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

Abstract

<p><strong>Abstract.</strong> 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.</p>

Processes Controlling the Composition and Abundance of Arctic Aerosol

“…Observations of negative nss‐SO 4 have been largely restricted to Antarctica, where relative contributions of anthropogenic sulfate can be very low and large areas of ocean are refrozen each year (Rankin & Wolff, ; Rankin et al, ). However, the potential for aerosol to be generated from frost flowers has been questioned in field and laboratory studies where frost flowers were not observed to produce aerosol even at relatively high wind speeds (Roscoe et al, ; Yang et al, ). Second, aerosol formation from blowing snow could also explain ambient sea salt seasonality and composition, though there are no observations to validate this hypothesis in the Arctic (J. Huang & Jaegle, ; Yang et al, ).…”

The ice–vapour interface during growth and sublimation

“…These concave features are especially well imaged next to bare surface areas, similar to our findings for sublimation of isolated crystals. In the final stages of sublimation, where the substrate exerts some influence on the ice morphology, the features meet in sharp ridges and tips/asperities, with radii down to the nanometre range (similar to the features at evaporating brine; Yang et al, 2017). The interaction with the solid substrate has some stabilizing effect on these features, which could be mechanical (immobilization), but also thermal (good heat conduction against beam damage).…”

Wintertime enhancements of sea salt aerosol in polar regions consistent with a sea ice source from blowing snow

“…Dry deposition of SSA over land accounting for particles growth under high humidity conditions follows the sizesegregated scheme described in Zhang et al (2001). The dry deposition velocity over the ocean is calculated based on the Slinn and Slinn (1980) deposition model for natural waters.…”