Experimental and Numerical Collision Efficiencies for Submicron Particles Scavenged by Small Raindrops

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Abstract
Experimental scavenging efficiencies were measured for freely falling drops of 0.40–0.85 mm diameter and charges of 10−5–10−3 esu supported by the vertical airstream of the UCLA cloud tunnel. The particles in the airstream of 0.4 µm radius and 1.5 gm cm−3 density were produced from indium acetylacetonate using a La Mer generator. Collection efficiencies of 10−4 to 10−3 determined by neutron activation analysis were used to provide a test of an expanded theoretical model of Beard and Grover where collision efficiencies are based on a numerical description of axisymmetric, steady-state flow about rigid spheres up to Reynolds number 400. Scavenging mechanisms were examined in order to determine the important forces on the particle. The coulomb force between a charged drop and charged particle was included in the equation of motion of the particle which moved in a gravitational field with a Stokes-Cunningham resistance in the imperturbed flow of the drop. Numerically evaluated collision efficiencies were found to increase for the smaller particles due to wake capture even without electrostatic effects. A comparison shows that the experimental results for negligibly charged drops scavenging submicron particles are predicted by the theoretical model to within a factor of 2. By use of the numerical results the inconsistencies of previous experiments are in part resolved in accounting for apparent electrostatic and wake effects.

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