In the past measurements of OClO have been used to determine the degree of chlorine activation in the Arctic and Antarctic lower stratospheres. In this study we have carried out box model calculations to investigate the behaviour of OClO concentrations at 50hPa for cold denoxified conditions, typical of the polar winter. The dependence of the OClO abundance on ClOx (ClO+2Cl2O2) and BrOx (BrO+BrCl), as well as the sensitivity to nitrogen species, latitude and temperature is investigated. The net production of OClO from the reaction ClO + BrO → OClO + Br, the major source of OClO, is found to be first order in BrO for BrOx values from 0 to 22 pptv, whereas it becomes pseudo zero order in ClO for ClOx greater than ∼400 pptv. This behaviour is due to the rate at which BrOx is tied up as BrCl at sunset. The insensitivity of the OClO concentration to high levels of ClO makes OClO a poor indicator for distinguishing between medium and high levels of ClO. OClO is only a qualitatively good indicator of chlorine activation but is a good quantitative indicator of BrO.
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