Lignin's ability to nucleate ice via immersion freezing
- Institute of Biogeochemistry and Pollutant Dynamics (IBP), ETH Zurich, Zurich, Switzerland
Uncertainties in current predictions for the atmosphere’s radiative balance are dominated by the impact of clouds. Ice nucleating particles (INPs) play a dominant role in the formation of mixed-phase clouds, however there is still a lack of understanding of how INPs interact with water in the freezing process. Detailed elucidations of the organic aerosol chemical composition from IN active atmospheric samples are scarce which is due to the analytical challenge of resolving their high complexity. We chose to reduce sample complexity by investigating the IN activity of a specific sub-component of organic aerosols, the biopolymer lignin. This approach facilitates connecting ice nucleating abilities to molecular properties. Ice nucleation experiments were conducted in our home-built Freezing Ice Nuclei Counter (FINC) to measure freezing temperatures in the immersion freezing mode which is the dominant IN mechanism in mixed-phase clouds. We find that lignin acts as an INP at temperatures relevant for mixed-phase cloud processes (e.g. 50% activated fraction at – 20 °C concentrated 20 mg C/L). Photochemistry and ozonation experiments were subsequently conducted to test the effect of atmospheric processing on lignin’s IN activity. We discovered that this activity was not susceptible to change under environmentally relevant conditions even though structural changes were introduced by monitoring UV/Vis absorbance. Additionally to atmospheric processing, laboratory treatments including heating, sonication and oxidation with hydrogen peroxide were done, where only the heating experiments had a decreasing effect on lignin’s IN activity. Based on these results, we present a thorough INP characterization of lignin, a specific organic matter subcomponent, and contribute to the understanding of how organic material present in the atmosphere can nucleate ice.
How to cite: Bogler, S. and Borduas-Dedekind, N.: Lignin's ability to nucleate ice via immersion freezing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-255, https://doi.org/10.5194/egusphere-egu2020-255, 2019
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Hi Sophie and Nadine
Nice poster! I am interested in lignin. Apart from what Yang has presented on solid surfaces, we are also doing related experiments with organic surfactants, such as hydroxy-substituted aromatics (orcinol and resorcinol), and we see interesting changes on the O-NEXAFS of water just underneath the surface, thus of water molecules in immediate vicinity of the organic molecules on the surface.
In your freezing experiments with lignin: how do you know whether freezing was initiated at the solution - vessel interface or the solution-air interface or on molecules in the bulk?
When lignin is dissolved in water, is there a size distribution? what is the monomer? I guess it is not desintegrating into the coniferyl-alcohol monomers, right?
Thanks
Markus
Hi Markus – thank you for your great questions and your interest in lignin!
In your freezing experiments with lignin: how do you know whether freezing was initiated at the solution-vessel interface or the solution-air interface or on molecules in the bulk?
We have had the same question! We’ve attempted to convince ourselves that the interface-solution and air-solution plays a negligible role according to these three points:
- First, the solution-vessel interface and the solution-air interface remain constant for all our experiments, as the sample volume per well is also constant throughout the series. The fact that more diluted solutions have a higher nm value, active sites per mg of carbon, suggest that the wall interfaces cannot be responsible for the majority of the IN activity. Instead, processes in the bulk phase are important.
- Second, when decreasing the volume, the surface area does decrease, so it’s still a possibility that the surface-air interface can play a role. However so far, experiments with and without film covers have given us the same results, so we can suggest that water-air partitioning at this interface doesn’t play a role. Still, we can’t at this point say anything with certainty about the solution-air interface directly.
- Third, validation experiences by my colleague Anna Miller (see poster D3126EGU2020-630) that lignin’s IN ability measured with other instruments is consistent and predictable, including for microfluidic devices. This intercomparing suggest that we are indeed measuring the IN activity of lignin inside an aqueous droplet. Nonetheless, we do not know how lignin is potentially interacting with the well surface of the PCR trays, but at this point in time we have done the control experiments to support (although not directly confirm) that we are measuring soluble lignin’s IN ability as a macromolecule acting as the ice templating surface.
When lignin is dissolved in water, is there a size distribution? what is the monomer? I guess it is not desintegrating into the coniferyl-alcohol monomers, right?
- Yes, the lignin we are using has a size distribution within its polymers (as is common for this biopolymer) and we hypothesize that it is aggregating in water. We could only measure sizes based on filtering experiments. They revealed that the distribution ranges at least from < 0.02 µm to > 0.2 µm. A good overview of sizes and shapes of lignin be found in the Book Lignin Chemistry and Applications, Chapter 2, p.42, Table 2.13 (https://www.sciencedirect.com/science/article/pii/B9780128139417000023?via%3Dihub).
- The monomeric composition of this lignin is also a complex mixture of different compounds with likely the most contribution from coniferyl alcohol. We found the commercial lignin we worked with to be rather recalcitrant towards chemical and atmospheric processing. Therefore, we hypothesize that it is not disintegrating into its monomers through dissolution only.
Please let us know if you have any follow-up questions!
Best,
Sophie and Nadine