Model study of stratospheric chlorine activation and ozone loss during the1996/1997 winter

Chlorine activation and ozone depletion in the Arctic winter stratosphere o f 1996-1997 have been studied with a newly developed stratospheric chemistr y-transport model (CTM). The chemistry scheme, using a Euler Backward Itera tive approximation method, includes a comprehensive set of reactions on ter nary aerosol and ice particles, which has been tested against a numerically exact solver. Tracer transports in the CTM are calculated from European Ce ntre for Medium-Range Weather Forecasts (ECMWF) meteorological analyses. Co mparisons have been made with O-3 and ClO measurements, and with ozone loss rates derived from observations during February and March 1997. ClO produc tion and ozone depletion are somewhat underestimated by the model. Furtherm ore, uncertainties regarding the aerosol phase are tested. Assuming nitric acid trihydrate (NAT particles to form at their melting point, while liquid aerosol is present simultaneously in the model, gives rise to the largest ClO production and the strongest ozone depletion. By correcting for an ECMW F temperature warm bias we obtain a similar large effect on calculated ClO production and ozone depletion for the 1996/1997 Arctic winter, whereas unc ertainties in the chlorine abundance seem less important. An average warm b ias of 1.3 K at polar stratospheric cloud temperatures on the 50 hPa model level reduces the calculated ozone depletion rates over February and March by 35%. Observations of ClO are reproduced when lower temperatures and maxi mum Cl-y abundance are assumed, but ozone depletion is slightly overestimat ed in that case.