Short note: Ozone depletion

Ozone layer

Ozone (O₃) in the stratosphere (15 to 35 km) is formed from oxygen by UV and absorbs harmful UV-B and UV-C radiation:

O₂ + hν (λ < 240 nm) → 2O
O  + O₂ → O₃
O₃ + hν (UV-B) → O₂ + O

The last step makes ozone a natural shield, absorbing 97 to 99 % of the sun's harmful UV.

Depletion

Ozone is destroyed faster than it forms, mainly by chlorine from CFCs (and bromine from halons) and, to a smaller extent, by NO from supersonic aircraft (NO + O₃ → NO₂ + O₂).

Because CFCs are so stable, they slowly rise to the stratosphere, where strong UV radiation (λ < 220 nm) breaks the C–Cl bond and releases chlorine atoms:

CCl₃F  + hν → •CCl₂F + Cl•
CCl₂F₂ + hν → •CClF₂ + Cl•

The free chlorine atom then destroys ozone in a chain reaction:

Cl• + O₃  → ClO• + O₂
ClO• + O  → Cl• + O₂
-------------------------------
Net:  O₃ + O → 2O₂        (Cl• is regenerated)

A single Cl atom can destroy about 10⁵ ozone molecules before it is removed (for example, as HCl by reaction with CH₄, or as ClONO₂ when ClO combines with NO₂).

Depletion is highest over Antarctica in spring, where polar stratospheric clouds help release chlorine, producing the ozone hole.

Effects

• Increased UV-B reaching the earth causes skin cancer, sunburn, cataract and weakening of the immune system.
• Damages crops and forests, reducing yield.
• Kills phytoplankton and other aquatic organisms, upsetting the marine food chain.
• Degrades plastics and paints; increases ground-level ozone and smog.
• Contributes to global warming (CFCs are greenhouse gases).

Control

The Montreal Protocol (1987) phased out CFCs, halons and carbon tetrachloride. Alternatives are HCFCs and HFCs (later restricted by the Kigali Amendment, 2016), hydrocarbons and ammonia as refrigerants. Nepal is a party to the Montreal Protocol.