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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Jul 2018

Research article | 03 Jul 2018

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This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.

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

Sarah Grawe1, Stefanie Augustin-Bauditz1,a, Hans-Christian Clemen2, Martin Ebert3, Stine Eriksen Hammer3, Jasmin Lubitz1, Naama Reicher4, Yinon Rudich4, Johannes Schneider2, Robert Staacke5, Frank Stratmann1, André Welti1,b, and Heike Wex1 Sarah Grawe et al.
  • 1Leibniz Institute for Tropospheric Research, Experimental Aerosol and Cloud Microphysics Department, Leipzig, Germany
  • 2Max Planck Institute for Chemistry, Particle Chemistry Department, Mainz, Germany
  • 3Darmstadt University of Technology, Institute of Applied Geosciences, Darmstadt, Germany
  • 4Weizmann Institute of Science, Department of Earth and Planetary Sciences, Rehovot, Israel
  • 5University of Leipzig, Felix Bloch Institute for Solid State Physics, Division of Nuclear Solid State Physics, Leipzig, Germany
  • anow at: Deutscher Wetterdienst, Hamburg, Germany
  • bnow at: Finnish Meteorological Institute, Helsinki, Finland

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 Cloud 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.

We 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 wet-generated 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-ray microanalysis, X-ray diffraction analysis) was performed with dry- and wet-generated particles. From these investigations, we conclude that anhydrous CaSO4 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 propertieswhich are potentially caused by hydration of CaSO4 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.

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Sarah Grawe et al.
Sarah Grawe et al.
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Short summary
In this study, coal fly ash particles immersed in supercooled cloud droplets were analyzed concerning their freezing behavior. Additionally, physico-chemical particle properties (morphology, chemical composition, crystallography) were investigated. In combining both aspects, components that potentially contribute to the observed freezing behavior of the ash could be identified. Interactions at the particle-water interface, that depend on suspension time and influence freezing, are discussed.
In this study, coal fly ash particles immersed in supercooled cloud droplets were analyzed...