Doctor of Philosophy (PhD)
Engineering Science (Interdepartmental Program)
In the natural process of wet deposition, gas-water interfaces play an important role in the transport of chemical contaminants in the atmosphere via fog, rain and cloud drops. Evidences from several other works point out deviations in gas-liquid partitioning as predicted by Henry’s law. Uptake and mass transfer of benzene, naphthalene, and phenanthrene was chosen to study in a falling droplet train apparatus. Higher droplet-to-vapor partition constant (KDV) was noted for diameters less than 200ìm and was attributed to surface adsorption and accumulation. Mass transfer of phenanthrene was dependent on gas-phase diffusion and mass accommodation at the interface, while the mass transfer of benzene was dependent on liquid phase diffusion and mass accommodation. Mass accommodation coefficients showed a negative dependence on temperature, resulting in lower partitioning at higher temperatures. In order to understand the influence of atmospheric oxidants such as ozone on mass transfer and uptake of organic vapors in water droplets, ozone was introduced into the modified droplet train apparatus. Ozone reacted with PAH vapor at the air-water interface, thereby decreasing the mass transfer resistance and increasing the rate of uptake of naphthalene into the droplet. A Langmuir-Hinshelwood reaction mechanism at the air-water interface satisfactorily described the surface reaction, where the surface reaction rate constant increased with decreasing droplet size. The presence of organic matter in the liquid phase resulted in a higher droplet-to-vapor partition constant due to both presence and absence of ozone in the reactor. Knowledge and observations from the laboratory scale setup were extended to field fogwater characterization. Various chemical properties and characteristics of fogwater were determined. Most of the chemical composition and make-up of fogwater was characterized due to near-surface local atmosphere. Concentration of certain pesticides and organic compounds were found in the fogwater far exceeding their aqueous solubility. The calculated KDV for the field samples were several orders of magnitude higher than bulk phase Henry’s constant prediction. Higher organic compound concentrations were observed in smaller-sized fogwater than in larger-sized droplets. These conclusions support our laboratory observation of higher partitioning due to surface adsorption and due to presence of organic matter in the aqueous phase.
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Raja, Suresh, "Transport and kinetics of aromatic hydrocarbons into micron-sized liquid droplets: with applications to atmospheric chemistry" (2005). LSU Doctoral Dissertations. 2738.
Kalliat T. Valsaraj