Thermodynamic modeling of atmospheric aerosols: predicting water content and solute activities

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Description

Accurate models of water and solute activities in atmospheric aerosols are central to predicting aerosol size, optical properties, and cloud formation. A powerful method has been recently developed (Dutcher et al. Journal of Physical Chemistry C 2011, 2012) for capturing the thermodynamic properties of multicomponent aerosols at low and intermediate levels of RH (< 90% RH) by applying the principles of multilayer adsorption to ion hydration in solutions. In that work, statistical mechanics was used to model adsorption of a solvent on to n energetically distinct layers in the hydration shell surrounding the solute molecule in aqueous mixtures. Here, we extend the model to the 100% RH limit and reduce the number of adjustable model parameters, allowing for a unified thermodynamic treatment for a wider range of atmospheric systems. The long-range interactions due to electrostatic screenings of ions in solution are included through a mole fraction based Pitzer-Debye-Hückel (PDH) term. Equations for the Gibbs free energy, solvent and solute activity, and solute concentration are derived, yielding remarkable agreement of the solute concentration and osmotic coefficients for solutions over the entire 0 to 100% RH range. The number of adjustable model parameters is reduced by relating the values of the energy of adsorption to each hydration layer to known short-range Coulombic electrostatic relationships. The effect of the PDH long-range and Coulombic short-range electrostatics on the mixing relationship is explored and new insights into the molecular relationships within atmospheric aerosols is discussed. Fields beyond atmospheric aerosol science, including geological and ocean solution thermodynamics, may benefit from the models developed in this work.