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https://doi.org/10.5194/acp-2020-589
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2020-589
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  25 Jun 2020

25 Jun 2020

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A revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

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

Sophie Bogler1 and Nadine Borduas-Dedekind1,2 Sophie Bogler and Nadine Borduas-Dedekind
  • 1Institute for Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, 8092, Switzerland
  • 2Institute for Atmospheric and Climate Sciences, ETH Zurich, Zurich, 8092, Switzerland

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.

Sophie Bogler and Nadine Borduas-Dedekind

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Sophie Bogler and Nadine Borduas-Dedekind

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Lignin’s ability to nucleate ice via immersion freezing and its stability towards physicochemical treatments and atmospheric processing S. Bogler and N. Borduas-Dedekind https://doi.org/10.3929/ethz-b-000422111

Sophie Bogler and Nadine Borduas-Dedekind

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Latest update: 23 Nov 2020
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Short summary
To study the role of organic matter in ice crystal formation, we investigated the ice nucleation ability of a sub-component of organic aerosols, the biopolymer lignin, using a drop freezing technique. We found that lignin is an ice active macromolecule with changing abilities based on dilutions. The effects of atmospheric processing and of physicochemical treatments on the ability of lignin to freeze in immersion freezing were negligible. Lignin is a recalcitrant ice nucleating macromolecule.
To study the role of organic matter in ice crystal formation, we investigated the ice nucleation...
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