Time evolution of steep diffusion fronts in highly viscous aerosol particles measured with Mie resonance spectroscopy.

Field and laboratory measurements indicate that atmospheric organic aerosol particles can be present in a highly viscous state. In contrast to liquid state particles, the gas phase equilibration to ambient relative humidity (RH) can be kinetically limited and governed by condensed phase diffusion. In water diffusion experiments on highly viscous single aerosol particles levitated in an electrodynamic balance, we observed a characteristic shift behavior of the Mie scattering resonances indicative of the changing radial structure of the particle, thus providing an experimental method to track the diffusion process inside the particle. Due to the plasticizing effect of water, theory predicts extremely steep, front-like water concentration gradients inside highly viscous particles exposed to a rapid increase in RH. The resulting quasi step-like concentration profile motivates the use of a simple core-shell model describing the morphology of the non-equilibrium particle during humidification. The particle growth and reduction of the shell refractive index can be observed experimentally as redshift and blueshift behavior of the Mie resonances, respectively. We can deduce the particle radius as well as a core-shell radius ratio from the measured shift pattern and Mie scattering calculations. Using both the growth information obtained from the Mie resonance redshift and thermodynamic equilibrium data, we can infer a comprehensive picture of the time evolution of the diffusion fronts in the framework of our core-shell model. The observed shift behavior of the Mie resonances provides direct evidence of very steep diffusion fronts caused by the plasticizing effect of water and a method to validate previous diffusivity measurements.