Development of the drop Freezing Ice Nuclei Counter (FINC), intercomparison of droplet freezing techniques, and use of soluble lignin as an atmospheric ice nucleation standard

Abstract

Abstract. Aerosol–cloud interactions, including the ice nucleation of supercooled liquid water droplets caused by ice-nucleating particles (INPs) and macromolecules (INMs), are a source of uncertainty in predicting future climate. Because INPs and INMs have spatial and temporal heterogeneity in source, number, and composition, predicting their concentration and distribution is a challenge requiring apt analytical instrumentation. Here, we present the development of our drop Freezing Ice Nuclei Counter (FINC) for the estimation of INP and INM concentrations in the immersion freezing mode. FINC's design builds upon previous droplet freezing techniques (DFTs) and uses an ethanol bath to cool sample aliquots while detecting freezing using a camera. Specifically, FINC uses 288 sample wells of 5–60 µL volume, has a limit of detection of −25.4 ± 0.2 ∘C with 5 µL, and has an instrument temperature uncertainty of ± 0.5 ∘C. We further conducted freezing control experiments to quantify the nonhomogeneous behavior of our developed DFT, including the consideration of eight different sources of contamination. As part of the validation of FINC, an intercomparison campaign was conducted using an NX-illite suspension and an ambient aerosol sample from two other drop freezing instruments: ETH's DRoplet Ice Nuclei Counter Zurich (DRINCZ) and the University of Basel's LED-based Ice Nucleation Detection Apparatus (LINDA). We also tabulated an exhaustive list of peer-reviewed DFTs, to which we added our characterized and validated FINC. In addition, we propose herein the use of a water-soluble biopolymer, lignin, as a suitable ice-nucleating standard. An ideal INM standard should be inexpensive, accessible, reproducible, unaffected by sample preparation, and consistent across techniques. First, we compared lignin's freezing temperature across different drop freezing instruments, including on DRINCZ and LINDA, and then determined an empirical fit parameter for future drop freezing validations. Subsequently, we showed that commercial lignin has consistent ice-nucleating activity across product batches and demonstrated that the ice-nucleating ability of aqueous lignin solutions is stable over time. With these findings, we present lignin as a good immersion freezing standard for future DFT intercomparisons in the research field of atmospheric ice nucleation.

Optical trapping and light scattering in atmospheric aerosol science

“…101,103,104 The heterogeneous freezing mechanisms allow the formation of ice at higher temperatures and are crucial to the formation of ice in mixedphase clouds. 34,74,94,99,105,106 However, a detailed microphysical understanding of these processes which allows representation in cloud and climate models remains elusive. 1,74,97,107,108 The two most important parameters for predicting ice nucleation are temperature and saturation ratio with respect to ice.…”

“…For example, soluble inorganic salts commonly found in atmospheric aerosol-in particular (NH 4 ) 2 SO 4 104,109,114 and NaCl 19 -have been shown to nucleate ice, contradicting the expectation that good IN must be insoluble. Soluble organic systems have also demonstrated IN activity, namely dissolved organic matter 106,118 and surfactants. 105 Submicron particles have also been shown to be effective IN, 34,94 in contrast with the expectation that good IN should be relatively large, i.e.…”

Integrated Science Teaching in Atmospheric Ice Nucleation Research: Immersion Freezing Experiments

“…Droplet freezing assays are a common practice in atmospheric INP research to measure the immersion freezing abilities of INPs as a function of temperature in a controlled setting. This technique has been widely applied to assess freezing properties of various sample types, including suspended dry powders and filter-collected ambient particle suspensions, from different environments. − The reproducibility of this assay is dependent on exclusion of background freezing artifacts . Additionally, substrate surfaces, variation in droplet size, and experimental variables can impact the ability to determine homogeneous freezing temperature, which is typically below ≈−35 °C .…”

Atmospheric Humic‐Like Substances (HULIS) Act as Ice Active Entities

“…Organic matter (OM) in water (Borduas‐Dedekind et al., 2019; Knackstedt et al., 2018; Moffett et al., 2018; Wilson et al., 2015), soil (Hill et al., 2016; O'Sullivan et al., 2014) or of biogenic origin (DeMott et al., 2016; Gute & Abbatt, 2018; Irish et al., 2019; Pummer et al., 2012, 2015) were reported to nucleate ice under mixed‐phase cloud conditions. These OM can be constituents from plants or microorganisms such as polysaccharides (Hill et al., 2016; Xi et al., 2021), ice active macromolecules, cellulose (Hiranuma et al., 2015, 2019), lignin (Bogler & Borduas‐Dedekind, 2020; Miller et al., 2020), or other unidentified substances (Hill et al., 2016; McCluskey, Hill, Humphries, et al., 2018; Steinke et al., 2020). These studies help to deepen our knowledge on the sources and ice nucleation properties of atmospheric organic INPs.…”