Studies of photo-chemistry of stratospheric chlorine monoxide and its influence of ozone loss at mid and high latitudes Abstract The aim of this thesis is to study open questions regarding stratospheric halogen induced ozone loss at both mid and polar latitudes . In-situ measurements of chlorine monoxide (C10) were performed employing the Jülich balloon-borne C10/BrO instrument at mid and high latitudes of the northern hemisphere . These C10 observations were analyzed and interpreted by model simulations wich the photo-chemical Boxmodel version of the Chemical Lagrangian Model of the Stratosphere (CLaMS) . The importance of C10 photo-chemistry wich respect to discrepancies in the present quantitative understanding of polar ozone chemistry and to unexplained causes for ozone loss at mid-latitudes is investigated . C10 measurements presented in this thesis were obtained at mid-latitudes an November 14, 1996 in Leon (North Spain) and an May 3, 1999 in Aire sur 1'Adour (South France) . At high latitudes C10 measurements were performed an January 27, 2000 and an March 1, 2000 in Kiruna (North Sweden) . Photo-chemical studies of diurnal variation of C10 mixing ratios are performed for all four flights investigated here . The observed diurnal variation of C10 mixing ratios as function of altitude is compared wich results of model simulations . In the frame of this thesis the model reproduction of C10 photo-chemistry can be verified and is confirmed by the C10 measurements up to approximately 31 km altitude at mid-latitudes and up to approximately 25 km altitude in the Arctic winter 1999/2000 . Further Jong term simulations are performed to study the temporal evolution of both chlorine partitioning and chlorine activation as well as chemical ozone loss during the Arctic winter 1999/2000 to study how far simultaneously mea sured C10 vertical profiles and the observed ozone loss could be described by model simulations . These model simulations extend over a period between early December 1999 and January 27, 2000 and March 1, 2000, respectively. Performing sensitivity tests the influence of polar stratospheric clouds and of denitrification observed in this winter an chlorine activation is studied . Further for high altitudes the influence of cosmic rays an stratospheric chlorine chemistry is analyzed . Results of model simulations are validated wich several in-situ and satellite measurements . The measured C10 vertical profile can be reproduced by model simulations within the error bars . The open question, if models can describe the vertical distribution of C10 mixing ratios and the simultaneously observed ozone loss, can be positively responded to the Arctic winter 1999/2000 .