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

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