Lignin's ability to nucleate ice via immersion freezing

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.