Evaporating brine from frost flowers with electron microscopy, and implications for atmospheric chemistry and sea-salt aerosol formation
Xin Yang1, Vilém Neděla2, Jiří Runštuk2, Gabriela Ondrušková3, Ján Krausko3, Ľubica Vetráková3, and Dominik Heger31British Antarctic Survey, Natural Environment Research Council, Cambridge, UK 2Environmental Electron Microscopy Group, Institute of Scientific Instruments of the CAS, Brno, Czech Republic 3Department of Chemistry, Faculty of Science, Masaryk University, Kamenice 5/A8, 625 00 Brno Research, and Centre for Toxic Compounds in the Environment (RECETOX), Masaryk University, Kamenice 5/A29, 625 00 Brno
Received: 13 Jan 2017 – Accepted for review: 19 Jan 2017 – Discussion started: 25 Jan 2017
Abstract. An environmental scanning electron microscope was used for the first time to obtain well-resolved images, in both temporal and spatial dimensions, of lab-prepared frost flowers (FFs) under evaporation within the chamber temperature range from −5 °C to −18 °C and pressures above 500 Pa. Our scanning shows temperature-dependent NaCl speciation: the brine covering the ice was observed at all conditions, whereas the NaCl crystals were formed at temperatures below −10 °C as the brine oversaturation was achieved. Finger-like ice structures covered by the brine, with a diameter of several micrometres and length of tens to one hundred micrometres, are exposed to the ambient air. The brine-covered fingers are highly flexible and cohesive. The exposure of the liquid brine on the micrometric fingers indicates a significant increase in the brine surface area compared to that of the flat ice surface at high temperatures, whereas the NaCl crystals can become sites of heterogeneous reactivity at lower temperatures. There is no evidence that, without external forces, salty FFs could automatically fall apart to create a number of sub-particles at the scale of micrometres as the exposed brine fingers seem cohesive and hard to break in the middle. The fingers tend to combine together to form large spheres and then join back to the mother body, eventually forming a large chunk of salt after complete dehydration. A present microscopic observation rationalizes several previously unexplained observations, namely, that FFs are not a direct source of sea salt aerosols and that saline ice crystals under evaporation could accelerate the heterogeneous reactions of bromine liberation.
Yang, X., Neděla, V., Runštuk, J., Ondrušková, G., Krausko, J., Vetráková, Ľ., and Heger, D.: Evaporating brine from frost flowers with electron microscopy, and implications for atmospheric chemistry and sea-salt aerosol formation, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-35, in review, 2017.