Aviation Emissions and Air Quality Handbook  7.  Conducting Dispersion Modeling for Criteria Air  Pollutants  7.1. Atmospheric Dispersion Modeling for Airports  7.1.1.  Introduction & Background  7.1.2.  EPA Guidance  70  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.  71  FAA,  Emissions and Dispersion Modeling System Policy for Airport Air Quality Analysis; Interim Guidance to  FAA Orders 1050.1D and 5050.4A , 18068 Federal Register/Vol. 63, No. 70/Monday, April 13, 1998/Notices,  Notably, dispersion modeling requirements and methodologies may vary by purpose and/or  locale, thus state and regional air quality regulatory agencies should be consulted on methodology, assumptions, and data sources utilized.  However, as discussed previously in  Section 5  ( Air Quality Assessment Models ), the FAA’s  EDMS is the required model for performing air quality assessments of aviation-related sources in  support of FAA projects/actions until EDMS is replaced by AEDT2b. 71  Coupled with  EPA’s principal guidance of conducting dispersion modeling and assessing the air impacts on nearby receptors is entitled and described as follows:  This guidance contains recommendations and supporting information on the selection and  applications of air quality models, determining background concentrations and the use of  meteorological data. This guidance also specifies dispersion models required to be used  for SIPs revisions and for New Source Review (NSR) and Prevention of Significant  Deterioration (PSD) programs.  In general terms, dispersion modeling is the process by which the dispersal of atmospheric  pollutants are simulated and assessed under effects of meteorological, terrain and other  influencing factors. Computer models such as  the American Meteorological Society/ EPA  Regulatory Model (AERMOD) have been developed and used for this purpose. The results of  this modeling allow for the prediction of pollutant concentrations at or near an source(s) and enable the comparison of these results to the NAAQS.  This section discusses the recommended guidance and approaches for conducting and reporting  atmospheric dispersion modeling of  aviation-related emissions in connection with FAA  projects/actions.  Appendix D  ( Atmospheric  Dispersion  Modeling )  provides  additional  information on dispersion modeling methods, data requirements, and other supporting materials.  This section addresses two t ypes of atmospheric dispersion modeling conducted in support of  FAA projects/actions: (i) atmospheric dispersion for airports and (ii) dispersion modeling.  • Appendix W to Part 51  - Guideline on Air Quality Models 70  57$_{ }$