A novel application of classical thermodynamics is presented to understand the distribution of aerosol forming material between the gas and aerosol phases in the polluted troposphere. The particular system studied involves NH4NO3 and its interactions with the environmental variables, temperature, relative humidity, droplet pH and aqueous (NH4)2SO4 concentration. In Chapter 1, the theoretical temperature dependence of the solid NH4NO3 dissociation constant is compared to ambient ammonia-nitric acid partial pressure products and general agreement is shown. Also, temperature is demonstrated to be a determining factor for ambient aerosol nitrate formation. Chapter 2 discusses how an urban aerosol can be chemically characterized and that the aqueous electrolytic aerosol solutions are very concentrated (> 8 molal). Thus, any attempt to model ion interactions in aerosol solutions must be able to represent the concentrated solution regime. The ammonia-nitric acid partial pressure product for concentrated NH4NO3-HNO3-H2O solutions is shown to be sensitive to relative humidity but not to pH (1-7) in Chapter 3. Since the ammonia-nitric acid partial pressure product is insensitive to pH, the NH4NO3 dissociation constant over NH4NO3-H2O solutions should typify the ammonia-nitric acid partial pressure product above slightly acidic solutions. The NH4NO3 dissociation constant temperature and relative humidity dependence is evaluated and compared to ambient data in Chapter 4. General agreement between the predictions and the data exists but the possible effect of additional solutes in aerosol droplets is evident. Since NH4NO3 and (NH4)2SO4 are present in atmospheric particles of similar size, it is appropriate to calculate the effect of (NH4)2SO4 on the relative humidity dependence of the NH4NO3 dissociation constant. Chapter 5 shows the presence of (NH4)2SO4 reduces the amount of ammonia and nitric acid in the gas phase and that the NH4NO3 dissociation constant is only 40% less for a 0.5 (NH4)2SO4 ionic strength fraction in aqueous solution. Also, methods for predicting the particle growth, the solution density and the refractive index of NH4NO3-(NH4)2SO4-H2O solutions are outlined in Chapter 5. Good accordance between experimental data and predictions is demonstrated indicating the possible applicability of these techniques to more complex multicomponent solutions.
In the Appendices, a density prediction technique for (NH4)2SO4-H2SO4-H2O solutions is presented since this aspect of ambient aerosols is not contained in the major thrust of this work.
