Maxwell–Stefan diffusion: a framework for predicting condensed phase diffusion and phase separation in atmospheric aerosol

• Published in: Atmospheric chemistry and physics Vol. 18; no. 3; pp. 1629 - 1642
• Main Authors: Fowler, Kathryn, Connolly, Paul J., Topping, David O., O'Meara, Simon
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 05-02-2018; Copernicus Publications
• Subjects: Activity coefficients; Aerosol composition; Aerosol effects; Aerosol particles; Aerosols; Air sampling; Atmospheric aerosols; Atmospheric models; Balancing; Binary systems; Coefficients; Composition effects; Diffusion; Folding; Frameworks; Interactions; Investigations; Phase separation; Physical properties; Relative humidity; Solubility; Viscosity; Water vapor; Water vapour
• ISSN: 1680-7324; 1680-7316; 1680-7324
• DOI: 10.5194/acp-18-1629-2018

The composition of atmospheric aerosol particles has been found to influence
their micro-physical properties and their interaction with water vapour in
the atmosphere. Core–shell models have been used to investigate the
relationship between composition, viscosity and equilibration timescales.
These models have traditionally relied on the Fickian laws of diffusion with
no explicit account of non-ideal interactions. We introduce the
Maxwell–Stefan diffusion framework as an alternative method, which explicitly
accounts for non-ideal interactions through activity coefficients. e-folding
time is the time it takes for the difference in surface and bulk
concentration to change by an exponential factor and was used to investigate
the interplay between viscosity and solubility and the effect this has on
equilibration timescales within individual aerosol particles. The e-folding
time was estimated after instantaneous increases in relative humidity to
binary systems of water and an organic component. At low water mole
fractions, viscous effects were found to dominate mixing. However, at high
water mole fractions, equilibration times were more sensitive to a range in
solubility, shown through the greater variation in e-folding times. This is
the first time the Maxwell–Stefan framework has been applied to an
atmospheric aerosol core–shell model and shows that there is a complex
interplay between the viscous and solubility effects on aerosol composition
that requires further investigation.