Diffusivity measurements of volatile organics in levitated viscous aerosol particles
Field measurements indicating that atmospheric secondary organic 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 gr...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/17/8453/2017/acp-17-8453-2017.pdf |
| Summary: | Field measurements indicating that atmospheric secondary organic
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 relative 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 <i>≲</i> 30 %,
corresponding to diffusivities < 10<sup>−14</sup> cm<sup>2</sup> s<sup>−1</sup> at
temperatures < 15 °C. The temperature dependence is
strong, suggesting a diffusion activation energy of about
300 kJ mol<sup>−1</sup>. 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). |
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| ISSN: | 1680-7316 1680-7324 |