The depletion of ozone layer is one of the Global Issues of environment related to atmosphere and air pollution. But what is ozone? How is it formed? What are its functions? …etc., are many questions that need to be answered here. Let us take up these questions one by one.
What is Ozone? How is it formed?
Ozone is one of the three allotropes of oxygen, an element in gaseous form. It is triatomic and less stable than oxygen. Its chemical formula is O3.
Ozone in the stratosphere is very important to life. It is formed by the action of the ultraviolet light from the sun on molecules of oxygen. However, it is mainly produced from oxygen containing molecules such as SO2, NO2, aldehyde etc. also when these molecules are exposed to ultraviolet radiations. Here is an example of the chemical reaction that takes place in the formation of ozone from NO2.
A large number of ozone molecules assemble around the earth to form the Ozone Layer which extends from 12 to 45 km above the earth surface. On an average it is about 230 Dobson units (DU) in thickness. DU is the unit which measures thickness of the ozone layer. It equals to 0.01 mm.
Importance of Ozone Layer
Ozone absorbs ultraviolet radiations so that much of it is never allowed to reach to the earth surface. The protective umbrella of ozone layer in the stratosphere protects the earth from harmful ultraviolet radiations. Ozone plays an important role in the biology and climatology on the earth’s environment. It filters out all the radiations that remain below 3000Å. Radiations below this wavelength are biologically harmful. Hence any depletion of ozone layer is sure to exert catastrophic impacts on life in the biosphere.
Ultraviolet Radiations and their Harmful Impacts
There are three types of ultraviolet radiations in the sunlight- ultraviolet-A, ultraviolet-B and ultraviolet-C radiations. The UV-A is a low energy radiation with wavelengths 400 to 315 nm (1nm= 10Å). It is not harmful to life. UV- B radiations that comprise 1- 5 percent of the total radiation is a short-wave radiation (315 to 280 nm) with high energy. It is harmful to life. The UV- C radiation is a radiation of shortest wavelength (280 to 100 nm) with highest quantum of energy. It has great power to damage life but the ozone layer does not allow it at all to pass through and to reach to the earth.
Harmful Impacts of Ultraviolet Radiations –
(1) UV radiation causes sun- eye- diseases (cataract), skin diseases, skin cancer and damage to immune system in our body.
(2) It damages plants and causes reduction in crop productivity.
(3) It damages embryos of fish, shrimps, crabs and amphibians. The population of salamanders is reducing due to UV-radiations reaching to the earth.
(4) UV- radiations damage fabrics, pipes, paints, and other non-living materials on this earth.
(5) It contributes in the Global Warming. If the ozone depletion continues, the temperature around the world may rise even up to 5.5 Celsius degrees.
Substances that cause depletion of Ozone Layer
Chlorofluorocarbons, methane, nitrous oxides (N2O), carbon tetrachloride (CCl4), methyl bromide (a soil fumigant and insecticide), aircraft emissions, n- propyl bromide and Halon- 1202 are major agents that cause depletion of ozone layer. Hence, these are called as Ozone Depleting Substances (ODS).
Chlorofluorocarbons are a group of aliphatic organic compounds. These are a family of synthetic chemicals that are mostly the compounds of chlorine, fluorine and carbon. These are stable, non-flammable, non-corrosive chemicals with a peculiar trade name Freon. This trade name has been registered by the E.I. du Pont de Nemours & Company. Being relatively non- toxic chemicals, these are easy and inexpensive to produce. These were first developed in 1930s but found widespread use only in the years following World War II.
During 1970s CFCs were linked to destruction of ozone layer due to which its manufacture has been banned in most of the countries of the world. Some important members of CFCgroup are dichlorodifluoromethane (Freon-12), trichlorofluoromethane (Freon- 11), chlorodifluoromethane (Freon- 22), dichlorotetrafluoroethane (Freon- 114) and trichlorotrifluoroethane (Freon- 113). On earth these chemicals are used extensively as aerosol- spray propellants, refrigerants, solvents and foam blowing agents.
How is the Ozone Layer Depleted?
Chlorofluorocarbons or Freons get accumulated in greater amounts at high altitudes and gradually reach to the stratosphere. Under the influence of intense short wave ultraviolet radiations, they release chlorine atoms. A single chlorine atom can react with more than, 100,000 molecules of ozone and can convert them into oxygen. Other ozone depleting substances like methane, nitrous oxide, methyl bromide etc. too, pass through a series of reactions under the influence of UV-radiations of sunlight and catalysts found in the air and help in the depletion of ozone layer.
Ozone Hole: The hole in the context of ozone depletion relates to thinning of the ozone layer in a certain area. The satellite measurements done in September 2000 revealed that the thinning of ozone layer in Antarctic had reached a record 28.3 million sq. km which was about one million sq. km greater than the record of 1998. Thinning of ozone in such a big area is rightly termed as ozone hole. The ozone hole in the Northern Latitudes has also been recorded. The ozone hole over Antarctica may expose not only the Antarctica but also a large area of the pacific and Atlantic oceans and South America as well.
Prevention and Control of Ozone Depletion
Banning the production and use of ozone depleting substances is one important way of preventing further depletion of the ozone layer in the stratosphere. On the other hand, alternatives to these chemical compounds should also be searched out so as to replace these chemicals.
Scientists of the University of California, U.S.A. devised a possible way of plugging the ozone hole by injecting alkanes or propanes into the atmosphere of Antarctica. The alkanes have the affinity of reacting with ozone destroying chlorine atoms. According to the scientists, about 50,000 tonnes of alkane or propane would have to be blown to check the ozone loss. These chemicals could be released from an altitude of about 15 km by a group of hundreds of large aircrafts.
Global Efforts
Since ozone depletion is a Global Environmental Problem, it requires strong global efforts and co- operations for its solution. The International Community is taking up strong efforts as a result of which global consumption of ozone depleting substances has decreased markedly
Following the UNEP’s Governing Council’s meeting to co- ordinate activities on protecting ozone layer in 1975, United States, Canada, Norway and Sweden banned the use of CFCs. The production capacity of the European Union (E U) was frozen allowing limited uses of aerosols. In March 1985, 28 countries of the world agreed on Vienna Convention for the protection of the ozone layer. In September 1987, different countries of the world adopted Montreal Protocol on substances that deplete ozone layer. By December 2001, 182 countries ratified the Vienna Convention and 181 the Montreal Protocol. By 2000, 96 chemicals were subject to control under the Montreal Protocol.
