What is Ozone layer?• It is a natural sunscreen in the stratosphere• It filters out harmful ultraviolet radiations from the sunlight & thereby protects various life forms on the earth.• It is a form of oxygen; ozone is continously being created by the absorption of short wave length UV radiations in the stratosphere.• Ozone is a poisonous gas made up of molecules consisting of three oxygen atoms.• The ozone layer effectively blocks almost all solar radiation of wavelengths less than 290 nanometres from reaching Earth’s surface, including certain types of ultraviolet (UV) and other forms of radiation that could injure or kill most living things.• The amount of ozone in the stratosphere varies naturally throughout the year as a result of chemical processes that create and destroy ozone molecules and as a result of winds and other transport processes that move ozone molecules around the planet.
• While stratospheric ozone which protects us from the sun is good, there is also ozone produced near the ground from sunlight interacting with atmospheric pollution in cities that is bad for human health.• It causes breathing problems for some people, and usually occurs in the summertime when the pollution over a city builds up during stagnant air conditions associated with high pressure areas.• The ozone layer above Antarctica will not return to its early 1980s state until about 2060, according to Nasa The ozone hole reached its maximum size on September 22, covering 8.2 million square miles (21.2 million square kilometers), or the area of the United States, Canada and Mexico combined. The average size of the 2012 ozone hole was 6.9 million square miles (17.9 million square kilometers). The September 6, 2000 ozone hole was the largest on record at 11.5 million square miles (29.9 million square kilometers).
The Chapman Cycle• The stratosphere is in a constant cycle with oxygen molecules and their interaction with ultraviolet rays.• This process is deemed a cycle because of its constant conversion between different molecules of oxygen. 1. An oxygen molecules is photolyzed by solar radiation, creating two oxygen radicals: hv + O2 → 2O. 2. Oxygen radicals then react with molecular oxygen to produce ozone: O2 + O. → O3 3. Ozone then reacts with an additional oxygen radical to form molecular oxygen: O3 + O. → 2O2 4.Ozone can also be recycled into molecular oxygen by reacting with a photon: O3 + hv → O + O.
Thinning of Ozone layer• The Antartic ozone hole was discovered by Dr Joe C. Farman & his colleagues.• Steep decline has been observed since mid 1970s with a record low concentration of 90 DU in early October of 1993.• The main reason for Ozone depletion is due to Chlorofluorocarbons (CFC).• CFCs are used as coolants in refrigerators & air conditioners, as propellants, cleaning solvents, sterilant, etc.• CFCs released in the troposphere reach the stratosphere & remain there for 65-110 years destroying O₃ molecules.• In 1985, scientists discovered that 50%(98% in some areas) of upper stratospheric ozone over Antartica was destroyed during the antartica’s spring & early summer.• Nitrous oxide emitted by supersonic aircrafts, during combustion of fossil fuel and use of nitrogen fertilizers & CFCs breaks ozone molecules.
Contd…. Chemistry of Ozone Depletion• CFC molecules are made up of chlorine, fluorine and carbon atoms and are extremely stable.• This extreme stability allows CFCs to slowly make there way into the stratosphere.• This prolonged life in the atmosphere allows them to reach great altitudes when photons are more energetic.• The following reaction displays how Cl atoms have an ozone destroying cycle: Cl + O3 → ClO + O2 ClO + O → Cl + O2 ________________ O3 + O → 2O2 : Overall reaction
Effects of Ozone depletion• Ozone depletion in thesratosphere will result in more UVradiation reaching the earthespecially UV-B(290-320nm)affecting DNA & photosyntheticreactions causing skin cancer.• Easy absorption of UV rays bythe lens & cornea of eye willresult in increase in incidents ofcataract.• Melanin producing cells ofepidermis will be destroyed by UV–rays resulting in immuno-suppression.• Fair people will be at a greaterrisk of UV exposure.• Phytoplanktons are sensitive toUV exposure.• Yield of vital crops like corn,rice, soyabean, cotton, bean, pea,sorghum & wheat will decrease.• Degradation of paints, plastics 7other polymers will result ineconomic loss.
Initiatives against the depletion of the ozone layerInter- governmental negotiations for aninternational agreement to phase out ozone-depleting substances started in 1981.In may 1985, British scientists published theirdiscovery of severe ozane depletion over theAntarctic.The discovery of the ozone ‘hole’ shocked theworld.It is regarded as one of the major environmentaldisasters of 20th Century.
Cont..The governments now recognized the need forstronger measures to reduce the production andconsumption of a number of CFCs and severalhalons.As a result, the Montreal protocol on sustancesthat Deplete the ozane layer was adopted inseptember 1987.In 2009 the Montreal Protocol became the firstUN treaty to achieve universal ratification(that is,by all the member countries), demonstrating theworld’s commitment to ozone protection, andmore broadly, to global environmentconservation.
Results So far…?As of the end of 2009, the parties to the protocolhad phased out the consumption of 98% of allthe chemicals controlled by the protocol.There was strong evidence that global ODSconcentrations were decreasing and that theglobal stratosphere had already experienced itshighest levels of ozone depletion from man-madehalocarbons.Outside the polar regions the ozone layer hasshown some initial signs of recovery.Arctic ozone levels are expected to return to pre1980 levels before 2050, while Antarctic ozone isexpected to do so in 2060-2075.
Cont..It is estimated that without the protocol, by theyear 2050 ozone depletion would have risen to atleast 50% in the northern hemisphere’s midlatitudes and 70% in the southern mid latitudes,about 10 times worse than current levels.The implications of this would have beenhorrendous, and have been estimated to include:20 million more cases of cancer, and 130 millionmore cases of eye cataracts, if there had been noMontreal Protocol.Ozone depleting substances are also potentgreenhouse gases, the protocol has deliveredsubstantial climate benefits too.
Montreal protocol The efforts of the world community to protect the ozone layer are a fascinating example of how humanity can act as one to face a common danger.• Ensure that improved scientific understanding can be incorporated quickly into decisions about the provisions of the treaty.• Promote universal participation by recognizing the ‘common but differentiated responsibility’ of developing and developed countries and ensuring the necessary financial and technogical support to developing countries.