About 90% of ozone of atmosphere resides in stratosphere (16 to 40 km) layer of atmosphere as ozone layer. It protects life on this planet from harmful U.V. radiations. In absence of this layer the U.V. radiations would have caused disastrous effect on life.
Ozone is formed by multi-step photochemical reaction:
O2 + UV (242 nm) 
O+O
O2 (g) + O O3
Thickness of ozone layer is measured in Dobson units (DU). It was observed iin mid 1970s that some human activities are destroying thickness of ozone layer. Ozone depletion is caused by following types of compounds.
(i) Chlorofluoro (Freon's) (CCI2F2)
(ii) Nitric oxide (NO)
(iii) Reactive hydroxyl radicals (OH)
Mechanism of ozone depletion:
(i) Chlorofluorocarbons:
CFC's are stable at earth surface but at high altitude of 15-30 km they decompose to form chlorine atoms in presence of U.V. radiations.
CCL2 F2 + hv CI0 + CCIF2
CL0 + O3 CIO0 + O2
O3 + hv O2 + O0
The Oxygen Atom Reacts With CIO to produce more CI0 leading to further loss of O3.
O + CIO0 CI0 + O2
CI0 + O3 CIO0 +O2
O +O3 2O2
CFC's are mainly used propellants in aerosol sprays, as coolant in refrigerating equipments.
(ii) Nitric oxide molecule:
The emissions of nitrogen oxide by high flying supersonic aircrafts and nuclear explosion have been found to be responsible for ozone depletion.
NO + CIO0 CI0 + NO2
CI0 + O3 CIO0 + O2
NO + O3 NO2 + O2
The CI0 is formed in chain reaction leading to ozone depletion actually 105 molecules of ozone are converted into oxygen by a single chlorine atom.
(iii) Hydroxyl radicals:
Hydroxyl ions produced by photo dissociation of water molecules in the atmosphere, which reacts with ozone molecules to produce HO2, which reacts with another O3 molecules to field hydroxyl ions again.
OH + O3 HO2 + O2
HO2 + O3 OH + 2O2
OH ion is again regenerated which leads to chain reaction.
Main causes of ozone depletion:
(i) Excessive use of chlorofluorocarbons (CFC's)
(ii) Excessive use of nitrogenous fertilizers.
(iii) Combustion of fossil fuels and organic matter.
(iv) Nuclear explosions.
(v) Supersonic aircrafts, rockets and space shuttles.
Effect of ozone depletion:
1. Exposures to U.V. radiations damages the cornea and lens and lens of eye causing blindness.
2. The excess U.V. radiation cause sun burn, skin cancer and various kinds of damages to animals and plants.
3. The genetic material DNA has capacity to absorb U.V. radiations which may cause DNA breakage inhibition and alteration of DNA's replication.
4. U.V. radiations causes leukemia and breast cancer.
5. It may cause lung cancer, lungs injury, dizziness, visual impairment.
6. Ozone depletion has strong effect on climate change indirectly.
7. Many micro-phytoplankton would die because of their exposure to U.V. radiation.
Measures for protecting ozone depletion:
In 1987 a global agreement was assigned by 30 countries in montreal for the reduction in production and consumption of ozone depleting substances.
Main objective:
(i) Phase out production and consumption of CFC's by 2010.
(ii) Phase out production and consumption of halogen as soon as possible.
(iii) Commitment to develop eco-friendly chemicals.
(iv) Scientific information, training and techniques must be given to developing of countries by developed countries.
Substitute for chlorofluoro carbons:
(i) As CFC's are hazardous ozone depleting substances so there was a need to find a safe alternative for CFC's.
(ii) Hydro- chloro- fluoro carbon (HCF'S) and HFC'S (hydro fluoro carbon) have proved to be a good substitute for CFC'S because of their low ODP (potential to release CI atmos and Their Life time in the atmosphere).