Pollutant Dispersion in a Developing Valley Cold-Air Pool

Pollutants are trapped and accumulate within cold-air pools, thereby affecting air quality. A numerical model is used to quantify the role of cold-air-pooling processes in the dispersion of air pollution in a developing region of enhanced cooling within an Alpine valley under decoupled stable conditions. Results indicate that the negatively buoyant downslope flows transport and mix pollutants into the valley to depths that depend on the temperature deficit of the flow and the ambient temperature structure inside the valley. Along the slopes, pollutants are generally entrained above the region of enhanced cooling and detrained within the region of enhanced cooling largely above the ground-based inversion layer. The ability of the region of enhanced cooling to dilute pollutants is quantified. The analysis shows that the downslope flows fill the valley with air from above, which is then largely trapped within the region of enhanced cooling and that dilution depends on where the pollutants are emitted with respect to the positions of the top of the ground-based inversion layer and region of enhanced cooling, and on the slope wind speeds. Over the lower part of the slopes, the concentrations averaged across the region of enhanced cooling are proportional to the slope wind speeds where the pollutants are emitted, and diminish as the region of enhanced cooling deepens. Pollutants emitted within the ground-based inversion layer are largely trapped there. Pollutants emitted farther up the slopes detrain within the region of enhanced cooling above the ground-based inversion layer, although some fraction, increasing with distance from the top of the slopes, penetrates into the ground-based inversion layer.