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.  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:  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  sources on or near the structure(s). Building downwash often leads to elevated concentrations downwind  of  affected sources.  72 Dry deposition  is the removal of gaseous or particulate material from the pollution plume by contact with the  ground surface or vegetation (or even water surfaces) through transfer processes such as absorption gravitational sedimentation.  Wet deposition  is the removal of pollution plume components by the action of rain.  73 Auer, August H., 1978:  Correlation of Land Use and Cover with Meteorological Anomalies . J. Appl. Meteor.,  17, 636–643 http://journals.ametsoc.org/doi/pdf/10.1175/1520- 0450%281978%29017%3C0636%3ACOLUAC%3E2.0.CO%3B2  74 Appendix W to Part 51 –  Guideline on Air Quality Models , http://www.ecfr.gov/cgi-bin/text- idx?SID=e6a5b817b94abf58460f48c032d9a39c&node=40:2.0.1.1.2.23.11.5.37&rgn=div9.  75$_{ }$ • 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.  D.2.2  Emission Source Release Characteristics  APPENDIX D$_{ }$