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CHAPTERFOUR. Responses to Public and Ag~encY Comments Letter and Comment No. Response atmospheric deposition of nitrogen, and the impact on visibility of particulate nitrate at receptors located at Class I azeas in the project vicinity. Since this CALMET/CALPUFF analysis was developed by the NPS, the same wind fields and modeling procedures were used for the Yankee Gulch Level 2 CALMET/CALPUFF analysis. The USDA Forest Service requested that EIS modeling analyses be performed for both Yankee Gulch sources alone (i.e., the ]?roposed Action analysis) and for multiple sources in the region (i.e., cumulative impacts analysis). The source emission for the Proposed Action a~ialysis included all Yankee Gulch sources of NO, and PM,o at both the Piceance and Parachute facilities. The cumulative sources modeled inclluded the Glenwood Springs Resource Area "reasonably foreseeable" scenario, which assumes that current NOx emissions from oil and gnu sources will double with foreseeable growth. The estimated annual NOx emissions for the "reasonably foreseeable" oil and gas development sources totaled 1,655 tPY• Ambient concentrations of atmospheric constituents were calculated by CALPUFF at discrete receptor locations for specific Class; I and sensitive Class II areas. The discrete receptors were placed on the closest boundary locations to the Yankee Gulch facilities for each Class I o:r Class II azea. Regional haze, caused by increased concentrations of primary particulate emissions and secondazy aerosols, is characterized by decreases in visual range and contrast of observed landscape features. For the purposes of assessing regional visibility degradation due to specific sources of air pollution, the primary focus is on the contribution to light extinction of secondary sulfate and nitrate, and primary particulate matter, and how the visibility is affected by changes to the extinction coefficient. Based on modeled concentrations, anextinction coefficient can be <:omputed and its effect on visibility assessed. The deciview visibility index is designed to be lineaz with humanly perceived changes under assumed commonly occurring conditions. Deciview increases with increasing visibility impairment. The deciview is a simple logarithmic transformation of the extinction coeflcient, and therefore is easily determined from measured or modeled concentrations. As a measure of potentially significant change in visibility, 1.0 deciview (i.e., just noticeable change) has been proposed by Pitchford and Mahn (1994) and was adopted by the Grand Canyon Visibility Transport Commission. A 1.0 deciview change (equivalent to a 10 :percent change in extinction coefficient) is a measure of a small but perceptible change under many circumstances. In addition, the USFS has established a'/:just 4-14