Scavenging of Aerosol Particles by a Failing Water Drop and Calculation of Washout Coefficients

Abstract

Collection efficiencies of falling water drops ranging from 0.094 to.O.254 cm in radius for silver chloride aerosol particles in nitrogen ranging from 0.15 to 0.6 μm in radius have been measured. The drops accelerated for distances ranging from 30 to 515 cm prior to entrance into the scavenging chamber. Results were extrapolated to terminal velocity. The results for the slower moving drops (30 cm acceleration distance) agreed roughly with the predictions of a theory based upon collection by Brownian diffusion. However, near terminal velocity, Brownian diffusion theory did not account for the results. Instead, for each value of particle radius a, the collection efficiency was a sharply increasing function of the impaction parameter P. Results by other workers for particles of different density agreed approximately with our values, provided comparisons were made at the same values of aρ½ where ρ is the particle density. Calculations of washout coefficient indicated that the half-life of 0.1-μm aerosol particles, is of the order of one day for moderate rainfall rates. However, this result is strongly dependent upon the particular raindrop size distribution.