Ozone depletion and the catalytic chemistry of CFCs

Chapman cycle — natural balance

• O₂ + hν (UV-C) → 2 O·
• O· + O₂ + M → O₃ + M
• O₃ + hν (UV-B) → O₂ + O· · O₃ + O· → 2 O₂

In the undisturbed stratosphere, ozone is made and destroyed at the same rate.

How CFCs destroy it

• CFCs are chemically inert in the troposphere, so they drift unbroken up to the stratosphere — that very stability is the reason they reach the ozone layer.
• There, strong UV splits them: CFCl₃ + hν → CFCl₂· + Cl·
• Cl· + O₃ → ClO· + O₂ and ClO· + O· → Cl· + O₂: the chlorine atom is regenerated, so it acts as a catalyst and one radical can remove up to 100,000 ozone molecules.
• Net reaction: O₃ + O· → 2 O₂ — the ozone is gone while the chlorine is untouched.

Why the hole is Antarctic

• In the polar winter the air is isolated and colder than −78 °C; polar stratospheric clouds form on whose surfaces inactive chlorine reservoirs (HCl, ClONO₂) are converted into photochemically active Cl₂.
• When sunlight returns in September, that chlorine is released all at once and the ozone column collapses — a seasonal hole, deepest in September–October.
• The Montreal Protocol (1987) and its amendments phased out CFCs and halons; the ozone layer is now recovering, with a full expected recovery near the middle of this century — the clearest example of successful international environmental action.