★ The upper stratosphere consists of considerable amount of ozone `color{red}((O_3))`, which protects us from the harmful ultraviolet (UV) radiations (`λ` 255 nm) coming from the sun.

★ These radiations cause skin cancer (melanoma) in humans. Therefore, it is important to maintain the ozone shield.

★ Ozone in the stratosphere is a product of UV radiations acting on dioxygen `color{red}((O_2))` molecules. The UV radiations split apart molecular oxygen into free oxygen `color{red}((O))` atoms. These oxygen atoms combine with the molecular oxygen to form ozone.

`color{red}(O_2 (g) overset(UV)→ O(g) + O(g))`

`color{red}(O (g) + O_2 (g) overset(UV) ⇌ O_3 (g))`

• Ozone is thermodynamically unstable and decomposes to molecular oxygen. Thus, a dynamic equilibrium exists between the production and decomposition of ozone molecules.

In recent years, a remarkable depletion in ozone layer has been reported because of the presence of certain chemicals in the stratosphere. The main reason of ozone layer depletion is believed to be the release of chlorofluorocarbon compounds (CFCs), also known as freons. These compounds are nonreactive, non flammable, non toxic organic molecules and therefore used in refrigerators, air conditioners, in the production of plastic foam and by the electronic industry for cleaning computer parts etc.

`color{green}("★ 𝐇𝐎𝐖 𝐂𝐅𝐂’𝐒 𝐃𝐄𝐏𝐋𝐄𝐓𝐄 𝐎𝐙𝐎𝐍𝐄 𝐋𝐀𝐘𝐄𝐑?")`

Once CFCs are released in the atmosphere, they mix with the normal atmospheric gases and eventually reach the stratosphere. In
stratosphere, they get broken down by powerful UV radiations, releasing chlorine free radical.

`color{red}(CF_2 Cl_2 (g) overset(UV)→ overset(•)Cl (g) + overset(•)CF_2 Cl (g))` ...................(i)

The chlorine radical then react with stratospheric ozone to form chlorine monoxide radicals and molecular oxygen.

`color{red}(overset(•)Cl(g) + O_3 (g) → Cl overset(•)O (g) + O_2 (g))` ..................(ii)

Reaction of chlorine monoxide radical with atomic oxygen produces more chlorine radicals.

`color{red}(Cl overset(•)O (g) + O(g) → overset(•)Cl (g) + O_2 (g))` ....................(iii)

The chlorine radicals are continuously regenerated and cause the breakdown of ozone. Thus, CFCs are transporting agents for continuously generating chlorine radicals into the stratosphere and damaging the ozone layer.

In summer season, nitrogen dioxide and methane react with chlorine monoxide (reaction iv) and chlorine atoms (reaction v) forming chlorine sinks, preventing much ozone depletion, whereas in winter, special type of clouds called polar stratospheric clouds are formed over Antarctica. These polar stratospheric clouds provide surface on which chlorine nitrate formed (reaction iv) gets hydrolysed to form hypochlorous acid (reaction (vi)). It also reacts with hydrogen chloride produced as per reaction (v) to give molecular chlorine.

`color{red}(Cl overset(•)O(g) + NO_2 (g) → ClO NO_2(g))` ................(iv)

`color{red}(overset(•)Cl (g) + CH_4(g) → overset(•)CH_3 (g) + HCl (g))` ..................(v)

`color{red}(ClONO_2 (g) + H_2O (g) → HOCl (g) + HNO_3 (g))` ...........................(vi)

`color{red}(ClONO_2 (g) + HCl (g) → Cl_2 (g) + HNO_3 (g))` ...................(vii)

When sunlight returns to the Antarctica in the spring, the sun’s warmth breaks up the clouds and `color{red}(HOCl)` and `color{red}(Cl_2)` are photolysed by sunlight, as given in reactions (viii) and (ix).

`color{red}(HOCl (g) overset(hv)→ overset(•)OH + overset(•)Cl(g)) ` ..............(viii)

`color{red}(Cl_2 (g) overset(hv)→ 2 overset(•)Cl (g)) ` .................................(ix)


The chlorine radicals thus formed, initiate the chain reaction for ozone depletion as described earlier.

With the depletion of ozone layer, more UV radiation filters into troposphere.

★ UV radiations lead to ageing of skin, cataract, sunburn, skin cancer, killing of many phytoplanktons, damage to fish productivity etc.

★ It has also been reported that plant proteins get easily affected by UV radiations which leads to the harmful mutation of cells.

★ It also increases evaporation of surface water through the stomata of the leaves and decreases the moisture content of the soil.

★ Increase in UV radiations damage paints and fibres, causing them to fade faster.