Abstract
Abstract. Glaciation in mixed-phase clouds predominantly occurs through the immersion-freezing mode where ice-nucleating particles (INPs) immersed within supercooled droplets induce the nucleation of ice. Model representations of this process currently are a large source of uncertainty in simulating cloud radiative properties, so to constrain these estimates, continuous-flow diffusion chamber (CFDC)-style INP devices are commonly used to assess the immersion-freezing efficiencies of INPs. This study explored a new approach to operating such an ice chamber that provides maximum activation of particles without droplet breakthrough and correction factor ambiguity to obtain high-quality INP measurements in a manner that previously had not been demonstrated to be possible. The conditioning section of the chamber was maintained at −20 ∘C and water relative humidity (RHw) conditions of 113 % to maximize the droplet activation, and the droplets were supercooled with an independently temperature-controlled nucleation section at a steady cooling rate (0.5 ∘C min−1) to induce the freezing of droplets and evaporation of unfrozen droplets. The performance of the modified compact ice chamber (MCIC) was evaluated using four INP species: K-feldspar, illite-NX, Argentinian soil dust, and airborne soil dusts from an arable region that had shown ice nucleation over a wide span of supercooled temperatures. Dry-dispersed and size-selected K-feldspar particles were generated in the laboratory. Illite-NX and soil dust particles were sampled during the second phase of the Fifth International Ice Nucleation Workshop (FIN-02) campaign, and airborne soil dust particles were sampled from an ambient aerosol inlet. The measured ice nucleation efficiencies of model aerosols that had a surface active site density (ns) metric were higher but mostly agreed within 1 order of magnitude compared to results reported in the literature.
Highlights
Atmospheric ice nucleation plays an important role in initiating precipitation in clouds that consist of a mixture of supercooled liquid water droplets and ice crystals and in catalyzing the formation of ice particles within high-altitude cirrus clouds (Lohmann and Feichter, 2005; Boucher et al, 2013)
The modified design allowed for the faster (≈ 30 min) accumulation of immersion-freezing data points to develop a continuous representation of the immersion-freezing behavior of ice-nucleating particles (INPs) compared to the traditional Compact Ice Chamber (CIC)-Pacific Northwest National Laboratory (PNNL) design, in which immersion freezing was investigated at discrete temperatures
Instead of investigating immersion freezing at discrete temperatures, immersion freezing was investigated by activating particles to droplets at high RHw followed by steady cooling under imposed ice-saturated conditions
Summary
Atmospheric ice nucleation plays an important role in initiating precipitation in clouds that consist of a mixture of supercooled liquid water droplets and ice crystals and in catalyzing the formation of ice particles within high-altitude cirrus clouds (Lohmann and Feichter, 2005; Boucher et al, 2013). Immersion-freezing measurements are commonly made using continuous-flow diffusion chamber (CFDC) devices (e.g., Rogers, 1988; Chen et al, 1998; Stetzer et al, 2008; Kanji and Abbatt, 2009; DeMott et al, 2010; Friedman et al, 2011; Chou et al, 2011; Jones et al, 2011; Kanji et al, 2013; Boose et al, 2016; Garimella et al, 2016, Schiebel, 2017; Zenker et al, 2017) These chambers consist of two ice-coated parallel walls held at different temperatures, and different ice supersaturations are achieved by regulating the temperature gradient. We present data to assess the performance from a newly developed CFDC in which all individual aerosol particles are activated to droplets, and these droplets are exposed to a sequence of supercooled temperatures This is accomplished by modifying the existing design of the Pacific Northwest National Laboratory (PNNL) ice chamber (e.g., Friedman et al, 2011; Kulkarni at al., 2012). Various INP proxies for mineral dust types that have previously shown ice nucleation ability over a wide span of supercooled temperatures were used to test the performance of the modified chamber
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