Thermodynamic Modeling of Organic Aerosol

Author: Tong, Chinghang

Year: 2008

Degree: Dissertation (Ph.D.)

Advisors: Seinfeld, John H.; Goddard, William A., III

Committee Members: Seinfeld, John H.; Blanco, Mario; Okumura, Mitchio; Goddard, William A., III

Option: Environmental Science and Engineering

DOI: 10.7907/3M7R-9620

Abstract

Modeling atmospheric aerosols containing a large organic fraction with unknown chemical composition and properties has been a constant challenge. The dissertation focuses on the theoretical treatment of the thermodynamic equilibrium of atmospheric aerosol involving organic species.

We present a vapor pressure estimation method, based on quantum chemistry methods, to predict the liquid vapor pressure, enthalpies of vaporization, and heats of sublimation of atmospheric organic compounds. Predictions are compared to literature data, and the overall accuracy is considered satisfactory given the simplicity of the equations. Quantum mechanical methods were also used to investigate the thermodynamic feasibility of various acid-catalyzed aerosol-phase heterogeneous chemical reactions. A stepwise procedure is presented to determine physical properties such as heats of formation, standard entropies, Gibbs free energies of formation, and solvation energies from quantum mechanics, for various short-chain aldehydes and ketones. Equilibrium constants of hydration reactions and aldol condensation are then reported; predictions are in qualitatively agreement with previous studies. We have shown that quantum methods can serve as useful tools for first approximation, especially for species with no available data, in determining the thermodynamic properties of multifunctional oxygenates.

We also present an atmospheric aerosol phase equilibrium model to determine the aerosol phase equilibrium of aqueous systems. Phase diagrams for a number of organic/water systems characteristic of both primary and secondary organic aerosols are computed. Effects of organics on the deliquescence behavior of electrolytes are also shown in the inorganic/organic/water phase diagrams.

Finally, we evaluate the performance of four recent activity coefficient models developed for inorganic-organic-water mixtures typical of atmospheric aerosols. Based on the comparison on water activities, it is found that models that include ion-organic mixture parameters (referred to as coupled models) do not necessarily produce more accurate predictions than those models that utilizes additive approaches (referred to as decoupled models). Since the chemical composition and physical properties of the organic fraction is largely unknown, the additive approaches of the decoupled models are more feasible than the coupled model.

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