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Ozone depletion
An ozone layer located 20-25 km above the earth's surface absorbs harmful ultraviolet radiation from the sun, protecting life on earth. Ultraviolet rays decompose oxygen into atomic oxygen, which then combines with molecular oxygen to form ozone. However, chlorofluorocarbons (CFCs) released into the atmosphere destroy ozone molecules. One chlorine atom can destroy up to 100,000 ozone molecules over time. The Montreal Protocol was formed to phase out the production of CFCs and other ozone-depleting substances.
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Ozone depletion
- 2. A layer ofozone (o3) is present in the stratosphere 20-25 km above the surface of the earth . This layer filters out harmful ultraviolet radiation from the sunlight, thus protects various life form on the earth . It acts like a natural sunscreen for the earth .
- 3. O2 + hvO + O Ultraviolet rays decomposes oxygen in to atomic oxygen by photolytic decomposition . The atomic oxygen thus farmed rapidly reacts with molecular oxygen to form ozone . O + O2 + O3 + ( M is a third body necessary to carry away the energy released in the reaction )
- 4. Ozone thus formeddistributes itself in the stratosphere and absorb harmful ultraviolet radiation (200-320 nm ) and it continuously converted back to molecular oxygen . O3 + hv O2 + O
- 5. The destruction ismostly caused by chlorofluoro-carbon (CFC) CCl3F UV CCl2F + Cl CCl2F2 UV CClF2 + Cl Cl + O3 ClO + O2 ClO + O Cl + O2
- 6. In previous reactionCl Just acts as a catalyst and reappears . It is estimated that one Cl atom can destroy 1,00,000 ozone molecules in one or two years . F C ClCl Cl F C Cl Cl Cl UV radiation hits CFC molecule O OO O Cl O O O Cl ClCl O O O Chlorine atom breaks away Chlorine atom takes one oxygen atom to creat chlorine monoxide Oxygen atom hits Chlorine monoxide molecule Chlorine atom is free and repeat the depletion process Chlorine atom hits Ozone molecule Two oxygen atoms form an oxygen molecule
- 7. CFC is acombination of carbon , hydrogen , fluorine and chlorine . Use of CFC : (1) Used as coolant in refrigerator and air Conditioners . (2) Propellants . (3) Cleaning solvents . (4) Stuffing of Mattresses . CFC released in the troposphere reach the stratosphere and remains there for 65-110 years destroying O3 molecules .
- 8. Ozone – depletingpotential (ODP) is the ratio of the impact on ozone caused by a chemical compared to the impact of a similar mass of CFC-11 .The ODP of CFC-11 is 1.0 .
- 9. Substance Uses Ozone Depleting Potential Globalwarming Potential Chloroflurocarbons (CFCs) Refrigerants, Cleaning solvents, aerosol propellants 0.6 – 1.0 4,680 – 10,72 Halons Fire extinguishers , Explosion protection 3 - 10 1,620 – 7,030 Carbon tetrachloride(CCl4) Production of CFCs , Solvent/diluents 1.1 1380 Methyl chloroform (CHCl3) Industrial solvent for cleaning , inks , correction fluid 0.1 144 Methyl Bromide (CH3Br) Fumigant used to control – Borne pests & diseases in crops prior 0.6 5 Hydro Chloroflurocarbons (HCFCs) Transitional CFC replacements used as refrigerants , solvents 0.01 – 0.5 76 - 2,270 Hydro Flurocarbons (HFCs) CFC replacements used as refrigerants , aerosol propellants , Solvents 0 122 – 14,130
- 10. Ozone depletion inthe stratosphere will result in more UV radiation reaching the earth especially UV-B (290-320 nm) . The UV-B radiation affects DNA and the photosynthetic chemicals . Any change in DNA can result in cancer . Easy absorption of UV rays by the lens and cornea of eye will result in increase in incidents of cataract . Melanin producing cells of the apidermis will be destroyed by UV-rays resulting in immunesuppression . Phytoplanktons are sensitive to UV exposure to UV exposure . Ozone depletion will result in decrease in their population thereby affecting the population of zooplankton , fish .
- 11. If the modeland measurement of ozone depletion and its possible effects are correct , it is highly essential to stop producing any ozone depleting chemicals now . Even after stopping the production of ozone depletion chemicals , the models indicated that we still will have to wait to 50 – 100 years for the ozone layer to return to 1985 levels , and another 100 – 200 years for full recovery . So the best methord to reduce ozone depleting potential like HCFC , hydrocarbons , ammonia etc . Montreal protocol was formed in 1987 and the participants agreed to freeze production of CFC at 1986 levels and to reduce production by 50% by 1999 .