AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Modeling of Solute Activities and Relative Humidity in Atmospheric Aerosols
CARI DUTCHER, Ge Xinlei, Anthony Wexler, Simon Clegg, University of California, Davis
Abstract Number: 390 Working Group: Aerosol Chemistry
Abstract Accurate predictions of water and solute activities in atmospheric aerosols to very low equilibrium relative humidities (RH) are central to predications of aerosol size, optical properties and cloud formation. A powerful method has been recently developed (Dutcher et al. JPC 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 found in solutions. In these works, statistical mechanics is 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-Huckel (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.