PREDICTING NO_x CONCENTRATION IN ALPINE VALLEYS USING APPLIED DISPERSION MODELING

摘要

This paper presents an applied method to model the dispersion of pollutants emitted in the basin of Alpine valleys. Frequency distributions of wind speed and direction, mixing height, and PGT stability class are analyzed for three meteorological stations located in Alpine valley basins. An applied Gaussian plume model is used to produce "dispersion matrices" (i.e., transfer functions), which give the annually averaged concentration impact per grid cell for a source of unit strength located in the center of the matrix. Dispersion matrices are computed for different source configurations and different local climatologies. These dispersion matrices are then used to disperse extensive inventories of NO_x emissions for the whole of Switzerland. The local wind conditions in alpine valleys are often determined either by the topographic forcing of the mesoscale circulation, or by thermally driven valley breezes. As a result, the circulation exhibits a complex pattern, which is neither homogeneous nor stationary, thus preventing the use of classical Gaussian approaches for modeling pollutant dispersion and the resulting ground-level concentrations. When modeling pollutant concentrations for entire states (Switzerland has a land surface area of approx. 41 000 square kilometers) using emission inventories, commonly applied dispersion models have to be used. Such dispersion models assume homogeneous and stationary conditions, and often take into account only one generic climatology, as the main goal is the prediction of the annual mean concentration only. This approach worked well in Switzerland for the prediction of annually averaged NO_x concentration throughout the country (SAEFL 1997), but NO_x concentrations from transit highway emissions in alpine valleys were clearly underestimated. A method has therefore been derived to better take into account the mean local flow patterns in alpine valleys. The main result are so-called dispersion matrices reflecting local climatology for alpine locations, for use within the Swiss PolluMap model, which is described in the next section. For a generic Alpine climatology and for each out of a set of several source configurations (traffic line source, elevated stacks, residential heatings, etc.), a distinct dispersion matrix is used.