A field study of particle scavenging by raindrops of different sizes using monodisperse trace aerosol

A field experiment was conducted to measure particle scavenging processes in the atmosphere during real precipitation events. Monodisperse aerosol particles (mean radii varying from 0.19 to 1.8 mu m) tagged by a fluorescent dye tracer are spread during several precipitation events in the atmosphere. Raindrops which cross the aerosol volume are collected according to size by freezing them in liquid nitrogen (Guttalgor method). A continuous decrease in the chemical concentration of the dye in the raindrops with increasing drop size is observed for the scavenging of particles with a radius of 1.8 mu m. For the scavenging of smaller particles a maximum of the chemical concentration of the dye tracer is found for drop radii between 0.175 and 0.35 mm. The concentration maximum shifts towards small drop radii with increasing aerosol particle radius. For the scavenging of aerosol particles smaller than 0.6 mu m radius the concentration maximum is found at a drop radius of 0.35 mm. Results of this study establish a relationship between the mean of the scavenged particle size and the most effective raindrop radius. Scavenging coefficients in the range 10(-6)-10(-9) s(-1) are derived as a function of drop size in field experiments They increase with raindrop radii between 0.1 and 0.6 mm and decrease with increasing particle size up to about 1.2 mu m. The scavenging coefficient then increases to larger particle sizes. Results are compared to theory which predicts this minimum at smaller particle sizes. (C) 1998 Elsevier Science Ltd. All rights reserved.