Das Projekt "Europaeisches Experiment zum Ozon in der Stratosphaere ueber der Arktis - Ballonwissenschaft" wird vom Umweltbundesamt gefördert und von Forschungszentrum Jülich GmbH, Institut für Chemie und Dynamik der Geosphäre durchgeführt. The mean total ozone amount derived from wintertime observations over high northern latitudes has been decreasing through the last two decades, as reported by the International Ozone Trends panel (1988). This phenomenon can not be described with time dependent numerical models of stratospheric chemistry that only consider homogeneous gas phase reactions. Results derived from experimental observations in the Arctic Winter stratosphere since 1987 (CHEOPS/AASE) suggest that Arctic air masses may be perturbed to an extend that is comparable to the conditions that lead to the formation of the 'ozone hole' in the Austral stratosphere. There is strong evidence that heterogeneous processes may lead to a strong enhancement of chlorine catalyzed ozone destruction cycles. However due to the considerable dynamic activity during Winter, Arctic air masses that are primed for depletion, or that are chemically depleted, may only extend over limited regions and persist over shorter time periods. This proposal is to perform a series of experimental measurements during the European Arctic Stratospheric ozone Experiment (EASOE), which is planned from early November 1991 to the end of March 1992. The observations will contribute to the primary objectives of EASOE: to study the structure and evolution of the Arctic polar vortex; to investigate the spatial and temporal extend of denitrification and dehydration processes: to search for the evolution and persistence of chemically perturbed air masses as well as for evidence of chemical ozone destruction. The proposed activities include balloon, aircraft, and ground based observations to measure vertical and horizontal profiles of many trace gases including long-lived trace gases and source gases (eg N2O, CH4, H2O, CO2, CFCs), of reservoir species (eg CIONO2, HCL, N2O5, HNO3), of reactive compounds (eg NO2, NO, NO3, OCIO), of aerosols and PSCs, and of the actinic flux. Regular ozone sonde launches will be made at two European sites. It is hoped that the results of the proposed work - in combination with those obtained from the other experimental and theoretical investigations during EASOE - will contribute to an improved understanding of the potential chemical perturbation of the Arctic stratosphere and of its immediate consequences. A proper description of chemical and dynamic processes and interactions will then allow to formulate atmospheric simulation models that are capable of making improved predictions of the future changes in polar stratospheric ozone. The proposed investigations, therefore, are considered part of an international cooperative effort to investigate the coupled response of the ozone/climate system to man-made perturbations by emissions of chlorine and bromine as well as greenhouse gases.