Improvements of droplet size distribution measurements with the Fast-FSSP (Forward Scattering Spectrometer Probe)

The basics of single particle measurements are discussed and illustrated with measurements of the droplet size distribution with an optical spectrometer, the Forward Scattering Spectrometer Probe (FSSP), and its improved version, the Fast-FSSP. The various sources of uncertainties are successively analyzed: the statistical significance of an incomplete sampling of the particle population, counting losses due to coincidence of particles and electronic dead time of the counter, artificial broadening of the size distributions by the coincidences, inhomogeneities of the sensitive volume of the probe, ambiguities of the Mie scattering curve, errors on the sampled volume, and, finally, the uncertainties on the size calibration of the instrument. It is demonstrated with examples of data collected with the Fast-FSSP that additional parameters, such as the pulse duration and the interarrival times between detections, are crucial for improving the sizing of the particles and the retrieval of the spatial evolution of the size distribution at the smallest scales.