APPENDIX D - Atmospheric Dispersion Modeling  APPENDIX D$_{ }$ D.1  Overview  D.2  Dispersion Modeling Data  D.2.1  Model Identification and Options  70  EPA, Preferred/Recommended Models, http://www.epa.gov/scram001/dispersion_prefrec.htm.  71  The term  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  74$_{ }$ AERMOD is typically executed using the regulatory default options (e.g., stack- tip downwash,  buoyancy-induced dispersion, and final plume rise), default wind speed profile categories,  default potential temperature gradients, and no pollutant decay. AERMOD also has the capability  to account for building downwash effects$^{71 }$and to employ gas or particle deposition or wet/dry  depletion of the plume. 72  The model predicts both short - (i.e., 1-, 3, 8, 24- hour) and long -term (i.e., annual) average  concentrations (in microgram per cubic meter or µg/m $^{3}$) at each receptor. These concentrations  may be presented as plot files and receptor files providing the results for both tabular graphical display.  AERMOD is a steady-state Gaussian dispersion model recommended by the EPA for short-range  (i.e., up to 50 kilometers) dispersion of air pollutant emissions from point, area, line, volumes sources over simple, intermediate, and complex terrain. 70  As such, AERMOD serves as  dispersion modeling component for the FAA’s EDMS/AEDT.  These concepts and elements are discussed individually in the following sections.  The fundamental concepts and elements of air dispersion modeling include the following:  This appendix discusses the methodology, typical input data, and endpoints associated  with  atmospheric dispersion modeling of airport-related sources.  Section 7.1  ( Atmospheric Dispersion  Modeling for Airports ) discusses the available guidance for preparing and reporting the results of  dispersion modeling for FAA projects/actions.  • Model identification and options  • Emission source release characteristics  • Meteorological data  • Spatial allocation  • Temporal profiles  • Topography data  • Building downwash  • Receptor locations  • NO$_{x }$to NO$_{2 }$conversion  • Background concentrations 