14.5 Ozone Depletion and Its Effects

Ozone is an allotropic form of oxygen consisting of three oxygen atoms. It is formed in atmosphere by the association of an oxygen atom with an oxygen molecule in the mid of stratosphere.

Ozone is present throughout the atmosphere. But its maximum concentration called ozone layer lies in stratosphere region about 25 to $30km$ away from the Earth's surface. This layer surrounds the globe and protects Earth like a shield from harmful ultraviolet radiations of sunlight as shown in figure 14.10. Otherwise, ultraviolet radiations would cause skin cancer. Thus ozone layer in stratosphere is beneficial for life on the Earth.

Under normal conditions ozone concentration in stratosphere remains nearly constant through a series of complex atmospheric reactions. Two reactions that maintain a balance in ozone concentrations are as follows:

But this ozone layer is being depleted through various chemical reactions.

However, chlorofluorocarbons (CFCs) (used as refrigerants in air conditioners and refrigerators) are major cause of depletion of ozone layer. These compounds leak in one way or other escape and diffuse to stratosphere. Ultraviolet radiations break the C- Cl bond in ${\mathrm{C}\mathrm{F}\mathrm{C}\mathrm{l}}_3$$\mathrm{CFCl}_3$ and generates chlorine free radicals as:

These free radicals are very reactive. They react with ozone to form oxygen as: