Measurements of the timescales for the mass transfer of water in glassy aerosol at low relative humidity and ambient temperature

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Abstract
The influence of glassy states and highly viscous solution phases on the timescale of aerosol particle equilibration with water vapour is examined. In particular, the kinetics of mass transfer of water between the condensed and gas phases has been studied for sucrose solution droplets under conditions above and below the glass transition relative humidity (RH). Above the glass transition, sucrose droplets are shown to equilibrate on a timescale comparable to the change in RH. Below the glass transition, the timescale for mass transfer is shown to be extremely slow, with particles remaining in a state of disequilibrium even after timescales of more than 10 000 s. A phenomenological approach for quantifying the time response of particle size is used to illustrate the influence of the glassy aerosol state on the kinetics of mass transfer of water: the time is estimated for the droplet to reach the halfway point from an initial state towards a disequilibrium state at which the rate of size change decreases below 1 nm every 10 000 s. This half-time increases above 1000 s once the particle can be assumed to have formed a glass. The measurements are shown to be consistent with kinetic simulations of the slow diffusion of water within the particle bulk. When increasing the RH from below to above the glass transition, a particle can return to equilibrium with the gas phase on a timescale of 10's to 100's of seconds, once again forming a solution droplet. This is considerably shorter than the timescale for the size change of the particle when glassy and suggests that the dissolution of the glassy core can proceed rapidly, at least at room temperature. Similar behaviour in the slowing of the mass transfer rate below the glass transition RH is observed for binary aqueous raffinose solution droplets. Mixed component droplets of sucrose/sodium chloride/water also show slow equilibration at low RH, illustrating the importance of understanding the role of the bulk solution viscosity on the rate of mass transfer with the gas phase, even under conditions that may not lead to the formation of a glass.
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