Das Projekt "Veraenderung der Leewellen von Wolken der polaren Stratosphaere ueber der skandinavischen Gebirgskette" wird vom Umweltbundesamt gefördert und von Universität Bonn, Physikalisches Institut durchgeführt. Heterogeneous reactions on polar stratospheric clouds (PSCs) initiate a long chain of chemical reactions which lead to the formation of the polar ozone holes. Necessary for the formation of PSCs are sufficiently low temperatures in the polar stratosphere. Meteorological statistics show the Arctic stratosphere to be on average too warm for PSC formation to occur on a large scale. lt is known that in the lee waves forming downwind of large mountain ranges, such as the Scandinavian mountains, temperatures may fall below PSC formation thresholds and PSCs will occur quite regularly even if the PSC formation temperature condjtion is not met in the overall stratosphere. Questions that arise concern the frequency of formation for PSCs in lee-wave fields and what are the meteorological conditions. How intense are such PSCs, what is their surface density? Are they of small spatial extent or will the lee wave field be the source region for stratiform PSCs covering wide areas? More generally, how will lee waves modify PSC fields that may be carried by the general circulation across a mountain ridge? Answers to these and other questions should be beneficial to computer models that predict the evolution of the ozone layer since reactions on PSCs are the first step towards ozone destruction. Initiating such models with erroneous amounts of PSCs (too much, too little, wrong time, altitude, surface area, or persistence) should lead to an equally erroneous prediction. The main objective is to perform field measurements down- and upstream of the Scandinavian mountain to search for generation, destruction, or modification of PSCs as the air parcels cross the mountain ridge. To achieve the objective we employ active remote sensing techniques deployed in two almost identical sets of instruments on each side of the Scandinavian mountains. Backscatter lidars will detect and characterize the stratospheric particulate load and identify PSCs. The ozone lidars will identify air masses from similarities in the ozone profiles. Temperature profiles will be measured by lidars using the rotational and vibrational Raman techniques. Lee waves will be identified from the ozone and temperature profiles. MST radars will perform wind measurements in the upper troposphere (which relates to lee wave generation) and a camera array will image the sky and connect lidar detection of PSCs and visual detection over a wider area. All instruments are located in a region where mountain lee waves tend to generate polar stratospheric clouds during the winter months. The detection of a chemically induced, large scale ozone hole in the spring of 1995 in the Arctic vortex was preceded by observations of intense and persistent polar stratospheric clouds in January 1995. The objective of this proposal contributes to a better understanding of the initial process that leads to modification of the stratospheric ozone layer, in particular the ozone hole in the northern hemisphere.