Taxipaths (i.e., the arrival path from the end of the landing roll to the terminal and the departure  path from the terminal to the runway end) are also designated based on field observations, airport  procedures, and/or airfield simulation modeling and included in the dispersion modeling  analysis. In this manner, a taxipath is a series of taxiways from runway end to aircraft servicing  location. Within these taxipaths, aircraft speeds can be designated as 5 knots within the terminal  areas, 10 knots within taxiways with tight turns, 15 knots for most remaining taxiways, and 35  knots for high speeds runway exits. The EDMS/AEDT default value is 15 knots.  Apron/gate areas define the locations of aircraft operations such as unloading and loading of  cargo and passengers. Thus, these locations also represent activities  associated with APU, GSE  and other supporting operations.  Temporal (or operational) profiles are used to describe the relationship of one time period another (i.e., the relationship of the activity during  1-hour to the activity during  a twenty-four  hour period). In EDMS/AEDT, these temporal profiles are used to represent varying levels of  activity as a fraction of a peak period (a scale of zero to one: unless values denoted as fraction of  peak values).  Notably, distinct temporal profiles can be developed for air carrier, cargo, commuter, general  aviation, and military aircraft operations during arrival and departure conditions for each quarter  hour of the day, day of the week, and month based on ASPM data.  The terrain in the vicinity of airports is usually flat because of the requirement for a runway, approach and climbout area. Some sources, such as training fires, stacks, and painting  operations, produce emission in the form of a buoyant plume. In such cases, topography may  play a role in altering the downwind dispersion of the plume and should be included in dispersion analysis.  AERMOD includes a terrain preprocessor, AERMAP 87 , which provides a physical relationship  between terrain features and the behavior  of air pollution plumes. AERMAP processes USGS  Digital Elevation Model (DEM) data and creates a file of elevation and hill- height scaling  factors. AERMAP generates location and height data for each receptor and source location. also provides information that allows the dispersion model to simulate the effects of air flowing  over hills or splitting flow around hills.  Building downwash  describes the effect that wind flowing over or around a structure(s) has on  plumes released from nearby sources (such as stacks). Essentially, structure(s) create a cavity of  recirculating winds in the area near the structure(s), and these cavities cause increased vertical  dispersion of plumes emitted from sources on or near the structure(s). Building downwash often  leads to elevated concentrations downwind of affected sources.  87 EPA,  User’s Guide for the AERMOD Terrain Preprocessor (AERMAP) , October 2004,  http://www.epa.gov/scram001/dispersion_related.htm.  D.2.8  Building Downwash  D.2.7  Topographical Data  D.2.6  Temporal Profiles  APPENDIX D$_{ }$ 82$_{ }$