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. Aerosol–cloud interactions dominate the uncertainties in currentpredictions of the atmosphere's radiative balance. Specifically, the icephase remains difficult to predict in mixed-phase clouds, where liquid waterand ice co-exist. The formation of ice in these clouds originates fromheterogeneous ice nucleation processes, of which immersion freezing is adominant pathway. Among atmospheric surfaces capable of forming a template for ice,mineral dust, biological material and more recently organic matter areknown to initiate freezing. To further our understanding of the role oforganic matter in ice nucleation, we chose to investigate the ice nucleation(IN) ability of a specific subcomponent of atmospheric organic matter, thebiopolymer lignin. Ice nucleation experiments were conducted in ourcustom-built freezing ice nuclei counter (FINC) to measure freezingtemperatures in the immersion freezing mode. We find that lignin acts as anice-active macromolecule at temperatures relevant for mixed-phase cloudprocesses (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 anon-linear effect in freezing temperatures; the number of IN sites per milligram ofcarbon increased with decreasing lignin concentration. We attribute thischange to a concentration-dependant aggregation of lignin in solution. Wefurther investigated the effect of physicochemical treatments on lignin's INactivity, including experiments with sonication, heating and reaction withhydrogen 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 backgroundlevel. Next, photochemical and ozone bubbling experiments were conducted totest the effect of atmospheric processing on lignin's IN activity. We showedthat this activity was not susceptible to changes under atmosphericallyrelevant conditions, despite chemical changes observed by UV–Vis absorbance. Our results present lignin as a recalcitrant IN-active subcomponent oforganic matter within, for example, biomass burning aerosols and brown carbon.They further contribute to the understanding of how soluble organic materialin the atmosphere can nucleate ice.

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