Comparisons of the Electrostatic Disdrometer with Impactor Slides

Abstract

Detailed comparisons of measurements from the electrostatic disdrometer and from soot-coated impactor slides show consistent differences in droplet size distributions determined by the two techniques. The disdrometer-derived distribution almost always shows highest concentrations in the first size interval and decreasing concentrations in successively larger size intervals, even in cases when the slides have recorded very few droplets in the smallest sizes. A comparison of the mean radii determined from the two sources for 36 different cases shows that the radii determined from the slides vary between 5.5 and 10 ?m, while those determined from the disdrometer vary only between 5.5 and 6.5 ?m. Also, as the mean droplet radius increases, the disdrometer measures increasingly higher droplet concentrations than the slides. Uncertainties and possible errors associated with both the impaction slide and disdrometer measurements are examined. From additional laboratory experiments it is concluded that the disdrometer does not properly size droplets which enter the orifice off center or at an appreciable angle relative to the axis of the orifice. A method for overcoming this problem is suggested.