Coal fly ash: Linking immersion freezing behavior and physico-chemical particle properties

Abstract

Abstract.To date, only a few studies have investigated the potential of coal fly ash particles to trigger heterogeneous ice nucleation in cloud droplets. The presented measurements aim at expanding the sparse dataset and improving process understanding of how physico-chemical particle properties influence the freezing behavior of coal fly ash particles immersed in water.Firstly, immersion freezing measurements were performed with two single particle techniques, i.e., the Leipzig Aerosol 5Cloud Interaction Simulator and the Spectrometer for Ice Nuclei. The effect of suspension time on the efficiency of the coal fly ash particles when immersed in a cloud droplet is analyzed based on the different residence times of the two instruments and employing both dry and wet particle generation. Secondly, two cold stage setups, one using microliter sized droplets (Leipzig Ice Nucleation Array) and one using nanoliter sized droplets (Weizman Supercooled Droplets Observation on Microarray setup) were applied. 10We found that coal fly ash particles are comparable to mineral dust in their immersion freezing behavior when being dry-generated. However, a significant decrease in immersion freezing efficiency was observed during experiments with wetgenerated particles in LACIS and SPIN. The efficiency of wet-generated particles is in agreement with the cold stage measurements. In order to understand the reason behind the deactivation, a series of chemical composition, morphology, and crystallography analyses (single particle mass spectrometry, scanning electron microscopy coupled with energy dispersive X-15 ray microanalysis, X-ray diffraction analysis) was performed with dry-and wet-generated particles. From these investigations, we conclude that anhydrous CaSO 4 and CaO, which, if investigated in pure form, show the same qualitative immersion freezing behavior as observed for dry-generated coal fly ash particles, contribute to triggering heterogeneous ice nucleation at the particle-water interface. The observed deactivation in contact with water is related to changes of the particle surface properties 1 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2018-583 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 3 July 2018 c Author(s) 2018. CC BY 4.0 License. which are potentially caused by hydration of CaSO 4 and CaO. The contribution of coal fly ash to the ambient population of ice nucleating particles therefore depends on whether and for how long particles are immersed in cloud droplets.