The ozone layer lies in the stratosphere which is situated 15-50 km above the earth’s surface. This layer contains 90 per cent of the earth’s ozone and is responsible for absorbing the ultraviolet-B radiations that come from the Sun. It is important that some ultraviolet-B radiations get absorbed because in higher concentration they harm human life, by causing skin cancer, cataract, and loss of immunity.
These radiations are also harmful to terrestrial and aquatic flora and fauna. Unfortunately, human activities have started to cause a lot of harm to the ozone layer. When nitric and sulphuric acids mix with the ozone in the air, they cause deterioration of the ozone layer.
When high concentrations of the protective ozone layer are disturbed over an area, we say that the ozone hole has been formed. So the ozone hole is the region of the ozone layer that has less than 220 Dobson units of the gas. Ozone is a bluish-coloured and pungent smelling gas. The first ozone hole was detected in 1985 over the Antarctica region.
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Ozone in the air can be measured using a chemically sensitive strip that changes colour in the presence of ozone. The thickness of ozone layer is measured in Dobson Unit (DU). DU measures the total amount of ozone in the atmosphere depending on how thick the layer of ozone is if it is compressed at 0 °C under 1 atmospheric pressure.
Ozone is formed in the atmosphere when high energy solar rays strike oxygen molecules and cause two oxygen atoms to split. The free oxygen atom then combines with an oxygen molecule (02) to form ozone (O3).
Causes of ozone depletion:
Chlorofluorocarbons (CFCs) that emanate from refrigerators, air conditioners, solvents and aerosols are the most common cause of the depleting ozone layer. These gases are broken down by solar radiation and release a chlorine atom that reacts with the ozone molecule. This forms chlorine oxide.
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CI + 03 -> CIO + o2
This is unstable and bonds with a single oxygen atom. The by-product of this reaction is a chlorine atom and another oxygen molecule.
CIO + O -> CI + o2
Oxygen atom also reacts with ozone and cause harm to the ozone layer.
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O + 03 -> 202
Other causes of destruction of the ozone layer are the hydrogen and nitrogen systems of the atmosphere. These are called the OH system and the NxO systems respectively. The chlorine system explained above is referred to as the CFCI3 system. Over North America, ozone level has fallen by 0.5 per cent, between the period 1975 to 1990, which has caused great concern to scientists.
Ozone destruction reaction by nitric oxide can be represented as follows:
NO + 03 N02 + 02
N02 + O —» NO + 02
O + O3 -> 202
There are many other gases like CO, C02 (indirect effect), CH4, CFCs, etc. which are responsible for the ozone depletion.
Harmful effects of ozone depletion:
Due to depletion of ozone in the ozone layer, more ultraviolet rays fall on the earths which are very harmful for all living things. The ultraviolet portion of light is divided into three bands from 200-400 nm wavelength, i.e. Ultraviolet-A (UV-A), Ultraviolet- B (UV-B) and Ultraviolet-C (UV-C). UV-A is comparatively less harmful than UV-B and UV-C which is completely absorbed by the ozone layer of the stratosphere.
Our body synthesizes vitamin D from UV-B band of ultraviolet radiation, which also causes cataract, blindness, skin cancer in human beings and mutational changes in phyto- and zooplankton. UV- A band causes destruction of plant DNA which affects photosynthesis and other metabolic activities. Ozone layer also absorbs heat of the sun. If it is depleted, the temperature of the stratosphere would increase and affect the rainfall pattern on the earth.
Efforts to Minimize Ozone Depletion:
In 1987, United Nations Convention on Ozone Lay was held in Montreal and Montreal Protocol f signed to protect the ozone layer and cut 50 per cent of CFC production.
Montreal Protocol was amended in 1990 to protect ozone layer by completely phasing out CFC and development of CFC alternatives.
One such alternative to CFC is hydrofluorocarbon (HFC) which is less harmful.
Developing countries that use less than 0.3 kg i CFCs per capita per year in comparison to 1 kg per capita in the developed nations have been exempted.