Ozone is produced in the upper atmosphere through absorption of a photon, hv of ultra violet light by an O2 molecule.
O2(g) K# -- > 2O(g)
O(g) + O2(g) -- > O3(g)
The ozone molecule formed has an excess of energy and dissociates back to O2 and O and it reacts with another molecule (M) such as CO2, N2 or O2, which causes the excess energy thus stabilizes the ozone molecule
O*3(g) + M(g) -- > O3(g)+ M*(g)
Factors affecting ozone layer
The ozone in the upper atmosphere is important in shielding us from the intense ultraviolet radiation coming from the sun. The so-called ozone shield is a shell about 30 km altitude which contains enough ozone to absorb short wavelength UV radiation (less than 300 nm). Hence ozone is considered to be 'earth's protective umbrella'. The absorption causes dissociation of O3 to reform O2.
O3(g) K# -- > O2(g) + O(g)
2O(g) -- > O·2(g)
O·2(g) + M(g) -- > O2(g)+ M·(g)
Existence of ozone shield owes to the life on the earth, since living tissues are very sensitive to wavelengths of ultraviolet absorbed by ozone. In recent years, the shield is damaged mainly by supersonic aircraft and chlorofluorocarbon products in the jet exhaust reduce ozone, and decreases its concentration in the shield.
Chlorofluorocarbons react with O3 and causes a hole in the ozone layer. CFC's are used as refrigerants and as propellants in some "aerosol sprays". The lifetime of CFCs are so long that in another decades, the extent of ozone depletion in the upper atmosphere will be tremendous. It is reported that the holes caused in the ozone layer over the Antarctic and Arctic ocean are due to the use of CFCs in aerosols and refrigerators. It is feared that this will allow an excessive amount of UV light to reach the earth which will cause skin cancer (melanoma) in human.
CFC -- > Cl(g)
Cl(g) + O3(g) -- > ClO(g) + O2(g)
ClO + O(g) -- > Cl(g) + O2(g)
O3(g) + O(g) -- > 2O2(g)
It is also seen that the oxides of nitrogen (from car exhausts) and the halogen can damage the ozone layer. Therefore the protecting shield of the earth must be protected by taking immediate steps over the control of pollution.