The part of the stratosphere at a height of c.22 km/14 mi in which the gas ozone (O3) is most concentrated. It is produced by the action of ultraviolet light from the Sun on oxygen (O2) in the air. The ozone layer shields the Earth from the harmful effects of solar ultraviolet radiation, but can be decomposed by complex chemical reactions, notably involving chlorofluorocarbons (CFCs), used as the pressurized propellant in some aerosol sprays, in refrigerating systems, and in the production of foam packaging. An area of 5075% depletion of total ozone has been called an ozone hole, defined geographically as the area in which the total ozone amount is less than 220 Dobson Units. (1 Dobson Unit is defined as 0.01 mm thickness at standard temperature and pressure of 0°C and 1 atmosphere pressure.) During the 1980s, reports of holes above the Arctic and Antarctic led to renewed efforts in Europe to reach agreement on an accelerated reduction in CFC consumption. The main focus of attention was in the Antarctic, where the hole, steadily increasing since the 1980s, was estimated to be c.27 million km² in September 2005. In 1987 the Montreal Protocol was signed by around 40 countries to limit the use of ozone-depleting substances (in force from 1990, since ratified by 180 countries), with the intention that consumption of CFCs would be frozen by 1999, reduced by 50% by 2005, and eliminated by 2010. Between 1986 and 1999, world global consumption of CFCs was reduced from 1·1 million to 150 000 tons, raising hopes that ozone levels would now begin to recover. In 2006 it was reported that the hole above the Antarctic appeared to have stopped widening and it is hoped that the area will recover fully over the next 60 years. The thinning over the Arctic is expected to recover sooner, sometime between 2030 and 2040.
The ozone layer, or ozonosphere layer (rarely used term), is the part of the Earth's atmosphere which contains relatively high concentrations of ozone (O3). The "Dobson unit", a convenient measure of the total amount of ozone in a column overhead, is named in his honor.
Origin of ozone
The photochemical mechanisms that give rise to the ozone layer were worked out by the British physicist Sidney Chapman in 1930. Ozone in the earth's stratosphere is created by ultraviolet light striking oxygen molecules containing two oxygen atoms (O2), splitting them into individual oxygen atoms (atomic oxygen); The ozone molecule is also unstable (although, in the stratosphere, long-lived) 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 region from about 10 to 50 km (32,000 to 164,000 feet) above Earth's surface. About 90% of the ozone in our atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 15 and 40 km, where they range from about 2 to 8 parts per million.
Ten percent of the ozone in the atmosphere is contained in the troposphere, the lowest part of our atmosphere where all of our weather takes place. Tropospheric ozone has two sources: about 10 % is transported down from the stratosphere while the remainder is created in situ in smaller amounts through different mechanisms.
Ultraviolet light and ozone
Although the concentration of ozone in the ozone layer is very small, it is vitally important to life because it absorbs biologically harmful ultraviolet (UV) radiation emitted from the Sun.
Depletion of the ozone layer allows more of the UV radiation, and particularly the more harmful wavelengths, to reach the surface, causing increased genetic damage to living organisms. Fortunately, where DNA is easily damaged, such as by wavelengths shorter than 290 nm, ozone strongly absorbs UV. At the longer wavelengths where ozone absorbs weakly, DNA damage is less likely. If there was a 10% decrease in ozone, the amount of DNA damaging UV increases, in this case, by about 22%. Considering that DNA damage can lead to maladies like skin cancer, it is clear that this absorption of the Sun's ultraviolet radiation by ozone is critical for our well being.
Distribution of ozone 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.
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. 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.
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. The high ozone concentrations at high latitudes are due to the accumulation of ozone at lower altitudes. 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 26 km (85,000 feet).
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.
The total column amount of ozone generally increases as we move from the tropics to higher latitudes in both hemispheres. 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). Indeed, the highest amounts of column ozone anywhere in the world are found over the Arctic region during the northern spring period of March and April. Meanwhile, the lowest amounts of column ozone anywhere in the world are found over the Antarctic in the southern spring period of September and October, owing to the ozone hole phenomenon.
Ozone depletion
The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), hydroxyl (OH), and atomic chlorine (Cl) and bromine (Br).
Regulation
On January 23, 1978 Sweden became the first nation to ban CFC-containing aerosol sprays that are thought to damage the ozone layer.
On August 2, 2003, scientists announced that the depletion of the ozone layer may be slowing down due to the international ban on chlorofluorocarbons, chemical compounds containing chlorine, fluorine and carbon. Three satellites and three ground stations confirmed that the upper atmosphere ozone depletion rate has slowed down significantly during the past decade. 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.
Compounds containing C-H bonds are being designed to replace the function of CFC's (such as HFC), since these 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.
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