Examining Soil-Dust as Immersion Freezing Ice-Nucleating Particles in Cloud Chamber and Microscopy Experiments

Ice crystal formation plays a crucial role in modulating the radiative properties of clouds and, thus, climate. Furthermore, over the continents most of the precipitation is initiated by the ice phase. Immersion freezing, where an ice-nucleating particle (INP) is engulfed in a supercooled aqueous solution, has been recognized to be a dominating ice formation pathway in mixed-phase clouds where supercooled liquid droplets and ice crystals coexist. Mixed-phase clouds form at temperatures well above the homogenous freezing temperatures of about -38 °C. While mineral dust is known to efficiently nucleate ice, it has been recognized that soil-dust particles associated with organic compounds can be responsible for initiating freezing at temperatures as high as about -5 ºC. Soil-dust can make up a significant fraction of the ambient particle population especially over the continents and, thus, can serve as a significant source of potent INPs. To further contribute to the research on soil-dust INPs, we conducted immersion freezing experiments in the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) cloud chamber covering a temperature range from -18 to -26 ºC using three types of soil samples. Those samples include a U.S. Department of Agriculture standard soil sample, a sample of surface soil collected from the Atmospheric Radiation and Measurement Southern Great Plains (ARM SGP) site, and a sample of airborne soil-dust also collected at the ARM SGP site. The latter two soil samples were collected to place respective INP measurements in context with a recent SGP aerosol-ice formation closure pilot study (AEROICESTUDY). The AIDA chamber experiments were accompanied by a suite of online INP instruments including the portable ice nucleation experiment (PINE) and continuous flow diffusion chamber (CFDC) and an offline INP freezing array, the ice nucleation spectrometer of the Karlsruhe Institute of Technology (INSEKT). The soil-dust particle size distribution (PSD) present in the chamber was measured prior to initiation of ice formation and the particle population was collected on substrates. In addition, ice crystal residuals were collected on substrates using a counterflow-virtual impactor system. This allowed to compare the particle population characteristics with the ice crystal residues, the latter representing the INPs. Particles and INPs collected on substrates were examined using offline single-particle micro-spectroscopic analyses including electron and X-ray microscopy to characterize their physicochemical properties. We present INP measurements by AIDA, PINE, CFDC, and INSEKT instrumentation. Combining measured soil-dust PSDs with different soil-dust immersion freezing parameterizations, we show first results of parcel model simulations predicting measured ice crystal number concentrations in comparison with measurements thereby evaluating the different parameterizations. Micro-spectroscopic particle and INP analyses demonstrate that while the soil-dust is dominated by inorganic-organic particles, the INPs are dominated by organic particles, reflecting a striking particle selection process for ice nucleation.