Two numerical integration methods identified as having features that provided significant improvements over the technique originally embedded in the Airshed Model have been evaluated. Of particular concern was the treatment of horizontal transport. In the evaluation of the schemes, the predictions resulting from the SHASTA method differed no more than about 20 percent from those generated using the original method. In addition, SHASTA posseses the better blend of computational speed and minimum error propagation. An objective analysis technique for obtaining a gridded, time-varying, fully three-dimensional wind field for the Airshed Model from available measurements has been developed. The technique accounts for urban heat island effects and should be directly applicable to relatively flat areas. A 42-step chemical kinetic mechanism for describing the chemical transformations of organics, NOx, O3, and SO2 and the production of sulfate, nitrate, and organic aerosols is presented. A unique feature of this mechanism is the explicit consideration given to the carbon bonds making up each organic molecule. An algorithm that relates the effective deposition velocity to the stability of the atmosphere and the type of surface has been implemented in the Airshed Model. Surface removal processes may significantly affect the concentrations of O3, NO2, and SO2.