Ozone Layer II

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Ozone Layer

The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of

ozone (O3). This layer absorbs 9799% of the Sun's high frequency ultraviolet light, which is

potentially damaging to the life forms on Earth.[1] It is mainly located in the lower portion of the

stratosphere from approximately 30 to 40 kilometres (19 to 25 mi) above Earth, though the

thickness varies seasonally and geographically.[2] 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 spectrophotometer (the

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 columnar density of ozone

overhead, is named in his honor.

Origin of ozone

Ozone-oxygen cycle in the ozone layer.

The photochemical mechanisms that give rise to the ozone layer were discovered by the British physicist Sidney

Chapman in 1930. Ozone in the Earth's stratosphere is created by ultraviolet lightstriking oxygenmoleculescontaining 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 (although, in th

stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O2 and an atom of atomioxygen, a continuing process called the ozone-oxygen cycle, thus creating an ozone layer in the stratosphere, the

region from about 10 to 50 kilometres (33,000 to 160,000 ft) above Earth's surface. About 90% of the ozone in ou

atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about 20 and 40 kilometre

(12 and 25 mi), where they range from about 2 to 8 parts per million. If all of the ozone were compressed to thepressure of the air at sea level, it would be only 3 millimeters thick.[3]
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Ultraviolet light and ozone

Levels of ozone at various altitudes and blocking ofultraviolet radiation.

UV-B energy levels at several altitudes. Blue line shows DNA sensitivity. Red line shows surface

energy level with 10% decrease in ozone

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

(400315 nm), UV-B (315280 nm), and UV-C (280100 nm). UV-C, which would be very harmful to

all living things, is entirely screened out by ozone at around 35 kilometres (115,000 ft) altitude. UV

B radiation can be harmful to the skin and is the main cause of sunburn; excessive exposure canalso 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.

Distribution of ozone in the stratosphere
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The thickness of the ozone layerthat is, the total amount of ozone in a column overheadvaries by a large fact

worldwide, being in general smaller near the equator and larger towards the poles. It also varies with season, beinin general thicker during the spring and thinner during the autumn in the northern hemisphere. The reasons for thi

latitude and seasonal dependence are complicated, involving atmospheric circulation patterns as well as solar

intensity.

Since stratospheric ozone is produced by solar UV radiation, one might expect to find the highest ozone levels ovthe 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 isfound in the mid-to-high latitudes of the northern and southern hemispheres, and the highest levels are found in thspring, not summer, and the lowest in the autumn, not winter in the northern hemisphere. During winter, the ozon

layer actually increases in depth. This puzzle is explained by the prevailing stratospheric wind patterns, known as

theBrewer-Dobson circulation. While most of the ozone is indeed created over the tropics, the stratosphericcirculation 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.

Brewer-Dobson circulation in the ozone layer.

The ozone layer is higher in altitude in the tropics, and lower in altitude in the extratropics, especially in the polarregions. 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 ozonerich 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.

The Brewer-Dobson circulation moves very slowly. The time needed to lift an air parcel from the tropical

tropopause near 16 to 20 kilometres (9.9 to 12 mi) is about 45 months (about 30 feet (9.1 m) per day). Eventhough ozone in the lower tropical stratosphere is produced at a very slow rate, the lifting circulation is so slow th

ozone can build up to relatively high levels by the time it reaches 26 kilometres (16 mi).

Ozone amounts over the continental United States (25N to 49N) 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 riseagain 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. However, the overall column amounts are greater in the northern hemisphere high latitudes than inthe southern hemisphere high latitudes. In addition, while the highest amounts of column ozone over the Arctic

occur in the northern spring (MarchApril), the opposite is true over the Antarctic, where the lowest amounts of

column ozone occur in the southern spring (SeptemberOctober).
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Ozone depletion

Main article: Ozone depletion

NASA projections of stratospheric ozone concentrations if chlorofluorocarbons had not been

banned.

The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), nitrous oxide (N2O),hydroxyl (OH), atomic chlorine (Cl), and atomicbromine (Br). While there are natural sources for all of these

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 chlorofluorocarbons (CFCs) andbromofluorocarbons.[4] These highly stable compounds are capable of surviving the rise to the stratosphere, where

Cl and Brradicals 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 thestratosphere results in the ozone molecules being unable to absorb ultraviolet radiation. Consequently, unabsorbed

and dangerous ultraviolet-B radiation is able to reach the Earths surface. Ozone levels over the northern

hemisphere have been dropping by 4% per decade. Over approximately 5% of the Earth's surface, around the nort

and south poles, much larger seasonal declines have been seen, and are described asozone holes.

In 2009, nitrous oxide (N2O) was the largest ozone-depleting substance emitted through human activities.[5]

Regulation

In 1978, theUnited States, Canada andNorway enacted bans onCFC-containing aerosol sprays that are thought tdamage 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 Antarcticozone 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]

Since that time, the treaty has been amended to ban CFC production after 1995 in the developed countries, and
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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 ispossible because of efforts to ensure that there will be substitute chemicals and technologies for all CFC uses.[6]

On August 2, 2003, scientists announced that the depletion of the ozone layer may be slowing down due to the

international ban on CFCs.[7]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 AmericanGeophysical 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

Compounds containing CH bonds (such as hydrochlorofluorocarbons, 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.[8]
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Ozone Layer II

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