A 120‐m 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 24‐hour 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 re‐suspended. There is thus a net flux of particles out of the airshed. (a) (2 points) Does settling follow Stokes’ or Newton’s Law? (b) (3 points) What is the range of settling speeds for these particles? Check on the Reynolds number to ensure that you use the correct formula. (c) (5 points) How long doesit take for the atmospheric PM concentration at ground level to drop below the compliance threshold?