In a previous study, a Regional Air Pollution Model was developed to simulate the effect of emissions from major point sources on air quality over long distances. This model was subsequently applied to the Northern Great Plains to assess the regional impact of existing and proposed energy developments in that area. This report continues that study by investigating methods for characterizing wind field and mixed layer diffusivity in mesoscale air quality modeling. After a review of pertinent theoretical and observational studies, the geostrophic wind and a scheme first proposed by Smagorinsky(1963) for horizontal eddy diffusivities were selected. A bicubic spline fit was used to interpolate the geopotential height data needed for implementation of this scheme. This method was used to compute the wind fields and horizontal diffusivities for the three time periods studied in the previous efforts, and the Regional Air Pollution Model was subsequently exercised using the new meteorological data.