Depletion of Ozone Layer
Ozone layer in stratosphere acts as one of the earth's most important life support systems.
The depletion of ozone layer in the stratosphere could cause an increased human cataract, skin cancer, depletion of plants and crops.
Due to human activity two types of compounds have been found responsible for depleting the ozone layer.
- Nitric Oxide(No)
This is produced in large amounts in the exhaust fumes from the engines of supersonic transport planes. NO reacts with ozone to form NO2, resulting in the decrease of O3.
- Chlorofluoro Carbons
Compounds containing chlorine, fluorine and carbon are commonly known as freons. These are introduced into the atmosphere from aerosol sprays, in which they function as propellants and from refrigerating equipments in which they act as coolants.
Mechanism of Depletion of Ozone Layer
In the first step they undergo photochemical decomposition, to give chlorine atoms.
The reactive chlorine atom free radical then destroy the ozone layer, through the following sequence of reactions, which are repeated over and again because chlorine atoms are re-generated in the second reactions.
These chlorine atoms are free to react with O3. Thus Ozone layer is deplated.The depletion of ozone layer, leading to ozone hole in the stratosphere is caused by the accumulation of Chlorofluro compounds and their decomposition in the stratosphere.
In generalized mechanism of Ozone depletion, the following reactions occur.
The free radical Chlorine (Cl ) reacts with ozone to form ClO. (g) and O2(g)
The Cl free radicals again attack O3 molecules and thus it is a chain mechanism of ozone depletion.
This occurs in stratosphere, but ozone depletion does not occur to such an extent of forming Ozone hole, except over Antarctica. This is because in other parts of stratosphere, except over Antarctica, the ClO and Cl radicals are scavenged (swept) by NO2 and CH4 so that chain mechanism of ozone depletion is not initiated.
In Antarctica, the conditions are different. During winter special type of clouds, called Polar Stratospheric clouds (PSC) are formed.
Type I clouds, which form at -770C, contain nitric acid trihydrate (HNO3 3H2O) and type - II clouds at -850C, contain ice. These clouds play important role in the formation of ozone hole as follows:
Chlorine nitrate in the reaction , (iv) reacts with water as well as with HCl from reaction (V) to form HOCl and Cl2 gas
The molecular Cl2 and HOCl (reactions VI and VII). Undergo photochemical reactions even under mild conditions.
This reactive Cl free radicals, thus formed, help in chain mechanism reaction, leading to Ozone depletion and hence Ozone hole (refer ii and iii) over Antarctica is formed.