Calibration of Knollenberg FSSP Light-Scattering Counters for Measurement of Cloud Droplets

Abstract

Measurement of cloud drop size distributions with the Knollenberg model FSSP-100 light-scattering counter can lead to artificial bumps or knees in the distributions at ∼0.6 μm and sometimes 2–4 μm radius if the manufacturer-supplied calibration is used. These artifacts are a consequence of the instrument having multivalued or slowly changing response in these regions of particle size. A modified calibration procedure is given that removes these artifacts so that the true droplet size distribution can be obtained. Measurement of slightly nonspherical particles with refractive indexes characteristic of those of atmospheric aerosols will generally lead to undersizing if the FSSP manufacturer-calibration is used, but likely by not more than a factor 2.