Heterogeneous chemistry and photochemistry of metal-containing aerosols and their impacts on atmospheric chemistry, metal mobilization, biogeochemical cycles and human health

Atmospheric aerosols have significant impact on the chemical balance of the atmosphere, biogeochemical cycles, the Earth's climate and human health. Although extensive studies have been performed to explore these effects, there is still considerable uncertainty regarding the global impacts of atmospheric aerosols.

For example, understand heterogeneous photochemistry on aerosols may be crucial to accurately predict the impact of aerosol loadings on atmospheric chemistry, yet few studies have been conducted. In this dissertation research, laboratory studies were performed to investigate heterogeneous photoreactions of HNO3 and O3 on typical components of metal containing aerosols. Results indicate that the presence of irradiation has the potential to change the reaction mechanism, kinetics, reaction extent, products, and product partitioning. The presence of water at different relative humidity also plays a key role in the rates of these reactions and the product distribution.

Increasing evidence has suggested that anthropogenic aerosols play a more important role in supplying soluble iron into open ocean water compared with mineral dust. Dissolution experiments to simulate atmospheric processing were performed to compare iron mobilization abilities of coal fly ash with Arizona test dust, a model for mineral dust aerosol. Results indicate that coal fly ash spheres, which is mainly composed of aluminosilicate glass, are unstable and disintegrate into irregular fragments during simulated cloud processing. The disintegration of spherical fly ash facilitates the release of iron and thus iron mobilization. In contrast, aluminosilicate mineral as the main component of Arizona test dust is relative stable and thus shows no significant change of morphology during simulated atmospheric processing. The iron solubility strongly depends on the source material, surface pH, types of acidic media, and the presence of solar irradiation.

Heavy metals in the environment have a negative effect on human health. A study on Pb mobilization from PbO particles following exposure to NO2 shows interaction of PbO particles with NO2 leads to an increase in Pb dissolution. These results point to the potential importance and impact that heterogeneous chemistry with trace gases can have on increasing solubility and therefore the mobilization of heavy metals such as lead in the environment.

Furthermore, as a transition metal, iron is capable of generating reactive oxygen species and contributing to oxidative stress. Collaboration work indicates that size and surface area of iron nanoparticles play a role in affecting bacteria growth, pathogenicity, and impairing the AMP activity. Nanoparticles, especially the smaller particles with large surface areas, may be harmful to human health as it relates to individuals susceptible to bacterial infections and/or colonization.

This thesis summarizes the above studies in Chapter 3 to Chapter 7. The research described herein provides a number of important issues where further studies are warranted. The last chapter suggests future directions for laboratory studies that have the potential to make an important contribution on understand the global impacts of atmospheric aerosols and heterogeneous chemistry.