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The phase state of atmospheric aerosols has significant impacts on global climate and atmospheric environment chemistry but remains indistinct. Previous study believed that deliquescence on hydration and efflorescence on dehydration for inorganics independent of circumstances. However, the present studies showed an unexpected phase transition of NaCl and (NH 4) 2 SO 4 when they mixed with ultraviscous gluconic aid. When gluconic acid and NaCl were mixed in equimolar ratio, no efflorescence was observed on dehydration, in contrast, a unique phenomenon of "efflorescence upon hydration" was observed clearly by optical microscope technology. When NaCl was replaced by (NH 4) 2 SO 4 , the similar phenomenon is also observed by optical images, which was further demonstrated by feature bands of solid (NH 4) 2 SO 4 in FTIR spectra. And Raman spectra intuitively identify that ammonium sulfate moves to the centre and become crystal in a single particle on hydration. In view of ultraviscous property of gluconic acid, the "efflorescence upon hydration" should be caused by slower water diffusion in the mixed particles. Meanwhile, we also explore gluconic acid/NaCl mixed system in different mole ratios. But no "efflorescence upon hydration" appeared. For 2:1 mixed system, the particles have no obvious morphological changes, exhibiting the behaviour similar to that of pure gluconic acid. Whereas for the system rich in NaCl, deliquescence and efflorescence both appear as usual, and the corresponding critical RHs are semblable with pure NaCl particle. Our data fill the database of phase transitions and hygroscopicity in the atmosphere and provide insights for future studies in ultraviscous polyhydroxy organic acid. [Display omitted] • The effect of ultraviscous gluconic acid on phase transitions of inorganic salts was first reported. • A unique phenomenon of "efflorescence upon hydration" was observed clearly by microscope technology. • The confocal micro-Raman spectroscopy was utilized to monitor the component distribution in a single particle. • The hygroscopic behaviour varied with the mole ratio of organic–inorganic mixture significantly. [ABSTRACT FROM AUTHOR]

The efflorescence transitions of aerosol particles have been intensively investigated due to their critical impacts on global climate and atmospheric chemistry. In the present study, we present a critical review of efflorescence kinetics focusing on three key issues: the efflorescence relative humidity (ERH) and the influence factors for aerosol ERH (e.g. particle sizes, and temperature); efflorescence processes of mixed aerosols, concerning the effect of coexisting inorganic and organic components on the efflorescence of inorganic salts; homogeneous and heterogeneous nucleation rates of pure and mixed aerosols. Among the previous studies, there are significant discrepancies for measured aerosol ERH under even the same conditions. Moreover, the interactions between organic and inorganic components remain largely unclear, causing efflorescence transition behaviours and chemical composition evolutions of certain mixed systems to be debatable. Thus, it is important to better understand efflorescence to gain insights into the physicochemical properties and characterize observed efflorescence characteristics of atmospheric particles, as well as guide further studies on aerosol hygroscopicity and reactivity. • The efflorescence kinetics of pure and mixed aerosols is systematically reviewed. • The interactions between organic and inorganic components in aerosols are discussed. • This review will help us understand the liquid-solid phase transition of aerosol particles. [ABSTRACT FROM AUTHOR]

Understanding the efflorescence process of atmospheric aerosols is a key step towards a complete description of their phase transition dynamics. Atmospheric aerosols comprise internally mixed organic and inorganic components, and the composition of the condensed phase strongly affects the overall efflorescence dynamics. Although the kinetics of efflorescence has been thoroughly investigated for aerosols containing inorganic salts, the influence of non-ideal mixing with water-soluble organic compounds (WSOCs) remains considerably controversial. In this work, we measure the efflorescence process of ammonium sulfate (AS) aerosol droplets mixed with four types of WSOCs – sucrose, glycerol, malonic acid, and citric acid – by using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The absorbance band of ammonium ions (δ-NH 4 + at 1441 cm−1) in the aqueous phase was used to calculate the crystallization ratio of AS and the heterogeneous nucleation rate. Adding WSOCs lowers the characteristic efflorescence relative humidity (ERH), broadens the ERH ranges, and reduces the terminal crystallization ratio of AS aerosol droplets. Amongst the various WSOCs studied here, the inhibiting effect on AS efflorescence decreases in the following order: sucrose > citric acid ≈ malonic acid > glycerol, mainly attributed to the mass transfer limitation caused by the viscosity of WSOCs. Sucrose and glycerol can completely suppress the crystallization of AS droplets at organic to inorganic ratio (OIR) of 1:1 and 4:1, respectively. This distinct observation suggests that atmospheric aerosols may skip efflorescence and evolve into a glassy state at low RH conditions. In order to systematically study and compare the inhibiting capabilities of four water-soluble organic compounds (WSOCs, Suc, Gly, CA, and MA stand for sucrose, glycerol, citric acid, and malonic acid, respectively) on the efflorescence behavior of ammonium sulfate (AS) aerosol droplets within a wide organic to inorganic ratio (OIR) range, the terminal crystallization ratio R (a), the peak heterogeneous nucleation rate J h e t ∗ (b), and the efflorescence relative humidity (ERH) range (c) of AS in the AS/WSOC aerosol droplets were determined by the FTIR method. [Display omitted] • Efflorescence kinetics of (NH 4) 2 SO 4 /organic aerosols is studied in wide OIR range. • Inhibiting effect of organics on (NH 4) 2 SO 4 efflorescence is compared. • Sucrose and glycerol are found to prevent (NH 4) 2 SO 4 from crystallizing. • Interactions between malonic acid, citric acid and (NH 4) 2 SO 4 are discussed. [ABSTRACT FROM AUTHOR]