Contrary to common misconception, ozone is not in the form of thick layer surrounding atmosphere. Equally untrue is another misconception that a hole is made in this ozone layer. To understand low ozone spread in atmosphere, one needs to know the structure and composition of atmosphere.
Depletion of Ozone
The distribution of ozone in ionosphere, mesosphere and stratosphere is being depleted. The concentration of ozone is gradually reducing. As the content of ozone is highest in ionosphere and the air itself being very thin, the depletion is negligible in ionosphere. But in mesosphere and stratosphere the air is thicker and ozone content is less. The depletion of ozone is of higher order in these layers. The so called hole in ozone layers 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 ozone layer is a global phenomena both in terms of cause and effect. The geographical limits of countries are not barriers to either dispersal of gases in layers of 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 chloroflouro carbon (CFC) chemicals evolved from various refrigerants, coolants and propellants are the primary reasons for depletion of ozone. CFC are a group of chlorine bearing gases of low specific gravity. They rise to stratosphere and mesosphere. Due to ionising 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.
The concept of ozone depletion is new. But study of international ozone trend is being made since 1988, when international ozone commission was established. Extensive ozone assessment facilities were established under International Middle Atmospheric Programme (IMAP) in India to study ozone along with number of other green house gases.
Destruction of ozone in the ozone layer
The ozone in the ozone layer protects the earth from harmful ultraviolet radiation by trapping these radiations. This ozone layer has been thinning gradually and poses potential health hazards for the future. The thinning of ozone layer has been attributed to the presence of chlorofluorohydrocarbons like CFCl3 and 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.
\[ CF_2Cl_2 + h\nu \rightarrow CF_2Cl + Cl \;\;\; (i)\]
Chlorine atom being highly reactive reacts with ozone (O3).
\[ Cl + O_3 \rightarrow ClO + O_2 \;\;\; (ii)\]
The monoxide of chlorine further reacts with another molecule of O3
\[ ClO + O_3 \rightarrow Cl + 2O_2 \;\;\; (iii)\]
The chlorine atom so obtained reacts with another ozone molecule. Hence, steps (ii) and (iii) are repeated again and again and, leads to the depletion of concentration of ozone.