Characterizing the Rate of Water Diffusion in Viscous Particles and Gels Containing Organic and Inorganic Material

CRAIG SHELDON, James F. Davies, University of California, Riverside

     Abstract Number: 8
     Working Group: Aerosol Chemistry

Abstract
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 viscous amorphous particles and phase separated core-shell and partially engulfed morphologies have been studied extensively. Recent work has demonstrated the importance of gel formation in particles containing mixtures of organic and inorganic material. A gel state is a porous solid network with liquid entrained in the pores, limiting the transport of material to slow percolation through the pores that penetrate throughout the particle. Unlike in the formation of viscous particles, gel formation is a phase transition and, as such, particles can exhibit a more distinct change in properties as RH decreases, increasing the importance of understanding the RH response of these systems.

Here, we focus on the rate of water diffusion in gel states as a proxy for molecular diffusion in general. We probe the response of a single levitated particle to instantaneous RH steps of 1% using a linear-quadrupole electrodynamic balance coupled with Mie resonance spectroscopy. By tracking the position of individual morphology-dependent resonances in the spectrum over the particle response, the rate of diffusion may be estimated from the half-life response and the particle radius.

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 an exploration of the water diffusion in organic aerosols and the influence of adding inorganic co-solutes, such as CaCl2 and ammonium sulfate, both of which have been shown to produce significant ion-molecular interactions that lead to large and unexpected changes in viscosity and diffusion rates. Our results show a slowing of water diffusion, correlating with changes in viscosity, in some systems, while in others the properties are decoupled, attributed to the formation of a gel state that imposes additional constraints on mass transport. Molecular diffusion is a controlling factor in the heterogeneous reactivity of aerosol particles and lifetime of chemicals in the atmosphere that is not fully accounted for in atmospheric models.