A Kinetic Study of the Heterogeneous Chemistry and Photochemistry of Ozone Dissociation on Oxide Surfaces

Transport of mineral dust over long distance raised the possibility that mineral dust aerosol can alter the chemical partitioning of atmospheric trace gases on global scale. One such important heterogeneous reaction is the decomposition of ozone, which is of particular interest due to the critical role ozone plays in atmospheric oxidation, its potential as a greenhouse gas, and its presence as a toxic tropospheric pollutant. In this study, the heterogeneous chemistry and photochemistry of ozone, O3, on components of mineral dust aerosol, including α-Fe2O3 (hematite), TiO2, and α-Al2O3, at different relative humidity were investigated using an environmental aerosol chamber. The rate and extent of O3 decomposition on the surfaces of these oxides is a function of the nature of the surface, presence of light and relative humidity. In the dark and dry conditions, only α-Fe2O3 exhibits catalytic decomposition toward O3. However, upon irradiation TiO2 is active toward O3 decomposition but α-Al2O3 remains inactive. In the presence of relative humidity, ozone decay on illuminated α-Fe2O3 as well as in the dark, is found to decrease, due to the competitive adsorption of water molecules and ozone for active surface sites. In contrast, the relative humidity dependence of ozone uptake on TiO2 in the presence of light is more complex with an increase in ozone uptake observed at low relative humidity and a decrease at higher relative humidity. A kinetic model to simulate reactions in the environmental aerosol chamber was built with the Kinetic Preprocessor, in order to obtain kinetic data regarding ozone uptake on these metal oxides under a variety of conditions. The application of this model to the environmental aerosol chamber results allows us to differentiate heterogeneous photochemistry from heterogeneous and homogeneous chemistry and extract valuable kinetic information, improving our understanding of ozone uptake on mineral dust aerosol.