A FIELD-STUDY OF PARTICLE SCAVENGING BY RAINDROPS OF DIFFERENT SIZES USING MONODISPERSE TRACE AEROSOL

A field experiment was conducted to measure particle scavenging proces ses in the atmosphere during real precipitation events. Monodisperse a erosol particles (mean radii varying from 0.19 to 1.8 mu m) tagged by a fluorescent dye tracer are spread during several precipitation event s in the atmosphere. Raindrops which cross the aerosol volume are coll ected according to size by freezing them in liquid nitrogen (Guttalgor method). A continuous decrease in the chemical concentration of the d ye in the raindrops with increasing drop size is observed for the scav enging of particles with a radius of 1.8 mu m. For the scavenging of s maller particles a maximum of the chemical concentration of the dye tr acer is found for drop radii between 0.175 and 0.35 mm. The concentrat ion maximum shifts towards small drop radii with increasing aerosol pa rticle 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 mea n of the scavenged particle size and the most effective raindrop radiu s. Scavenging coefficients in the range 10(-6)-10(-9) s(-1) are derive d as a function of drop size in field experiments They increase with r aindrop radii between 0.1 and 0.6 mm and decrease with increasing part icle size up to about 1.2 mu m. The scavenging coefficient then increa ses to larger particle sizes. Results are compared to theory which pre dicts this minimum at smaller particle sizes. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.