Application of linear polarized light for the discrimination of frozen and liquid droplets in ice nucleation experiments

[摘要] We report on the development and test results of the new optical particlecounter TOPS-Ice (Thermo-stabilized Optical Particle Spectrometer for thedetection of Ice). The instrument uses measurements of the cross-polarizedscattered light by single particles into the near-forward direction(42.5° ± 12.7°) to distinguish between spherical andnon-spherical particles. This approach allows the differentiation betweenliquid water droplets (spherical) and ice particles (non-spherical) havingsimilar volume-equivalent sizes and therefore can be used to determine thefraction of frozen droplets in a typical immersion freezing experiment. Weshow that the numerical simulation of the light scattered on non-sphericalparticles (spheroids in random orientation) considering the actualscattering geometry used in the instrument supports the validity of theapproach, even though the cross-polarized component of the light scattered byspherical droplets does not vanish in this scattering angle. For theseparation of the ice particle mode from the liquid droplet mode, we use thewidth of the pulse detected in the depolarization channel instead of thepulse height. Exploiting the intrinsic relationship between pulse height andpulse width for Gaussian pulses allows us to calculate the fraction of frozendroplets even if the liquid droplet mode dominates the particle ensemble. Wepresent test results obtained with TOPS-Ice in the immersion freezingexperiments at the laminar diffusion chamber LACIS (Leipzig Aerosol CloudInteraction Simulator) and demonstrate the excellent agreement with the dataobtained in similar experiments with a different optical instrument. Finally,the advantages of using the cross-polarized light measurements for thedifferentiation of liquid and frozen droplets in the realistic immersionfreezing experiments are discussed.

[学科分类] 几何与拓扑