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CNAPTERFOUR Responses to Public and Agency Comments Letter and Comment No. Response noticeable change 0.5 deciview Limit of Acceptable Change (LAC) to evaluate visibility impacts at the Flat Tops Class I azea. The IWAQM analysis procedure recommends that Mt. Zirkel background visibility data be used as the baseline against the change in extinction in the Flat Tops Wilderness Class I azea. The background visual range at Mt. Zirkel is 16.69 inverse meters (Mm"') (234 ktn visual range) based on the "mean of the best 20 percentile" data as provided by USFS. The relative humidity correction factor was the same value (2.27) that was used to reconstruct the annual average extinction from the Interagency Monitoring of Protected Visual Environments (IMPROVE) measured atmospheric constituent data. The IWAQM analysis procedure states that it is important to apply consistent relative humidity adjustment factors to both the source contribution to light extinction and the background light extinction. The highest CALPUFF-calculated 24-hour extinction from Yankee Gulch NO, emissions (both the Piceance and Parachute facilities) was 0.194 Mm"' at the Flat Tops Class I azea. The highest 24-hour extinction from PM,a emissions was 0.017 Mm"'. Therefore, adding the two impacts together (regazdless of spatial or temporal matching), the overall maximum extinction at the Flat Tops Class I azea is 0.211 Mm"'. Given the background extinction of 16.69 Mm"',the extinction change is 1.26 percent or 0.126 deciview. Since this value is not greater than the 1.0 deciview just noticeable change threshold, the Yankee Gulch facilities would not significantly effect the visibility at the Class I area. The highest 24-hour extinction from cumulative source NOS emissions (both Yankee Gulch and oil and gas sources) was calculated at the Flat Tops Class I azea as 1.536 Mm"'. Since PM,o emission data were not available for the oil and gas cumulative sources, the highest 24-hour extinction from Yankee Gulch PM,o emissions (0.017 Mm'') was added to the cumulative NO, impacts. The overall cumulative extinction at the Flat Tops Class I area is 1.553 Mm"'. Given the background extinction of 16.69 Mm"', the extinction change is 9.3 percent or 0.9 deciview. Since this value is not greater than 1.0 deciview change (equivalent to a small but perceptible change under many circumstances), the maximum predicted cumulative visibility impacts aze not expected to be perceptible. The cumulative visibility impacts at the sensitive Class II azeas from NOr and PM,o Yankee Gulch Project emissions and other oil and gas NO, emissions were also calculated. The deciview change for Colorado National Monument is 0.13 deciviews, and for Dinosaur National Monument is 0.16 deciviews. 4-15