EVALUATION, IMPROVEMENT, AND APPLICATION OF MODELS OF ENVIRONMENTAL FATE AND TRANSPORT OF ATMOSPHERE-SURFACE EXCHANGEABLE POLLUTANTS (ASEPs)

Abstract

Certain toxic, semivolatile chemicals, also known as atmosphere-surface exchangeable pollutants (ASEPs) are emitted into the environment from anthropogenic activities and natural sources. This dissertation focused on the (i) evaluation and improvement of dry particle deposition and atmosphere-surface exchange parameterizations of ASEPs for use in chemical transport models (CTMs), and (ii) application of a dynamic multimedia model to examine recovery of Lake Superior from historical inputs of polychlorinated biphenyl (PCB) compounds (a class of ASEPs) contamination. Current knowledge has been inadequate to propose quantitative measures of the relative performance of available dry particle deposition parameterizations. In Chapter 1 of this dissertation, five dry deposition parameterizations were evaluated against field observations across five land use categories, in terms their ability to reproduce observed deposition velocities, Vd (accuracy), the influence of imprecision in input parameter values on the modeled Vd (uncertainty), identification of the most influential parameter(s) (sensitivity), and complexity. Based on the evaluation, a recommendation for the superior parameterization for use in CTMs is made. In most CTMs, current parameterizations of atmosphere-terrestrial surface exchange of elemental mercury (Hg0) lack critical evaluation against field measurements. In Chapter 2 of this dissertation, an extensive evaluation of most commonly used resistance-based dry gaseous deposition and soil re-emission parameterizations (base model) of Hg0 was performed using a direct comparison to micrometeorological flux measurements from two ecosystems. This evaluation elucidated two major shortcomings of the base model: significant overestimation of leaf Hg uptake in summer month and an inability of capture measured nighttime net depositions. A step-wise model calibration was performed to adjust certain stomatal, non-stomatal, and soil re-emission parameters of the base model, which enabled an improved prediction of measured net exchange fluxes and growing-season leaf Hg accumulation. Based on the evaluation, generic recommendations for improvement in modeling Hg0 exchange for CTMs were made. Despite the U.S. production ban on PCBs in 1979, the measured concentrations in Lake Superior fish exhibited only a slow decline over the last 20 years. Sediment recycling of PCBs is often invoked to explain this slow recovery in fish PCB concentrations. In Chapter 3 of this dissertation, a dynamic multimedia model was applied to investigate the observed leveling-off of fish PCB concentrations in Lake Superior. Using historical to present-day PCB emissions (1930-2013) as the primary input in the multimedia model, the long-term trends in predicted PCB concentrations in different environmental media (water, sediment, and biota) were compared with available measurements for