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Appendix G <br />Air Quality Analysis Modeling Report <br />4.2 Receptor Grid <br />A receptor grid using receptor rings was created, in accordance with guidance from the <br />Interagency Workgroup on Air Quality Modeling (IWAQM). Receptor grids were created for <br />each Class I area. Receptor rings were positioned so that they coincided with the distances from <br />the source to the Class I area boundaries. Two receptor rings were placed for each Class I and <br />sensitive Class II Area, one at the distance coincident with the nearest Class I area or sensitive <br />Class II boundary, and the other at the farthest Class I area or sensitive Class II boundary. All <br />receptor rings used in this far-field assessment are shown in Figure 4-2, for the five Class I areas <br />and two sensitive Class II areas included in this analysis. Although not shown on Figure 4-2, <br />receptors are spaced at one-degree intervals around each ring, per IWAQM guidance. All <br />receptors are elevated to the average elevation for the area of analysis and from the model's <br />"point of view," the area of analysis is considered to lie along each point of the ring (i.e., each <br />360 directions). A total of 720 receptors were modeled for each Class I or sensitive Class II area. <br />The modeling domain was extended approximately 25 km beyond the farthest receptor to allow <br />for puffs to pass the receptor rings and then potentially move back toward the emission source, <br />thereby reducing edge effects. <br />4.3 CALPUFF/CALPOST/POSTUTIL Model Options and Inputs <br />For this analysis, CALPUFF ran in a screening mode (known as Tier 2 or CALPUFF-Cite) as <br />outlined in the USEPA document Interagency Workgroup on Air Quality Modeling (IWAQM) <br />Phase 2 Summary Report and Recommendations for Modeling Long Range Transport Impacts <br />(USEPA, 1998). This methodology bypasses the need to generate a full three-dimensional wind <br />field with CALMET. Instead, an ISCST3 single-station meteorological field is used. Results <br />from aCALPUFF-Cite analysis are considered to be conservative assessments of air quality <br />impacts, because a number of assumptions are made that tend to over-predict air quality impacts. <br />In some cases, aCALPUFF-Cite analysis can predict much larger impacts than those obtained <br />with a complete CALPUFF analysis (using athree-dimensional wind field generated by <br />CALMET). <br />Table 4-1 provides a summary of several CALPUFF-Cite and CALPOST modeling options and <br />inputs utilized in this analysis, including: <br />The full chemistry option was turned on (MCHEM =1, MESOPUFF II scheme). <br />The deposition option was turned on (MWET = 1 and MDRY = 1). <br />Method six (6) was selected for estimating light extinction (MVISBK); therefore, <br />monthly relative humidity adjustment factors are needed by CALPOST for each <br />analysis area (Class I or sensitive Class II). The monthly relative humidity adjustment <br />factors (f (RH)) were obtained from FLAG guidance for the sensitive Class II Areas <br />and from the "Seasonal FLAG Screening Analysis Spreadsheet" prepared by the BLM <br />for Class I Areas. The recommended FLAG natural background aerosol concentrations <br />for the western portion of the United States were input to CALPOST. The options and <br />scaling parameters selected for POSTUTIL, conformed to the Federal Land Managers <br />(FLM) modeling guidance. <br />G-17 <br />DBMS 529 <br />