CHEMISTRY - Ozone Layer

<b>Formation of Ozone Layer :- </b> More than three billions years ago, before the evolution of life on the earth, it is believed that the atmosphere consisted mainly of methane, nitrogen, corbon dioxide, ammonia, hydrogen and water vapours and there was no oxygen. When plants started growing, they released oxygen during photosynthesis. In the stratosphere (11-50km), the oxygen started partially being converted into ozone. Now at an altitude of 25-30 km, we have a layer in which the concentration of ozone is about 10 ppm. It is called ozone layer. This region is, therefore, also called ozonosphere. It does not allow the harmful ultraviolet radiations coming from the sun to reach the surface of the earth and thus protects lifes on the earth. That is why this ozone layer is also called a earth's protective umbrella.
The formation of ozone in the stratosphere takes place in two steps. In the first step, the ultraviolet radiations coming from the sun have the sufficient energy to split the dioxygen into two oxygen atoms. In the second step, the oxygen atoms react with more dioxygen to form ozone.
The ozone thus formed absorbs the ultraviolet  radiations and is again broken into dioxygen and an oxygen atom. Heat is given out in this reaction which warms up the stratosphere.
In this way, "ozone cycle" is completed in the stratosphere.

<b> Depletion of Ozone Layer :-</b> Due to human activity, two types of compounds have been found to be most responsible for depleting the ozone layer and creating a hole into it. These two ozone-depleting agent are :-

1. Nitric Oxide(NO) :- Whch may be produced at the ground level due to human activity or natural sources or is produced in large amounts in the exhaust gases by the engine of supersonic transport planes and introduce directly into the stratosphere. NO reacts with ozone thereby decreasing the amount of ozone and forms nitrodioxide which reacts with oxygen atoms available in the stratosphere due to decomposition of ozone and oxygen producing back NO. Thus, no NO is consumed but ozone gets depleted.

2. Chlorofluorocarbons (CFC):- i.e, compounds containing chlorine, fluorine, and carbon commonly known as freons. These are introduced into the atmosphere from aerosol sprays in which function they as propellants and from refrigerating equipments, in which they act as coolants. They are also used as solvents and as blowing agent for plastic foams. They are chemically unreactve, non-toxic and odourless. However, they have a very long life time,i.e, they stay in the atmosphere for years and ultimately reach the upper layer where they start decposing in the presence of UV radiation coming from the sun.

<b> Effects of Depletion of Ozone layer :-</b>

* The most serious effect of the depletion f the ozone layer or the development of ozone hole is that the ultraviolet radiations coming from the sun can pass through the stratosphere and reach the surface of the earth. This type of radiations is known to be a cancer causing agent. It has been found that with increase in the exposure to ultraviolet radiation, the chance for occurence of skin cancer increasing.

* Exposure of eye to ultraviolet radiations damages the cornea  and lens of the eye and may cause cataract or even blindless.

* Exposure of plants to ultraviolet radiation adversely affect the plant proteins and results in the reduction of chlorophyll and harmful mutation

* Ozone depletion has a very strong effect on climate.

* Ozone depletion, if not controlled, would cause ecology disturbances, which would adversely affect man and animals.