Presented by:Hira Sumbal (15)

Aarfa Tufail (11) Nawal Khalid (25) shereen Rehman (31)

Ozone hole

“It is a naturally occurring layer in stratosphere extended from 15 to 30 km above the Earth protecting from ultraviolet radiation which damage the life on earth.”

Ozone Layer

“The thinning of ozone layer in stratosphere over Antarctic due to ozone layer depletion caused by several anthropogenic processes is called ozone hole.

Ozone hole

This phenomena was first discovered by Joe Farman, a British physicist in in May 1985.

the study of Farman and his colleagues reveals that the levels of ozone over Antarctica has fallen 40% between 1975 and 1984

They demonstrated that the hole was not a natural occurrence but the result of reactions triggered by chlorofluorocarbons (CFCs) in the stratosphere.

Discovery of Ozone hole

CFCs contains chlorine, fluorine and carbon atoms.

They are emitted from industries, refrigerator, air conditioner, aerosols etc.

1 million tones of CFCs are released annually.

Major constituent in ozone depletion

Once free, the CFCs float up to the ozone layer and remain there for 75 to 100 years, destroying the ozone molecules.

CFCs

Factors:

(1)Polar vertex.(2)Polar Stratospheric clouds.(3)Effect of active chlorine.

These are factors which triggers the formation of ozone hole over Antarctica.

Why ozone hole is only formed over Antarctica?

Polar vertex is one of the major reason of ozone hole. During winter season in Antarctica, strong wind blows in continent called polar vertex-this isolates the air over Antarctic from rest of the world.

Why ozone hole is only formed over Antarctica?

For rapid ozone destruction, stratospheric clouds have to form in ozone layer. These clouds are only formed in Antarctic where winter is cold enough for these clouds to form widely through the centre of the ozone layer.

Why ozone hole is only formed over Antarctica?

Polar vertex-cold circulating wind, formation of stratospheric clouds and 6 months of darkness over Antarctic, all provide ideal conditions for chlorine to break from the CFCs. This active chlorine, bromine and other ozone depleting substances destroys the ozone 1% per day during spring causing the formation of ozone hole.

Why ozone hole is only formed over Antarctica?

One chlorine atom is capable of fragmentation of 1,000 to 10,000 to 1 million of ozone molecules.

Why ozone hole is only formed over Antarctica?

The concentration of ozone in the atmosphere is measured in "Dobson Units", the average concentration of ozone in the atmosphere is about 300 Dobson Units. The ozone hole is considered to be wherever the concentration drops below 220 Dobson Units.

Ozone layer depletion over Antarctic during summer season

Why ozone hole is only formed over Antarctica?

Recent research has shown that chlorine monoxide can react with stratospheric nitrogen dioxide to form chlorine nitrate (ClONO2). When chlorine nitrate is in the presence of trace amounts of HCl or absorbed on ice particles in stratospheric clouds, additional redox reactions can occur in which chlorine nitrate produce Cl2 or HOCl. HCl(g) + ClONO2(g) Cl2(g) + HNO3

H2O(g) + ClONO2(g) HOCl(g) + HNO3(g)

Why ozone hole is only formed over Antarctica?

Both Cl2 and HOCl undergo cleavage reactions by even weak sunlight to give reactive chlorine atoms. When the sun finally rises after the long polar night, relatively large amounts of Cl2 and HOCl are present and rapidly generate high levels of chlorine atoms.

HCl(g) + ClONO2(g) Cl2(g) + HNO3

H2O(g) + ClONO2(g) HOCl(g) + HNO3(g)

Why ozone hole is only formed over Antarctica?

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