Ozone (O3)

Ozone if formed by the action of sunlight on oxygen. Ozone forms a layer at about 50 Km from the earth’s surface. On the surface it is considered to be a polluting gas. However the ozone layer present 50 km above the surface of the earth is important as it protects us from harmful ultraviolet radiations coming from the sun.

Ozone in the upper layer gets destroyed by sunlight into oxygen and gets regenerated again regenerates which are a natural process.

2 O3 → 3 O2

Ozone Layer depletion

It takes place when the destruction of the ozone takes place at a greater rate than the regeneration. This is fatal as it allows the harmful UV radiations to enter the earth’s atmosphere.

In 1970s, scientists discovered that certain chemicals called Chloroflurocarbons (CFCs) which were used as refrigerants or in aerosol spray propellants, posed a threat to the ozone later. The CFC molecules are virtually indestructible, until they reach the stratosphere where UV radiations break them down to release chlorine atoms.

Ozone can be destroyed by a number of free radical catalysts.

These free radical catalysts are found in certain stable organic compounds, especially Chloroflurocarbons (CFCs), which may find their way to the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action of ultraviolet light, e.g.

CFCl3 + electromagnetic radiation → Cl· + ·CFCl2

The Cl and Br atoms can then destroy ozone molecules. A chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. The chlorine monoxide (i.e., the ClO) can react with a second molecule of ozone (i.e., O3) to yield another chlorine atom and two molecules of oxygen.

Chemical reactions:

Cl· + O3 → ClO + O2 

The chlorine atom changes an ozone molecule to ordinary oxygen

ClO + O3 → Cl· + 2O2

The ClO from the previous reaction destroys a second ozone molecule and recreates the original chlorine atom, which can repeat the first reaction and continue to destroy ozone.