Diffusivity measurements of volatile organics in levitated viscous aerosol particles

• Published in: Atmospheric chemistry and physics Vol. 17; no. 13; pp. 8453 - 8471
• Main Authors: Bastelberger, Sandra, Krieger, Ulrich K., Luo, Beiping, Peter, Thomas
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 11.07.2017; Copernicus Publications
• Subjects: Accuracy; Activation energy; Aerosol particles; Aerosols; Analytical methods; Aqueous solutions; Aromatic compounds; Aromatic hydrocarbons; Atmospheric chemistry; Atmospheric research; Breakdown; Chemistry; Coefficients; Diffusion; Diffusion coefficients; Diffusion effects; Diffusivity; Dye dispersion; Evaporation; Experiments; Fluid flow; Gas flow; Humidity; Long-range transport; Matrices (mathematics); Organic chemistry; Organic compounds; Oxidants; Oxidizing agents; Particulates; Pollutants; Pollution dispersion; Polycyclic aromatic hydrocarbons; Polyethylene glycol; Properties; Relative humidity; Resonance; Secondary aerosols; Spectroscopy; Studies; Sucrose; Sugar; Temperature; Temperature dependence; Temperature effects; Ternary systems; Translation; Viscosity; VOCs; Volatile organic compounds; Volatility
• DOI: 10.5194/acp-17-8453-2017
• ISSN: 1680-7324; 1680-7316

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 ≲ 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).