Abstract
Abstract. 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.
Highlights
As relative humidity (RH) decreases, LLPS occurs at ∼ 79.3 % RH, detected by the sudden appearance of schlieren, which will be discussed in detail below
Determination of deliquescence relative humidity (DRH) is always prone to uncertainties, not like the separation relative humidity (SRH) and efflorescence relative humidity (ERH), because the occurrence of ammonium sulfate (AS) crystal dissolution is not easy to be clearly judged from the images and the turning point of humidification curve, in view of the continuous water uptake by organic coating
For organic–inorganic mole ratios (OIRs) = 1 : 1, 1 : 2 and 1 : 4 particles, a secondary LLPS in the inner phase, as a result of more concentrated AS inclusions formation, is exhibited with water release, and it becomes more obvious with decreasing RH and increasing sulfate fractions
Summary
Field and laboratory studies showed that atmospheric particulate matters far away from local sources is basically the internal mixtures of organic and inorganic species (Middlebrook et al, 1998; Murphy et al, 2006; Lee et al, 2002), which is established by gas-phase diffusion (Marcolli et al, 2004) and gas-particle partitioning (Zuend et al, 2010) of semivolatile organic compounds. Mixed organics and ammonium sulfate (AS) particles can be regarded as model organic–inorganic mixed systems and have been previously chosen in numerous laboratory studies
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