Catalytic ozone destruction (Cl, NO, HO_x)

X + O₃ → XO + O₂; XO + O → X + O₂ (net: O₃ + O → 2 O₂); each X atom destroys ~10⁴–10⁵ O₃ before removal. Active X: Cl (CFC source, Molina–Rowland), Br, NO, OH. Basis of the Montreal Protocol.