Articles | Volume 17, issue 10
Atmos. Chem. Phys., 17, 6291–6303, 2017
Atmos. Chem. Phys., 17, 6291–6303, 2017

Research article 23 May 2017

Research article | 23 May 2017

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 Ondrušková3,4, Ján Krausko3,4, Ľubica Vetráková3,4, and Dominik Heger3,4 Xin Yang et al.
  • 1British 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, Czech Republic
  • 4Research Centre for Toxic Compounds in the Environment (RECETOX), Masaryk University, Kamenice 5/A29, 625 00 Brno, Czech Republic

Abstract. An environmental scanning electron microscope (ESEM) 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 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 100 µm, 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; the NaCl crystals formed 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. The 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.

Short summary
A unique environmental electron microscope was used for monitoring the evaporation of salty frost flowers. We observe a cohesive villous brine surface layer facilitating the formation of NaCl microcrystals at temperatures below −10°C as the brine oversaturation is achieved. This finding confirms the increased surface area and thus also the enhanced heterogeneous reactivity; however, no support for the easiness of fragmentation to produce aerosols can be provided.
Final-revised paper