Influence of valley winds on transport and dispersion of airborne pollutants in the Freiburg-Schauinsland area

The change of the concentration of trace gases in a natural environment depends on both chemical and meteorological conditions. To quantify the contribution of meteorological processes, such as transport and dispersion, chemical and meteorological processes must be assessed separately. In June 1996 the Schauinsland Ozone Precursor Experiment (SLOPE) was carried out to investigate the behavior of ozone and its precursors as a function of time in a valley between Freiburg in the Rhine Valley and Schauinsland in the Black Forest [Volz-Thomas et nl., this issue]. The main purpose of this contribution is to calculate the influence of transport and dispersion on the change of the concentration of airborne pollutants in a propagating air mass. Therefore (1) an inert tracer, SF6, was released, and (2) meteorological measurements were performed to investigate the valley wind regimes and the evolution of the mixed layer. Here are the main results of the experiment: Clear-sky conditions caused upvalley winds to be generated in the various valleys in the morning. These upvalley winds led to the transport of polluted air and SF6 from Freiburg through the Zarten Basin and the side valley (GroBes Tal) up to the Schauinsland hill. The propagation through the GroBes Tal took 90 +/- 5 min, which gives a propagation rate of approximately 1.3 m s(-1). The dilution factor for SF6 between the entrance and the outlet of the valley was approximately 9. This dilution factor could not only be explained by the continuous growth of the boundary layer during the propagation time of the tracer from the entrance to the outlet of the valley, but additionally by flow splitting and mountain venting [Fiedler et al., this issue].