Ozone layer
Ozone is produced in the upper atmosphere through absorption of a photon, hv of ultra violet light by an
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.
UV
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.
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