Rain Scavenging of Zinc Sulfide Particles

Abstract

Several scavenging experiments were conducted using artificial rain in the outdoors with zinc sulfide tracer particles. The measured collection efficiencies for raindrops are compared to the theoretical predictions of Langmuir, and to the measurements with freely falling drops made by Kinzer and Cobb, Walton and Woolcock, and McCully et al. The four main characteristics of the efficiencies found are 1) higher efficiencies for drops of about 0.4 mm diameter, 2) efficiencies greater than 1.0 with all drops for particles larger than 13 microns, 3) a rapid increase in efficiency with particle size in the general region of ten microns diameter, and 4) an apparent minimum for all particle sizes with drop sizes of about 0.8 mm diameter. The explanation offered utilizes the classical inertial impaction of particles on the leading side of the drops with additional wake effects. Particles that narrowly miss the drops are drawn into the wake, fall therein with respect to the drop, and impact on the upper side of the drop. This favors the collection of the larger particles. The vortices in the wakes of the larger drops are more quickly shed and the chance of wake collection is reduced.