Reductions of Antarctic ozone due to synergistic interactions of chlorine and bromine

The vertical column density of ozone observed in October over Antarctica has fallen precipitously over the past 10 yr. The concentration at Halley Bay (76° S 27° W), expressed conventionally in Dobson units (DU) (1 DU = 10−3atmos. cm = 2.7×1016molecules cm−2), has dropped from about 300 DU in 1975 to <200 DUin 1984 (ref. 1). Values in 1985 were even lower, comparable with the lowest values recorded anywhere on Earth2. We suggest here that the loss of O3in Antarctica may be attributed to catalysis of O3recombination by a scheme in which the rate-limiting step is defined by the reaction ClO+BrO→Cl+Br+O2. Concentrations of NO2must be low and heterogenous reactions involving particles in the polar stratospheric clouds must be an important element of the relevant chemistry. Industrial sources make important contributions to the contemporary budgets of both BrO and ClO and are likely to grow significantly in the future.