Physical Characterization of Frozen Saltwater Solutions Using Raman Microscopy

Ice is an important but poorly understood atmospheric reaction medium. Reactions in ice and at air-ice interfaces are often modeled using rate constants measured in liquid aqueous solution, despite evidence that reactivity in these two media can be very different. This approach may be valid at high ionic strengths (e.g., in sea ice) as a result of the formation of liquid brine. However, recent experiments indicate uneven solute distribution at ice surfaces, suggesting that liquid water does not completely wet ice surfaces at environmentally relevant solute concentrations. We have investigated the distribution of liquid solution, solid ice, and solid salt (NaCl·2H₂O, "hydrohalite") at the surface of frozen aqueous sodium chloride (NaCl) solutions and frozen seawater using Raman microscopy. At temperatures above the eutectic temperature (-21.1 °C), the ice surfaces were incompletely wetted, except occasionally at the highest temperatures (approximately -5 °C). Liquid water at the surface took the form of either isolated patches or channels, depending upon the salt concentration and sample temperature; liquid fractions ranged from approximately 11 to 85%. Three-dimensional ("volume") maps showed similar liquid fractions and channel widths at all depths investigated (up to 100 μm) as well as at the surface for each sample composition. Below -21.1 °C, no liquid was observed in any sample. Instead, hydrohalite was observed with surface coverages ranging from 13 to 100% depending upon the salt concentration; surface coverage was independent of temperature between -30 and -22 °C. Accounting for the presence of two distinct reaction environments at the surface of salty ice might improve predictions of physical and chemical processes in snow-covered regions.