STRATOSPHERIC OCLO MEASUREMENTS AS A POOR QUANTITATIVE INDICATOR OF CHLORINE ACTIVATION

In the past measurements of OClO have been used to determine the degre e of chlorine activation in the Arctic and Antarctic lower stratospher es. In this study we have carried out box model calculations to invest igate the behaviour of OClO concentrations at 50hPa for cold denoxifie d conditions, typical of the polar winter. The dependence of the OClO abundance on ClOx (ClO+2Cl(2)O(2)) and BrOx (BrO+BrCl), as well as the sensitivity to nitrogen species, latitude and temperature is investig ated. 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 similar to 400 pptv. This behaviour is due to t he rate at which BrOx is tied up as BrCl at sunset. The insensitivity of the OClO concentration to high levels of CIO makes OClO a poor indi cator for distinguishing between medium and high levels of ClO. OClO i s only a qualitatively good indicator of chlorine activation but is a good quantitative indicator of BrO.