Date of Degree
PhD (Doctor of Philosophy)
Vicki H. Grassian
Particulate matter in the atmosphere emitted from various natural and anthropogenic sources is important due to their effects on the chemical balance of the atmosphere, the Earth's climate, human health and biogeochemical cycles. Although there have been many studies performed to understand the above effects, there still remains substantial uncertainty associated with processes involved and thus it is difficult for current atmospheric chemistry and climate models to reconcile model results with field measurements. Therefore, it is important to have better agreement between models and observations as the accuracy of future atmospheric chemistry and climate predictions depends on it.
In this research, a greater understanding of the role of mineral dust chemistry was pursued through focused laboratory studies in order to better understand fundamental processes involved. In particular, studies to further understand the photochemistry of adsorbed nitrate, an important inorganic ion associated with particulate matter exposed to gas-phase nitrogen oxides, were conducted using Al2O3, TiO2 and NaY zeolite to represent non-photoactive components, photoactive components and aluminosilicate respectively, present in mineral dust. These studies reveal that photochemistry of nitrate adsorbed on mineral dust is governed by wavelength of light, physicochemical properties of dust particles and adsorption mode of nitrate. Gas phase NO2, NO and N2O are the photolysis products of nitrate on oxide particles under dry conditions. In contrast, nitrate adsorbed on zeolite is converted mainly to adsorbed nitrite upon irradiation. This nitrite yield is decreased with increasing relative humidity. Gas phase N2O is the main photolysis product of nitrate adsorbed in zeolite in the presence of co-adsorbed ammonia. Water adsorbed on semiconducting TiO2 can be photochemically converted to hydroxyl radicals. These hydroxyl radicals can be involved in surface mediated as well as gas phase oxidation reactions in the presence of cyclohexane. Another focus of this dissertation was to investigate the oxidation of sulfur dioxide oxidation in the presence of mineral aerosol, particularly, coal fly ash (FA), γ-Fe2O3 and Arizona test dust (AZTD), a model for mineral dust aerosol. Depending on the temporal evolution of Fe(II), we proposed that S(IV) oxidation in the presence of FA and γ-Fe2O3 initially occurs through a heterogeneous pathway and a homogeneous pathway is also possible over later time scales. S(IV) oxidation in the presence of AZTD appears to be mostly heterogeneous and does not lead to iron dissolution. Overall, these studies suggest that the rate, extent and products of atmospheric S(IV) oxidation can be highly variable and heavily dependent upon the nature of aerosol sources, thereby precluding simple generalizations about this reaction when modeling atmospheric processes involving diverse mineral dust aerosols. With the recent development in nanotechnology, nanoparticles are becoming a major fraction of atmospheric particulate matter. These particles can undergo aging under ambient conditions at any stage of their life cycle. This impacts the fundamental properties of these materials and therefore the behavior in the environment and interactions with biomolecules and biological systems. ZnO and CuO nanoparticles form adsorbed carbonate phases upon exposure to CO2 and water vapor. These carbonates become more solvated as the relative humidity is increased. Presence of carbonate phases on ZnO particles increases their water solubility. Thus, overall the work reported in this dissertation provides insights into heterogeneous and multiphase atmospheric chemical reactions in the presence of mineral aerosol and atmospheric aging of nanoparticles.
Atmosphere or the air surrounding us contains a lot of solid and liquid particles called aerosols. These aerosols include soil particles that get into the atmosphere by wind activity and very small particles of man-made materials. These particles in the atmosphere encounter different gases, water vapor and sunlight, and participate in interesting atmospheric chemistry. Atmospheric gases such as carbon dioxide, nitrogen dioxide and sulfur dioxide interact with these particles and generate coatings of different chemical species. A major focus of this dissertation was to understand the effect of sunlight on the interactions of atmospheric gases with aerosol particles. I found that chemical species on aerosol particle surfaces formed by the interaction with nitrogen containing gases are converted to different chemical species in the presence of sunlight. Mostly, these species formed in the presence of sunlight are gases. Some of the aerosol particles can absorb sunlight and use that energy to split water molecules generating hydroxyl radical. Aerosol particles present in clouds can convert pollutant gas sulfur dioxide into sulfate. I also found that interaction of carbon dioxide with man-made metal containing particles such as zinc oxide and copper oxide lead to changes in the particle surfaces. These changes affect their dissolution in water. Results presented in this dissertation help to better understand the effect of aerosols on the Earth’s atmospheric processes and climate.
publicabstract, Mineral dust, Nanoparticle aging, Nanoparticles, Nitrate, Photochemistry, Sulfur dioxide
Copyright 2016 Aruni Gankanda