7.1.3.  Dispersion Modeling Methods  Aviation Emissions and Air Quality Handbook  Typically, the first step in dispersion modeling is to compile detailed information on emission sources being modeled. This information includes the source emission rates (including  temporal variations), release characteristics, location coordinates , and source layout. For an  airport, these emission source data may take the form of  an aircraft fleet mix, runway and  taxiway locations, gate assignments, stack height, and airport operational profiles (see  Appendix  D ,  Airport Dispersion Modeling , for more information).  Second, representative meteorological information is required, typic ally obtainable from  publicly-available databases; this data  includes wind speed, wind direction,  and atmospheric  stability (i.e., surface roughness, albedo, and Bowen Ratio). For those locations with variable  terrain, local topographic data may  also be required. Receptors are similarly identified and  located as representing the locations where pollutant concentrations are computed.  Finally, appropriate background concentrations are added to the  computed concentrations to  represent the contributions from all  other emission sources within the study area.  Special  consideration should be given  to the estimation of NO 2  concentrations to account for the  conversion of nitric oxide (NO) to NO$_{2 }$ (see  Appendix D ,  Airport Dispersion Modeling , for  more information).  For clarity, this multi- step process of setting up and running a dispersion model such AERMOD for aviation-related sources is summarized as follows:  59$_{ }$ • Tabulate results  - Representative background concentrations should be added to the  model predicted concentrations.  • Determine background concentrations  - Background concentrations are typically  obtained from a representative background monitoring site not affected by the  modeled  emission source(s).  • Select receptor site locations  - The locations at which concentrations are estimated are  known as receptors. Generally receptors are located where the general public is likely to  have continuous access.  • Select terrain data  - Digital elevation data is used to determine elevations for sources and  receptors using AERMAP and account for wind and plume behavior associated with  terrain features.  • Select meteorological data  - Develop meteorological data such as wind speed, wind  direction, turbulence indices, temperature, and relative humidity using the AERMET  processor and surface/upper air data from the nearest representative monitoring station.  • Select model options  - Model options include averaging periods, pollutant types, urba n  vs. rural setting, receptor layout, and data output format.  • Select emission source characteristics  - Develop source release conditions for point, area,  line, and volume sources including emission rate, stack height and diameter, exhaust  temperature and exit velocity, source length and width, and volume height above ground  level. Also, source locations are to be provided to create a spatial relationship between  sources and receptors. 