Efflorescence kinetics of aerosols comprising internally-mixed ammonium sulfate and water-soluble organic compounds. (1st April 2022) - Xu, Yeming
Abstract: 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-NH4 + 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. Graphical abstract: 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. Image 1 Highlights: Efflorescence kinetics of (NH4 )2 SO4 /organic aerosols is studied in wide OIR range. Inhibiting effect of organics on (NH4 )2 SO4 efflorescence is compared. Sucrose and glycerol are found to prevent (NH4 )2 SO4 from crystallizing. Interactions between malonic acid, citric acid and (NH4 )2 SO4 are discussed.