Physical Characterization of Frozen Saltwater Solutions Using Raman Microscopy

Abstract

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•2H 2 O, "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.

The morphology of ice and liquid brine in an environmental scanning electron microscope: a study of the freezing methods

“…1) (Gao et al, 2017). The concentrations are proxies for natural salt occurrence in coastal snow (5 mM) (Beine et al, 2012;Douglas et al, 2012), up to the concentration in sea water (0.5 M) (Massom et al, 2001;Thomas, 2017). Prior to the measurement, the solutions had been filtered through a 0.45 µm filter to exclude impurities that might interfere with the microscopic observations.…”

“…The tested concentrations ranged over 2 orders of magnitude. The values within 500-50 mM define the concentration of NaCl in seawater, and therefore they are also descriptive of fresh sea ice in the given context (Massom et al, 2001;Thomas, 2017). NaCl concentrations reaching up to 160 mM were detected immediately next to a highway treated against road icing; 50 mM of a salt solution can thus be considered a concentration potentially found farther from roads or in their close vicinity when the salt was already partly flushed away (Notz and Worster, 2009;Labadia and Buttle, 1996).…”

pH-dependent production of molecular chlorine, bromine, and iodine from frozen saline surfaces

“…Alternatively, XO can react with HO 2 to form HOX (Reaction R3) or NO 2 to form XONO 2 . Gas-phase HOX can heterogeneously react with salt-laden surfaces, including sea-salt aerosol particles (McConnell et al, 1992) and the "disordered interface" (often referred to as a quasiliquid or quasi-brine layer) that exists on frozen saline surfaces (Bartels-Rausch et al, 2014;Cho et al, 2002) to produce X 2 , effectively returning two halogen radicals to the gas phase. Additionally, this mechanism is enhanced under acidic conditions, confirmed by laboratory studies of aqueous (Fickert et al, 1999) and frozen solutions (e.g., Abbatt et al, 2010;Sjostedt and Abbatt, 2008;Wren et al, 2013) and by field observations (Pratt et al, 2013).…”

Supplementary material to "pH-Dependent production of molecular chlorine, bromine, and iodine from frozen saline surfaces"

“…When repeated for the other experiments at pH = 4.7, it is found that at least 16% of the original Iremains unreacted after similarly extended limits of integration. This suggests that all of the Iin our frozen samples may not be completely excluded to the disordered interface, and may exist within the ice bulk or inaccessible brine channels throughout the ice, and that differences in integration production amounts can originate from differences in Idistribution during freezing (Bartels-Rausch et al, 2014;Malley et al, 2018). Tables Table S1: Integrated I 2 production amounts prior to irradiation or addition of O 3 from low pH experiments involving samples with an OH precursor.…”