(a) The problem of depletion is less serious at poles because $\;NO_{2}\;$ solidifies and is not available for consuming $\;CIO^{.}\;$ radicals\[\] (b) The problem of depletion is more serious at poles because ice crystals in the clouds over poles act as catalyst for photochemical reactions involving the decomposition of ozone by $\;CI^{.}\;$ and $\;CIO^{.}\;$ radicals \[\] (c) Frenos ,chlorofluorocarbons , are inert chemically ,they do not react with ozone in stratosphere \[\](d) Oxides of nitrogen also do not react with ozone in stratosphere
