Modeling water content and solute activities of atmospheric aerosols at extremely low relative humidities: extension of the Brunauer-Emmett-Teller (BET) adsorption isotherm equations

Description

Multilayer adsorption isotherm models describe a lattice adsorption of a solvent (e.g., water) molecule onto sites on a substrate (e.g., an electrolyte). The seminal adsorption model, Brunauer-Emmett-Teller (BET), successfully reproduces solute concentrations of solutions at water activities (aw, equivalent to the equilibrium relative humidity) of <0.4 to 0.5 using only two parameters: (1) the number of adsorption sites and (2) the energy of adsorption of the solvent directly on to the solute. The Guggenhein-Anderson-deBoer (GAB) model applies to solutions of aw <0.7 to 0.8 by adding a single additional energy of adsorption to approximately account for the extended hydration shell surrounding the solute molecule. In this work, statistical mechanics is used to modify the BET and GAB model to include distinct energies of adsorption of the solvent on to n layers in the hydration shell. Equations for the excess Gibbs energy, solute activity, and solute concentration are derived. The inclusion of additional hydration layers of distinct energy results in remarkable agreement of the solute concentration and osmotic coefficients for solutions of aw as high as 0.9 to 0.95. New insights into solute and water activity for single- and multi-component systems are presented. In particular, the model can be shown to be consistent with models commonly used in atmospheric science for estimating water contents of aerosols containing more than one solute. The model will be incorporated into the Extended Aerosol Inorganics Model (E-AIM) and made available on the web.

Clegg, SL, AS Wexler, and P Brimblecombe. Extended Aerosol Inorganics Model (E-AIM). http://www.aim.env.uea.ac.uk/aim/aim.php.