Immersion freezing measurements were performed with CFA samples from four anonymous German coal-fired power plants. Three instruments were used to achieve good coverage of the temperature range between the first occurrence of heterogeneous ice nucleation and the homogeneous freezing limit. Firstly, the Leipzig Aerosol Cloud Interaction Simulator (LACIS ), a laminar flow tube where single, size-selected particles are immersed in airborne droplets, was used. Here, the influence of suspension time of CFA particles in water could be investigated by employing dry particle generation (aerosolization of dry sample) and wet particle generation (atomization of CFA-water suspensions). Secondly, two cold stage setups, one using microliter sized droplets (Leipzig Ice Nucleation Array; LINA ) and one using nanoliter sized droplets (WeIzman Supercooled Droplets Observation on Microarray setup; WISDOM ) were applied.
We found that CFA 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 the cold stage measurements. In order to understand the reason behind the deactivation, a series of chemical composition, morphology, and crystallography analyses (e.g., 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, which shows the same qualitative immersion freezing behavior as was observed for dry-generated CFA particles and is known to occur as a coating on submicron CFA , triggers heterogeneous ice nucleation at the CFA-water interface. The observed deactivation is likely caused by CaSO4 being dissolved with time which leads to the adsorption of hydrated ions on the CFA surface and hinders direct interaction with the surrounding water molecules .
 M. Brands, M. Kamphus, T. Böttger, J. Schneider, F. Drewnick, A. Roth, J. Curtius, C. Voigt, A. Borbon, M. Beekmann, A. Bourdon, T. Perrin, and S. Borrmann. Characterization of a newly developed aircraft-based laser ablation aerosol mass spectrometer (ALABAMA) and first field deployment in urban pollution plumes over Paris during MEGAPOLI 2009. Aerosol Science and Technology, 45(1): pages 46–64, 2011.
 J. Chen, Z. Wu, S. Augustin-Bauditz, S. Grawe, M. Hartmann, X. Pei, Z. Liu, D. Ji, and H. Wex. Ice nucleating particle concentrations unaffected by urban air pollution in Beijing, china. Atmospheric Chemistry and Physics, 2017:1–33, 2017.
 M. Enders. The CaO distribution to mineral phases in a high calcium fly ash from Eastern Germany. Cement and Concrete Research, 26(2): pages 243–251, 1996.
 R. C. Flagan and J. H. Seinfeld. Fundamentals of Air Pollution Engineering, Chapter: Removal of Particles from Gas Streams, pages 391–478. Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1988.
 S. Grawe, S. Augustin-Bauditz, S. Hartmann, L. Hellner, J. B. C. Pettersson, A. Prager, F. Stratmann, and H. Wex. The immersion freezing behavior of ash particles from wood and brown coal burning. Atmospheric Chemistry and Physics, 16, pages 13911–13928, 2016.
 S. Hartmann, D. Niedermeier, J. Voigtländer, T. Clauß, R. A. Shaw, H. Wex, A. Kiselev, and F. Stratmann. Homogeneous and heterogeneous ice nucleation at LACIS: Operating principle and theoretical studies. Atmospheric Chemistry and Physics, 11, pages 1753–1767, 2011.
 D. Havlíček, R. Přibil, and O. Školoud. The chemical and mineralogical composition of the water-soluble fraction of power-plant ash and its effect on the process of crystallization of water. Atmospheric Environment, Vol. 27A, No. 5, pages 655–660, 1993.
 K. Kandler, N. Benker, U. Bundke, E. Cuevas, M. Ebert, P. Knippertz, S. Rodrguez, L. Schütz, and S. Weinbruch. Chemical composition and complex refractive index of Saharan mineral dust at Izaña, Tenerife (Spain) derived by electron microscopy. Atmospheric Environment, 41(37): pages 8058–8074, 2007.
 N. Reicher, L. Segev, and Y. Rudich. The WeIzmann Supercooled Droplets Observation on a Microarray (WISDOM) and application for ambient dust. Atmospheric Measurement Techniques, 11(1): pages 233–248, 2018.
 T. Sievert, A. Wolter, and N. B. Singh. Hydration of anhydrite of gypsum (CaSO4.II) in a ball mill. Cement and Concrete Research, 35(4): pages 623–630, 2005.
 N. S. Umo, B. J. Murray, M. T. Baeza-Romero, J. M. Jones, A. R. Lea-Langton, T. L. Malkin, D. O’Sullivan, L. Neve, J. M. C. Plane, and A. Williams. Ice nucleation by combustion ash particles at conditions relevant to mixed-phase clouds. Atmospheric Chemistry and Physics, 15, pages 5195–5210, 2015.