Crystallization kinetics of sea-salt aerosols studied by high-speed photography

Abstract

Morphological changes with decreasing relative humidity (RH) of supersaturated sea-salt aerosol droplets on a quartz substrate were observed using a high-speed video-camera. Stable gypsum (CaSO 4 ·2H 2 O) or the metastable hemihydrate (CaSO 4 ·0.5H 2 O) were precipitated as the RH decreased. The dynamic process of crystal growth under steady-state humidity was studied by controlling the RH; the metastable hemihydrate was precipitated at 70.5%-77.1% RH, and the apparent crystal growth rate was between 1. 42 Supersaturated aerosols are common in nature. The achievement of a supersaturated state is considered to be the result of the suppression of both homogeneous and heterogeneous nucleations in aerosols with tiny volumes [1]. The crystallization dynamics of supersaturated aerosols and the formation of a metastable solid-phase are basic problems in crystal growth and atmospheric science. In the supersaturated state, the crystallization of aerosol droplets is mainly affected by kinetic factors, resulting in metastable solids instead of thermodynamically stable ones [2-4]. There have been few experimental investigations of the nucleation mechanisms of supersaturated aerosols and the kinetics of crystal growth because of a lack of effective methods. Recently, Ciobanu et al. [5] analyzed the distribution of the initial growth sites of ammonium sulfate crystals using a high-speed video-camera and found that the vicinity of the liquid/vapor interface, rather than the droplet, is the preferred nucleation site. Hindmarsh et al.[6] observed surface and volume nucleation phenomena in undercooled sucrose solution droplets with a high-speed video-camera and analyzed the surface nucleation mechanism. Sea-salt aerosols (SSAs), which are an important component of marine aerosols, are produced predominantly by bubble bursting and the formation of spume droplets as a result of the action of the wind on the ocean. Although SSAs have been found to precipitate stable gypsum or the metastable hemihydrate in the high-humidity range [7], the formation conditions of CaSO 4 ·0.5H 2 O or CaSO 4 ·2H 2 O crystals are unclear, especially with regard to their growth kinetics; the growth kinetics is important in understanding the phase-separation process as well as the nucleation of NaCl crystals [8].High-speed photography is one of the most effective methods for the investigation of instantaneous processes. The combination of a high-speed video-camera and an optical microscope can give a time resolution of 2 s and a spatial resolution of 1 m [9]. In this paper, we study the morphological changes with decreasing relative humidity (RH) of supersaturated SSA droplets to investigate the nucleation mechanism and kinetics of crystal growth of SSAs. Dynamic images of calcium sulfate crystallization are captured using a high-speed video-camera so that the crystal growth rate

Nucleation Pathway of Calcium Sulfate Hemihydrate (Bassanite) from Solution: Implications for Calcium Sulfates on Mars

“…These bassanite nucleation studies highlight the key role of hydration effects during the crystallization of calcium sulfate phases, which are likely to change substantially as the activity of water is significantly reduced at high salt/low water contents. Another observation pointing in the same direction is the formation of bassanite during the evaporation of droplets of CaSO 4 solutions at room temperature 15,16 . The relative humidity (RH < 80%) and the time-dependent availability of water appear to be the controlling factors for phase selection in this system.…”

Calcium sulfate precipitation pathways in natural and engineered environments

“…Also, bassanite can form spontaneously when droplets of CaSO 4 solutions are rapidly evaporated (i.e. when the level available hydration water is rapidly decreased) at ambient conditions (Qian et al, 2012;Shahidzadeh et al, 2015). Experiments at 60 °C were monitored over periods of up to two years of reaction, but the precipitate remained phase-pure gypsum (according to PXRD, with its detection limit of around 1%) in all studied cases (Ossorio et al, 2014).…”

Temperature-Dependent Phase Transitions of Aqueous Aerosol Droplet Systems in Microfluidic Traps

“…This highlights the fact that while speculative, differences in droplet size, shape, substrates, as well as the other factors discussed above could all potentially be contributing factors while comparing the transition temperatures between the methods presented here. The rate of dehydration has also been shown to be a major factor affecting the morphology of efflorescing particles in previous studies on seawater , as well as studies on pharmaceutical compounds , and could be the biggest cause of the different crystal structures at the same temperature seen here and needs to be investigated further.…”

“…Some studies have also reported a different efflorescence crystal structure in seawater depending on the rate of decrease of RH due to different number of water molecules trapped in the dried particle. 62,63 Here, we investigate a range of model aerosol systems in the range −20 to +25 °C that covers a significant portion of tropospheric temperature (−60 to +25 °C) for O/C ratios between 0.38 and 0.67, which are below the critical O/C < 0.92 for LLPS. We varied the organic to inorganic ratio (OIR) of the systems to estimate the effect of the organic and inorganic components on the LLPS morphology temperaturedependence.…”

“…Another study on particles suspended in an electrodynamic balance (EDB) reported the transition to be in a similar temperature range. Some studies have also reported a different efflorescence crystal structure in seawater depending on the rate of decrease of RH due to different number of water molecules trapped in the dried particle. , …”