A Method for Separating and Quantifying Organic and\nInorganic Ice Nucleating Substances Based on Density Gradient Centrifugation

Ice nucleating substances (INSs)\ninfluence the properties and frequencies\nof ice and mixed-phase clouds in the atmosphere and, hence, the climate\nand the hydrological cycle. INSs can be classified as inorganic (e.g.,\nmineral dust, volcanic ash) or organic (e.g., bacterial cells, cell-free\nproteins). While the properties of both INS classes have been studied\nin the laboratory, the amounts in the atmosphere are still poorly\nconstrained. Here, we demonstrate a new method for separating and\nquantifying inorganic and organic INSs. First, INS suspensions were\nseparated into a high-density isolate containing inorganic INSs and\na low-density isolate containing organic INSs using density gradient\ncentrifugation, and then, INSs were quantified in each isolate using\na droplet freezing assay. K-feldspar and Snomax, inorganic and organic\nINSs respectively, were used to test our method. The average K-feldspar\nINS recovery in the high-density isolate was 54%, with no evidence\nof K-feldspar INSs in the low-density isolate. The average Snomax\nINS recovery in the low-density isolate was 27%, with small amounts\nof Snomax contaminating the high-density isolate. A mixture of K-feldspar\nand Snomax was successfully separated, with recoveries comparable\nto those observed for K-feldspar and Snomax individually. Recoveries\non the order of 30% or greater are acceptable, especially if they\nare known and can be accounted for since the uncertainties associated\nwith the concentrations of organic and inorganic INSs in the atmosphere\ncan span several orders of magnitude.