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<br />This haze layer could be most perceptible whe~ viewed from a <br />location near the edge of the valley or' a};love the Valley <br />floor (e.g. along U.S. Highway 160, from the I wilderness, or <br />from a ski run) where a white ish haze layer would result in a <br />change in contrast between the darker backgro~d terrain and <br />light sky.** <br /> <br />Two parameters were evaluated to estimate the visual effects <br />of the haze layer, optical thickness of tM surface layer <br />visual range reduction. For both paramete~s, a one hour <br />particulate concentration of approxiJrately 23! ug/m per cubic <br />foot was combined with an assumed background qoncentration of <br />18 ug/m per cubic foot for a total one hi:lUr particulate <br />concentration of 41 ug/rn per cubic foot (wl\.ich would have <br />occurred if Alternatives Two and 'lhree had been developed on <br />January 12 at 0900 and January 21 at 1700, 1984). Equations <br />from EPA's "Workbook for EstiIPating Visibility Impairment" <br />(Lattirrer and Ireson, 1980) were used to ccjlcu1ate optical <br />thickness of the polluted layer (41 ug/rn, per cubic foot <br />TSP). The change in contrast (change in haz~) was estimated <br />to be a value of 0.05. Based on the limits I of human visual <br />perception, Latimer and Ireson (1980) sugges~ a significance <br />threshold criteria of 0.1 for change in haze. Thus, it <br />appears that the particulate levels estimate(! for Wolf Creek <br />Valley would not result in a significant change in the <br />sky/terrain contrast due to haze in the valley. <br /> <br />For a TSP of 41 ug/m per cubic foot, the ci:bange in visual <br />range was estimated at 1.4 percent. Thresliold significance <br />criteria for percent change in visual range !are supported at <br />5 to IO percent, so visual range reducti~n would not be <br />significant. : <br /> <br />From the calculations performed, the visibiJlity effects are <br />not thought to be significant. There :is considerable <br />uncertainty, hCMever, in estimating visual a~r quality and in <br />assessing what effects may be perceived by individuals as <br />significant. For this reason, the vis~bi1ity analysis <br />performed for this study was designed to )be conservative, <br />thus tending to over-estimate the actual v~sibi1ity inpact. <br />However, one source of uncertainty not addr~ssed here is the <br />effect of atmos];heric moisture (humid~ty) on aerosol <br />formation and thus visual air quality. !The simultaneous <br />occurrence of high humidity and increased particulate levels <br />in Wolf Creek Valley could result in visual impairment which <br />could be perceived as SOlely coming from maJ'l's activities. <br /> <br />** COntrast is the perceived difference in ~ight intensity <br />and color. <br /> <br />188 <br />