Answer:
- In the ozone layer, ozone O3 is a natural toxic gas formed by 3 atoms of oxygen. It has the maximum concentration in the Stratosphere at an altitude of 16 to 25 km from the Earth's surface, which is also called the 'Ozonosphere'.
- It protects life on Earth like a natural 'sunscreen' by absorbing harmful ultraviolet rays (UV-B) of the Sun. Its thickness is measured in Dobson Units (DU). For measurement, the Dobson spectrophotometer and TOMS mounted on NASA's Nimbus-7 satellite are used. A condition where the thickness of the ozone layer in the stratosphere falls below 200 DU is called an ozone hole.
Primary causes of ozone depletion:
- Man-made Ozone Depleting Substances (ODS) release active chlorine and bromine radicals upon reaching the stratosphere, which deplete ozone:
- Chlorofluorocarbons (CFCs): Used in refrigerators, AC, aerosol sprays, foaming agents, and as a solvent for cleaning electronic equipment.
Reaction:
Sunlight
CF2Cl2 → CF2Cl + Cl*
Cl* + O3 →ClO* + O2 and
ClO* + O → Cl* +O2 - Effect: One active chlorine atom can destroy approximately 1 lakh ozone molecules.
- Halons: Bromine-containing compounds used in fire extinguishing equipment; one active bromine atom is capable of destroying approximately 4 lakh ozone molecules.
- Other chemical compounds: Carbon tetrachloride, chloroform, methyl bromide, nitrous oxide, and sulphate.
- Polar Stratospheric Clouds (PSC): Clouds of ice and acid particles HNO3, H2SO4 formed during polar winter provide a favorable surface for activating chlorine.
Reaction:
+ -
H + Cl → HCl
-
ClO + NO2 → ClNO3
Key international initiatives for protection:
Vienna Convention (1985):
- The first non-binding international framework prepared for the protection of the ozone layer, under which consensus was reached on research and data sharing.
Montreal Protocol (1987):
- A legally binding and most successful environmental agreement introduced for the phased reduction in the production and consumption of ODS.
- Signed on 16 September 1987 (in commemoration of which 'World Ozone Day' is celebrated every year on 16 September) and came into force from 1 January 1989. India joined it in 1992.
Copenhagen Amendment (1992 - MoP 4):
- A deadline was set to completely eliminate the use of CFCs (by 2000 for developed countries and by 2010 for developing countries). India banned its use as early as 2008.
Kigali Agreement (2016 - MoP 28):
- A binding amendment to phase down HFCs (which do not harm ozone but have high global warming potential) by 80–85%. It came into effect from 1 January 2019.
- As a result of the successful implementation of the Montreal Protocol and the amendments under it, continuous recovery is being recorded in the ozone layer.