Pollutant Dispersion in a Developing Valley Cold - Air Pool

Pollutants are trapped and accumulate within cold-air pools, thereby affecting air 5 quality. A numerical model is used to quantify the role of cold-air-pooling processes in 6 the dispersion of air pollution in a developing cold-air pool within an alpine valley under 7 decoupled stable conditions. Results indicate that the negatively buoyant downslope flows 8 transport and mix pollutants into the valley to depths that depend on the temperature deficit 9 of the flow and the ambient temperature structure inside the valley. Along the slopes, pollu10 tants are generally entrained above the cold-air pool and detrained within the cold-air pool, 11 largely above the ground-based inversion layer. The ability of the cold-air pool to dilute pol12 lutants is quantified. The analysis shows that the downslope flows fill the valley with air from 13 above, which is then largely trapped within the cold-air pool, and that dilution depends on 14 where the pollutants are emitted with respect to the positions of the top of the ground-based 15 inversion layer and cold-air pool, and on the slope wind speeds. Over the lower part of the 16 slopes, the cold-air-pool-averaged concentrations are proportional to the slope wind speeds 17 where the pollutants are emitted, and diminish as the cold-air pool deepens. Pollutants emit18 ted within the ground-based inversion layer are largely trapped there. Pollutants emitted 19 farther up the slopes detrain within the cold-air pool above the ground-based inversion layer, 20 although some fraction, increasing with distance from the top of the slopes, penetrates into 21 the ground-based inversion layer. 22