Morphologies of mixed organic/inorganic/aqueous aerosol droplets

Abstract

Despite major progress in the understanding of properties of tropospheric aerosol particles, it remains challenging to understand their physical state and morphology. To obtain more detailed knowledge of the phases, phase transitions and morphologies of internally mixed organic/inorganic aerosol particles, we evaluated liquid-liquid phase separation (LLPS), deliquescence relative humidity (DRH) and efflorescence relative humidity (ERH) of 33 organic/ammonium sulfate (AS)/H2O systems from our own and literature data. The organic fraction consists of single compounds or mixtures with up to ten aliphatic and/or aromatic components with carboxylic acid, hydroxyl, carbonyl, ether, and ester functionalities, covering O : C ratios between 0.29 and 1.33. Thirteen out of these 33 systems did not show LLPS for any of the studied organic-to-inorganic mixing ratios, sixteen underwent LLPS showing core-shell morphology, and four showed both core-shell and partially engulfed configurations depending on the organic-to-inorganic ratio and RH. In all cases the organic fractions of the systems with partially engulfed configurations consisted of dicarboxylic acids. AS in mixed organic/AS/H2O particles deliquesced between 70 and 84% RH. AS effloresced below 58% RH or remained in a one-liquid-phase state. AS in droplets with LLPS always showed efflorescence with ERH between 30 and 50% RH, providing clear evidence that the presence of LLPS facilitates AS efflorescence. Spreading coefficients of the organic-rich phase on the AS-rich phase for systems containing polyethylene glycol 400 (PEG-400) and a mixture of dicarboxylic acids are in agreement with the optically observed morphologies of droplets deposited on the hydrophobic substrate. Analysis of high resolution elastic Mie resonance spectra allowed the detection of LLPS for single levitated droplets consisting of PEG-400/AS/ H2O, whereas LLPS was difficult to detect in (2-methylglutaric acid + 3-methylglutaric acid + 2,2-dimethylsuccinic acid)/AS/H2O. Measured Mie spectra of PEG-400/AS/H2O at 93.5% and at 80.9% RH agreed with computed Mie spectra for a homogeneous and a core-shell configuration, respectively, confirming the results obtained from droplets deposited on a hydrophobic substrate. Based on the presented evidence, we therefore consider the core-shell morphology to be the prevalent configuration of liquid-liquid-phase-separated tropospheric organic/AS/H2O particles.

Surface tension prevails over solute effect in organic-influenced cloud droplet activation

“…At this point, we make the simplifying assumption that the liquid-liquid interfacial energy contribution per unit area ("interfacial tension") is small compared to the surface tension and, as a first-order approximation, can be neglected. This assumption is supported by available measurements for organic-inorganic LLPS systems containing oxygenated organic compounds, which indicate that the liquid-liquid interfacial tension value is typically one to two orders of magnitude smaller than the surface tension (i.e., typically smaller than 10 mN m -1 , often smaller than 2 mN m -1 ) 66,67 .…”

Airborne observations of IEPOX-derived isoprene SOA in the Amazon during SAMBBA

“…The conditions during B744 and B745 were drier than B749 and B750, with the respective measured relative humidities in the boundary layer ranging from approximately 45 to 70 % instead of 60 to 80 %. The former humidity range is within the metastable regime for pure ammonium sulfate and according to the work of Song et al (2013), internally mixed ammonium sulfate and organic particles could effloresce under these conditions. As such, it is possible that the inorganic matter during B749 and B750 existed as a solid or as a non-aqueous organic liquid, which would inhibit the formation through the neutral mechanism of Nguyen et al (2014).…”

“…As well as the phase effect on inorganic ion chemistry described by Nguyen et al (2014), the organic fraction is known to potentially exhibit exotic behaviours such as forming amorphous states and separations from the aqueous phase (Virtanen et al, 2010;Song et al, 2013), which may affect how the particles interact with the gas phase. This is a difficult concept to test with the data available, as there are no measurement data on the hygroscopicity of the particles or the exact phase of the organic fraction.…”

Physicochemical Characteristics of Individual Aerosol Particles during the 2015 China Victory Day Parade in Beijing

“…For example, various "bulk" sample analyses conducted in the laboratory have confirmed the liquid-liquid phase separation do exist for the secondary particles [41,46,47]. The presence of the organic core and inorganic shell in individual particles have been confirmed in the laboratory by using optical microscopy and Raman spectroscopy [38,41,42], atomic force microscopy with infrared spectroscopy [48], and environmental scanning electron microscopy and scanning transmission X-ray microscopy [49].…”

“…The OM coating can change a host particle's early hygroscopic properties [36,37]. They can influence the RH at which crystallization and dissolution of sulfate solids occur in atmospheric particles [38], change the partitioning of semi volatile organic compounds between the gas and particle phase [39,40], and alter the scattering and absorption of solar radiation. In atmospheric chemistry, the heterogeneous reactions of gas-phase species (such as ozone or N 2 O 5 ) on the surface of aerosol particles play an important role in the removal of species that affect the oxidative balance of the atmosphere [41].…”