Ozone layer has been a protective thin band in the stratosphere (above troposphere) which tends to shield the earth from the harmful ultra-violet rays coming from the sun.
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In 1970, it was found that this ozone layer was attacked by chlorofluorocarbons (CFCs) which are released into atmosphere by refrigeration units, air conditioning systems, aerosol sprays and cleaning solvents.
The process of ozone destruction happens as follows: Chlorofluorocarbons release chlorine which breaks ozone into oxygen.
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As chlorine is not affected by its interaction with ozone, chlorine continues to destroy ozone over a period of time. Ozone level has been declined by about two per cent worldwide, with larger declines over parts of North and South America, Europe, Australia and New Zealand. A study undertaken in 1992 indicates that ozone layer is thinning even more than previously thought.
British scientists based in Antarctica found in late 1980s that in every southern spring (September-October), 50-95 percent of stratospheric ozone is destroyed at a height of 15-24 km above Antarctica, creating pockets which have been described as the ozone hole. Countries like Australia, New Zealand, South Africa and parts of South America, where the ozone layer is depleted, are at a greater risk than the rest of the world.
As ozone filters out harmful UV radiation, less ozone means higher UV levels at the surface.
The more is the depletion, the larger is the increase of incoming Ultraviolet rays. Ultra-violet has been linked to skin cancer, cataracts, damage to materials like plastics, and harmful to certain crops and marine organisms. Although some ultra-violet reaches the surface even without ozone depletion, its harmful effects will increase as a result of this problem.
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Countries around the world including United States have seen the threats created by ozone depletion and agreed to a treat called the Montreal Protocol (1981). This protocol will help humans to stop making and using ozone-eating chemicals.
In 1963, scientists have reported a large hole in the ozone layer over Antarctica where ozone level dropped by 30 per cent.
The ozone hole covers an area as large as USA. Chlorofluorocarbons were considered to be prime suspect for causing ozone depletion. It was also established that one molecule of CFC is capable of destroying 1 lakh ozone molecules. Subsequently, a similar hole was discovered over the thickly populated northern hemisphere.
A study by NASA (National Aeronautics and space Administration, USA) scientist revealed that the area of ozone over northern hemisphere decreased by 3 per cent between 1969 and 1986.
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The overall reduction in the 03 layer is now estimated to be about 8 per cent. Under the auspices of the United Nations Environment Pogramme (UNEP), 334 countries have signed an agreement at Montreal, Canada in 1986 called “Montreal Protocol”. The scientists have agreed to reduce the production and use of CFCs up to 50 per cent by the year 1999. In a second meeting at Helsinki held in 1989, it was agreed upon a total phase out of CFCs by 2000 A.D.
Ozone in Troposphere
Small traces of ozone are present in troposphere too. This ozone behaves as a greenhouse gas and contributes to global warming. Ozone in the atmosphere plays a dual role.
Its presence in the stratosphere protects life on earth by arresting ultra-violet radiation and its presence in troposphere affects life on earth as a greenhouse gases.