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

Abstract

Abstract. Aerosol–cloud interactions dominate the uncertainties in current predictions of the atmosphere's radiative balance. Specifically, the ice phase remains difficult to predict in mixed-phase clouds, where liquid water and ice coexist. The formation of ice in these clouds originates from heterogeneous ice nucleation processes, of which immersion freezing is a dominant pathway. Among atmospheric surfaces capable of templating ice, mineral dust, biological material, and more recently organic matter are known to initiate freezing. To further our understanding of the role of organic matter in ice nucleation, we chose to investigate the ice nucleation (IN) ability of a specific sub-component of atmospheric organic matter, the biopolymer lignin. Ice nucleation experiments were conducted in our home-built Freezing Ice Nuclei Counter (FINC) to measure freezing temperatures in the immersion freezing mode. We find that lignin acts as an ice active macromolecule at temperatures relevant for mixed-phase cloud processes (e.g. 50 % activated fraction up to −18.8 °C at 200 mg C L−1). Within a dilution series of lignin solutions, we observed a non-linear effect in freezing temperatures; the number of IN sites per mg carbon increased with decreasing lignin concentration. We attribute this change to a concentration-dependant aggregation of lignin in solution. We further investigated the effect of physicochemical treatments on lignin's IN activity, including experiments with sonication, heating and reaction with hydrogen peroxide. Indeed, harsh conditions such as heating to 260 °C and addition of 1 : 750 g of lignin to mL of hydrogen peroxide were needed to decrease lignin's IN activity to the instrument's background level. Next, photochemistry and ozonation experiments were conducted to test the effect of atmospheric processing on lignin's IN activity. We showed that this activity was not susceptible to changes under atmospherically relevant conditions, despite chemical changes observed by UV/Vis absorbance. Our results present lignin as a recalcitrant IN active subcomponent of organic matter within for example biomass burning aerosols and brown carbon, and contribute to the understanding of how soluble organic material in the atmosphere can nucleate ice.

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

“…Note that this parameterization is specifically valid for lignin solutions of 20 mg C L −1 . In fact, lignin is a macromolecule suggested to adopt different solution aggregation properties depending on its concentration (Bogler and Borduas-Dedekind, 2020), and thus our paramaterization is specifically for concentrations of 20 mg C L −1 , equivalent to 40 mg of lignin per L. The n m parameterization by Wilson et al (2015) of sea-surface microlayer organic matter is included for reference (Fig. 6a).…”

“…Lignin's recalcitrance towards atmospheric processing has recently been demonstrated by Bogler and Borduas-Dedekind (2020). Indeed, photochemical exposure, ozonation, heating, sonication and hydrogen peroxide treatments had negligible effects on lignin's ice nucleation ability.…”

“…Here, we show that commercial lignin, a complex organic polymer from the cell wall structure of vascular plants (Ciesielski et al, 2020), can serve as a reproducible standard for ice nucleation across different immersion freezing techniques. Indeed, lignin is a water-soluble macromolecule with an ice nucleating activity, thereby qualifying it as an INM (Pummer et al, 2015;Bogler and Borduas-Dedekind, 2020;Steinke et al, 2019). Furthermore, lignin and its oxidation products are abundant in the atmosphere, emitted for example during biomass burning and agricultural harvesting (Myers-Pigg et al, 2016;Shakya et al, 2011).…”

“…Lignin is also produced as a by-product of the industrial kraft process, in which wood is converted to wood pulp and subsequently used for paper products (Harkin, 1969). Recent research has shown lignin to be ice active, albeit with colder freezing temperatures than leaf litter and agricultural dust (Steinke et al, 2019;Bogler and Borduas-Dedekind, 2020). Several other studies have shown that plant materials, which may have included lignin, can be ice active in immersion freezing (e.g., Conen et al, 2016;Felgitsch et al, 2018;Suski et al, 2018;Gute and Abbatt, 2020).…”

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

“…Here, we show that commercial lignin, a complex organic polymer from the cell wall structure of vascular plants (Ciesielski et al, 2020), can serve as a reproducible standard for ice nucleation across different immersion freezing techniques. Indeed, lignin is a water-soluble macromolecule with an ice nucleating activity, thereby qualifying it as an INM (Pummer et al, 2015;Bogler and Borduas-Dedekind, 2020;Steinke et al, 2019). Furthermore, lignin and its oxidation https://doi.org/10.5194/amt-2020-414 Preprint.…”