In Polar regions, stratospheric clouds provide the surface for chlorine nitrate and hypochlorous acid, which react further to give molecular chlorine. Molecular chlorine and $HOCl$ are photolysed to give chlorine-free radicals.

$ClON{{O}_{2(g)}}+{{H}_{2}}{{O}_{(g)}}\to HOC{{l}_{(g)}}+HN{{O}_{3(g)}}$

$ClON{{O}_{2(g)}}+HC{{l}_{(g)}}\to C{{l}_{2(g)}}+HN{{O}_{3(g)}}$

$HOC{{l}_{(g)}}\xrightarrow{hv}\overset{\centerdot }{\mathop{O}}\,{{H}_{(g)}}+{{\overset{\centerdot }{\mathop{Cl}}\,}_{(g)}}$

$C{{l}_{2(g)}}\xrightarrow{hv}{{\overset{\centerdot }{\mathop{Cl}}\,}_{(g)}}+{{\overset{\centerdot }{\mathop{Cl}}\,}_{(g)}}$

The chlorine-free radicals lead to the decomposition of ozone as:

${{\overset{\centerdot }{\mathop{Cl}}\,}_{(g)}}+{{O}_{3(g)}}\to Cl{{\overset{\centerdot }{\mathop{O}}\,}_{(g)}}+{{O}_{2(g)}}$

Hence, a chain reaction is initiated. The chlorine-free radical is continuously regenerated, thereby depleting the ozone layer. This phenomenon is known as the as 'ozone hole'.

Effects of depletion of ozone layer

The ozone layer protects the Earth from the harmful $UV$ radiations of the sun. With the depletion of the layer, more radiation will enter the Earth's atmosphere. $UV$ radiations are harmful because they lead to the ageing of skin, cataract, skin cancer, and sunburns. They cause death of many phytoplanktons, which leads to a decrease of fish productivity. Excess exposure may even cause mutation in plants.

Increase in $UV$ radiations, decreases the moisture content of the soil and damages both plants and fibres.