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Atmospheric aerosols consisting of organic and inorganic components may undergo liquid–liquid phase separation (LLPS) and liquid–solid phase transitions during ambient relative humidity (RH) fluctuation. However, the knowledge of dynamic phase evolution processes for mixed organic–inorganic particles is scarce. Here we present a universal and visualized observation of LLPS, efflorescence and deliquescence transitions as well as hygroscopic growth of laboratory-generated mixed 1, 2, 6-hexanetriol / ammonium sulfate (AS) particles with different organic–inorganic mole ratios (OIR = 1:4 , 1:2 , 1:1 , 2:1 and 4:1) with high time resolution (0.5 s) using an optical microscope operated with a video camera. The optical images suggest that an inner AS solution phase is surrounded by an outer organic-rich phase after LLPS for all mixed particles. The LLPS mechanism for particles with different OIRs is found to be distinct; meanwhile, multiple mechanisms may dominate successively in individual particles with a certain OIR, somewhat inconsistently with previously reported observations. More importantly, another phase separation in the inner AS solution phase, defined as secondary LLPS here, is observed for OIR = 1:1 , 1:2 and 1:4 particles. The secondary LLPS may be attributed to the formation of more concentrated AS inclusions in the inner phase and becomes more obvious with decreasing RH and increasing AS mole fraction. Furthermore, the changes in size and number of AS inclusions during LLPS are quantitatively characterized, which further illustrate the equilibrium partitioning process of organic and inorganic components. These experimental results have significant implications for the revelation of complex phase transitions of internally mixed atmospheric particles and evaluation of liquid–liquid and liquid–solid equilibria in thermodynamic models. [ABSTRACT FROM AUTHOR]

The hygroscopic behaviors and phase changes of inorganic aerosols have been widely explored, but little is known on the hygroscopicity of soluble carbonates. The hydrated states of solid Na 2 CO 3 particles in an air environment remain largely unclear. In this work, the hygroscopic growth, hydrated form transformations, and influence of internal Li 2 CO 3 on phase transitions of Na 2 CO 3 particles are investigated in linear and pulsed relative humidity (RH) changing modes by the vacuum Fourier transform infrared (FTIR) technique. For pure Na 2 CO 3 , aqueous droplets effloresced to a mixture of anhydrous Na 2 CO 3 and Na 2 CO 3 ·H 2 O with the initial efflorescence relative humidity (ERH) of 50.8%, probably concerning the formation of Na 2 CO 3 ·10H 2 O in the conversion from aqueous to anhydrous Na 2 CO 3 . A reverse process is presented during the three-stage deliquescence transition beginning at ∼60.1% RH; i.e., anhydrous Na 2 CO 3 transforms into aqueous Na 2 CO 3 and Na 2 CO 3 ·10H 2 O in stage I, Na 2 CO 3 ·10H 2 O dissolves to aqueous Na 2 CO 3 in stage II, and Na 2 CO 3 ·H 2 O dissolves into aqueous Na 2 CO 3 in stage III. For internally mixed Na 2 CO 3 /Li 2 CO 3 particles, a double salt, LiNaCO 3 , is found in mixed crystalline phases for the first time, leading to the eutonic composition with Na 2 CO 3 . The experimental observations point to the excess of LiNaCO 3 and complete consumption of Na 2 CO 3 in eutonic composition formation, which results in the absence of Na 2 CO 3 hydrates during phase transitions. The results provide key data for model simulations of hygroscopic properties and phase transitions of Na 2 CO 3 as well as mixed soluble carbonates.

Water-uptakes of pure sodium carbonate (Na 2 CO 3 ), pure β-alanine and internally mixed β-alanine/Na 2 CO 3 aerosol particles with different mole ratios are first monitored using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) technique. For pure Na 2 CO 3 aerosol particles, combining the absorptions at 877 and 1422 cm -1 with abrupt water loss shows the efflorescence relative humidity (ERH) of 62.9%-51.9%. Upon humidifying, solid Na 2 CO 3 firstly absorbs water to from Na 2 CO 3 ·H 2 O crystal at 72.0% RH and then deliquesces at 84.5% RH (DRH). As for pure β-alanine particles, the crystallization takes place in the range of 42.4%-33.2% RH and becomes droplets at ~88.2% RH. When β-alanine is mixed with Na 2 CO 3 at various mole ratios, it shows no efflorescence of Na 2 CO 3 when β-alanine to Na 2 CO 3 mole ratio (OIR) is 2:1. For 1:1 and 1:2 β-alanine/Na 2 CO 3 aerosols, the ERHs of Na 2 CO 3 are 51.8%-42.3% and 57.1%-42.3%, respectively. While β-alanine crystal appears from 62.7% RH for 2:1 and 59.4% RH for both 1:1 and 1:2 particles and lasts to driest state. On hydration, the DRH is 44.7%-75.2% for Na 2 CO 3 with the OIR of 1:1 and 44.7%-69.0% for 1:2 mixture, and those of β-alanine are 74.8% for 2:1 mixture and 68.9% for two others. After the first dehumidification-humidification, all the water contents decrease despite of constituent fraction. And at ~92% RH, the remaining water contents are 92%, 89% and 82% at ~92% RH, corresponding to OIR of 2:1, 1:1 and 1:2 mixed system, respectively., (Copyright © 2019. Published by Elsevier B.V.)