Ice nucleation activities of agricultural soil derived mineral and organic particles in mixed-phase cloud conditions

Heterogeneous ice nucleation processes and the concentration of ice nucleating particles (INPs) influence the formation of ice cloud, thus affecting cloud radiative properties and precipitation. Agricultural soil dust containing mixed mineral and organic components is a potential source of INPs in the atmosphere, but the factors controlling the ice nucleation efficiency of agricultural soils are not yet well-understood. In this study we investigated soils sampled from Richland, WA (semi-arid site) and the DOE's Southern Great Plains research site in Oklahoma (arable site). Ice nucleation experiments were performed using a continuous flow diffusion chamber, simulating immersion freezing under mixed-phase cloud conditions. Physicochemical properties of individual INPs and particle populations were characterized using multimodal micro-spectroscopy and chemical imaging techniques. Our results show that the mineral components of the soil are more dominant in the ice crystal residues than organic components. Furthermore, we separated out the dissolved organic fraction of soil material by dialysis and performed ice nucleation experiments on the extracted material. We find that the organic components extracted from SGP soil cause immersion freezing more efficiently than organic components extracted from Richland soil. This result suggest that characterization of dissolved soil organic matter is crucial to better understand the ice nucleation process. To this end, we utilized aerosol mass spectrometry and ultra-high resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR) for molecular characterization of soil organic matter. The physicochemical properties of mineral and organic particles derived from agricultural soils may be useful for predicting ice nucleation.