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Matthew E. Wise, Ph.D.

Advisor: Margaret A. Tolbert, Department of Chemistry and Biochemistry
Funding: Department of Energy

Laboratory Studies of Sulfate Aerosols at Upper Tropospheric and Lower Stratospheric Temperatures and Compositions

Atmospheric aerosols can affect the Earth’s radiation bal-ance directly by scattering and absorbing solar radiation and indirectly by modifying cloud properties. In addition, aerosols can act to catalyze atmospheric reactions that would otherwise occur only slowly in the gas phase. The phase and water content of the aerosols are two of the most important parameters that determine how the particles im-pact atmospheric chemistry and climate. The phase and growth behavior of model inorganic aerosols is well estab-lished. However, recent work indicates that essentially all real atmospheric aerosols are complex mixtures of many components. This dissertation examines the growth and phase change behavior of sulfate aerosols internally mixed with other components. The first chapter of the dissertation focuses on the deliquescence and growth of ammonium sulfate/organic aerosols representative of one of the most common types of tropospheric particulate. For the systems studied, it was found that soluble organics in the particles contributed to water uptake. In addition, the soluble or-ganic material lowered the deliquescence relative humidity compared to pure ammonium sulfate, implying that the aerosols would be found in a liquid state over a broader range of atmospheric conditions. Together, these studies suggest that soluble organics enhance the reactivity and cloud forming ability of inorganic aerosols. The second chapter of the thesis focused on the ability of internally mixed ammonium sulfate/organic aerosols to serve as nu-cleation centers for cirrus clouds. Here, it was found that the freezing temperature of each system was identical, for a given water activity of the solution, even though the so-lutions contained varying fractions of inorganic and or-ganic components. Thus the soluble organics studied do not impede ice formation, in contrast to an earlier predic-tion. The final chapter of the thesis focused on strato-spheric sulfate aerosols. Recent field studies have shown that these aerosols contain metals characteristic of meteor-ites. These metals, whether present as soluble ions or in-soluble mineral inclusions, have an unknown effect on the formation of polar stratospheric clouds. Here he measured the solubility of metals in sulfuric acid and showed that dissolved metal ions enhanced freezing. This finding may provide a mechanism for the formation of polar strato-spheric clouds and provide clues to predicting future cloud abundances and their impact on stratospheric ozone.