How Many Years of Modeling Are Necessary?
All refined modeling is completed over a five year period using five years of sequential meteorological data. All concentrations calculated by the model are based on a one-hour value averaged over the requisite time period. The modeled concentrations are then compared to the applicable standard. The monthly, quarterly, Prevention of Significant Deterioration (PSD) increments, annual standards and all Annual Average Concentrations (AAC) may never be exceeded, so the first highest value is examined for making the comparison. The short-term National Ambient Air Quality Standards (NAAQS) set as 1 hour, 3 hour, 8 hour, and 24 hour increments, may be exceeded once per calendar year. Modeled results are given as the highest second-highest value, which is typically found by examining model output based on 5 years of meteorological data. The second highest modeled impact for each year of data is calculated by the model. The highest of those values is selected as the short term modeled impact.
