A method for separating and quantifying organic and inorganic ice nucleating substances in atmospheric samples based on density gradient centrifugation

03 May 2024, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Ice nucleating substances (INSs) influence the properties and frequencies of ice and mixed-phase clouds in the atmosphere, and hence, 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 technique. Inorganic K-feldspar and organic Snomax INSs 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 less than 100% can be explained by losses of INSs to vessel walls, accidental mixing of the different density layers during pipetting, and incomplete collection of material during pipetting.

Keywords

ice-nucleation

Supplementary materials

Title
Description
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Title
SPINDL Manuscript_Supplement
Description
Explanations of sedimentation time for density gradient centrifugation; sedimentation time for differential centrifugation washing step; correction of freezing point depression, tables of INS types; estimated sedimentation time during density gradient centrifugation; estimated sedimentation time during centrifugation washing step, and figures of experimental setup; recovered IN activity; INS concentrations of processed and unprocessed suspensions; INS recoveries after centrifugation.
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