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Sunday, September 12, 2010

Ozone Layer: Its depletion, consequences, and protection

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Ozone is an unstable blue gas having pungent odour. Chemically, it is an allotrope of Oxygen which is an element in the gaseous form. It has three oxygen atoms in its single molecule and in the language of Chemistry; its molecular formula is 03. It is used as a powerful oxidant, bleach, and water purifier. It is also used to treat industrial wastes.

Where is ozone found?
If found in the troposphere; ozone acts as a powerful pollutant. But, when found in the stratosphere, it acts like a friend of the earth because it shields most of the ultra violet radiations and does not allow them to pass on towards the same. In stratosphere, it is found in the form of a dense layer called as the Ozone Layer or the Ozone Belt. Thus, the Ozone Belt in the stratosphere acts like a Protective Umbrella of the earth. Let us see, how this gas is formed in the atmosphere.

How is ozone formed?
Ozone is formed in the stratosphere when oxygen molecules Photo dissociate after absorbing an UV Photon of shorter wavelength(less than 240 nm) to produce two oxygen atoms. Ozone is mainly produced from oxygen containing molecules such as Sulphur dioxide, Nitrogen Oxides, etc. also when these molecules are exposed to ultraviolet radiations. In Chemistry, a molecule is the particle of any substance that can remain in a free state. But, what is an atom? Well, an atom is the smallest particle of a substance that can not usually remain in a free condition. Two or more atoms combine to form a molecule. Through the foregoing lines, we came across another term, allotrope. One of the two or more different forms of molecules of an element is called as an allotrope.
A large number of ozone molecules assemble around the earth to form the Ozone Layer which extends from 13 to 48 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.

Ozone depleting substances
Chlorofluorocarbons (CFCs or Freons), 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).
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 molecule absorbs UV light between 310 and 200 nm. The ozone molecule absorbs oxygen atom to form two molecules of Oxygen, and the Ozone cycle continues.
• Ozone is destroyed by a number of free radicals catalysts –like Hydroxyl radical, Nitric oxide radical, and Bromine through natural and anthropogenic sources.

Effects of the Depletion of Ozone Layer

I. General Effects
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. The Ultraviolet radiation is one of the most harmful radiations contained in the sunlight. Ozone layer in the stratosphere absorbs these radiations and does not allow it to reach to the earth.

The depletion of Ozone layer may lead to UV exposures that may cause a number of biological consequences like Skin Cancer, damages to vegetation, and even the reduction of the population of planktons (in the oceanic Photic zone).

Some of the remarkable effects of the UV radiations or the effects of depletion of the Ozone Layer are mentioned below.

(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.

II.Specific Effects

The specific effects of depletion of Ozone Layer have been observed on Human Society, Agriculture, Plants and Animals etc. These effects have been summarized as below-

A. Effects of Ozone Depletion on Human Society

(i).The flux of ultra violet radiation in the biosphere is increased due to ozone depletion. It has seriously harmful effects on human societies like formation of patches on skin and weakening of the human immune system.

(ii). It may cause three types of skin cancer like basal cell carcinoma, squamous cell carcinoma and melanoma. A 10 per cent decrease in stratospheric ozone has been reported to cause 20 to 30 per cent increase in cancer in human society. Each year, about 7000 people die of such diseases each year in USA. About 10 percent increase in skin cancer has been reported in Australia and New Zealand.

(iii).Exposure to UV radiations damages skin of the sun-bathing people by damaging melanocyte-cells or by causing sun-burns due to faster flow of blood in the capillaries of exposed areas.

(iv).Exposure to UV radiations due to ozone depletion may cause leukemia and breast cancer.

(iv).Exposure of UV radiation to human eye damages cornea and lens leading to Photo keratitis, cataract and even blindness.

(v).The Ambient Ozone Exposure may cause Emphysema, bronchitis, asthma and even obstruction of lungs in human beings.

(vi).Exposure to radiations due to ozone depletion has been reported to cause DNA breakage, inhibition and alteration of DNA replication and premature ageing in human beings.

B. Effect of Ozone Depletion on Agriculture
(i). Radiations reaching to the earth due to ozone depletion cause severe damage to plants including crops. As per reports, ultra violet radiations reaching to the earth cause losses up to 50 per cent in European countries.

(ii).The radiation reaching to the earth due to the depletion of the ozone layer cause visible damages in plants. They adversely affect the rate of photosynthesis that finally results into decrease in the agricultural production.

(iv).The UV radiation enhances the rate of evaporation through stomata and decreases the moisture content of the soil. This condition adversely affects the growth and development of crop plants and reduces the crop yield.

(v). The ozone reduction adversely affects the weather pattern which in turn affects the crop production by encouraging plant injuries and disease development.

(vi). The UV radiation reaching to the earth surface alters the global balance between radiation and energy. This condition of imbalance causes seasonal variations that further reduce the crop production.

(vii). A number of economically important plant species such as rice, depend on cyanobacteria residing in their roots for the retention of nitrogen. These bacteria are sensitive to UV light and they are hence, are killed instantly.

C. Effects of Ozone Depletion on other Plants and Animals
(i).The ozone layer depletion causes climatic alterations that cause physiological changes in plants and animals. The change in the energy balance and radiation may affect the survival and stability of living organisms.

(ii).The depletion of ozone layer may cause changes in thermal conditions of the biosphere. It may affect type, density and stability of vegetation which in turn may affect different bio-geo-chemical cycles operating in nature. Interruption in these cycles damages important process of ecosystem leading to dangerous conditions for plants and animals.

(iii).The depletion of ozone layer causes death of plankton- populations in fresh as well as marine waters .This condition seriously affects the transfer of materials in ecosystems. The recent researches gave analyzed a widespread extinction of planktons 2 million years ago that coincided with the nearby supernova. Planktons are particularly susceptible to effects of UV light and are vitally important to the marine food webs.

The Ozone Hole
The hole in the context of ozone depletion relates to thinning of the ozone layer in a certain area. Here, the word hole is considered as a hole in the ground which in the context of ozone layer is thinning of ozone in a certain area up to certain depth as measured by scientists. In fact, ozone hole is an area where the ozone concentration drops to an average of about 100 Dobson Units. The word ‘Dobson’ has been taken from the name of the famous scientist and climatologist G. M. B. Dobson, who observed the ozone hole for the first time in 1956, over Halley Bay.

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.

The ozone hole over Antarctica was first discovered by Farman, Gardiner and Shanklin in 1985. They jointly declared their findings through a paper published in the May issue of Nature (an important International Journal) in 1985. The entire scientific community was shocked to know their findings.

On the basis of observations made through a network of ground based Dobson Spectrophotometer, an International Panel of scientists confirmed that the Ozone Layer was being depleted at all latitudes out side the tropics. Out of a big group of scientists across the world, Crutzen, Molina, and Rowland were awarded the Nobel Prize in Chemistry for their work on Stratospheric Ozone, in 1995.The scientific assessment of ozone depletion is going on across the world since 1981, under the sponsorship of the United Nations Environment Programme (UNEP), and the most recent measurement was done during the year 2006. Here are the comparative pictures showing the Ozone Holes over Antarctica during the spring seasons of two different years.

Why is the Ozone Hole over Antarctica, usually formed during spring months?

A circulation pattern of gases traps the ozone over the South Pole for several months but not during winter. This circulation pattern is called as Antarctic Polar Vortex. Within this vortex, the substantial ozone loss was detected for the first time during 1980. During extreme cold conditions, the polar winters are dark and continue up to three months without solar radiations. This leads to the decrease in temperature. The polar vortex traps air and contributes in further falling temperature which goes down up to -80 0c. The low temperature forms cloud particles that contain nitric acid and ice. These clouds provide surfaces for chemical reactions that lead to ozone depletion. During the Antarctic winters and springs, reactions that take place on the surface of the Polar Stratospheric Clouds (PSCs) convert pollutants into free radicals such as Cl and ClO.

These clouds can also remove NO2 from the atmosphere by converting it to nitric acid. It prevents the newly formed ClO from being converted back into ClONO2. The role of sunlight is the fundamental reason why the Antarctic ozone depletion is greatest during spring. The Antarctic ozone depletion is caused during September to early December. Over 50 per cent of the lower stratospheric ozone is destroyed during the period of the Antarctic Spring.

Prevention and Control of Depletion of the Ozone Layer
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 tones 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 for Controlling the Depletion of the Ozone Layer
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. The General Assembly of the United Nations voted to designate September 16 as the World Ozone Day, to mark the signing of the Montreal Protocol, the 16th September, 1987.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.

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