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 CaSO4). 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.
Highlights
Seawater spray is the dominant source of atmospheric aerosol over oceans and in coastal areas
We suggest that the supersaturated NaCl solution droplets effloresced into polycrystalline NaCl dihydrate particles while maintaining the envelope shape of the droplet
We report series of single droplet efflorescence experiments with micron-sized droplets of NaCl and sea salt analogue (Instant Ocean) solutions suspended in the electrodynamic balance (EDB) at constant temperature and humidity
Summary
Seawater spray is the dominant source of atmospheric aerosol over oceans and in coastal areas. The nucleation of NaCl dihydrate in sea spray aerosol may be further influenced by the presence of inorganic (CaSO4, MgCl2, MgSO4, KMgCl3) and organic substances Some of these salts (e.g., gypsum) have lower solubility than NaCl and would precipitate at humidity values higher than the ERH for micron size pure NaCl solution droplets.. A reliable CNT-based parameterization is, missing due to the absence of experimental data on key parameters such as diffusivities of water and ionic species and interfacial energies for the crystalline phase in contact with supersaturated solute in the low temperature range, i.e., below 240 K To mitigate this problem, we use a humidified electrodynamic balance (EDB) coupled with a Raman microscope to measure the volume specific nucleation rates ( called nucleation rate coefficients) of NaCl and NaCl dihydrate in suspended droplets of aqueous NaCl solution at different temperatures. We extend this concept to the case of heterogeneous nucleation of NaCl dihydrate in a multicomponent system of inorganic salts, simulating the sea salt aerosol
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