Observations on hygroscopic growth and phase transitions of mixed 1, 2, 6-hexanetriol ∕ (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> particles: investigation of the liquid–liquid phase separation (LLPS) dynamic process and mechanism and secondary LLPS during the dehumidification
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Published:2021-06-29
Issue:12
Volume:21
Page:9705-9717
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ISSN:1680-7324
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Container-title:Atmospheric Chemistry and Physics
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language:en
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Short-container-title:Atmos. Chem. Phys.
Author:
Ma Shuaishuai, Chen Zhe, Pang ShufengORCID, Zhang Yunhong
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.
Funder
National Natural Science Foundation of China
Publisher
Copernicus GmbH
Subject
Atmospheric Science
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