FSSP characterization with monodisperse water droplets

Abstract

A droplet generator was used to calibrate and study some features of the forward-scattering spectrometer probe (FSSP). The generator produces a stable jet of monodisperse droplets with the rate of 50-500 droplets per second and a velocity between 0.4 and 1.3 m s-1. The estimated standard deviation of the diameter of the droplets is not larger than 1%. Because of the low rate of generated droplets, there were no coincidence problems during the measurements, that is, no multiple droplets in the laser beam. The results of FSSP calibration in its operating ranges 0 and 1 are presented in this paper. The experiments revealed minor differences (<15%) to the size calibration given by the manufacturer. A comparison of the measurements with theoretical Mie curves yielded good agreement. Furthermore, the effects of the laser beam inhomogeneity on the droplet sizing were studied. Scanning the droplet jet perpendicular and parallel to the axis of the laser beam of the FSSP showed that underestimates of the droplet size up to 9 μm measured by the FSSP may occur. The undersizing depends on the position, in which the droplets cross the sample volume, and the size of the droplets. This contributes to artificial broadening of the size spectra by the FSSP. An algorithm for droplet spectra retrieval due to light inhomogeneity and particle velocity is discussed. The results obtained during this study were obtained using one specific FSSP owned by the Institute for Tropospheric Research. However, the range of effects will certainly be similar for other FSSPs.