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CRAPTERFOUR Responses to Public and Agency Comments Letter and Comment No. Response compressors installed at N0, rates much [5 to 10 times] greater than aze currently in operation", and "Although such a replacement is theoretically possible, it is unlikely either the operators would consider, or that the State of Colorado would permit, such a replacement with much dirtier equipment " Therefore, the " 36% increased visibility impairment to the Flat Tops Wilderness area" is simply a gross overestimation based upon the unlikely USDA-Forest Service belief that industry would replace existing compressors with older equipment which could be 10 times dirtier, and that the air regulatory agencies would allow it. The cumulative air quality impact analysis conducted for the Final Supplemental EIS was based upon the best emission estimates available at that time. These included nitrous oxide emissions from the Yankee Gulch Project estimated at neazly 74 tons per yeaz (tpy). It is now known these emission levels would be higher. Therefore, subsequent to publishing the Yankee Gulch Project Draft EIS, a modeling analysis of the impacts from Yankee Gulch sources was conducted using the CALMET/CALPUFF modeling system and methodologies described in the recent document Interagency Workgroup on Air Quality Modeling (i'WAQM) Phase 2 Summary Report and Recommendations for Modeling Long-Range Transport Impacts (EPA 1998). A similaz CALMET/CALPUFF modeling analyses in the study area was performed in 1998 by Mr. John Vimont of the National Pazk Service (NPS) for the BLM's Draft EIS for potential natural gas development within the neazby Glenwood Springs Resource Area (BLM 1998a). For this analysis, CALMET was run for the year 1990 using surface and upper air data from Grand Junction, surface data from 10 Remote Automatic Weather Stations sites, and azchived data. The wind field grid has a resolution of 5 km per grid cell and covered an azea of 400 km in the east-west direction and 300 km in the north-south direction, centered on Rifle, Colorado. The CALPUFF model was run to evaluate the effects of NOx emissions, as the emissions of SOz and primary particles from the GSRA development were small and considered not significant. This analysis was prepazed for several levels of development. The worst case was based on the potential to relocate older compressor units in Colorado that do not meet current New Source Standards, and the best case was based on units meeting best available control technology (BACT) emission levels. The "reasonably foreseeable" scenario is based on units meeting the current emission estimates presented in the EPA emission factor Handbook (AP-42). It was determined that this additional analysis should be based on the reasonably foreseeable development alternatives. Air quality impact parameters included annual average NOZ concentrations, annual 4-13