A 120m thick layer of air over a polluted city contains aerosol particles with diameters ranging from 1.0 to 1.7 m particles with a total concentration of 105 g/m3, which is in excess of the 24hour PM2.5 threshold of 65 g/m3 . (PM stands for Particle Matter, and the 2.5 subscript refers to particles 2.5 m in diameter or smaller.) The air inside this layer is constantly well mixed by winds, while the air above 120 m is perfectly clean and still. Because particles are heavier than air (particle density = 2500 kg/m3 whereas air density = 1.20 kg/m3 ), particles fall and settle on the ground, where they stick and are not resuspended. There is thus a net flux of particles out of the airshed