8.3 Ozone
Ozone (O₃) plays dual roles: stratospheric ozone protects from UV, while tropospheric ozone is a pollutant and greenhouse gas.
Stratospheric Ozone
Chapman cycle describes natural ozone formation and destruction:
O$_2$ + h$\nu$ → 2O (λ < 240 nm)
O + O$_2$ + M → O$_3$ + M
O$_3$ + h$\nu$ → O$_2$ + O (λ < 320 nm)
Ozone Hole
CFCs release chlorine which catalytically destroys ozone:
Cl + O$_3$ → ClO + O$_2$
ClO + O → Cl + O$_2$
Net: O₃ + O → 2O₂ (one Cl destroys ~100,000 O₃)
Montreal Protocol (1987) phased out CFCs. Ozone layer recovering.