The selection of the appropriate dispersion coefficients depends on the land use within three kilometers (km) of the source. The land use typing is based on the classification method defined by Auer$^{73}$; using pertinent U.S. Geological Survey (USGS) 1:24,000 scale (7.5 topographic maps of the area. If the Auer land use types of heavy industrial, light -to-moderate industrial, commercial, and compact residential account for 50 percent or more of the total area, the EPA Guideline on Air Quality Models 74 recommends using urban dispersion coefficients; otherwise, the appropriate rural coefficients are to be used. Generally, airports are located in urban areas; however, the immediate area is often characterized by large areas of pavement, low buildings, and open space. Thus, rural coefficients may be most appropriate for most airports. 

## D.2.2 Emission Source Release Characteristics 

Dispersion models require several types of data for each emission source to simulate the emission release characteristics. Depending on the source category (e.g., taxiway, roadways, stationary sources), AERMOD constructs point, area, line, or volume sources. Point stationary) sources, with buoyant plumes due to thermal energy and momentum, are used to model stacks from boilers, turbines, generators, and cooling towers. Area sources are used to model emissions from aircraft gate aprons (i.e., aircraft at startup, GSE operations, and APU activity), aircraft taxiing, queuing, accelerating on the runway, and in climb- out and approach modes. Roadways are classified as line sources or series of area sources. Volume sources are used to model any source that has an area and height element such as fuel storage facilities. 

Depending on the source type, some additional information about the source is required to model the dispersion of the emissions. This additional information includes the following: 

• Source layout, including location and height; 

• For stack emissions, parameters including location, stack gas temperature, exit velocity, stack diameter and stack height above ground; 

• For mobile sources, link length and width; and 

• For area sources, coordinates defining the shape and height above ground. 

EDMS/AEDT spatially allocates APU and GSE emissions to a defined aircraft apron. These operations can be designated as an area source with a height of 1.5 meters and an initial vertical distribution of 3 meters. 

Each parking garage should be treated as one area source per garage level, which is stacked to simulate a parking garage. The number of levels in each garage and the average separation between levels (e.g., approximately 4 meters or 13 feet) is based on site-specific information and 

