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

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.