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

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 co-exist. 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 forming a template for 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 subcomponent of atmospheric organic matter, the biopolymer lignin. Ice nucleation experiments were conducted in our custom-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 milligram of 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. Only harsh conditions such as heating to 260 ∘C and addition of a mixture with a ratio of 1 : 750 of grams of lignin to millilitres of hydrogen peroxide were able to decrease lignin's IN activity to the instrument's background level. Next, photochemical and ozone bubbling 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. They further contribute to the understanding of how soluble organic material in the atmosphere can nucleate ice.