Heterogeneous chemistry and photochemistry of atmospherically relevant gases on oxide surfaces

Metal oxides in the atmosphere emitted from various natural and anthropogenic processes alter the chemical balance of the Earth's atmosphere due to heterogeneous and photochemical processes with atmospheric trace gases. Therefore, understanding the heterogeneous chemistry and heterogeneous photochemistry of atmospheric trace gases on these oxide surfaces has become vital to precisely predict the effect of mineral dust loading on the Earth's atmosphere. Among the various components of mineral dust, light absorbing oxides play a significantly important role during the daytime.

The work reported herein has focused mainly on TiO₂ and Α-Fe₂O₃. These are light adsorbing components found in atmospheric mineral dust. Apart from being a component of mineral dust, TiO₂ is heavily used in a number of industrial applications ranging from uses in self-cleaning, water purification to cosmetics. These applications have led to their presence in the atmosphere as anthropogenic dust particles and in contact with the atmosphere as a stationary phase. Iron-containing particles are transferred to the atmosphere mainly from wind and volcanic activities in the form of iron-containing mineral dust and volcanic ash aerosols. Α-Fe₂O₃ is the most stable iron containing compound found in the Earths' crust which constitutes in significant amounts in mineral dust. The presence of these oxide surfaces in the atmosphere can play a major role in heterogeneous chemistry and photochemistry.

In this dissertation research, transmission FTIR spectroscopy and X-ray photoelectron spectroscopy are used to probe the details of heterogeneous chemistry and photochemistry of CO₂, SO₂, NO₂, HCOOH, and HNO₃ on titanium dioxide and hematite surfaces. Adsorption sites, surface speciation and surface species stability have been determined from analysis of FTIR and XPS spectra. Isotope labeling experiments were also carried out in order to obtain mechanistic information about the details of surface hydroxyl group reactivity on these oxide particle surfaces. Furthermore, heterogeneous photochemical reactions of adsorbates from atmospheric trace gas adsorption on TiO₂ and Α-Fe₂O₃ were investigated under the conditions pertinent to troposphere. The role of adsorbed water on the stability of adsorbed species that form as a result of heterogeneous reactions and the effect of relative humidity on photochemistry on these oxide particles surfaces has also been investigated due to its important implications in the atmospheric chemistry of oxide surfaces. The research adds to our overall scientific understanding of the molecular level details of heterogeneous chemistry and photochemistry of light absorbing components in the atmosphere.