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5.1 AIR QUALITY • Results of the ambient air quality modeling analyses • PSD increment consumption 5.1.5.2 Model Selection The screening and refined air quality impact analyses were performed using the American Meteorological Society/Environmental Protection Agency Regulatory Model Improvement Committee (AERMIC) model,also known as AERMOD (current version 04300). The AERMOD model is a steady-state,multiple-source,Gaussian dispersion model designed for use with stack emission sources situated in terrain where ground elevations can exceed the stack heights of the emission sources (i.e. complex terrain).9 The model is capable of estimating concentrations for a wide range of averaging times (from 1 hour to 1 year). Inputs required by the AERMOD model include the following: • Model options • Meteorological data • Source data • Receptor data Model options refer to user selections that account for conditions specific to the area being modeled or to the emissions source that needs to be examined. Examples of model options include use of site-specific vertical profiles of wind speed and temperature;consideration of stack and building wake effects; and time-dependent exponential decay of pollutants. The model supplies recommended default options for the user for some of these parameters. AERMOD uses hourly meteorological data to characterize plume dispersion. The representativeness of the data is dependent on the proximity of the meteorological monitoring site to the area under consideration,the complexity of the terrain,the exposure of the meteorological monitoring site,and the period of time during which the data are collected.The meteorological data used in this analysis were collected at the Woodley Island NWS. 5.1.5.3 Good Engineering Practice Stack Height Analysis For the purposes of modeling, a stack height beyond what is required by Good Engineering Practices (GEP) is not allowed (40 CFR Part 60 §51.164). However,this requirement does not place a limit on the actual constructed height of a stack.GEP as used in modeling analyses is the height necessary to ensure that emissions from the stack do not result in excessive concentrations of any air pollutant in the immediate vicinity of the source as a result of atmospheric downwash,eddies, or wakes that may be created by the source itself,nearby structures,or nearby terrain obstacles. In addition,the GEP stack height modeling restriction assures that any required regulatory control measure is not compromised by the effect of that portion of the stack that exceeds the GEP height. The EPA guidance ("Guideline for Determination of Good Engineering Practice Stack Height," Revised 6/85) for determining GEP stack height indicates that GEP is the lesser of 65 meters or Hg,where Hg is calculated as follows: Hg= H + 1.5L 9 AERMOD was adopted in November 2005 as a guideline model by EPA as a replacement for ISCST3.AERMOD incorporates an improved downwash algorithm as compared to ISCST3(Federal Register,November 9,2005;Volume 70, Number 216, Pages 68218-68261). 5.1-34 SAC/371322/082410013(LEC_5.1_AIR_QUALITY.DOC)