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52.8 OZONE LAYER AND ITS DEPLETION

FORMATION AND BREAKDOWN OF OZONE

Upper stratosphere about 50 km above the earth’s surface, consists of considerable amount of ozone (O3) which protects us from the harmful ultraviolet (u.v) radiation (λ. = 255 nm),
coming from the sun. Ozone in the upper stratosphere is a product of u.v. radiation acting on dioxygen (O2) molecules according to following reactions.

O2 (g) + u.v. → O(g) + O(g)

                       u.v.

O (g) + O2(g)  O3(g)

 

Ozone is energetically unstable. It absorbs u. v. radiation to give dioxygen along with liberation of heat which raises the temperature of stratosphere.

            u.v.

O3 (g)  O2(g) + O

 

O + O3 (g) O2 (g) + Heat

 

In this way, the dynamic equilibrium existing between the production and decomposition of ozone molecules causes ozone layer to absorb u. v. radiation and prevent most of them
to reach earth’s surface.

DEPLETION OF OZONE LAYER

The equilibrium between formation and decomposition of ozone has been upset by the influx of several substances into the atmosphere which react with ozone to destroy it. The rate at which ozone is being destroyed is much faster than the rate at which it is being formed. The factors which accelerate the ozone depletion are as follows.

1.      Effects of Chlorofluorocarbons (Freons)

A very serious threat to the existence of ozone in the stratosphere comes from the use of chlorofluorocarbons (CFC) such as Freon-l (CFCI3) Freon-12 (CC12F2), Freon-22 (CHCI2F); Freon-113 (CC12FCCIF2) and Freon-114 (CClF2CCIF2), etc. CFC’s find a wide and varied application due to their properties like non-corrosiveness, non-flammability, low toxicity and high chemical stability. They are used as refrigerants, propellants in aerosol sprays, foaming agents in plastic manufacturing, fire extinguishing agents and solvents for cleaning electronic and metallic components, etc.

Once these CFCs enter the atmosphere by gradual evaporation from their source, they do not react with any substance due to their chemical inertness and thus cannot be eliminated from the atmosphere. They upfloat the atmosphere unchanged and eventually go to the stratosphere.

In stratosphere, they absorb the u. v. radiation and break up librating chlorine free radicals. The chlorine free radicals react with O3 to form chlorine monoxide radicals (ClO) which
combines with atomic oxygen to form O2.

 

CF2CL2 (g) → CF2Cl + Cl

Cl + O3 → ClO + O2

ClO + O → O2 + Cl

Net process O3 + O → O2 + O2

 

Thus, we find that chlorine radicals which converts O3 to O2 is regenerated at the end. A single chlorine radical can, thus, destroy millions of O3 molecules which eventually
causes depletion of ozone. This depletion of ozone layer over south pole refers to ozone hole. It was first reported in 1980 by scientists working in Antarctica.

 

2. Effect of Depletion of Ozone Layer

Depletion of ozone layer poses severe threat to mankind.

As a result of decreased concentration of ozone in stratosphere, the influx of u.v. radiation reaching the surface of the earth would increase. Medical scientist have pointed out that every 1 % decrease in ozone concentration would lead to 2% increase in risk to skin cancer (melanoma) in humans due to exposure to u.v. radiation.

 

  • u. v. radiation lead to ageing of skin, cataract sunburns
    and also tend to damage the immune system which
    may lead to increased viral infections.
  • Aquatic animals and aquatic plants also get damaged by their excessive exposure to u.v. radiation. u.v. radiation kill many micro phytoplanktons, and damage
    the fish productivity.
  • It has also been reported that plant proteins get easily affected by u.v. radiations which leads to the harmful mutation of cells. It also increases evaporation of
    surface water through the stomata of the leaves and decreases the moisture content of the soil. Increased u.v. radiations damages paints and fibres, causing them
    to fade faster.