A high-speed particle phase discriminator (PPD-HS) for the classification of airborne particles, as tested in a continuous flow diffusion chamber

Abstract

A new instrument, the High-speed Particle Phase Discriminator (PPD-HS), developed at the University of Hertfordshire, for sizing individual cloud hydrometeors and determining their phase is described herein. PPD-HS performs an in situ analysis of the spatial intensity distribution of near-forward scattered light for individual hydrometeors yielding shape properties. Discrimination of spherical and aspherical particles is based on an analysis of the symmetry of the recorded scattering patterns. Scattering patterns are collected onto two linear detector arrays, reducing the complete 2-D scattering pattern to scattered light intensities captured onto two linear, one-dimensional strips of light sensitive pixels. Using this reduced scattering information, we calculate symmetry indicators that are used for particle shape and ultimately phase analysis. This reduction of information allows for detection rates of a few hundred particles per second.Here, we present a comprehensive analysis of instrument performance using both spherical and aspherical particles generated in a well-controlled laboratory setting using a vibrating orifice aerosol generator (VOAG) and covering a size range of approximately 3-32 µm. We use supervised machine learning to train a random forest model on the VOAG data sets that can be used to classify any particles detected by PPD-HS. Classification results show that the PPD-HS can successfully discriminate between spherical and aspherical particles, with misclassification below 5 % for diameters > 3 µm. This phase discrimination method is subsequently applied to classify simulated cloud particles produced in a continuous flow diffusion chamber setup. We report observations of small, near-spherical ice crystals at early stages of the ice nucleation experiments, where shape analysis fails to correctly determine the particle phase. Nevertheless, in the case of simultaneous presence of cloud droplets and ice crystals, the introduced particle shape indicators allow for a clear distinction between these two classes, independent of optical particle size. From our laboratory experiments we conclude that PPD-HS constitutes a powerful new instrument to size and discriminate the phase of cloud hydrometeors. The working principle of PPD-HS forms a basis for future instruments to study microphysical properties of atmospheric mixed-phase clouds that represent a major source of uncertainty in aerosol-indirect effect for future climate projections.

Retrieving ice nucleating particle concentration and ice multiplication factors using active remote sensing validated by in situ observations

“…This causes the ice crystals to grow at the expense of the evaporation of cloud droplets which is referred to as the Wegener-Bergeron-Findeisen process (Wegener, 1911;Bergeron, 1935;Findeisen, 1938). The complexity of phase partitioning adds to difficulties simulating MPCs in models (e.g., McCoy et al, 2016;Matus and L'Ecuyer, 2017) where experimental observations could reduce uncertainties (see e.g., Baumgardner et al, 2011;Mahrt et al, 2019). In the evolution of a MPC, the ice phase plays an important role as it controls precipitation initiation and consequently cloud life time (see e.g., Field and Heymsfield, 2015;Heymsfield et al, 2020).…”

Development and characterization of the Portable Ice Nucleation Chamber 2 (PINCii)

“…This results in large droplets being counted in the same size channels as ice crystals; thus, the particle phase can no longer be reliably determined. Some phase-discriminating detectors exist that directly determine the phase during detection (Nicolet et al, 2010;Garimella et al, 2016;Mahrt et al, 2019), but these have other experimental challenges that limit their use. Moreover, for the purposes of a general introduction, the use of the simplified evaporation section-to-OPC coupling is considered to be best practice.…”

Development and characterization of the Portable Ice Nucleation Chamber 2 (PINCii)

“…This results in large droplets being counted in the same size channels as ice crystals, and thus particle phase can no longer be reliably determined. There exist some phase discriminating detectors where phase can be directly determined during detection (Nicolet et al, 2010;Garimella et al, 2016;Mahrt et al, 2019), but these have other experimental challenges limiting their use. Moreover, for the purposes of a general introduction, the use of the simplified evaporation section-to-OPC coupling is considered to be best practice.…”