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Investigation of Dispersion and Micrometeorology Under Spatially Inhomogeneous Conditions

  • Author(s): Qian, Wenjun
  • Advisor(s): Venkatram, Akula
  • et al.
Abstract

This dissertation summarizes the results from a study to develop and evaluate models to estimate dispersion of pollutants in boundary layers whose properties change with downwind distance. Such boundary layers occur at the interface between rural and urban areas, and water and land bodies.

I first developed a method to estimate the meteorological inputs required to apply the current generation of dispersion models, such as AERMOD, to urban areas. This method uses measurements made at a single level on a tower located in an urban area, and is based on the assumption that similarity methods applicable to spatially homogeneous conditions are locally valid even in an inhomogeneous urban area. I show that under unstable conditions, measurements of temperature fluctuations improve upon commonly used energy balance methods to estimate heat flux. Also, any bias in heat flux estimates has a minor effect on the prediction of surface friction velocity and turbulent velocities.

I examined a method to estimate urban micrometeorology using measurements made on a tower located in an upwind suburban area. I applied an internal boundary layer model to trace the evolution of the boundary layer as it traveled from the suburban measurement location to the urban location of interest. Estimates from the model were observations made during a field study conducted in Riverside, CA. Estimates of friction velocity compare well with urban measurements only when the variation of friction velocity with height within the urban canopy was accounted for.

I examined the performance of two steady-state dispersion models in explaining concentrations measured during field studies designed to study low wind speed conditions typical of urban areas. One model is based on the numerical solution of the two-dimensional mass conservation equation and the other is AERMOD, which accounts for low wind speeds by including wind meandering. The numerical method performs better than AERMOD through a justifiable description of the interaction between dispersion and the gradient of the wind speed near the surface. Including wind meandering, which occurs under low wind speeds, improves the performance of the numerical model. As part of this study, I developed a method to improve estimates of surface friction velocity during low wind speeds.

The last model is applicable to elevated sources, such as power plants, situated close to shorelines. It is designed to be compatible with AERMOD (Cimorelli et al., 2005), the USEPA's regulatory model that is currently designed for spatially homogenous conditions. The semi-empirical shoreline dispersion model accounts for plume entrainment by the thermal internal boundary Layer (TIBL), whose height increases with distance from the shoreline. I show that AERMOD can be modified to account for two-dimensional shoreline effects, and this modified model performs as well in explaining observations as dispersion models specifically designed for shoreline sources. I also developed a method to generate meteorological inputs that are compatible with the current structure of AERMOD's inputs.

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