Single Particle Measurements of the Optical Properties of Small Ice Crystals and Heterogeneous Ice Nuclei

Abstract

Dust aerosol and ice crystals are two major types of nonspherical particles in the atmosphere which have significant roles in cloud-aerosol interactions and the radiative budget. The presence of dust and ice often coincide in the atmosphere because dust particles are efficient ice nuclei. The size and composition dependence of the scattering properties of dust and ice are needed to assess their individual contributions to the optical scattering of sunlight. Here we present a new measurement technique used to determine the single particle forward scattering, backscattering, and depolarization ratio (at a wavelength of 680 nm) for representative nonspherical atmospheric particles. The Texas A&M University Continuous Flow Diffusion Chamber (CFDC) was used as an ice crystal generator to produce ice crystals via both homogenous and heterogeneous nucleation mechanisms under well-controlled laboratory conditions. Optical scattering properties of mineral dusts and small ice crystals (0.6 mm to 50 mm optical diameter) were measured by the Droplet Measurement Technologies, Inc. (DMT) Cloud Aerosol Spectrometer with Polarization (CASPOL). Significant differences between the optical properties of single dusts and ice particles of the same size were observed. Differences between the optical signatures of homogeneously and heterogeneously nucleated ice crystals were not statistically significant. Our results suggest that atmospheric ice crystals can be identified and quantified independently from the dust particles on which they form based on analysis of their backscatter and depolarization signals.

Using depolarization to quantify ice nucleating particle concentrations: a new method

“…In previous TAMU CFDC studies, either an OPC (Climet, Inc.) or the CASPOL were employed (Glen and Brooks, 2014;McFarquhar et al, 2011). During FIN-02, the CASPOL was the chosen detector.…”

“…CFDC-CASPOL measurements were taken under conditions that approached those needed for homogeneous freezing, thus generating higher concentrations of ice crystals in the absence of activated liquid droplets. These measurements are detailed in Glen and Brooks (2014). For these measurements, the sample flow was conditioned with a pre-cooler, which was set to −10 • C to remove excess moisture and the CFDC was operated at −55 ± 0.2 • C and 51 ± 2.3 % SS i (−11 ± 1.5 % SS w ).…”

“…The TAMU CFDC was custom built in our laboratory at Texas A&M University and has been operated in previous laboratory and field campaigns to take temperature-and supersaturation-resolved INP concentration measurements (Glen and Brooks, 2014;McFarquhar et al, 2011). Additional details on CFDC and CFDC-CASPOL instrument design and operation are provided in our previous work (Glen andBrooks, 2013, 2014;Glen, 2014).…”

“…The depolarization ratio derived from CASPOL measurements is defined as follows (Glen and Brooks, 2014):…”

“…The number of INPs present in a cloud can dictate its optical properties throughout the ice nucleation process Murray et al, 2012). Several previous studies have designed new analysis methods for ice chambers that utilize the depolarization ratio measured by optical particle counters (OPCs) (Glen and Brooks, 2014;Nicolet et al, 2010;Clauss et al, 2013;Garimella et al, 2016). Nicolet et al (2010) accurately quantified ice crystals in the presence of water droplets in a chamber by using the peak intensity of the depolarization ratio to discriminate between ice crystals and droplets with the Ice Optical DEtector (IODE).…”