Abstract
Abstract. We report on the development and test results of the new optical particle counter TOPS-Ice (Thermo-stabilized Optical Particle Spectrometer for the detection of Ice). The instrument uses measurements of the cross-polarized scattered light by single particles into the near-forward direction (42.5° ± 12.7°) to distinguish between spherical and non-spherical particles. This approach allows the differentiation between liquid water droplets (spherical) and ice particles (non-spherical) having similar volume-equivalent sizes and therefore can be used to determine the fraction of frozen droplets in a typical immersion freezing experiment. We show that the numerical simulation of the light scattered on non-spherical particles (spheroids in random orientation) considering the actual scattering geometry used in the instrument supports the validity of the approach, even though the cross-polarized component of the light scattered by spherical droplets does not vanish in this scattering angle. For the separation of the ice particle mode from the liquid droplet mode, we use the width of the pulse detected in the depolarization channel instead of the pulse height. Exploiting the intrinsic relationship between pulse height and pulse width for Gaussian pulses allows us to calculate the fraction of frozen droplets even if the liquid droplet mode dominates the particle ensemble. We present test results obtained with TOPS-Ice in the immersion freezing experiments at the laminar diffusion chamber LACIS (Leipzig Aerosol Cloud Interaction Simulator) and demonstrate the excellent agreement with the data obtained in similar experiments with a different optical instrument. Finally, the advantages of using the cross-polarized light measurements for the differentiation of liquid and frozen droplets in the realistic immersion freezing experiments are discussed.
Highlights
Tered by spherical droplets does not vanish in this scattering liquid water and dramatically enhance the rate of angle
During the first measurements of fice conducted with TOPS-Ice at Leipzig Aerosol Cloud Interaction Simulator (LACIS), we have discovered that it is beneficial to use the signal width distribution and not the signal height distribution of pulses recorded by PMT C for the droplet and ice particle size range investigated
The following iteration procedure is applied to the data: at the initial step, a normal distribution with parameters N0, σ0 and μ0 is fitted to the droplet mode of the smoothed PWDC, where N0 is the area under the curve, σ0 is the standard deviation and μ0 is the mean value of the normal distribution
Summary
The experimental setup for the immersion freezing experiments is described. The particle conditioning, i.e., the generation of droplets with a single immersed solid particle and the freezing of these droplets, takes place in the cloud simulator LACIS, which is described in Sect. All particles moving along the axis of the LACIS flow tube experience the same humidity and temperature conditions. In the experiments described LACIS was operated in immersion freezing mode (Niedermeier et al, 2010); i.e., the seed particles were first activated to supercooled droplets, and the droplets were cooled down to the temperature where some of them freeze. For a certain amount of these supercooled droplets, the immersed particles act as IN, leading to heterogeneous ice nucleation and freezing of the droplet. The number of frozen droplets divided by the number of frozen and liquid droplets yields the ice fraction fice, which can be considered as the probability of heterogeneous freezing as a function of temperature, particle type, particle size, and time
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