In $1980 \mathrm{~s}$ atmospheric scientists working in Antarctica reported about depletion of ozone layer commonly known as ozone hole.

In summer season, nitrogen dioxide and methane react with chlorine monoxide (reaction $IV$) and chlorine atoms (reaction $V$) forming chlorine sinks. It preventing much ozone depletion.

In winter, special type of clouds called polar stratospheric clouds are formed over Antarctica.

These clouds provide surface on which chlorine nitrate formed (reaction $IV$) gets hydrolysed toform hypochlorous acid (reaction $VI$). It also reacts with hydrogen chloride a producedmolecular chlorine.

$\mathrm{Cl} \dot{\mathrm{O}}_{(\mathrm{g})}+\mathrm{NO}_{2(\mathrm{~g})} \rightarrow \mathrm{ClO} \mathrm{NO}_{2(\mathrm{~g})}$

$\dot{\mathrm{C}} \mathrm{l}_{(\mathrm{g})}+\mathrm{CH}_{4(\mathrm{~g})} \rightarrow \dot{\mathrm{C}} \mathrm{H}_{3(\mathrm{~g})}+\mathrm{HCl}_{(\mathrm{g})} \quad \ldots . .$ $(iv)$

$\mathrm{ClONO}_{2(\mathrm{~g})}+\mathrm{H}_{2} \mathrm{O}_{(\mathrm{g})} \rightarrow \mathrm{HOCl}_{(\mathrm{g})}+\mathrm{HNO}_{3(\mathrm{~g})} \ldots . .$ $(vi)$

$\mathrm{ClONO}_{2(\mathrm{~g})}+\mathrm{HCl}_{(\mathrm{g})} \rightarrow \mathrm{Cl}_{2(\mathrm{~g})}+\mathrm{HNO}_{3(\mathrm{~g})} \quad \ldots . .$ $(vii)$

In $\mathrm{spring}$, sunlight returns to the Antarctica. The sun's warmth breaks up the could and $HOCl$and $\mathrm{Cl}_{2}$ are photolysed by sunlight, as given in reactions$ (viii)$ and $(ix)$.

$\mathrm{HOCl}_{(\mathrm{g})} \stackrel{h v}{\longrightarrow} \dot{\mathrm{O}} \mathrm{H}_{(\mathrm{g})}+\dot{\mathrm{C}} \mathrm{l}_{(\mathrm{g})} \quad \ldots \ldots .$ $(viii)$

$\mathrm{Cl}_{2(\mathrm{~g})} \stackrel{h v}{\longrightarrow} 2 \dot{\mathrm{C}} \mathrm{l}_{(\mathrm{g})}$ $\ldots \ldots$ $(ix)$

Thus, the chlorine radicals formed. It initiate the chain reaction for ozone depletion.