Variabilities and uncertainties in characterising water transport kinetics in glassy and ultraviscous aerosol.

Andrew M J Rickards,Jonathan P Reid,Rachael E H Miles,Young-Chul Song,Thomas C Preston

doi:10.1039/c4cp05383d

Abstract

We present a comprehensive evaluation of the variabilities and uncertainties present in determining the kinetics of water transport in ultraviscous aerosol droplets, alongside new measurements of the water transport timescale in sucrose aerosol. Measurements are performed on individual droplets captured using aerosol optical tweezers and the change in particle size during water evaporation or condensation is inferred from shifts in the wavelength of the whispering gallery mode peaks at which spontaneous Raman scattering is enhanced. The characteristic relaxation timescale (τ) for condensation or evaporation of water from viscous droplets following a change in gas phase relative humidity can be described by the Kohlrausch-Williams-Watts function. To adequately characterise the water transport kinetics and determine τ, sufficient time must be allowed for the particle to progress towards the final state. However, instabilities in the environmental conditions can prevent an accurate characterisation of the kinetics over such long time frames. Comparison with established thermodynamic and diffusional water transport models suggests the determination of τ is insensitive to the choice of thermodynamic treatment. We report excellent agreement between experimental and simulated evaporation timescales, and investigate the scaling of τ with droplet radius. A clear increase in τ is observed for condensation with increase in drying (wait) time. This trend is qualitatively supported by model simulations.

Highlights

The conditions that lead to the formation of inhomogeneities in aerosol particle composition are well-established
We have provided a simple measure of the kinetics of the size change following a step change in the gas phase relative humidity (RH) as the time required for the particle to progress halfway towards its final limiting state at long time.[12]
We have considered the factors that lead to variability in measurements of the water transport kinetics and the uncertainties, both experimental and predictive, that must be considered when interpreting measurements of evaporation and condensation processes in ultraviscous and glassy aerosol

Summary

Introduction

The conditions that lead to the formation of inhomogeneities in aerosol particle composition are well-established. The mass transport of water in single viscous aerosol droplets has been studied using the electrodynamic balance technique,[1,17] optical tweezers[12,15,25] and a Bessel beam trap.[26] From the time-dependent response of particle size to change in relative humidity (RH), the compositional dependence of diffusion coefficients can be estimated and the mechanisms of condensation and evaporation explored and compared. The distinct fingerprints of wavelengths (or size parameters) of WGM structure can be used to infer gradients in RI and, concentration gradients within particles, and the particle size Such an approach is currently intractable when attempting to fit the large data sets required to investigate the slow water transport in viscous aerosol over long times. We will occasionally use limiting values of 0.4 and 0.6 to indicate the range of behaviour in the response function that might be expected

Uncertainties in modelling water transport in viscous aerosol

Key features of water transport kinetics in ultraviscous and glassy aerosol

Findings

Conclusions