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Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2017-210
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
14 Mar 2017
Review status
This discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
Diffusivity measurements of volatile organics in levitated viscous aerosol particles
Sandra Bastelberger, Ulrich K. Krieger, Beiping Luo, and Thomas Peter Institute for Atmospheric and Climate Science, ETH Zürich, 8092 Zürich, Switzerland
Abstract. Field measurements indicating that atmospheric secondary aerosol (SOA) particles can be present in a highly viscous, glassy state have spurred numerous studies addressing low diffusivities of water in glassy aerosols. The focus of these studies is on kinetic limitations of hygroscopic growth and the plasticizing effect of water. In contrast, much less is known about diffusion limitations of organic molecules and oxidants in viscous matrices. These may affect atmospheric chemistry and gas-particle partitioning of complex mixtures with constituents of different volatility. In this study, we quantify the diffusivity of a volatile organic in a viscous matrix. Evaporation of single particles generated from an aqueous solution of sucrose and small amounts of volatile tetraethylene glycol (PEG-4) is investigated in an electrodynamic balance at controlled humidity (RH) and temperature. The evaporative loss of PEG-4 as determined by Mie resonance spectroscopy is used in conjunction with a radially resolved diffusion model to retrieve translational diffusion coefficients of PEG-4. Comparison of the experimentally derived diffusivities with viscosity estimates for the ternary system reveals a breakdown of the Stokes-Einstein relationship, which has often been invoked to infer diffusivity from viscosity. The evaporation of PEG-4 shows pronounced RH and temperature dependencies and is severely depressed for RH ≲ 30 %, corresponding to diffusivities < 10−14 cm2 s−1 at temperatures < 15 °C. The temperature dependence is strong, suggesting a diffusion activation energy of about 300 kJ mol−1. We conclude that atmospheric volatile organic compounds can be subject to severe diffusion limitations in viscous organic aerosol particles. This may enable an important long-range transport mechanism for organic material, including pollutant molecules such as polycyclic aromatic hydrocarbons (PAHs).

Citation: Bastelberger, S., Krieger, U. K., Luo, B., and Peter, T.: Diffusivity measurements of volatile organics in levitated viscous aerosol particles, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-210, in review, 2017.
Sandra Bastelberger et al.
Sandra Bastelberger et al.
Sandra Bastelberger et al.

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Short summary
We present quantitative condensed phase diffusivity measurements of a volatile organic (tetraethylene glycol) in highly viscous single aerosol particles (aqueous sucrose). The condensed phase diffusivity exhibits a strong temperature and humidity dependence. Our results suggest that diffusion limitations of volatile organics in highly viscous organic aerosol may severely impact gas-particle partitioning under cold and dry conditions.
We present quantitative condensed phase diffusivity measurements of a volatile organic...
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