Temperature-dependent formation of NaCl dihydrate in levitated NaCl and sea salt aerosol particles

Abstract

Recent laboratory studies indicate that the hydrated form of crystalline NaCl is potentially important for atmospheric processes involving depositional ice nucleation on NaCl dihydrate particles under cirrus cloud conditions. However, recent experimental studies reported a strong discrepancy between the temperature intervals where the efflorescence of NaCl dihydrate has been observed. Here we report the measurements of the volume specific nucleation rate of crystalline NaCl in the aqueous solution droplets of pure NaCl suspended in an electrodynamic balance at constant temperature and humidity in the range from 250 K to 241 K. Based on these measurements, we derive the interfacial energy of crystalline NaCl dihydrate in a supersaturated NaCl solution and determined its temperature dependence. Taking into account both temperature and concentration dependence of nucleation rate coefficients, we explain the difference in the observed fractions of NaCl dihydrate reported in the previous studies. Applying the heterogeneous classical nucleation theory model, we have been able to reproduce the 5 K shift of the NaCl dihydrate efflorescence curve observed for the sea salt aerosol particles, assuming the presence of super-micron solid inclusions (hypothetically gypsum or hemihydrate of CaSO 4 ). These results support the notion that the phase transitions in microscopic droplets of supersaturated solution should be interpreted by accounting for the stochastic nature of homogeneous and heterogeneous nucleation and cannot be understood on the ground of bulk phase diagrams alone.

Heterogeneous Ice Nucleation Ability of NaCl and Sea Salt Aerosol Particles at Cirrus Temperatures

“…The complex relationship of the magnitude of δ with the size, shape, and refractive index for aspherical particles and, furthermore, with the size and position of the solid core in internally mixed solid‐liquid particles (Sun et al, ) might explain why after recompression (period V) the depolarization ratio of the recrystallized AW particles was higher than in period I, whereas the opposite trend was observed for the recrystallized sodium chloride particles. Recent temperature‐dependent efflorescence experiments with levitated aqueous NaCl and SSA particles revealed that below a temperature of about 240 K exclusively the dihydrate phase of NaCl crystallized from the solution droplets (Peckhaus et al, ). For both the NaCl and AW recrystallization experiments, we therefore assumed a 100% conversion of anhydrous NaCl to NaCl · 2H 2 O, given that the maximum temperature encountered in the recompression periods of the recrystallization experiments was only 233 K.…”

“…Some constituents of sea salt like CaSO 4 ·0.5H 2 O and CaSO 4 ·2H 2 O have a lower solubility than NaCl and are expected to precipitate first from initially aqueous SSA solution droplets when the RH is reduced (Xiao et al, 2008). Some other constituents like MgSO 4 , MgCl 2 , and KMgCl·6H 2 O have a higher solubility than NaCl and remain in solution even after the crystallization of NaCl in the SSA particles (Ault, Zhao, et al, 2013;Ault, Moffet, et al, 2013;Cziczo & Abbatt, 2000;Peckhaus et al, 2016;Schill & Tolbert, 2014;Tang, 1976;Xiao et al, 2008), which occurs at a similar RH compared to the crystallization of NaCl in purely aqueous NaCl particles (Koop, Kapilashrami, et al, 2000;Tang et al, 1997). In such mixed-phase SSA particles consisting of a brine layer of dissolved salts around a solid core of predominantly NaCl, heterogeneous ice nucleation can only proceed via immersion freezing and not via deposition nucleation as for entirely crystalline NaCl particles (Schill & Tolbert, 2014).…”

“…Two crystalline forms of NaCl can form upon efflorescence: anhydrous NaCl and NaCl dihydrate (NaCl · 2H 2 O). NaCl · 2H 2 O is the thermodynamically stable solid phase below 273.3 K (Martin, ) and was experimentally observed to be formed upon efflorescence of aqueous NaCl solution droplets at temperatures lower than about 240 ± 5 K (Peckhaus et al, ; Wagner et al, ; Wise et al, ).…”

Heterogeneous ice nucleation ability of aerosol particles generated from Arctic sea surface microlayer and surface seawater samples at cirrus temperatures

“…Below 240 ± 5 K, the formation of sodium chloride dihydrate (NaCl • 2H2O) is favoured over anhydrous NaCl (Wagner et al, 2012;Wise et al, 2012;Peckhaus et al, 2016). AIDA ice nucleation experiments with partly deliquesced inorganic sea salt particles that contained a solid core of NaCl • 2H2O instead of anhydrous NaCl showed that the temperature limit below which the immersion freezing mode of the particles initiated was shifted to much higher temperatures (Wagner et al, 2018).…”

A continuous flow diffusion chamber study of sea salt particles acting as cloud nuclei: deliquescence and ice nucleation

“…Another uniqueness of this region is that several phases of NaCl-water combination can be found, which are (1) liquid solution (supersaturated solution, from >DRH), (2) ice + dihydrate (homogeneously frozen), (3) anhydrous NaCl (inactivated, never > DRH), (4) dihydrate (Na-Cl•2H 2 O, marked as DH in Fig. 7) (Wagner et al, 2012;Wise et al, 2012;Peckhaus et al, 2016) and (5) ice + anhydrous NaCl (depositional freezing on NaCl particles).…”