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Appendix 1: Nature of the Visibility Problem . . A person's judgment of aesthetic damage is related to the ability to see form, texture, and color of scenes at various distances. Visibility can be quan- tified directly in terms of human judgments or in terms of indices, such as light extinction, that can be related to judgments. Visibility can be Ascribed quantitatively in terms of the following indices: · Contrast the relative brightness of various features in a scene; · Discolorationshifts in the wavelength distribution of light as it moves through the atmosphere; · Visual rangethe farthest distance at which an observer is able to distin- guish a large black object against the horizon sky; and · .Extinctton coefficient- the fraction of light that is attenuated per unit of distance as a light beam traverses the atmosphere. A decrease in visual range from 130 to 110 km produces a noticeable change in the contrast (relative brightness of various features within a scene) and coloration (distribution in wavelengths of received light) of a view only 30 km away. Contrast and coloration yield the best correlation with subjective human evaluations of visual air quality. Of the four visibility indices, the extinction coefficient is the one that is most directly related to the composition of the atmosphere. The total amount of light extinction is the sum of scattering and absorption of light by particles and gases. The extinction coefficient, BeXt, in conventional units of Mm~i [~106 m)~ i, thus comprises four additive components, BeXt = Bsg + Bag + Bsp + Bap Bsg = light scattering by gas molecules. Gas scattering is almost entirely attributable to oxygen and nitrogen molecules in the air, and it is often referred to as Rayleigh or natural "blue-sky" scatter. It is 43
44 · APPENDIX1 essentially unaffected by pollutant gases, such as SO2, because of their low relative concentrations. Bag = light absorption by gas molecules. Nitrogen dioxide (NO2) is the only significant atmospheric trace gas that absorbs visible light. Bsp = light scattering by particles. Bs usually is dominated by fine parti- cles, because the scattering ef~ciency/unit particle mass exhibits a pronounced peak in the range of 0.1-1.0 ~m. Many pollutant aerosol species occur in this size range. Bap = light absorption by particles. Bap arises almost entirely from black carbon particles. . · In the absence of particles, visibility is limited by the Rayleigh scattering of air molecules. Under such conditions, the nominal visual range, which is one component of visibility, is on the order of 330-400 km. Fluctuations of mass concentration up to a few tenths of a ~g/m3 should cause little perceptible change in the optical properties of the atmosphere, except perhaps under conditions of high RH. At an aerosol mass of a few tenths of a ~g/m3, aero- so} extinction starts to become significant compared with Rayleigh scattering. Increases in aerosol concentrations above this level could cause significant decreases in visual range and shifts in color and contrast. RH affects visibility significantly especially for aerosols that are hydroscopic, such as those con- taining SO4=. The size of hydroscopic aerosols can change dramatically with changes in RH. However, the factors controlling growth are quite complex and very sensitive to the composition of the aerosol. During WHITED, the mean fine particle concentration at Hopi Point was reported as about 1.6 g/m3; of this, SO4= contributed about 0.5 ~g/m3. Past estimates (Trijonis et al., 1989) suggest the following annual average apportionment of BCX' in the region of GCNP: Bsg = 10 Mew. Bsp (natural) = 7 Maw. Bsp (regional anthropogenic average) = 8 Mm~~. These values suggest a natural background extinction (Bs + Bsp (natural)) of about 17 Mm~ out of the current regional average olg about 25 Mm~ . Accordingly, pollution aerosols from all sources have on the average resulted In about a 50% increase in light extinction above natural conditions. During haze episodes, pollution aerosols can produce extinction values 2 or more times higher than natural levels (Trijonis et al., 1989), so that pollution effects dominate natural contributions. Under such circumstances, visibility will be severely degraded.
