Heterogeneous Ice Nucleation of Super-Cooled Water Droplets By Biomass Burning Organic Aerosols and Bioaerosols: The Role of Hydrophobic Interfaces

With the increase of wildfires, there are more biomass burning organic aerosols present in the atmosphere, capable of impacting cloud formation and precipitation patterns. Recent evidence from our group and the community suggests that organic aerosols can catalyze heterogeneous ice nucleation.^1–3 We are investigating the mechanism by which atmospheric biomass burning organic aerosols impact mixed-phase cloud composition.

An important component of organic biomass aerosols is lignin, a macromolecule which provides strength and structure to vascular plants. Lignin has been measured as a notably recalcitrant component of organic aerosols following biomass burning events.^4,5 To elucidate the role of chemistry and size of biomass burning organic aerosols in ice nucleation, we synthesized nanoparticles from commercially available Kraft lignin via a facile nanoprecipitation process.^6,7 The nanoparticles were centrifugally separated by size, characterized by nanoparticle tracking analysis (NTA) and by transmission electron microscopy (TEM), then tested for their freezing ability in our home-built Freezing Ice Nuclei Counter (FINC).⁸ Next, the freezing mechanism and location of onset freezing for lignin were investigated using a high-speed camera on a cryo-microscope.⁹ Cylindrical droplets, between two glass slides, were frozen to localize the onset location of freezing at the air-water interface (AWI) or in the bulk of the droplets. Most recently, we are developing a cryo-microscope technique capable of identifying freezing in micrometer-sized droplets in oil emulsions.

With these techniques in hand, our results suggest that lignin, whether dissolved or as nanoparticles ranging in size from 50 – 500 nm in diameter, is ice active up to -15 ºC. Normalizing the freezing data to mass and surface area suggests that aggregation must occur for lignin and lignin nanoparticles to nucleate ice. Moreover, the high-speed videos suggest that lignin’s ice nucleation activity is higher closer to the air-water interface of a droplet, indicating that hydrophobic interactions could be responsible for the aggregation of lignin and adsorption at the interface, similar to the behavior of surfactants. A decrease in surface tension of droplets containing lignin further underlines this proposed mechanism. These findings help understand how lignin, and by extension biomass burning organic aerosols, are able to nucleate ice and hence impact the ice crystal concentration in mixed-phase clouds.

(1) Bogler, S.; Borduas-Dedekind, N. Lignin’s Ability to Nucleate Ice via Immersion Freezing and Its Stability towards Physicochemical Treatments and Atmospheric Processing. Atmospheric Chem. Phys. 2020, 20 (23), 14509–14522. https://doi.org/10.5194/acp-20-14509-2020.

(2) Knopf, D. A.; Alpert, P. A.; Wang, B. The Role of Organic Aerosol in Atmospheric Ice Nucleation: A Review. ACS Earth Space Chem. 2018, 2 (3), 168–202. https://doi.org/10.1021/acsearthspacechem.7b00120.

(3) Tobo, Y.; DeMott, P. J.; Hill, T. C. J.; Prenni, A. J.; Swoboda-Colberg, N. G.; Franc, G. D.; Kreidenweis, S. M. Organic Matter Matters for Ice Nuclei of Agricultural Soil Origin. Atmos Chem Phys 2014, 14 (16), 8521–8531. https://doi.org/10.5194/acp-14-8521-2014.

(4) Shakya, K. M.; Louchouarn, P.; Griffin, R. J. Lignin-Derived Phenols in Houston Aerosols: Implications for Natural Background Sources. Environ. Sci. Technol. 2011, 45 (19), 8268–8275. https://doi.org/10.1021/es201668y.

(5) Myers-Pigg, A. N.; Griffin, R. J.; Louchouarn, P.; Norwood, M. J.; Sterne, A.; Cevik, B. K. Signatures of Biomass Burning Aerosols in the Plume of a Saltmarsh Wildfire in South Texas. Environ. Sci. Technol. 2016, 50 (17), 9308–9314. https://doi.org/10.1021/acs.est.6b02132.

(6) Lievonen, M.; Valle-Delgado, J. J.; Mattinen, M.-L.; Hult, E.-L.; Lintinen, K.; Kostiainen, M. A.; Paananen, A.; Szilvay, G. R.; Setälä, H.; Österberg, M. A Simple Process for Lignin Nanoparticle Preparation. Green Chem. 2016, 18 (5), 1416–1422. https://doi.org/10.1039/C5GC01436K.

(7) Zou, T.; Nonappa, N.; Khavani, M.; Vuorte, M.; Penttilä, P.; Zitting, A.; Valle-Delgado, J. J.; Elert, A. M.; Silbernagl, D.; Balakshin, M.; Sammalkorpi, M.; Österberg, M. Experimental and Simulation Study of the Solvent Effects on the Intrinsic Properties of Spherical Lignin Nanoparticles. J. Phys. Chem. B 2021, 125 (44), 12315–12328. https://doi.org/10.1021/acs.jpcb.1c05319.

(8) Miller, A. J.; Brennan, K. P.; Mignani, C.; Wieder, J.; David, R. O.; Borduas-Dedekind, N. 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. Atmospheric Meas. Tech. 2021, 14 (4), 3131–3151. https://doi.org/10.5194/amt-14-3131-2021.

(9) Bieber, P.; Borduas-Dedekind, N. High-Speed Cryo-Microscopy Reveals That Ice-Nucleating Proteins of Pseudomonas Syringae Trigger Freezing at Hydrophobic Interfaces. Sci. Adv. 2024, 10 (27), eadn6606. https://doi.org/10.1126/sciadv.adn6606.