Dispersion Modeling uses mathematical formulations to characterize the atmospheric processes that disperse a pollutant emitted by a source. It is used to predict concentrations at selected downwind receptor locations based on emissions and meteorological input. The air quality models are used to determine compliance with National Ambient Air Quality Standards (NAAQS), and other regulatory requirements such as New Source Review (NSR) and Prevention of Significant Deterioration (PSD) regulations. These models are addressed in Appendix A of EPA's Guideline on air quality models to provide consistency and equity in the use of modeling within the air quality management system. These guidelines are periodically revised to ensure that new model developments or expanded regulatory requirements are incorporated. It serves to assist in the design of effective control strategies to reduce emissions of harmful air pollutants.