The morphology of mixed organic/inorganic particles can strongly influence the physicochemical properties of aerosols but remains relatively less examined in particle formation studies. The morphologies of inorganic seed particles grown with either -pinene or limonene secondary organic aerosol (SOA) generated in a flow tube reactor were found to depend on initial seed particle water content. Effloresced and deliquesced ammonium sulfate seed particles were generated at low relative humidity (<15% RH, dry) and high relative humidity (~60% RH, wet) and were also coated with secondary organic material under low growth and high growth conditions. Particles were dried and analyzed using SMPS and TEM for diameter and substrate-induced diameter changes and for the prevalence of phase separation for organic-coated particles. Effloresced inorganic seed particle diameters generally increased after impaction, whereas deliquesced inorganic seed particles had smaller differences in diameter, although they appeared morphologically similar to the effloresced seed particles. Differences in the changes to diameter for deliquesced seed particles suggest crystal restructuring with RH cycling. SOA-coated particles showed negative diameter changes for low organic growth, although wet-seeded organic particles changed by larger magnitudes compared to dry-seeded organic particles. High organic growth gave wide ranging diameter percent differences for both dry- and wet-seeded samples. Wet-seeded particles with organic coatings occasionally showed a textured morphology unseen in the coated particles with dry seeds. Using a flow tube reactor with a combination of spectrometry and microscopy techniques allows for insights into the dependence of aerosol particle morphology on formation parameters for two seed conditions and two secondary organic precursors.
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Exploring the hygroscopicity, water diffusivity, and viscosity of organic–inorganic aerosols – a case study on internally-mixed citric acid and ammonium sulfate particles
Internally-mixed aerosol particles containing organic molecules and inorganic salts are prevalent in the atmosphere, arising from direct emission ( e.g., from the ocean) or indirect production by condensation of organic vapors onto existing inorganic particle seeds. Aerosol particles co-exist with water vapor and, under humid conditions, will exist as dilute aqueous solution particles that can be well described by thermodynamic models. Under low humidity conditions, the increase in solute concentrations leads to molecular interactions and significant non-ideality effects that drive changes in important physical properties, such as viscosity and phase state, that are not predicted using simple models. Here, we explore a model system containing ammonium sulfate (AS) and citric acid (CA). We measure the hygroscopicity, viscosity, and rate of water diffusion in particles across a range of RH conditions and organic fractions to better understand the influence of organic–inorganic mixtures on particle properties. We report the RH dependence of these properties and explore the applicability of commonly used methods that connect them together, such as the Stokes–Einstein relationship and thermodynamic modelling methods. We show that at low RH, the addition of AS to CA leads to a reduction in the amount of water as indicated by the radial growth factor at a fixed RH, while observing an increase in the viscosity over several orders of magnitude. Contrary to the viscosity, only minor changes in water diffusion were measured, and analysis with the fractional Stokes–Einstein relationship indicates that changes in the molecular matrix due to the presence of AS could explain the observed phenomena. This work reveals that small additions of electrolytes can drive large changes in particle properties, with implications for chemical reactivity, lifetime, and particle phase that will influence the environmental impacts and chemistry of aerosol particles.
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- NSF-PAR ID:
- 10431710
- Date Published:
- Journal Name:
- Environmental Science: Atmospheres
- Volume:
- 3
- Issue:
- 1
- ISSN:
- 2634-3606
- Page Range / eLocation ID:
- 24 to 34
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2EA00116K
