Sodium Chloride Dihydrate Crystals: Morphology, Nucleation, Growth, and Inhibition

Abstract

Sodium chloride powders have a strong tendency to cake at freezing temperatures, which is caused by the formation of sodium chloride dihydrate solid bridges between the powder particles. Using the crystallographic structure, the morphology of the dihydrate crystals was determined. In addition, the growth rate of the most important facets of these crystals was measured as well as the influence of a number of anticaking agents and related additives on the growth rate. A likely candidate for the inhibition of dihydrate growth was found: a mixture of iron(III) and L-tartaric acid, which probably forms a metal−organic coordination complex. The nucleation rate of the dihydrate was found to be extremely low and to increase with time. This is probably caused by the formation of a metastable prenucleation phase, which inhibits the nucleation of dihydrate crystals. The influence of the additives on the nucleation rate was found to be limited.

The Horizontal Ice Nucleation Chamber (HINC): INP measurements at conditions relevant for mixed-phase clouds at the High Altitude Research Station Jungfraujoch

“…Additionally, we acknowledge that the fraction of particles > 0.3 µm reaches a maximum at higher RH w than theoretically expected, which can also be attributed to the sizing and counting uncertainty of the OPC, which is most pronounced at these small particle sizes, when the wavelength of the laser (780 nm) is similar to the diameter of detectable particles. Due to the generation method of NaCl at T > 273 K and immediate exposure of the NaCl to the respective temperature and humidity when the aerosol particles are injected in HINC, we believe that we deliquesce NaCl anhydrate and not NaCl dihydrate (Bode et al, 2015). For 200 nm (NH 4 ) 2 SO 4 particles, deliquescence and hygroscopic growth was also observed at RH w = 82-85 %, consistent with the (NH 4 ) 2 SO 4 deliquescence at a RH w = 82-84 % (Cziczo and Abbatt, 1999).…”

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

“…Halite crystallizes in the cubic crystal structure, whereas hydrohalite does so in the monoclinic one. The hydrohalite crystals rapidly recrystallize to anhydrous halite and brine at temperatures of > 0.11 • C; the reverse recrystallization of halite to hydrohalite is slow even in contact with a saturated sodium chloride solution (Bode et al, 2015).…”

Answer to RC2

“…In addition to laboratory studies, which aim to understand the physical processes of ice nucleation and determine key aspects of aerosols acting as INPs, it is crucial to quantify the total number concentration of ambient INPs in an environment relevant for clouds containing ice and to address the question of their variability in space and time. Several studies exist from airborne platforms (e.g., Bigg, 1967;Rogers et al, 1998;Prenni et al, 2009;DeMott et al, 2010;Avramov et al, 2011;Schrod et al, 2017) and ground-based observations (e.g., DeMott et al, 2003b;Chou et al, 2011;Ardon-Dryer and Levin, 2014;Mason et al, 2016;Boose et al, 2016a, b) quantifying the number concentration of INPs and identifying their potential sources. Typically, filter sampling with subsequent offline freezing methods, and online measurements with continuous-flow-diffusion chambers (CFDCs) are used as INP measurement techniques.…”

“…However, this comes at the cost of a low temporal resolution since the sampling times of the filters often are on the order of a few hours or longer. CFDCs measure INP concentrations in real time with a higher temporal resolution, on the order of a few to tens of minutes (e.g., Rogers, 1988;Rogers et al, 2001;Chou et al, 2011), but their total sampling volume is lower, and their sensitivity to detect INPs is limited at low concentrations (Boose et al, 2016a). This in particular is challenging at low supercooling or in areas where INP concentrations are lower than 0.1-1 per standard liter (std L −1 ; normalized to standard T of 273 K and pressure, p, of 1013 hPa).…”

“…This variation is attributed to previous INP activation and subsequent fallout from advected air masses prior to reaching the JFJ, leaving an air mass which is depleted in INPs upon arrival to the JFJ during wintertime Stopelli et al, 2015). Also at the JFJ, INP measurements were performed at T = 241 K during winters 2012, 2013 and 2014 with the CFDC Portable Ice Nucleation Chamber (PINC; Boose et al, 2016a). INP concentrations were sampled in the deposition nucleation mode in winters 2012-2014 and also in the condensation freezing mode in winter 2014.…”