Aerosols are generated from a wide range of natural and anthropogenic sources. The physical state of an aerosol particle is primarily determined by the relative humidity (RH) and temperature of the environment and the particle composition. As RH decreases, aerosol particles can transition to a wide variety of physical states and morphologies that uniquely impact light scattering and mass transport. The properties of amorphous viscous states and phase separated core-shell and partially engulfed morphologies have been studied extensively However, recent work has demonstrated the importance of gel formation in particles containing mixtures of organic and inorganic material. A gel state will limit the transport of materials to slow percolation through pores that penetrate throughout the particle. Unlike in the formation of viscous particles, gel particles exhibit a more distinct transition as RH decreases, increasing the importance of understanding the RH response of these systems.<br/>To characterize the rate of diffusion in gel states, this study will look at the response of a levitated particle to instantaneous RH steps of 1% using an electrodynamic balance coupled with MIE resonance spectroscopy. By selecting individual resonant peaks in the spectrum and tracking them over the full particle response, the rate of diffusion may be estimated from the half-life response (t<sub>1/2</sub>) and the particle radius.<br/>This method provides a simplification over other methods for characterizing water diffusion by reducing the extent of perturbation in RH, allowing D to be assumed constant over the course of the measurement. We present a validation of this approach by comparison to other methods and explore the rate of water diffusion in a range of chemical systems containing mixtures of oxygenated organic molecules and salts.

Characterizing the rate of water diffusion in gel and ultraviscous containing organic and inorganic

ACS Spring 2022

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