] phenomenon, the lowest amounts of column ozone found anywhere in the world are over the Antarctic in the southern spring period of September and October. |
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[[Image:Nimbus ozone Brewer-Dobson circulation.jpg|thumb|Brewer-Dobson circulation in the ozone layer.]] |
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The ozone layer is higher in altitude in the tropics, and lower in altitude in the extratropics, especially in the polar regions. This altitude variation of ozone results from the slow circulation that lifts the ozone-poor air out of the troposphere into the stratosphere. As this air slowly rises in the tropics, ozone is produced by the overhead sun which photolyzes oxygen molecules. As this slow circulation bends towards the mid-latitudes, it carries the ozone-rich air from the tropical middle stratosphere to the mid-and-high latitudes lower stratosphere. The high ozone concentrations at high latitudes are due to the accumulation of ozone at lower altitudes. |
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The Brewer-Dobson circulation moves very slowly. The time needed to lift an air parcel from the tropical tropopause near {{convert|16|to|20|km}} is about 4–5 months (about {{convert|30|ft|m}} per day). Even though ozone in the lower tropical stratosphere is produced at a very slow rate, the lifting circulation is so slow that ozone can build up to relatively high levels by the time it reaches {{convert|26|km}}. |
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Ozone amounts over the continental [[United States]] (25°N to 49°N) are highest in the northern spring (April and May). These ozone amounts fall over the course of the summer to their lowest amounts in October, and then rise again over the course of the winter. Again, wind transport of ozone is principally responsible for the seasonal evolution of these higher latitude ozone patterns. |
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The total column amount of ozone generally increases as we move from the tropics to higher latitudes in both hemispheres. However, the overall column amounts are greater in the northern hemisphere high latitudes than in the southern hemisphere high latitudes. In addition, while the highest amounts of column ozone over the Arctic occur in the northern spring (March–April), the opposite is true over the Antarctic, where the lowest amounts of column ozone occur in the southern spring (September–October). |
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==Ozone depletion== |
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{{Main|Ozone depletion}} |
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[[File:Future ozone layer concentrations.gif|thumb|400px|NASA projections of stratospheric ozone concentrations if chlorofluorocarbons had not been banned.]] |
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The ozone layer can be depleted by free radical catalysts, including [[nitric oxide]] (NO), [[nitrous oxide]] (N<sub>2</sub>O), [[hydroxyl]] (OH), atomic [[chlorine]] (Cl), and atomic [[bromine]] (Br). While there are natural sources for all of these [[Chemical species|species]], the concentrations of chlorine and bromine have increased markedly in recent years due to the release of large quantities of man-made [[organohalogen]] compounds, especially [[chlorofluorocarbon]]s (CFCs) and [[bromofluorocarbon]]s.<ref>{{cite web|url=http://www.eia.doe.gov/oiaf/1605/archive/gg97rpt/chap5.html|title=Halocarbons and Other Gases|accessdate=2008-06-24|author=Energy Information Administration/Emissions of Greenhouse Gases in the United States 1996|date=2008-06-24}}</ref> These highly stable compounds are capable of surviving the rise to the [[stratosphere]], where Cl and Br [[Radical (chemistry)|radicals]] are liberated by the action of ultraviolet light. Each radical is then free to initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone molecules. The breakdown of ozone in the stratosphere results in the ozone molecules being unable to absorb ultraviolet radiation. Consequently, unabsorbed and dangerous ultraviolet-B radiation is able to reach the Earth’s surface. 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 seasonal declines have been seen, and are described as [[ozone hole]]s. |
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In 2009, [[nitrous oxide]] (N<sub>2</sub>O) was the largest ozone-depleting substance emitted through human activities.<ref>[http://www.noaanews.noaa.gov/stories2009/20090827_ozone.html NOAA Study Shows Nitrous Oxide Now Top Ozone-Depleting Emission, NOAA, August 27, 2009]</ref> |
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===Regulation=== |
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In 1978, the [[United States]], [[Canada]] and [[Norway]] enacted bans on [[Chlorofluorocarbon|CFC]]-containing [[aerosol spray]]s that are thought to damage the ozone layer. The European Community rejected an analogous proposal to do the same. In the U.S., 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.{{Citation needed|date=November 2010}} Since that time, the treaty has been amended to ban CFC production after 1995 in the developed countries, and later in developing. Today, over 160 countries have signed the treaty. Beginning January 1, 1996, only recycled and stockpiled CFCs will be available for use in developed countries like the US. This production phaseout is possible because of efforts to ensure that there will be substitute chemicals and technologies for all CFC uses.<ref>[http://www.epa.gov/ozone/science/q_a.html]</ref> |
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On August 2, 2003, scientists announced that the depletion of the ozone layer may be slowing down due to the international ban on CFCs.<ref>{{cite news |url=http://www.ecozine.co.uk/OzoneLayer.htm|title=Ozone_Layer. |accessdate=2010-11-09 |author=EcoZone |date=2005-01-01 |archiveurl = http://web.archive.org/web/20060830060353/http://www.ecozine.co.uk/OzoneLayer.htm <!-- Bot retrieved archive --> |archivedate = 2010-11-09 | location=unknown | work=ecozone}}</ref> 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. CFCs have very long atmospheric lifetimes, ranging from 50 to over 100 years, so the final recovery of the ozone layer is expected to require several lifetimes. |
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Compounds containing C–H bonds (such as [[hydrochlorofluorocarbon]]s, or HCFCs) have been designed to replace the function of CFCs. These replacement compounds are more reactive and less likely to survive long enough in the atmosphere to reach the stratosphere where they could affect the ozone layer. While being less damaging than CFCs, HCFCs can have a negative impact on the ozone layer, so they are also being phased out.<ref>{{cite web|url=http://www.epa.gov/ozone/defns.html#hcfc|title=Ozone Depletion Glossary|accessdate=2008-09-03|author=US EPA|date=2008-09-03}}</ref> |
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==See also== |
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*[[OzonAction Programme]] |
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==References== |
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{{reflist}} |
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==Further reading== |
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* Seinfeld, John H.; Pandis, Spyros N. (1998). ''Atmospheric Chemistry and Physics: From Air Pollution to Climate Change''. John Wiley and Sons, Inc. ISBN 0-471-17816-0. |
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==External links== |
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{{commons category|Ozone layer}} |
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*[http://www.ccpo.odu.edu/SEES/ozone/oz_class.htm Stratospheric ozone: an electronic textbook] |
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* http://www.unep.org/ozone/Public_Information/4Aii_PublicInfo_Facts_OzoneLayer.asp |
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* NASA. ''Studying Earth's Environment From Space.'' June 2000. (accessed November 3, 2010) http://www.ccpo.odu.edu/~lizsmith/SEES/index.html. |
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{{earthsatmosphere}} |
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{{DEFAULTSORT:Ozone Layer}} |
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[[Category:Atmosphere]] |
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[[Category:Oxygen]] |
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[[Category:Ozone depletion]] |
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[[af:Osoonlaag]] |
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[[am:የኦዞን ንጣፍ]] |
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[[ar:أوزونوسفير]] |
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[[ast:Capa d'ozonu]] |
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[[bg:Озонов слой]] |
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[[ca:Capa d'ozó]] |
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[[cs:Ozonová vrstva]] |
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[[cy:Haen osôn]] |
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[[da:Ozonlaget]] |
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[[de:Ozonschicht]] |
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[[et:Osoonikiht]] |
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[[es:Capa de ozono]] |
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[[eo:Ozona tavolo]] |
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[[eu:Ozono geruza]] |
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[[fa:لایه اوزون]] |
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[[fr:Couche d'ozone]] |
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[[gl:Ozonosfera]] |
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[[ko:오존층]] |
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[[hr:Ozonski omotač]] |
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[[id:Lapisan ozon]] |
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[[it:Ozonosfera]] |
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[[he:שכבת האוזון]] |
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[[ka:ოზონის ფენა]] |
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[[kk:Озоносфера]] |
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[[ht:Kouch ozòn]] |
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[[lt:Ozono sluoksnis]] |
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[[hu:Ózonréteg]] |
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[[mk:Озонска обвивка]] |
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[[ml:ഓസോൺ പാളി]] |
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[[mr:ओझोनचा पट्टा]] |
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[[ms:Lapisan ozon]] |
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[[mn:Озоны давхарга]] |
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[[my:အိုဇုန်းလွှာ]] |
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[[nl:Ozonlaag]] |
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[[ne:ओजोन तह]] |
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[[ja:オゾン層]] |
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[[no:Ozonlaget]] |
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[[nn:Ozonlaget]] |
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[[oc:Jaç d'ozòn]] |
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[[om:Ozone layer]] |
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[[pl:Ozonosfera]] |
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[[pt:Ozonosfera]] |
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[[ro:Stratul de ozon]] |
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[[qu:Achiksamaytu p'istu]] |
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[[ru:Озоновый слой]] |
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[[si:ඕසෝන් වියන]] |
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[[simple:Ozone layer]] |
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[[sl:Ozonski plašč]] |
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[[sr:Озонски омотач]] |
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[[fi:Otsonikerros]] |
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[[sv:Ozonlagret]] |
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[[ta:ஓசோன் படலம்]] |
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[[th:ชั้นโอโซน]] |
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[[tr:Ozonosfer]] |
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[[uk:Озоносфера]] |
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[[vi:Lớp ôzôn]] |
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[[zh:臭氧层]] |
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Revision as of 14:34, 16 September 2011
The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone