The magnitude of emissions associated with urban activities will increase as urban population increases. Prolonged exposure to air pollution can negatively affect humans and the environment. The goal of this research project is to examine how changes in urban emissions affect the concentration of chemical species above and downwind of Mexico City using the National Center for Atmospheric Research Master Mechanism (MM). First, the photolysis of nitrogen dioxide (NO₂) was simulated. The result shows for a short time span an analytical solution can be used to approximate the rate of change of NO₂ concentration when photolyzed, but for a longer time span a larger set of reactions must be taken into account. Next, the MM's simulation of photochemical reactions above and downwind of Mexico City was compared to concentration measurements of sampled air pollutants compiled in the MIRAGE-Mex field campaign database. Then a sensitivity test was conducted to reduce the urban emission of nitric oxide (NO) by reducing the NO urban emission by a factor of eight. The results of the MM simulation indicate significant disparities between air pollutant concentration partitioning as determined by the MM and air pollutant concentration measurements made above Mexico City. Overall, the MM does not yet reasonably simulate photochemistry above Mexico City. The results from this research project can be used to better understand which initial concentrations and values in the input files for MM need to be changed in order to obtain reasonable simulations of photochemistry above Mexico City.