Examples Of Visibility Impairment

The camera ean be an effective tool in capturing the visual impact that pollutants have on a visual resource. Figures 12a to 12d show the effect that various levels of uniform haze have on a Glacier National Park vista. These photographs were taken of the Garden Wall from across Lake McDonald. Figures 13 and 14 show similar hazes of vistas at Mesa Verde and Bryce Canyon National Parks. The Chuska Mountains in Figure 13 are 95 km away, Navajo Mountain in Figure 14 is 130 km distant. This photograph should be compared with Figure la, a photograph of Navajo Mountain taken on a day in which the particulate concentration in the atmosphere was near zero.

Under stagnant air mass conditions aerosols can be "trapped'1 and produce a visibility condition usually referred to as layered haze. Figure 15 shows Navajo Mountain viewed from Bryce Canyon National Park with a bright layer of haze that extends from the ground to about halfway up the mountain. Figure 16 is a similar example of layered haze but with the top portion of the mountain obscured.

Figure 12 Effect of regional or uniform haze on a Glacier National Park vista. The view is of the Garden Wall from across Lake McDonald. Atmospheric particulate concentrations associated with photographs («), (b), (e), and (d) correspond to 7.6, 12,0, 21.7, and 65.3 (ig/m . See ftp site for color image.

Figure 12 Effect of regional or uniform haze on a Glacier National Park vista. The view is of the Garden Wall from across Lake McDonald. Atmospheric particulate concentrations associated with photographs («), (b), (e), and (d) correspond to 7.6, 12,0, 21.7, and 65.3 (ig/m . See ftp site for color image.

Figure 17 is a classic example of plume blight. In plume blight instances, specific sources such as those shown in Figure 18 emit pollutants into a stable atmosphere. The pollutants are then transported in some direction with little or no vertical mixing.
Figure 13 Effects of uniform haze on the Chuska Mountains as seen from Mesa Verde National Park. The atmospheric particulate concentration on the day this photograph was taken corresponded to 1 (ig/rn . See ftp site for color image.
Figure 14 Uniform haze degrades visual air quality at Bryce Canyon National Park. The 130-ktn distant landscape feature is Navajo Mountain, Atmospheric particulate concentration on the day this photograph was taken is 3 fig/mJ. See ftp site for color image.

Figure 15 Navajo Mountain as seen from Bryce Canyon, showing the appearance of layered haze. The pollutants are trapped in a stable air mass thai extends from the ground to about half-way up the mountain side. Sec ftp site for color image.

Figure 16 Photograph of Navajo Mountain similar to Figure 15 but with a suspended haze layer that obscures the top portion of the mountain. See ftp site for color image.

Figure 17 Classic example of "plume blight," The thin, dark plume on Navajo Mountain results from a point source emitting particulate matter into a stable atmosphere. See ftp site for color image.
Figure 18 Example of one kind of point source that emits pollutants into the atmosphere. See ftp site for color image.

Figures 19, 20, 21, and 22 show other layered haze conditions that frequently occur at Grand Canyon and Mesa Verde National Parks. At Mesa Verde (Figure 22), much of the pollution comes from urban areas and the Four Corners and San Juan Power Plants, while at the Grand Canyon layered hazes arc associated with smoke and nearby coal-fired power plants.

Figures 23 and 24 show the appearance of plumes containing carbon. In both of these cases the pollutants are being emitted from forest fires. However, Figure 23 shows the appearance of a specific forest fire plume, while Figure 24 shows the effect of viewing a vista through a concentration of particles containing carbon. In this instance, the vista is the north wall of the Grand Canyon as seen from the top of

Figure 19 Smoke trapped by an inversion layer in the Grand Canyon. During the winter months, inversions are quite common in almost all parts of the United States. See ftp site for color image.

San Francisco Peaks in northern Arizona. Noticc the overall "graying" and reduction of contrast of the distant scenic features. Remember that carbon absorbs all wavelengths of light and scatters very little. Thus the scene will always tend to be darkened.

Figure 20 Example of power plant emissions trapped in an air inversion layer in the Grand Canyon. See ftp site for color image.
Figure 21 Effects of inversion layer in Grand Canyon. In this case, a cloud has formed within the canyon walls. See lip site for color image.
Figure 22 Effects of layered haze trapped in Front of the Chuska Mountains as viewed from Mesa Verde National Park. This condition occurs 30 to 40% of the time during winter months. See ftp site for color image.
Figure 23 Forest fire plume exemplifying the appearance of carbon particles and demonstrating the effect of lighting. Where the plume is illuminated, it appears gray, but identical particles in the shadow of the plume appear dark or almost black. See ftp site for color image.

Figure 24 Example of how light-absorbing particles (in this case carbon) affect the ability to see a vista. Carbon absorbs all wavelengths of light and generally causes a "graying" of the overall scene. Shown here is the north wall of the Grand Canyon as seen from the top of the San Francisco Peaks in northern Arizona. See ftp site for color image.

Figure 24 Example of how light-absorbing particles (in this case carbon) affect the ability to see a vista. Carbon absorbs all wavelengths of light and generally causes a "graying" of the overall scene. Shown here is the north wall of the Grand Canyon as seen from the top of the San Francisco Peaks in northern Arizona. See ftp site for color image.

Figure 25 shows the effects of illumination on the appearance of power plant plumes. The two plumes on the left are particulate plumes, while the two plumes on the right consist of water droplets. The plume on the far right, which is illuminated by direct sunlight, appears to be white. The second, identical water droplet plume, which is shaded, appears dark. The amount of illumination can have a significant effect on how particulate concentrations appear.

Figure 26 demonstrates how the effect of nitrogen dioxide gas (N02), in combination with varied background illumination, can combine to yield a very brown atmospheric discoloration. If a volume of atmosphere containing N02 is shaded and if light passes through this shaded portion of the atmosphere, the light reaching the eye will be deficient in photons in the blue part of the spectrum. As a consequence, the light will appear brown or reddish in color. However, if light is allowed to shine on. but not through, that same portion of the atmosphere, scattered light reaches the observer's eye and the light can appear to be gray in nature. Both of these conditions are shown in Figure 26. On the right side of the photo clouds shade the mixture of NO; and particulates. The same atmosphere, illuminated because the cloud cover is not present, appears almost gray in the middle portion of the photograph.

Effects of illumination are further illustrated in Figures 11a, and lib. Figure 27 is an easterly view of the La Sal Mountains in southeastern Utah as seen from an elevated point that is some 100 km distant. The photograph in Figure 27a was taken at 9:00 A.M.. while the photograph shown in Figure lib was taken later in the day.

Figure 25 The effect of illumination on the appearance of plumes. The two plumes on the right are identical in terms of [heir chemical makeup, in that they are primarily water droplets. However, the far right plume is directly illuminated by the sun and the plume second from the right is shaded. The first plume appears white and the second appears almost black. The two plumes on the left are fly-ash plumes. See ftp site for color image.

Figure 25 The effect of illumination on the appearance of plumes. The two plumes on the right are identical in terms of [heir chemical makeup, in that they are primarily water droplets. However, the far right plume is directly illuminated by the sun and the plume second from the right is shaded. The first plume appears white and the second appears almost black. The two plumes on the left are fly-ash plumes. See ftp site for color image.

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Figure 26 (see color insert) The brown discoloration resulting from an atmosphere containing nitrogen dioxide (N02) being shaded by clouds but viewed against a clear blue sky. Light scattered by particulate matter in the atmosphere can cominate light absorbed by NOj, causing a gray or blue appearing haze (left side of photograph). See dp site for color image.

These photographs show how these views, or vistas, appear when obscured by a layer of haze. In the first view the haze layer appears white, but the same air mass viewed later in the day has a dark gray appearance. This effect is entirely due to the geometry involved with the observer and the sun. In the first view the sun is low in the eastern sky. Consequently, the photons reaching the observer have been scattered in the forward direction. Because the haze appears white, we can conclude that the particles must be quite large in comparison to the wavelength of light. The assumption that particles are large is further reinforced by their appearance when the sun is behind the observer as shown in Figure 21b. For scattered photons to reach the observer, they would have to be backscattered from the particles. Because the haze appears dark, we can conclude that there is very little back scattering, which is consistent with the large particle hypothesis.

The angle at which the sun illuminates a vista or landscape feature (sun angle) plays another important role. Figures 28a to 28d exemplify this effect. The view is from Island in the Sky, Canyonlands National Park, looking out over Canyonlands with its many colorful features toward the 50 km distant La Sal Mountains. Figure 28ii shows how the canyon appears when it is in total shadow (6:00 A.M.). Figures 2%a to 28c show a progressively higher sun angle until in Figure 2%d the scene is entirely illuminated. In each case, the air quality is the same. The only change is in the angle with which the sun illuminated the vista. There are primarily two reasons for the apparent change in visual air quality. First, at higher sun angles, there is less scattering of light by the intervening atmosphere in the direction of the observer. Second, the vista reflects more light: consequently, more image-forming information (reflected photons from the vista) reaches the eye. The contrast detail and scene are enhanced.

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Figure 27 Photographs show how the same haze trapped in an inversion layer looks under forward and backscatter conditions. In (a) under forward scattering conditions (morning), the haze appears white; in (6) ihc identical haze, viewed in the afternoon during backscatter conditions, is dark or gray. Because most of the light energy is scattered in the forward direction (white haze), it can be concluded that the particles must be quite large in comparison to the wavelength of light. See fip site for color image.

Figure 27 Photographs show how the same haze trapped in an inversion layer looks under forward and backscatter conditions. In (a) under forward scattering conditions (morning), the haze appears white; in (6) ihc identical haze, viewed in the afternoon during backscatter conditions, is dark or gray. Because most of the light energy is scattered in the forward direction (white haze), it can be concluded that the particles must be quite large in comparison to the wavelength of light. See fip site for color image.

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Figure 28 Four photographs showing the effect of shifting sun angle on the appearance of a vista as seen from Island in the Sky, Canyonlands National Park, In each photograph, the air quality is the same. In (a) (6:00 A.M.) the sun angle—observer- vista geometry results in a large amount of scattered air light (forward scattering) added to the sight path, but minima! amount of imaging light reflected from the vista. Figures 28b and 28c show a progressively higher sun angle until in Figure 28t/, the scene is entirely illuminated. Scattered light is minimized and reflected; imaging light is at a maximum. See ftp site for color image.

Figure 28 Continued

Homeowners Guide To Landscaping

Homeowners Guide To Landscaping

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