A photochemical grid model is a computer model used by meteorologists and environmental scientists to simulate what might happen during air pollution episodes under various weather conditions. A grid system is employed in which the area of study—for instance, a city—is cut up into thousands of cells, each usually a couple miles or kilometers wide and long; the cells also have a third dimension (height), varying in depth according to the altitude that scientists wish to study. These models simulate vertical and horizontal air movements, increases in various gases and particles from sources ranging from buildings and cars to plants and animals, and chemical reactions occurring in the atmosphere; they are very useful in predicting effects on ozone levels. A photochemical grid model is thus not the same as a meteorological model, but it does use this meteorological tool to study how pollution increases, dissipates, and affects certain areas. Photochemical grid models can be used to simulate how making different decisions affecting pollutant outputs would affect air quality. For example, if city officials decided to limit commuter traffic into the downtown area by ten percent, they could simulate how levels of carbon monoxide would be reduced.