HAVE U EVER WONDERED Y DE DEPLETION OF OZONE LAYER IS TAKING PLACE ONLY IN THE ANTARCTIC???

NO???
WELL....HERE'S DE REASON....
firstly let us find out de surface chemistry takin place xactly in the formation and depletion of ozone..
Ozone depletion describes two distinct, but related observations: a slow, steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere (the ozone layer) since the late 1970s, and a much larger, but seasonal, decrease in stratospheric ozone over Earth's polar regions during the same period. The latter phenomenon is commonly referred to as the ozone hole. In addition to this well-known stratospheric ozone depletion, there are also tropospheric ozone depletion events, which occur near the surface in polar regions during spring.
The detailed mechanism by which the polar ozone holes form is different from that for the mid-latitude thinning, but the most important process in both trends is catalytic destruction of ozone by atomic chlorine and bromine.^[1] The main source of these halogen atoms in the stratosphere is photodissociation of chlorofluorocarbon (CFC) compounds, commonly called freons, and of bromofluorocarbon compounds known as halons. These compounds are transported into the stratosphere after being emitted at the surface.^[2] Both ozone depletion mechanisms strengthened as emissions of CFCs and halons increased.

Image of the largest Antarctic ozone hole ever recorded (September 2006).

In the figure above,the purple n blue coloured portion refers to region where dere is least amount of ozone which result in large amount of UV rays to transpire through de region...
green,yellow n red portions refer to regions where dere is more ozone,diz does'nt allow ozone transpires through de region...
The ozone hole of 2006 is the most severe ozone hole (ozone hole was the largest ever observed,11.4 million square miles (29.5 million square kilometres)) observed to date. NASA's Aura satellite observed a low value of 85 Dobson Units (DU) on Oct. 8 in a region over the East Antarctic ice sheet. Dobson Units are a measure of ozone amounts above a fixed point in the atmosphere.
NOW...
for understandin de surface chemistry in de formation n depletion of ozone it is neccessary to understand de layers different layers of earth's surface,namely
1)Troposphere
2)Stratosphere and
3)Ionosphere or Mesosphere.


I posted u a sketch to help u understand different layers of earth surface better...

Layers of the atmosphere (not to scale)

Three forms (or allotrope) of oxygen are involved in the ozone-oxygen cycle: oxygen atoms (O or atomic oxygen), oxygen gas (O₂ or diatomic oxygen), and ozone gas (O₃ or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photo dissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This produces two oxygen atoms. The atomic oxygen then combines with O₂ to create O₃. Ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O₂ and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process which terminates when an oxygen atom "recombines" with an ozone molecule to make two O₂ molecules: O + O₃ → 2 O₂
The overall amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination.


Ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH·), the nitric oxide radical (NO·), atomic chlorine (Cl·) and bromine (Br·). All of these have both natural and man made sources; at the present time, most of the OH· and NO· in the stratosphere is of natural origin, but human activity has dramatically increased the levels of chlorine and bromine. These elements are found in certain stable organic compounds, especially chlorofluorocarbons (CFCs), which may find their way to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action of ultraviolet light, e.g. ('h' is Planck's constant, 'ν' is frequency of electromagnetic radiation)
CFCl₃ + hν → CFCl₂ + Cl
The Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle,^[4] a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. The chlorine monoxide (i.e., the ClO) can react with a second molecule of ozone (i.e., O₃) to yield another chlorine atom and two molecules of oxygen. The chemical shorthand for these gas-phase reactions is:
Cl + O₃ → ClO + O₂
ClO + O₃ → Cl + 2 O₂
The overall effect is a decrease in the amount of ozone. More complicated mechanisms have been discovered that lead to ozone destruction in the lower stratosphere as well.
A single chlorine atom would keep on destroying ozone (thus a catalyst) for up to two years (the time scale for transport back down to the troposphere) were it not for reactions that remove them from this cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate (ClONO₂). On a per atom basis, bromine is even more efficient than chlorine at destroying ozone, but there is much less bromine in the atmosphere at present. As a result, both chlorine and bromine contribute significantly to the overall ozone depletion. Laboratory studies have shown that fluorine and iodine atoms participate in analogous catalytic cycles. However, in the Earth's stratosphere, fluorine atoms react rapidly with water and methane to form strongly bound HF, while organic molecules which contain iodine react so rapidly in the lower atmosphere that they do not reach the stratosphere in significant quantities. Furthermore, a single chlorine atom is able to react with 100,000 ozone molecules. This fact plus the amount of chlorine released into the atmosphere by chlorofluorocarbons (CFCs) yearly demonstrates how dangerous CFCs are to the environment.

NOW CUMIN TO DE MAIN CONCERN...

The Antarctic Ozone hole

What is it?

The antarctic ozone hole is an area of the antarctic stratosphere in which the recent (since about 1975) ozone levels have dropped to as low as 33% of their pre-1975 values. The ozone hole occurs during the antarctic spring, from September to early December, as strong westerly wind start to circulate around the continent and create an atmospheric container. In this container over 50% of the lower stratospheric ozone is destroyed.

What is so special about Antarctic conditions?

Polar regions get a much larger variation in sunlight than anywhere else, and during the 3 months of winter spend most of time in the dark without solar radiation. Temperatures hover around or below -80'C for much of the winter and the extremely low antarctic temperatures cause cloud formation in the relatively ''dry''stratosphere. These Polar Stratospheric Clouds (PSC's) are composed of ice crystals that provide the surface for a multitude of reactions, many of which speed the degradation of ozone molecules.
The ozone hole is caused by the effect of pollutants in the atmosphere destroying stratospheric ozone. During the Antarctic winter something special happens to the Antarctic weather.

Firstly, strong winds blowing around the continent form, this is known as the "polar vortex" -  this isolates the air over Antarctica from the rest of the world.

Secondly, special clouds form called Polar Stratospheric Clouds(PSC'S). Clouds don't normally form in the stratosphere and these turn out to have the effect of concentrating the pollutants that break down the ozone, so speeding the process up

Polar stratospheric clouds at about 80,000 feet altitude. These are the highest flying of all clouds and only occur in polar regions where the temperature in the upper atmosphere dips below minus 100F. They are sometimes called "nacreous clouds" as they are coloured like the nacre of mother-of-pearl with coloured bands that move the position of cloud and observer

ELABORATED EXPLANATION:

1. As mid-May brings on the onset of winter, the antarctic stratosphere cools and descends closer to the surface. The Coriolis effect (caused by the earths rotation) sets up a strong westerly circulation around the south pole, forming an oblong vortex which varies in size from year to year. Current theory [Tuck 1989] holds that the vortex is like a semi-sealed reaction vessel with most of the antarctic air staying trapped inside the vortex. As temperatures in the lower stratosphere cools below -80'C, Polar Stratospheric Clouds (PSC's) start to form.

2. Most of the Antarctic stratospheric chlorine ends up in reservoir compounds such as ClONO2 or HCl. Reservoir compounds are so named because they hold the atmospheric chlorine in an inactive form but can react later, usually after a hit by ultraviolet radiation, and release reactive chlorine molecules. On the surface of the PSC crystals, nitrogen compounds are readily absorbed and chlorine reservoir compounds are converted to far more reactive compounds such as Cl2 and HOCl.

3. The small amounts of visible light during the antarctic winter are sufficient to convert much of the atmospheric Cl2 to ClO:

• Cl2 + light ---> 2 Cl
• Cl + O3 ---> ClO + O2

Ordinarily much of the ClO would be captured by atmospheric NO2 and returned to the ClONO2 reservoir, but the polar clouds have absorbed most of the Nitrogen compounds such as NO2.

4. Spring brings an increase of ultraviolet light to the lower antarctic stratosphere, providing the energy needed needed for the rapid catalytic break-down of ozone by ClO and its dimer ClOOCl. Another mechanism involving Bromine adds another 33% to the depletion total. Over 50% of the stratospheric ozone is destroyed by these two mechanisms, most of the damage occurring in the lower stratosphere.

5. Towards the end of spring (mid-December) the warming temperatures cause the vortex to break up; ozone-rich air from the surrounding area comes flooding in and masses of ozone-depleted air go wandering off, temporarily lowering the ozone in areas of South America and New Zealand by up to 10%. 

DIZ IS DE ENTIRE SCENARIO....

GUYS DIZ MY FIRST POST....

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