16AEROSOL Developing a technique for separating ice nucleating substances via density gradient centrifugation

There is evidence that many different types of materials can act as INSs, with mineral dust and biological materials currently believed to be two of the most effective and relevant classes of INSs to the atmosphere. However, when and where each type of ice nuclei is a significant fraction of the INSs in the atmosphere remains unresolved. Several existing methods are currently used to both quantify and characterize INSs in the atmosphere which range from single particle analysis techniques like particle aerosol analysis by laser mass spectrometry and scanning electron microscopy paired with energy dispersive X-ray spectroscopy to bulk analyses such as heat treatment and treatment with (NH₄)₂SO₄. Here we develop a complementary technique to detect and characterize INSs based on buoyant density.

Density gradient centrifugation involves separating components of a solution, in this case mineral dust particles and biological material, by buoyant density into layers using centrifugal force and media of known density. Once the two components are separated, droplet freezing experiments can be done on each aliquot separately to study the low- and high-density INSs present independently. Seeing as different classes of INPs are known to have significantly different densities with mineral dusts ≥ 2.5 g cm^-3, biological materials between 0.9 and 1.75 g cm^-3, an understanding of an INSs density should provide insight into its identity.

To test our proposed protocol, we are using several test materials including bacterial cultures, Snomax (a commercial snow inducer made of freeze-dried components of P. syringae), fungal cultures, organics such as lignin and fulvic acid, and mineral dusts. We have performed the proposed density gradient centrifugation protocol on suspensions of each material individually, as well as several 2-component INS mixtures (e.g. K-feldspar and Snomax). Most of the bacterial cell, fungal cell and Snomax INSs were successfully recovered in the low-density isolates with minimal evidence of INSs in the high-density isolates. Similarly, most of the mineral dust INSs were successfully recovered in the high-density isolates with minimal evidence of INSs in the low-density isolates. 2-component INS mixtures have also been successfully separated. These preliminary results suggest that there is strong potential in using density gradient centrifugation to separate INSs based on density.