Considerable progress have been made in recent years to establish mixing rules for viscosity (and diffusivity) of organic aerosol compounds and even model viscosity of complex organic mixtures based on functional group contributions. However, we know much less about the viscosity of atmospherically relevant organic/inorganic mixtures. Here, we will present an experimental approach to measure ion diffusivities in a surrogate system for aged atmospheric aerosol and estimate viscosity using the Stokes-Einstein relationship. In the figure, we show preliminary data from an evaporation experiment at constant temperature. Relative humidity (RH) is changed in 4 steps and evaporation rates of the semivolatile ammonium nitrate inorganic compound is monitored using Mie-resonance spectroscopy. At dry conditions (15 % RH) conditions, this evaporation rate is slowed down by a factor of about 40 relative to the rate at 30 % RH due to slow diffusion of the nitrate and ammonium ions to the surface. We will present ion diffusivity coefficients based on the data and compare estimates of viscosity from these coefficients with simple mixing rule predictions.

Evaporation experiment of a single NH<sub>4</sub>NO<sub>3</sub>/sucrose/H<sub>2</sub>O particle levitated in an electrodynamic balance at 288 K with a dry mass ratio of AN to sucrose equal to 1:1 at the beginning of the experiment. <b>(a)</b> Relative humidity (RH); <b>(b)</b> <b>left axis</b>: Mie resonance spectra, intensity color coded, <b>right axis</b>: radius of the particle deduced from Mie resonance spectra (dark yellow); <b>(c)</b> evaporation rates at constant RH obtained from of a linear regression to dr<sup>2</sup>/dt calculated from (b) (black horizontal lines, dashed line to guide the eye), note that the evaporation rates are plotted on a log scale.

Evaporation experiment of a single NH₄NO₃/sucrose/H₂O particle levitated in an electrodynamic balance at 288 K with a dry mass ratio of AN to sucrose equal to 1:1 at the beginning of the experiment. (a) Relative humidity (RH); (b) left axis: Mie resonance spectra, intensity color coded, right axis: radius of the particle deduced from Mie resonance spectra (dark yellow); (c) evaporation rates at constant RH obtained from of a linear regression to dr²/dt calculated from (b) (black horizontal lines, dashed line to guide the eye), note that the evaporation rates are plotted on a log scale.

Ulrich Krieger

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