Plant-derived biopolymers initiate heterogeneous ice nucleation via particulate interfaces immersed in supercooled droplets

Organic matter can initiate heterogeneous ice nucleation in supercooled water droplets, thereby influencing atmospheric cloud glaciation. Atmospheric organic matter includes biopolymers, which are emitted as primary bioaerosols, biomass burning aerosols, soil dust and sea spray aerosols and can nucleate ice in the absence of a solid surface like mineral dust. There is evidence that biopolymers could form aggregates in solution, thereby creating ice-nucleating sites. However, the submicron size and heterogeneity of these aggregates creates challenges in studying and predicting their ice-nucleating ability. Here, we characterized self-assembled nanoparticles of cellulose and lignin and of two newly identified ice-nucleating biopolymers, namely xylan and laminarin. Our freezing ice nuclei counter (FINC) instrument measured the median ice nucleation temperatures of aqueous cellulose, lignin, xylan and laminarin samples to be –22.9 °C, –22.0 °C, –14.2 °C, and –20.0 °C, respectively. Furthermore, a nanoparticle tracking technique detected and quantified particles in the biopolymer solutions, with mean diameters between 132 nm and 267 nm. A positive trend between size and nucleation temperatures suggests that the biopolymers initiate freezing via particulate interfaces immersed in the supercooled droplets. Finally, we determined ice-active site densities normalized to quantitatively measured surface area and mass of the nanoparticles, to demonstrate how the biopolymers self-assemble in solution and subsequently nucleate ice in supercooled droplets. The mechanism by which biopolymers nucleate ice leads to improved predictive capabilities to estimate the impact of organic aerosols on climate.