How is the ozone hole formed? What is the Montreal Protocol? Explain.

(D) The ozone found in the lower atmosphere (troposphere) is considered 'bad' ozone as it harms plants and animals. The 'good' ozone is found in the upper atmosphere,known as the stratosphere,which acts as a shield by absorbing ultraviolet $(UV)$ radiation from the sun. $UV$ rays are highly damaging to organisms because they are absorbed by $DNA$ and proteins,causing their chemical bonds to break. The thickness of the ozone layer in a column of air from the ground to the top of the atmosphere is measured in Dobson units $(DU)$. Ozone is continuously produced by the action of $UV$ rays on molecular oxygen $(O_2)$ and simultaneously degraded into molecular oxygen in the stratosphere. There should be a balance between production and degradation of ozone in the stratosphere. However,this balance has been disrupted due to the enhancement of ozone degradation by chlorofluorocarbons (CFCs). CFCs are widely used as refrigerants. When CFCs released from the lower atmosphere reach the stratosphere,$UV$ rays act on them,releasing chlorine atoms. These chlorine atoms degrade ozone into molecular oxygen,leading to the thinning of the ozone layer,commonly referred to as the 'ozone hole'. The Montreal Protocol was signed in Montreal,Canada,in $1987$ (effective in $1989$) to control the emission of ozone-depleting substances,especially CFCs,to protect the ozone layer.