# 8.11.2.1.2: Ozone Layer and Ozone Hole

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Contrary to a common misconception, ozone is not in the form of a thick layer surrounding the atmosphere. Equally untrue is another misconception that there is a hole in this ozone layer. To understand how ozone spreads in the atmosphere, one needs to know the structure and composition of the atmosphere.

## Depletion of Ozone

Ozone in the ionosphere, mesosphere, and stratosphere is being depleted. The concentration of ozone is gradually being reduced. As the content of ozone is highest in the ionosphere with the air itself being very thin, the depletion is negligible in the ionosphere. But in the mesosphere and stratosphere, the air is thicker and the ozone content is less. The depletion of ozone is of a higher order in these layers. The so-called hole in the ozone layer simply means that above some continents (specifically Antarctica, Asia, and parts of South America), the mesosphere and stratosphere have lost their original level of ozone content.

The depletion of the ozone layer is a global phenomenon in terms of both cause and effect. The geographical limits of countries are not barriers to either dispersal of gases in the layers of the atmosphere or depletion of gases. The causes for depletion may arise in any country. The effects (in terms of depletion) may arise in any other country. The effects (in terms of ozone depletion) need not be exactly above the country causing the depletion.

## Causes of Depletion

It is now established that chlorofluorocarbon (CFC) chemicals evolved from various refrigerants, coolants, and propellants are the primary reason for the depletion of the ozone. CFCs are a group of chlorine-bearing gases of low specific gravity. They rise to the stratosphere and mesosphere. Due to ionizing solar radiation in these layers (which is the primary reason for production of ozone), fresh chlorine gas is produced from CFCs. This nascent chlorine gas has the capacity to react with ozone and bring down the level of ozone substantially.

## Destruction of ozone in the ozone layer

The ozone in the ozone layer protects the earth from harmful ultraviolet radiation by trapping this radiation. The ozone layer has been thinning gradually and poses potential health hazards for the future. The thinning of the ozone layer has been attributed to the presence of chlorofluorohydrocarbons (e.g., $$\ce{CFCl3}$$ and $$\ce{CF2Cl2 }$$) in the atmosphere. These chemicals have been used as aerosol propellants and cooling mixtures in refrigerators. How these chemicals affect ozone concentration is illustrated with the equations given below.

$\ce{CF_2Cl_2 + h\nu \rightarrow CF_2Cl^{\cdot} + Cl^{\cdot} }\label{i}$

Chlorine atoms, being highly reactive, react with ozone ($$\ce{O3}$$):

$\ce{Cl^{\cdot} + O_3 \rightarrow ClO^{\cdot} + O_2}\label{ii}$

The monoxide of chlorine further reacts with another molecule of $$\ce{O3}$$:

$\ce{ClO^{\cdot} + O_3 \rightarrow Cl^{\cdot} + 2O_2 } \label{iii}$

The chlorine atom so obtained reacts with another ozone molecule. Hence, steps \ref{ii} and \ref{iii} are repeated again and again, and lead to the depletion of the concentration of ozone.