Evaporating brine from frost flowers with electron microscopy and implications for atmospheric chemistry and sea-salt aerosol formation

摘要

An environmental scanning electron microscope (ESEM) was used for thefirst time to obtain well-resolved images, in both temporal and spatialdimensions, of lab-prepared frost flowers (FFs) under evaporation within thechamber temperature range from −5 to −18 °C andpressures above 500 Pa. Our scanning shows temperature-dependent NaClspeciation: the brine covering the ice was observed at all conditions,whereas the NaCl crystals were formed at temperatures below −10 °Cas the brine oversaturation was achieved. Finger-like ice structures coveredby the brine, with a diameter of several micrometres and length of tens to100 µm, are exposed to the ambient air. The brine-coveredfingers are highly flexible and cohesive. The exposure of the liquid brine onthe micrometric fingers indicates a significant increase in the brine surfacearea compared to that of the flat ice surface at high temperatures; the NaClcrystals formed can become sites of heterogeneous reactivity at lowertemperatures. There is no evidence that, without external forces, salty FFscould automatically fall apart to create a number of sub-particles at thescale of micrometres as the exposed brine fingers seem cohesive and hard tobreak in the middle. The fingers tend to combine together to form largespheres and then join back to the mother body, eventually forming a largechunk of salt after complete dehydration. The present microscopic observationrationalizes several previously unexplained observations, namely, that FFsare not a direct source of sea-salt aerosols and that saline ice crystalsunder evaporation could accelerate the heterogeneous reactions of bromineliberation.