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 of INPs' and INMs' 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 Nucleation Counter (FINC), a droplet freezing technique (DFT), for the quantification of INP and INM concentrations in the immersion freezing mode. FINC's design builds upon previous 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 −25.37 ± 0.15 °C with 5 μL, and has an instrument temperature uncertainty of ±0.5 °C. We further conducted freezing control experiments to quantify the non-homogeneous 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 with two other drop-freezing instruments: ETH's DRoplet Ice Nuclei Counter Zurich (DRINCZ) and 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 compare its freezing temperature across different drop-freezing instruments, including on DRINCZ and LINDA, and determine an empirical fit parameter for future drop freezing validations. Second, we show that commercial lignin has a consistent ice nucleating activity across product batches. Third, we demonstrate that the ice nucleating ability of aqueous lignin solutions are stable over time. With these findings, we aim to show that lignin can be used as a good immersion freezing standard in future technique intercomparisons in the field of atmospheric ice nucleation.

Microphysical investigation of the seeder and feeder region of an Alpine mixed-phase cloud

“…The predominant ice particle shape was obtained from LDR measurements of the cloud radar and the ice crystal images observed by HOLIMO. For this case, the particle shapes from Mitchell (1996) were used, assuming "hexagonal plates" for ice crystals smaller than 600 µm in diameter and "aggregates of planar polycrystals in cirrus clouds" for ice particles larger than 600 µm in diameter. For a particular ice crystal shape, the whole lookup table is searched for matching measurement values within the margins of the corresponding measurement errors.…”

Lignin's ability to nucleate ice via immersion freezing and its stability towards physicochemical treatments and atmospheric processing

“…Lignin is a subcomponent of organic matter in aerosols and soils and is capable of nucleating ice in mixed-phase cloud conditions. Specifically, n m values for lignin solutions ranged from 1 to 10 5 IN sites (mg C) −1 between −7.6 to −26.2 • C, representing 1-2 orders of magnitude lower values than dissolved organic matter samples Knackstedt et al, 2018;Moffett et al, 2018), seasurface microlayer samples (Irish et al, 2017;Wilson et al, 2015) and plant extracts (Gute and Abbatt, 2018;Steinke et al, 2020). However, lignin concentrations in the atmosphere have been estimated to be up to 150 ng m −3 after events related to biomass burning (Myers-Pigg et al, 2016).…”

“…Organic aerosols are ubiquitous in the environment (Jimenez et al, 2009) and their ice-nucleating ability is highly variable and depends on their chemical composition (Knopf et al, 2018). Recently, dissolved organic matter from lakes and rivers have been identified as efficient soluble INPs (Borduas-Dedekind et al, 2019;Knackstedt et al, 2018;Moffett et al, 2018). The analytical challenge, however, of resolving the chemical composition of complex organic matter hinders our ability to identify the specific functional group, chemical moieties or conformations acting as a surface to form a template for ice.…”

“…The production of lake, river or sea spray aerosol transfers this biological material including lignin and other biogenic macromolecules into the atmosphere (Axson et al, 2016;Knackstedt et al, 2018;Meyers-Schulte and Hedges, 1986;Olson et al, 2019;Slade et al, 2010;Zark and Dittmar, 2018). The aerosols containing this complex organic matter have been shown to nucleate ice at temperatures relevant for mixed-phase clouds (Borduas-Dedekind et al, 2019;Knackstedt et al, 2018;Moffett et al, 2018). (2) Soils can be a source of lignin-containing organic matter to the atmosphere.…”

Towards parameterising atmospheric concentrations of ice-nucleating particles active at moderate supercooling

“…The presence of ice in clouds is important for precipitation initiation (Mülmenstädt et al, 2015;Heymsfield et al, 2020). Ice-nucleating particles (INPs) affect clouds and their development by generating primary ice at temperatures between 0 and −38 • C. The difficulty of understanding and thus predicting the atmospheric INP concentration ([INP]) originates from observational challenges related to field measurement techniques (Cziczo et al, 2017), the large variety of potential sources (Kanji et al, 2017), and the wide range in atmospheric abundances from 10 −6 to 10 3 L −1 (Petters and Wright, 2015).…”