Effects of initial active chlorine concentrations on the Antarctic ozone spring depletion

Several one‐dimensional model experiments have been conducted in order to simulate the observed downward trend of total ozone and the shape of ozone profiles in the Antarctic during the austral spring. The results underline the fact that the O 3 depletion is quadratically sensitive to the amount of active chlorine, which we have expressed as the fraction ( F ) of Cl x that is in active form by polar sunrise. For F = 1, this result is largely independent of the occurrence of heterogeneous chemistry after polar sunrise since all of the processing has already occurred. The main O 3 loss is due to formation and photolysis of Cl 2 O 2 , the mechanism first suggested by Molina et al. [1987]. By comparison, using Br x levels similar to currently measured values, it was found that the BrO + ClO reaction plays a less important role in O 3 depletion. The depth of the O 3 minimum in spring appears to be related to the vertical extent of the region chemically processed by polar stratospheric clouds as well as the absolute levels of active Cl x ; more extensive vertical regions result in larger bite outs of the O 3 profile. This may account for the difference in O 3 minimum between 1986 and 1987. Simulations involving downward vertical transports are unable to reproduce observed decreases in total O 3 . We speculate that low trace gas mixing ratios of species such as N 2 O may be remnants of the previous summer and fall conditions, rather than signatures of downward transport during the springtime ozone depletion.