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Protecting Visibility in National Parks and Wilderness Areas 4 Haze Formation and Visibility Impairment To develop an effective strategy for ameliorating the effects of human activities on visibility, the complex processes that form haze and impair visibility must be understood. The primary visibility attributes—light extinction, contrast, discoloration, and visual range—can be quantitatively measured, and despite some limitations in knowledge about visibility, changes in those attributes can be related to changes in the chemical and physical properties of the atmosphere. This chapter presents the current scientific understanding of the processes involved in haze formation and visibility impairment. In this chapter we discuss Some of the fundamental factors that relate to haze and visibility; The role of meteorological processes in haze formation; Experimental strategies for monitoring visibility; The modeling of the relationship between aerosol properties and visibility; Issues related to quality assurance and quality control. The measurement techniques used to characterize the components that affect visibility are reviewed in Appendices A and B; Appendix B discusses techniques used to relate the human perception of visibility degradation to physical measurements.
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Protecting Visibility in National Parks and Wilderness Areas FUNDAMENTALS OF VISIBILITY AND RELATED MEASUREMENTS Fundamental Processes in Visibility If an observer is to see an object, light from that object must reach the observer's eye. The perceived visual character of the image depends on the light emitted from or reflected by the object and on the subsequent interaction of that light with the atmosphere. When an observer views a distant object, the light reaching the observer is weakened by two processes: absorption of energy or scattering by gases or particles in the atmosphere. These two processes are referred to collectively as extinction and are depicted in Figure 4-1. Transmitted light is not the primary factor that determines visibility. The visibility of a distant object also is affected by light from extraneous sources (e.g., sun, sky, and ground) that is scattered toward the observer by the atmosphere (Figure 4-1). This extraneous light is referred to as air light. The air light behind an object provides backlighting and causes the object to stand out in silhouette (iv in Figure 4-1); the air light between the observer and an object tends to reduce the contrast of the object and to mute its colors (v in Figure 4-1). Air light can be an important element of a view; it can have a positive as well as a negative effect on perception. The appearance of the daytime sky is due the scattering of sunlight by gases and particles in the atmosphere. If there were no scattering (or if there were no atmosphere), the daytime sky would be black, allowing the stars to be seen during the day. Air light also provides diffuse light to the surface below; without air light, objects viewed on Earth would have the deep shadow effects seen in photographs of the Moon. Haze affects the quality and quantity of air light because absorption and scattering are wavelength dependent. That dependence accounts for the deep blue color of the sky in pristine areas, as well as the gray color of smog. Air light is proportional to extinction and, like extinction, depends on particle concentrations. Unlike extinction, air light also depends on viewing angle; particles scatter preferentially in forward directions, so that haze tends to appear brighter in the direction of the sun. The extinction coefficient, bext, is a key measure of atmospheric trans-
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Protecting Visibility in National Parks and Wilderness Areas FIGURE 4-1 Elements of daytime visibility. The atmosphere modifies an observer's view of a distant object, in this case a tree illuminated by the sun. The paths illustrate: (i) light from the target that reaches the observer; (ii) light from the target that is scattered out of the observer's line of sight; (iii) light absorbed by gases or particles in the atmosphere; (iv, v) light from the sun or sky that is scattered by the intervening atmosphere into the observer's line of sight; process iv causes the object to stand out in silhouette while v reduces the contrast of the object. Source: EPA, 1979. parency and is the measure most directly related to the composition of the atmosphere. It is a measure of the fraction of light energy dE lost from a collimated beam of energy E in traversing a unit thickness of atmosphere dx: dE =-bextEdx. The extinction coefficient has dimensions of inverse length (e.g., m-1). The extinction coefficient comprises four additive components: where bsg = light scattering by gas molecules. Gas scattering is almost entirely attributable to oxygen and nitrogen molecules in the air and often is referred to as Rayleigh or natural "blue-sky" scatter. It is essentially unaffected by pollutant gases.
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Protecting Visibility in National Parks and Wilderness Areas bag = light absorption by gases. Nitrogen dioxide (NO2) is the only common atmospheric gaseous species that significantly absorbs light. bsp = light scattering by particles. This scattering usually is dominated by fine particles, because particles 0.1–1.0 µm have the greatest scattering efficiency. Many pollutant airborne particles are in this size range. bap = light absorption by particles. Absorption arises nearly entirely from black carbon particles. The extinction coefficient usually is given in units of Mm-1 or km -1. The extinction coefficient for visible light in the ambient atmosphere can range from as little as 10-2 km-1 in pristine deserts to as much as I km-1 in polluted urban areas. The behavior of light in the sky is a complex process that depends on many factors. It is because of this complexity that the sky presents such a fascinating spectacle to the observer. However, this complexity also makes it difficult to characterize the visual environment, especially when human perceptions are involved. Nonetheless, techniques are available to characterize the optical properties of the atmosphere and to identify and quantify the determinants of visual air quality that are directly affected by pollutant emissions. Visibility Measurements There is no standard approach to measuring and quantifying optical air quality. Instruments for these purposes are commercially manufactured specialty items and are not widely available. EPA has no instrument standards, and uniformity is lacking in field measurements. Consequently, the regulatory community is uncertain which methods should be used. Visibility instruments usually measure either: the energy scattered out of the direct path of the beam or the energy that remains in the beam after it passes through the atmosphere. The nephelometer shown in Figures 4-2a and 4-2b is based on the measurement of scattered light; the transmissometer measures transmitted light (see Appendix B). These two instruments are fundamentally different not only in what they measure, but also in the way data are obtained and can be used.
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Protecting Visibility in National Parks and Wilderness Areas FIGURE 4-2a Approaches to the measurement of extinction. The nephelometer consists of a light-tight container that is fitted with a light source and a photodetector (represented by the sensor in the figure). The interior of the instrument is painted black and contains baffles so that the detector is not directly illuminated by the source; the detector only sees the light that is scattered from the light path. Ambient air is drawn through the instrument; the increase in the signal from the detector (compared to the signal obtained with clean, filtered air) is proportional to the scattering component of the extinction coefficient. The nephelometer provides a point measurement, and the data obtained with it can be compared directly with other physical and chemical measurements made at the site (e.g., gas and aerosol concentration and composition and particle-size distribution). In contrast, transmissometers measure over long path lengths, at least several km (in clean air, typically 15 km), thereby yielding measurements of the mean transmittance over a long distance. Because of heterogeneities in the atmosphere, it is difficult to relate transmissometer data to chemical and physical measurements, which usually can be made only at one point or, at best, a few points. Relationship between Particle Concentrations and Visibility Visibility impairment is approximately proportional to the product of airborne particle concentration and viewing distance (Figure 4-3). Consequently, relatively low particle concentrations can affect visibility substantially, as shown in the following example. A dark mountain at a distance of 100 km may be clearly visible in clean air, assuming an
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Protecting Visibility in National Parks and Wilderness Areas FIGURE 4-2b Radiance difference techniques are based on the teleradiometric measurement of adjacent bright and dark targets located several km or more from the detectors (here represented by a sensor). In transmissometry, the target radiance difference is commonly obtained by using a single, switched light source which serves as both the bright (on) and dark (off) targets. With the light off (a), the detector sees only air light; with the light on (b), the detector sees, in addition to air light, the light transmitted through the atmosphere from the source, some of which is scattered in transit. The difference in the measured radiances depends solely on the transmittance of the intervening atmosphere. (The radiances of both targets will be affected to the same degree by air light.) The average extinction coefficient (absorption and scattering combined) can be calculated if one knows the radiance difference of the targets. average extinction coefficient of about 0.015 km-1; under such conditions, the mountain's contrast with the background sky will be about 20%. If the particle concentration increases sufficiently to increase the extinction by 0.015 km-1, the contrast will fall below the threshold for detection (about 5%) and the mountain will no longer be visible. An extinction increment of this magnitude can be produced by a relatively small concentration of fine particles, about 3–5 µg/m3 of particles with diameters between 0.1 and 1.0 µm. Concentrations of a few µg/m3 are not unusual, even in remote regions. At these concentrations, particles usually constitute only a small
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Protecting Visibility in National Parks and Wilderness Areas FIGURE 4-3 Visibility impairment and contaminant concentration. All other things being equal, visibility impairment depends on the product of path length and average concentration along the sight path, not on the average concentration. In this figure the two sight paths have the same total particulate mass (represented by shaded areas) along the sight path. The upper portion of the figure represents a hazy urban environment, while the lower represents a relatively clean natural setting. A photon from the target has the same chance of reaching the observer in either of the two situations; also the same number of extraneous photons (air light) will be scattered into the observer's line of sight. The radiation received by the observer is therefore the same whether the intervening atmosphere is deep and relatively clean or shallow and relatively turbid. In practice, the 'other things' of our qualifier are seldom all equal, because different extinction coefficients usually arise from differing proportions of particles and gases which could have differing scattering and absorption characteristics. Nonetheless, the simple dependence of visibility on the product of concentration and distance is a useful approximation. Among other things, it explains why the most transparent atmospheres are the most sensitive to contamination. fraction of the total trace materials (gases and particles) found in the atmosphere, even in relatively clean regions. Sulfate (SO42), nitrate (NO3-), and organic carbon are usually the most important airborne particle fractions on a mass basis, and they are the trace materials that usually reduce visibility the most. The sulfur in 1 µg/m3 of ammonium sulfate aerosol is equivalent to 0.2 ppb of sulfur dioxide (SO2). This
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Protecting Visibility in National Parks and Wilderness Areas SO2 gas-phase equivalent is low compared with the concentration found in a typical urban region. (The National Ambient Air Quality Standards permit 24-hour-average SO2 concentrations of up to 140 ppb. If this quantity of SO2 were converted to ammonium sulfate aerosol, the resulting concentration would be 700 µg/m3.) Indeed, the equivalent gas phase SO2 concentration calculated in this example for 1 µg/m3 of SO42is below the detection threshold of the instruments normally used to monitor compliance with SO2 air-quality standards (see Appendix B). In contrast, transmissometers easily and accurately can measure the light extinction produced by several µg/m3 of particles while nephelometers can do the same by fractions of a µg/m3 (see Appendix B). Similar conclusions hold for the gas-phase equivalents of typical nitrate and organic carbon particle concentrations. Empirical Relationships between Airborne Particles and Visibility The components of extinction (i.e., particle and gas scattering and absorption) and their relationship to visibility have been well characterized in a wide range of environments. These empirical relationships are shown in Figures 4-4 a-d. The relationship between visual range and the scattering coefficient (as measured with an integrating nephelometer) is shown for an urban area (Seattle) in Figure 4-4a and for an area near Shenandoah National Park in Figure 4-4b. If atmosphere and illumination are uniform, visual range and extinction in theory are inversely proportional. Because scattering is almost always the dominant component of atmospheric extinction, visual range should be inversely related to scattering as well. Figures 4-4a and 4-4b empirically confirm this expectation for hazy conditions, where sightpaths are relatively short and air masses are fairly uniformly mixed. Figures 4-4c and 4-4d illustrate the relationship between atmospheric light extinction, as measured with nephelometers, and particle concentrations. Figure 4-4c (for Seattle) and Figure 4-4d (for an area outside of Shenandoah National Park) show that scattering is approximately proportional to total particle mass. Figure 4-5 shows the fraction of the non-Rayleigh extinction attributable to the various components of scattering and absorption. In all
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Protecting Visibility in National Parks and Wilderness Areas FIGURE 4-4a Empirical visibility relationships. Light scattering coefficient as a function of prevailing visibility (visual range attained over at least half of the horizon circle), in Seattle. Data are from summer 1968 at humidities below 65%. Visual range was determined by human observer; light scattering was measured by unheated nephelometer and is corrected to a photopic spectral response. Both scales are logarithmic; sloping line indicates the theoretical Koschmieder (1924) relationship V = 3.9/bext for a contrast threshold of 0.02. Source: Reprinted from Atmospheric Environment 3:543–550, H. Horvath and K.E. Noll, ''The relationship between atmospheric light scattering coefficient and visibility,'' 1969, with permission from Pergamon Press Ltd., Headington Hill Hall, Oxford, OX3 OBW, UK. cases, fine-particle scattering is the dominant contributor to light extinction; this is especially true for eastern locations. In the West, coarse-particle scattering (usually soil dust) and particle absorption also contribute significantly. In all regions, gases have a minor role. The only atmospheric trace gas contributing to visible extinction is nitrogen dioxide (NO2), which has a broad absorption band at the blue end of the spectrum; consequently, when NO2 concentrations are high, the atmosphere has a distinct
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Protecting Visibility in National Parks and Wilderness Areas FIGURE 4-4b Visual range as a function of the light scattering coefficient, just outside Shenandoah National Park. Data were obtained during non-overcast conditions when the atmosphere was well mixed during mid-summer 1980. Visual range was determined by human observation of mountain peaks aligned to the southwest of the site; easily identified peaks were available at 2–3 km intervals from 5 to 24 km. Light scattering was measured by unheated nephelometer and is plotted on a reciprocal scale, in accordance with the Koschmieder (1924) relationship V 1/bext. Source: Adapted from Ferman et al., 1981. red-brown color. However, NO2 is relatively reactive, and its concentration is generally small, except in urban areas near emissions sources. Therefore it usually is a small contributor to regional optical air quality. Aerosol Chemistry and Particle Size Distributions The optical effects of atmospheric aerosols depend on the chemical composition and size distribution of the airborne particles. Particle size distributions in the atmosphere change with time; the size distribution is determined by the characteristics of the particles emitted directly by a
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Protecting Visibility in National Parks and Wilderness Areas FIGURE 4-4c Particle mass concentration as a function of light scattering coefficient, in Seattle. Data are from winter 1966, averaged over periods of 2 hours to 24 hours. Light scattering was measured by unheated nephelometer, mass by a total filter located in the nephelometer outlet. Both scales are logarithmic. Source: Reprinted from Atmospheric Environment 1:469–478, R.J. Charlson, H. Horvath, and R.F. Pueschel, "The direct measurement of atmospheric light scattering coefficient for studies of visibility and pollution," 1967, with permission from Pergamon Press Ltd., Headington Hill Hall, Oxford, OX3 OBW, UK. source, the subsequent formation of airborne particles by reactions of the emitted gases (especially SO2), and processes that remove the particles and gases from the atmosphere. Those processes are sensitive to variations in the composition of the emissions and to meteorological conditions, including sunlight intensity, temperature, humidity, and the presence of clouds, fog, or rain. Primary airborne particles are those emitted directly from a source—for example, soot, fly ash, and soil dust, but a major portion of the fine-particle mass fraction (particles with diameters between 0.1 and 1.0 µm) usually is formed in the atmosphere by the conversion of species emitted
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Protecting Visibility in National Parks and Wilderness Areas FIGURE& 4-18 Allocation& procedure& for& light& extinction in& the& RESOLVE& study.& Data& are& averaged& over three& sites& in& the& Mojave& Desert.& Source:& Trijonis& et& al.,& 1987,& 1988.
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Protecting Visibility in National Parks and Wilderness Areas perceptual indices and human judgments of VAQ. Statistical analysis of observer data can be used to establish relationships between perceptual cue judgments (e.g., clarity of objects) and judgments of overall VAQ. The human judgments of cues and overall VAQ can be derived from field observations or from judgments of photographs as described in Appendix B. Studies have attempted to establish relationships between judgments of the VAQ of natural scenes and various atmospheric and vista parameters, such as mountain/sky contrast, solar angle, extinction coefficient, sky color, and percent cloud cover (e.g., Maim and Pitchford, 1989; Malm et al., 1980; Maim et al., 1981; Latimer et al., 1981; Middleton et al., 1983a, 1984; Hill, 1990; Ely et al., 1991). Summaries of many of these study findings are given in Trijonis et al. (1990). A major implication of this research is that a small number of variables (e.g., sun angle, cloud cover, and scene composition) play a dominant role in judgments of overall VAQ or scenic beauty. One example of this approach is given by Maim and Pitchford (1989), who suggested using a quadratic detection model to predict the change in atmospheric particle concentrations that would be required to evoke a just-noticeable-change (JNC) in the appearance of contrast-related landscape features in photographs. The change resulting from a new level of emissions could then be expressed as the number of JNCs between an earlier appearance and the appearance under current conditions. It should be emphasized that calculations of detection thresholds and JNCs are assessments of changes in information content in a scene and, as such, they are not necessarily good indicators of human judgments of overall VAQ. For instance, a change of 10 JNCs in a scene with low overall contrast might not be judged to have the same effect as a 10-JNC change in a high-contrast scene. Also, the relationships between JNC and human judgments have not been established under realistic field conditions; similarly the relationship of JNC to other optical parameters has not been studied. The relationship between emission changes and visibility effects can be described for current conditions using the modeling framework described above. However, predictions of the effects of changes in emissions under a variety of atmospheric conditions are more difficult because the human response to visibility changes must be quantified. Perhaps the easiest way to document the effect of changes on a scenic resource is through photography. By making optical measurements concurrently with color photographs, it should be possible to establish a
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Protecting Visibility in National Parks and Wilderness Areas TABLE 4-4 Fine-Particle Scattering Efficiencies Used in the RESOLVE Study Scattering efficiencies (m2/g) Methodology Organics Sulfates Elemental Carbon Soil Dust Multiple regression analysis (based on routine RESOLVE data at the three receptor sites) Ordinary least squares 3.7 5.0 0.6b 0.4 Corrected least squaresa 3.8 5.1 -1.8b 0.5 Literature review (20 studies with the following adjustments for consistency) Nephelometer calibration Airport contrast = 5% Nephelometer λ = 530 mm Organics = 1.50C Relative humidity = 40% 2–3 3–6 (2–3)c 1–2 1/4 data base that would show pictorially the correspondence between measured values and the appearance of the scenic resource. Such a data base could capture a wide variety of atmospheric conditions; however, it would not necessarily reflect changes in emissions. An alternative to taking photographs in conjunction with optical mea-
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Protecting Visibility in National Parks and Wilderness Areas Scattering efficiencies (m2/g) Methodology Organics Sulfates Elemental Carbon Soil Dust Mie Theory (for Mojave Desert data) Ouimette and Flagan (1982) 2.5d 3.2 NA 1.4 RESOLVE: DRUM sampler data NA 3.2 NA 1.4 Interactive Mie Theory (based on Detroit size distributions. Sloane,1986) 3.8 4.7 (3.8)c 1.3 Consensus 3 1/4 4 1/4 1 1/2 1 1/4 a As in White and Macias (1987a), and White (1989b). b Difference from zero not statistically significant. c Elemental carbon grouped with organic carbon. d Mie theory based on volume size distribution for all material, not just organics. Source: Trijonis et al., 1988. surements is to use image processing techniques (Williams et al., 1980; Maim et al., 1983; Larson et al., 1988). This method uses atmospheric optical models that simulate the effects of pollutants on a scene. With such an approach, the consequences of a variety of atmospheric conditions and emission scenarios can be represented pictorially; these pictures then could be judged by observers for their VAQ and acceptability. This approach is promising, but the ability of simulations to reproduce the effects obtained in real photographs has not been thoroughly tested (Larson et al., 1988).
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Protecting Visibility in National Parks and Wilderness Areas EXPERIMENTAL DESIGN, QUALITY ASSURANCE, AND QUALITY CONTROL The quality of data acquired in intensive or routine measurement programs depends, to a large extent, on the design of suitable measurement strategies and on the implementation of appropriate quality assurance plans. Issues that need to be considered in establishing measurement strategies include sampling periods and locations, sampling and analytical methodologies, choice of instrumentation, and coordination of activities among participants. Quality-assurance plans require that a significant portion of the budget for a given project be used for replicate analyses, collocated sampling, blanks (no sample collected), and the quantification of analytical capabilities including precision, accuracy, and detection limits. Numerous factors have led to compromises in experimental design and quality assurance for visibility monitoring programs. For example, budgets are always limited and quality-assurance programs are expensive. If a significant portion of the available funds is invested in quality assurance, then the scope of work (number and diversity of sites, number of substrates to be analyzed, etc.) must be reduced. Because of limited funds, for example, independent system audits have not been incorporated in the IMPROVE network (Pitchford and Joseph, 1990). As a result, when an anomaly is detected, it is not clear if the trend is real or if it is an artifact of the techniques used; an important example, discussed earlier, is the sharp increase in the organic carbon concentration trends in the IMPROVE data. Because of similarities in the sampling and analytical methods used in the current (1988-present) IMPROVE network to those employed at similar sites in the earlier Western Particulate Monitoring Network (1979–1986) and the NPS National Fine Particle Monitoring Network (1982–1986), an opportunity may exist to greatly extend the temporal coverage for certain variables and sites. However there is a concern about the compatibility of some measurements across the different monitoring programs. For example, during the past two decades visibility measurements have been made with nephelometers, telephotometers, cameras, and transmissometers. Because the data obtained with these various techniques are not equivalent, the existing record provides limited information on temporal and spatial trends. These examples under-
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Protecting Visibility in National Parks and Wilderness Areas score the importance of designing long-term measurement strategies around techniques that have been thoroughly tested and accepted by the research community. A greater effort should be made (with associated funding commitments) to test the compatibility (or lack thereof) of current and historical monitoring efforts through additional methods intercomparisons, filter analysis, and statistical assessments. Priority should be given to establishing an independent scientific advisory committee with oversight responsibility for national visibility monitoring networks including IMPROVE. Such oversight could help to eliminate errors that may leave large gaps in the historical record, would help to ensure a consensus on measurement and sampling strategies, and would facilitate drawing on the broad experience that is available nationally. The continuity of data records is vitally important. In this regard the committee expresses concern about the future of airport visual range measurements which are now made by human observers. As discussed in Chapter 2, these human observations have provided most of the information that has been used to assess long-term visibility trends and to relate these trends to changes in emissions. Despite the subjectivity and inherent variability of individual observers, the population of observers changes less over the decades than do the instrument technologies involved in other pollution-related measurements. However, these airport visual range measurements will soon be stopped and will be replaced by a different technique. As part of a comprehensive and long-planned modernization program (NRC, 1991a), the National Weather Service, Federal Aviation Administration, and Department of Defense intend to replace the present network of human observers by a network of instrumental visibility monitors. The new Automated Surface Observing System (ASOS), due to be in place by 1995, should provide better spatial coverage, higher time resolution, and improved standardization. Unfortunately, the new measurements will also be more narrowly focused on aviation needs. The principal limitation of the ASOS measurements for haze studies is that they will not record variations in range during good visibility conditions. Only three visual range values in excess of 4 miles will be reported: 5 mi, 7 mi, and > 10 mi (J.T. Bradley, pers. comm., 1991, NOAA/NWS). However, in most regions the visual range is greater than 10 miles most of the time. Thus, this measurement technique will
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Protecting Visibility in National Parks and Wilderness Areas only provide useful data under extreme conditions such as those associated with the worst regional haze events in the East. Consequently, if there is any change in visibility conditions in the central or western parts of the nation (where most national parks and wilderness areas are located), the changes will not be observed until the degradation is severe. The ASOS sensor will be a forward-scattering (40°) pulsed visible-light, open-air monitor. This instrument, like the candidates over which it was chosen, has been field tested against human observers and transmissometers (Bradley and Imbembo, 1985; Bradley, 1989) to assess its operational response to distinct weather classes (fog, rain, snow, and haze). There is as yet no commitment to continuing studies such as those currently planned for temperature and precipitation measurements (NWS, 1991); such studies are important for documenting the effect of the change-over on climate data continuity. The nation's existing visibility monitoring network has been implemented largely by the National Park Service. Its approach has been aggressive and innovative. However, the NPS group is too small to have in-house expertise in all aspects of work pertaining to visibility monitoring, airborne particle sampling and analysis, and data interpretation. EPA personnel have a broad range of experience with the chemical measurements made in visibility sampling networks, and ought to be involved. Lamentably, EPA funding for visibility monitoring and research has been insignificant (see Figure 3-1) and, as a result, EPA's participation has been minimal. In summary, the nation's visibility measurement program needs to establish a balance between innovation and standardization, and between the scope and the quality of work. It is important that routine monitoring networks provide data that are comparable over decades. In order to achieve these objectives, an independent science advisory panel with EPA sponsorship should be established. This would help to ensure a wider participation among the scientific community on important decisions regarding visibility monitoring and research. We are particularly concerned that the historical record of visual range measurements made at airports by human observers will be interrupted by the new Automated Surface Observing System, which will not provide useful visibility information. We recommend that airports should be equipped with integrating nephelometers that are sensitive enough to measure the range of haze levels encountered in the atmo-
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Protecting Visibility in National Parks and Wilderness Areas sphere. Nephelometer data are closely correlated with visual range measurements made by trained observers. It is essential to have a continuous record of such data during the 1990s in order to determine the effect on visibility of the acid rain controls that will be implemented in response to the 1990 Clean Air Act Amendments. SUMMARY AND CONCLUSION Nature has contrived to maximize the effect of anthropogenic activities on visibility. As a result of physical and chemical processes in the atmosphere, a large fraction of anthropogenic primary and secondary airborne particles accumulate in the 0.1 to 1.0 µm diameter size range where removal mechanisms are least efficient. Because these particles have sizes comparable to the wavelength of sunlight, scattering and absorption are at a maximum. Thus, anthropogenic particles tend to accumulate in the size range that contributes most to haziness per unit mass. As discussed in Appendix A, a great deal is known about the processes that produce visibility-impairing particles. However, there are some major gaps in the understanding of visibility impairment. For example, although organic particles can contribute significantly to visibility impairment, especially in the West, there is poor understanding of the concentration and composition of atmospheric organic materials in the particle and the vapor phase (Appendix B). There is also poor knowledge of the relative importance of primary and secondary organic carbon species and of their anthropogenic and natural sources (Appendix A). Furthermore, there are major problems associated with collecting representative organic carbon particulate samples. Also, important insights about atmospheric transport and transformations of visibility-impairing particles would be possible if instrumentation for measuring concentrations of particulate species on a continuous basis were available (Appendix B). The current understanding of visibility is based on information from a variety of sources including historical data on airport visual range, data from routine state and national visibility networks, and intensive, short-term field programs. While these measurements have provided a good qualitative picture of visibility, there remain important gaps in
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Protecting Visibility in National Parks and Wilderness Areas measurement protocols. Inadequate attention has been paid to ensuring self-consistency between measurements using different sampling or analysis methods. As a result, it is sometimes difficult to determine whether the observed trends are real or are due to changes in measurement procedures. Also, there are no EPA-recognized performance standards for aerosol and optical sampling instruments. Many of the instruments used for visibility monitoring are expensive and are not readily available. This has led to a lack of uniformity in field measurements and to uncertainty within the regulatory community as to which sampling methods should be used. As discussed in Appendix B, adopting the integrating nephelometer as the instrument of choice for routine measurements of haze in monitoring networks would go a long way towards implementing one valuable standard for optical measurements. The planned transition from human to automated airport visibility monitoring has unfortunate implications for visibility monitoring. Most existing information about historical haze trends is from airport data. The new automated instruments are designed to measure the very poor visibility conditions that are of primary concern for aviation safety, but will provide little or no information on haze under typical visibility conditions. We recommend that the proposed instrumentation be supplemented with integrating nephelometers which would permit measurements of light scattering coefficients under typical visibility conditions. Intercomparisons have shown that light scattering coefficients measured with integrating nephelometers are closely correlated with human observer visual range data. The addition of nephelometers to airport instrumentation would ensure that haze levels are monitored over a broad and representative geographic scale, thereby providing important information on spatial and temporal trends of regional haze. It is especially important that such trends be documented during the coming decade, so that the effect of acid rain controls on haze levels can be qualified. Emissions reductions being implemented to reduce acid rain provide atmospheric scientists and regulators with an unparalleled experiment of opportunity. It would be a serious and potentially costly error to fail to record key data. A large fraction of submicron, visibility-impairing particles is produced in the atmosphere. This is a major obstacle to assessing the effects of new or existing pollution sources on visibility impairment. While laboratory studies have provided a great deal of information about
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Protecting Visibility in National Parks and Wilderness Areas rates and chemical mechanisms of gas-to-particle conversion, it is often difficult to apply this knowledge to the atmosphere. Atmospheric transformations can occur in clear air or in clouds. In-cloud processes depend on the availability of oxidants and on the frequency and duration of cloud processing. Clear air chemical transformations depend on sunlight intensity, and on the blend of NOx and organic gases. Because these phenomena are so complex, they are difficult to characterize either empirically or theoretically. For example, if clouds or fog are involved and if all the H2O2 reacts with SO2, then the conversion of a large fraction of the SO2 would be a reasonable estimate because of the rapid oxidation in droplets; if clouds or fog are not involved, then little SO2 would be oxidized in transit because of the slow rates of homogeneous oxidation. Clearly, a better understanding of atmospheric conversion processes is needed to link emissions adequately to their effect on visibility. Visual air quality goals are usually stated in terms of some readily measurable quantity such as visual range or extinction coefficient. Ultimately, these criteria are, or should be, related to the human visual perception of what is desirable or acceptable. People base their judgments of visual air quality on a variety of perceptual cues, and the relative importance of these cues varies with the setting. A better understanding is needed of the factors that affect perception. It is also important to communicate visually the possible results of a visibility improvement program (or its absence). One promising technique is the use of computer visibility models to generate photographic representations of scenes under various conditions. However, more research is needed to establish the general validity of this approach. The phenomena that lead to visibility impairment are reasonably well understood, particularly when compared with many other environmental issues which have much larger uncertainties. Nevertheless, a number of scientific and technical issues need to be resolved in order to reduce uncertainties in the understanding of the relationships between human activities and visibility.
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Representative terms from entire chapter: