To study the role of organic matter in ice crystal formation, we investigated the ice nucleation ability of a subcomponent of organic aerosols, the biopolymer lignin, using a droplet-freezing technique. We found that lignin is an ice-active macromolecule with changing abilities based on dilutions. The effects of atmospheric processing and of physicochemical treatments on the ability of lignin solutions to freeze were negligible. Thus, lignin is a recalcitrant ice-nucleating macromolecule.

For characterizing atmospheric ice nuclei, we present (1) the development of our home-built droplet freezing technique (DFT), the Freezing Ice Nuclei Counter (FINC), (2) an intercomparison campaign using NX-illite and an ambient sample with three DFTs, and (3) the application of lignin as a soluble and commercial ice nuclei standard with four DFTs. We further compiled the growing number of DFTs in use for atmospheric ice nucleation since 2000, to which we add FINC.