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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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Direct measurement of the viscosity of ternary aerosol mixtures
Secondary organic aerosols contribute a large fraction to atmospheric aerosols. The phase states of secondary organic aerosols influence heterogeneous and multiphase chemistry in the atmosphere and thus climate. In previous studies we have used the dual tandem differential mobility analyzer technique to characterize the temperature- and humidity-dependent viscosity and glass transition temperature of suspended particles. However, the technique requires high particle number concentrations, is a complex setup, is expensive, and measurements are time consuming. Here we demonstrate a new simplified and more cost-effective method to obtain similar data. The technique was used to measure the temperature where the viscosity is ∼107 Pa s for submicron particles composed of binary and ternary mixtures of the sucrose/tartaric acid/citric acid system. Sucrose, tartaric acid and citric acid are taken as proxies for viscous organic aerosol components in the atmosphere. A subset of data were compared to measurements with the dual-tandem differential mobility analyzer method. Results show good agreement between the two techniques. The same mixed chemical systems were modeled using an updated version of the parametric phase diagram model described in Kasparoglu et al. (2021, https://doi.org/10.5194/acp-21-1127-2021) as well as the predictions with the viscosity module of the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients model (AIOMFAC-VISC). Results show that appropriately parameterized mixing rules are suitable to describe these mixtures. We anticipate that the new technique will accelerate discovery of aerosol phase transitions in aerosol research.
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- PAR ID:
- 10484189
- Publisher / Repository:
- Royal Society of Chemistry
- Date Published:
- Journal Name:
- Environmental Science: Atmospheres
- Volume:
- 3
- Issue:
- 3
- ISSN:
- 2634-3606
- Page Range / eLocation ID:
- 595 to 607
- 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/D2EA00160H
