Question 12.4: Calculate the fraction of an aerosol of 10 µm diameter parti...
Calculate the fraction of an aerosol of 10 µm diameter particles that would be washed out of the atmosphere in 1 h by rainfall of 1 mm h^{−1}, assuming that the raindrops are all of the same radius, namely (i) 100 µm and (ii) 1000 µm. (Hints: Use Figure 12.1 or the method in Problem 1 to calculate the sedimentation velocity v of the raindrops. Assume that the stopping distance of a 10 µm diameter particle impacting on a droplet moving at velocity v is S_{0} = 3 × 10^{4}v where S_{0} is measured in m and v is in m s^{−1}. The efficiency of impaction c_{p} of the aerosol on the droplet may be assumed similar to that for impaction on cylinders of the same diameter. Then the rainfall rate enables you to calculate how often a raindrop passes through a fixed point in the atmosphere, and if this happens n times per second, the washout coefficient of the particles by the rain drops is nc_{p}.)
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