“The ozone layer” refers to the ozone within stratosphere, where over 90% of the earth’s ozone resides. Ozone is an irritating, corrosive, colourless gas with a smell something like burning electrical wiring. In fact, ozone is easily produced by any high-voltage electrical arc (spark plugs, Van de Graaff generators, Tesla coils, arc welders). Ozone is a tri-atomic form of oxygen, i.e., each molecule of ozone has three oxygen atoms and is produced when oxygen molecules (O2) are broken up by energetic electrons or high energy radiation. Thus, it is formed naturally in the upper levels of the Earth’s atmosphere by high-energy ultraviolet radiation from the Sun. As, the radiation breaks down oxygen molecules, releasing free atoms, some of which bond with other oxygen molecules to form ozone. About 90 per cent of all ozone formed in this way lies between 15 and 55 kilometres above the Earth’s surface – the part of the atmosphere called the stratosphere. Hence, this is known as the ‘ozone layer’. Even in the ozone layer, ozone is present in very small quantities; its maximum concentration, at a height of about 20-25 kilometres, is only ten parts per million.
Depletion of stratospheric ozone (O3), as commonly known as ‘the hole in the ozone layer’, is an issue of international concern. Most ozone is found in the stratosphere (upper part of the atmosphere), more than 10 to 16 kms from the surface of the Earth. The natural distribution of ozone around the Earth is not uniform, as seasonal winds and formation patterns contribute to lower concentrations at the equator and higher concentrations at the poles. Ozone in the stratosphere protects life on Earth as it limits penetration of ultraviolet radiation through the atmosphere, but it is considered a pollutant in the troposphere (close to the ground). The amount of ozone in the atmosphere is measured in Dobson units (DU). One DU is about twenty-seven million molecules per square centimetre. The average thickness of the atmospheric ozone layer at any place varies from month to month, but is generally between 260 and 330 DU.
Mechanism of Ozone hole
The criticality of ozone layer can be understood from the fact that, only 10 or less of every million molecules of air is ozone. The majority of these ozone molecules reside in a layer between 10 and 40 kilometres above the surface of the Earth known as stratosphere. Each spring in the stratosphere over Antarctica (spring in the southern hemisphere is from September through November.), atmospheric ozone is rapidly destroyed by chemical processes. As winter arrives, a vortex of winds develops around the pole and isolates the polar stratosphere. When temperatures drop below -78°C, thin clouds form of ice, nitric acid, and sulphuric acid mixtures. Chemical reactions on the surfaces of ice crystals in the clouds release active forms of CFCs. Ozone depletion begins, and the ozone “hole” appears.
Over the course of two to three months, approximately 50% of the total column amount of ozone in the atmosphere disappears. At some levels, the losses approach 90%. This has come to be called the Antarctic ozone hole. In spring, temperatures begin to rise, the ice evaporates, and the ozone layer starts to recover.
Thus, ozone “hole” is a reduction in concentrations of ozone high above the earth in the stratosphere. The ozone hole is defined geographically as the area wherein the total ozone amount is less than 220 Dobson Units. The ozone hole has steadily grown in size and length of existence over the past two and half decades. Now, the size of ozone hole over Antarctica is estimated to be about 30 million sq. km.
It has been observed that, man-made chlorines, primarily chlorofluorocarbons (CFCs), contribute to the thinning of the ozone layer and allow larger quantities of harmful ultraviolet rays to reach the earth.
Human activities are mostly responsible
Human activity is by far the most prevalent and destructive source of ozone depletion, while threatening volcanic eruptions are less common. Human activity, such as the release of various compounds containing chlorine or bromine, accounts for approximately 75 to 85 percent of ozone damage. Perhaps the most evident and destructive molecule of this description is chlorofluorocarbon (CFC). CFCs were first used to clean electronic circuit boards, and as time progressed, were used in aerosols and coolants, such as refrigerators and air conditioners. When CFCs from these products are released into the atmosphere, the destruction begins. As CFCs are emitted, the molecules float toward the ozone rich stratosphere. Then, when UV radiation contacts the CFC molecule, this causes one chlorine atom to liberate. This free chlorine then reacts with an ozone (O3) molecule to form chlorine monoxide and a single oxygen molecule (O2).
UV-B (the higher energy UV radiation absorbed by ozone) are generally accepted to be a contributory factor to skin cancer. In addition, increased surface UV leads to increased troposphere ozone, which is a health risk to humans. The increased surface UV also represents an increase in the vitamin D synthetic capacity of the sunlight. The cancer preventive effects of vitamin D represent a possible beneficial effect of ozone depletion. In terms of health costs, the possible benefits of increased UV irradiance may outweigh the burden. In other words, a thinning of the ozone layer is the key factor in the greenhouse effect, and exposes life on Earth to excessive ultra violet radiation, which can increase skin cancer and cataracts, reduce immune-system responses,
Environmental Effects of Ozone Depletion – As discussed, ozone acts as a shield to protect the Earth’s surface by absorbing harmful ultraviolet radiation. If this ozone becomes depleted, then more UV rays will reach the earth. Exposure to higher amounts of UV radiation could have serious impacts on human beings, animals and plants, such as the following:
(a) Harm to human health:
* More skin cancers, sunburns and premature aging of the skin.
* More cataracts, blindness and other eye diseases: UV radiation can damage several parts of the eye, including the lens, cornea, retina and conjunctiva.
* Cataracts (a clouding of the lens) are the major cause of blindness in the world. A sustained 10% thinning of the ozone layer is expected to result in almost two million new cases of cataracts per year, globally.
* Weakening of the human immune system (immunosuppression). Early findings suggest that too much UV radiation can suppress the human immune system, which may play a role in the development of skin cancer.
(b) Adverse impacts on agriculture, forestry and natural ecosystems:
* Several of the world’s major crop species are particularly vulnerable to increased UV, resulting in reduced growth, photosynthesis and flowering. Many agricultural crops are sensitive to the burning rays of the sun, including the world’s main food crops, rice, wheat, corn and soyabean.
* Many species of crops like sweet corn, soyabean, barley, oats, cow peas, carrots, cauliflower, tomato, cucumber, peas and broccoli are highly sensitive to UV-B radiation. As a result, food production could be reduced by 10% for every 1% increase of UV-B radiation.
* The effect of ozone depletion on the Indian agricultural sector could be significant.
* Only a few commercially important trees have been tested for UV (UV-B) sensitivity, but early results suggest that plant growth, especially in seedlings, is harmed by more intense UV radiation.
(c) Damage to marine life:
* In particular, plankton (tiny organisms on the surface layer of oceans) are threatened by increased UV radiation. Plankton are the first vital step in aquatic food chains.
* Decreases in plankton could disrupt the fresh and saltwater food chains, and lead to a species shift.
* Species of marine animals in their growing stage, including young fish, shrimp larvae and crab larvae, have been threatened in recent years by the growing UV-B radiation under the Antarctic ozone hole. Loss of biodiversity in our oceans, rivers and lakes could reduce fish yields for commercial and sport fisheries.
(d) Animals:
* In domestic animals, UV over exposure may cause eye and skin cancers.
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