Formation of the Antarctic ozone hole by the CIO dimer mechanism

The discovery in 1986 of an extremely large concentration (∼1 part per 109) of chlorine monoxide (CIO) at low altitudes in the spring stratosphere over Antarctica^1,2, and measurements showing the formation of OC1O at night³, provided strong evidence that the evolution of the Antarctic ozone 'hole⁹ is chemically driven by chlorine. Here we use new measurements⁴ of the low-altitude CIO profile, made during September 1987, along with detailed observa-tions of ozone depletion⁵ over McMurdo Station during the same period, to show that both the rate and altitude range of ozone depletion can be quantitatively accounted for by a mechanism in which the CIO dimer⁶ is the important intermediary in the catalytic destruction of ozone. An alternative bromine mechanism appears capable of contributing only 5-15% to the ozone loss rate.