Protecting Visibility in National Parks and Wilderness Areas (1993) / Chapter Skim
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4 Haze Formation and Visibility Impairment
4 Haze Formation and Visibility Impairment
Pages 81-142
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From page 81... ...
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.
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From page 82... ...
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, beat' is a key measure of atmospheric trans
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From page 83... ...
The extinction coefficient comprises four additive components: bed = bSg + bag + bSp + bap 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.
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From page 84... ...
This scattering usually is dominate~by fine particles, because particles 0.~-~.0 ,um have the greatest scattering efficiency. Many pollutant airborne particles are in this size range.
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From page 85... ...
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.
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From page 86... ...
(The radiances of both targets will be affected to the same degree by air light.) The average extinction coefficient (absorption and scattering combined)
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From page 87... ...
, 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 ~ ,ug/m3 of ammonium sulfate aerosol is equivalent to 0.2 ppb of sulfur dioxide (SON.
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From page 88... ...
Empirical Relationships Between Airborne Particles and Visibility The components of extinction (i.e., particle anti gas scattering and absorption) and their relationship to visibility have been well character~zed in a wide range of environments.
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From page 89... ...
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.
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From page 90... ...
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 emitter!
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From page 91... ...
Pueschel, "lithe 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)
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From page 92... ...
For further discussion and a more quantitative explanation of the transformations of gases into airborne particles, see Appendix A The effects of primary particle emissions and chemical transformations on atmospheric particle size distributions are illustrated in Figure 4-7 (see Appendix B for particle size measurement techniques)
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From page 93... ...
White, nThe components of atmospheric light extinction: A survey of ground-level budgets," 1990, with permission from Pergamon Press Ltd., Headington Hill Hall, Oxford, OX3 OBW, UK.
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From page 94... ...
Ozone (03) ~ PROTECTING VISIBILITY IN NATIONAL PARKS Ozonle(O3)
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From page 95... ...
9s \ \ I.lI: bit 'at' 0 .
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From page 96... ...
In the next section, we show that these long-lived accumulation-mode particles have the greatest effect per unit mass on visibility and that it is for this reason that visibility impacts can often extend over large regions. Particle Optics and Visibility The optical properties of airborne particles are affected by several factors, among them particle size.
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From page 97... ...
By comparing this figure with Figure 4-9, which shows particle scattering efficiency as a function of particle size, one sees that none of the common removal mechanisms is very effective in removing particles that are the most efficient scatterers of light. This fact accounts for the accumulation and persistence of large hazy air masses.
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From page 98... ...
Therefore, airborne particles, which tend to accumulate in the size range 0.1 to 1.0 Am diameter, can have a large effect on visibility. Particles much larger than the wavelength of visible light show a sharply decreasing scattering efficiency with increasing particle size.
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From page 99... ...
Some Experimental Difficulties in Aerosol Chemistry Studies Because of the relationship of visibility to various airborne particles
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From page 100... ...
and their size distributions, it is important that aerosol composition and particle-size distributions be measured accurately in visibility programs. In practice, such measurements are difficult.
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From page 101... ...
As discussed in Appendix B there are many other cases where gas-phase species react with the sampling medium to yield erroneously high particle concentrations.
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From page 103... ...
Because most of the aerosol particulate mass consists of hydroscopic materials (e.g., sulfuric acid, ammonium sulfate, ammonium bisulfate, and ammonium nitrate) , the size of the airborne particles depends on He relative humidity.
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From page 104... ...
104 oaf/\ /d/\ In \ I \ ~n AN I I 1 Z 1 O ~O Oda/da _ O Lr)
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From page 106... ...
For example, high SO42- concentrations in the Adirondack Mountains most often are associated with transport from polluted regions to the south and southwest of New York (Calvin et al., 1978~. In the Shenandoah Valley, 78-86% of the light extinction is attributed to anthropogenic airborne particles, most of which originate in the Midwest (Ferman et al., 198 I)
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From page 107... ...
Thus, He effect of a particular source on a receptor region can vary tremendously, depending on ambient atmospheric conditions. It is clear that the meteorological conditions associated with reduced visibility in national parks and wilderness areas in the West are different from those in the East.
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From page 108... ...
Vertical mixing subsequently redistributes the pollutants into a thicker layer of more homogeneous haze. In contrast, reduced mixing combined with low wind speeds can increase the likelihood of formation of valley fogs; valley fogs are often an important factor in pollution episodes, because many industrial sources are located in valleys, near water resources.
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From page 109... ...
However, the composition of the resulting aerosol changes because of He aqueous-phase chemistry that takes place in cloud droplets, principally because of reactions with atmospheric gases such as SO2 and HNO3. Airborne particles are likely to cycle through clouds many times before Hey are removed by precipitation, and the composition and size of the particles changes with each cycle.
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From page 110... ...
All estimates show that emissions of soil dust are higher in the arid Southwest than in other parts of the United States. STRATEGIES FOR VISIBILITY MEASUREMENT PROGRAMS The preceding sections discussed visibility measurement techniques
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From page 111... ...
This section focuses on systems and procedures used in field measurement programs and different strategies for establishing a visibility monitoring program. Criteria for Monitoring Programs In monitoring programs, the optical properties measured are those that are closely related to human visual perception.
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From page 112... ...
Wind measurements should represent the wind flow at the height of the emission plume, which usually requires aloft measurements of winds aloft. Examples of Visibility Measurement Programs The Interagency Monitoring of Protected Visual Environments Program In response to Section 169A of the 1977 Clean Air Act Amendments, EPA promulgated regulations for a visibility monitoring strategy for Class ~ areas for states that have not incorporated such strategies in their state implementation plans (STPs)
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From page 113... ...
to collect data that are useful for assessing progress toward the national visibility goal, and (4) to promote He development of improved visibility monitoring technology and the collection of visibility data (Pitchford and Joseph, 1990~.
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From page 115... ...
HAZE FORMATION AND VISIBILITY 115 T',','.~,,',W,'.~.-.l'..'...'.P.,"'d."'i'c,'l"e..'.~'' ' ts""M=''' ~th''~t ' cy"''' ito' n'"o ....".2..'..'9,"',=.."""""'' ' i '' ' i' ' ts'' ' O' )
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From page 116... ...
It would be far preferable, however, to measure particulate carbon more accurately and directly, for reasons to be discussed below. Transmissometers are used to measure extinction coefficients in the IMPROVE network.
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From page 117... ...
, IC, and TOR to provide aerosol composition data for receptor modeling and extinction budget analysis. The state regulatory agencies of Washington, Oregon, and California measure extinction as part of their visibility monitoring programs.
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From page 118... ...
COH tape sampler measurements as an indicator of light absorption in urban areas; and 3) ambient air hygrometer measurements of relative humidity.
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From page 119... ...
Visual range is estimated from the slides by densitometric radiometry. As a result of these measurement programs, Oregon and Washington have adopted restrictions on sources that impair visibility in their Class ~ areas.
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From page 120... ...
However, certain measurements, such as SO2 and SO42- concentrations and fine particle mass were made routinely at most sites. Other measurements, such as organic and elemental carbon concentrations, size-resolved aeroso} chemical composition, aerosol water content, cloud water chemistry, and aerosol optical measurements, were made at selected sites.
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From page 121... ...
. ~5 ° ~O 2S SO 75 ~ 00 ~ 2S FIGURE 4-16 Ihe locations of meteorological measurement sites in the experimental area for the 1990 NGS Visibility Study.
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From page 126... ...
Receptor Sltes R1-R3 - AQ Visibility and Tracer - Rasea~h Sites B1-B7 ~ C1 AD and Tour · - Regional Chareacrization Sass - Sultur and Tracer ~1 Tracer Sampans She 1; Iu 3 IN FIGURE 4-17 The locations of tracer and air-quality monitonug sites for the 1990 NGS Visibility Study. Source: Adapted from Richards etal., 1991.
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From page 127... ...
The scattering, absorption, and extinction coefficients for atmospheric particles can be calculated from measurements of the size-resolved chemical composition made at a given location and time. The procedure involves converting airborne particle measurements to number distributions; these are then multiplied by particle projected areas and by single particle scattering, absorption, or extinction efficiencies and integrated over the particle size distribution.
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From page 128... ...
Researchers have used this theoretical approach to investigate the contributions of various species of atmospheric particles to extinction (e.g., Ouimette and Flagan, 1982; Hasan and Dzubay, 1983; Sloane, 1983; Sloane 1984; SIoane and Wolff, 1985~. In each of these studies, number distributions of airborne particles were calculated from cascade impactor measurements of size-resolved chemical mass distributions.
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From page 129... ...
However, the current understanding of secondary atmospheric particles usually is inadequate to permit definitive calculations of secondary particle size distributions. Changes in size distributions can be estimated, but these estimates introduce uncertainties that are difficult to quantify.
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From page 130... ...
that accurately describes the observed total extinction may be inaccurate in apportioning this total among individual species (White, 19861. The assumed linear relationship would be invalid, for example, if the mean particle size were found to depend on mass concentrations.
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From page 131... ...
Table 4 - presents the "consensus" fine-particle scattering efficiencies used in RESOLVE. Perceptual Air-Quality Modeling i, The preceding sections discussed models that deal with the physical processes involved in the interaction of light with particles.
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From page 132... ...
132 Q _ ~ _ + ~Q _ ~n- _ + m 11 ._ ~n ._ ~a _ ~7 ~ UJ-8 J `: o ~ 0 ~ u~ .E ,: 3 ~ 0 u°' , O: ~ ia LL CD 30 u=, E lr O Q ~ C`' 0 ·8 L[J G o ~: Z O cn _ C ·8 c,)
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From page 133... ...
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; Maim et al., 1980; Maim et al., 1981; Latimer et al., 1981; MiddIeton et al., 1983a, 1984; Hill, 1990; Ely et al., 1991~. Summaries of many of these study findings are given in Trijonis et al.
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From page 134... ...
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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From page 135... ...
.:::.: surements Is to use image processing techniques (Williams et al., 1980; Maim et al., 1983; Larson et al., 1988~. This method uses atmospheric optical moclels that simulate the effects of pollutants on a scene.
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From page 136... ...
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.
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From page 137... ...
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.
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From page 138... ...
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.
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From page 139... ...
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. ~ to 1.0 ,um diameter size range where removal mechanisms are least efficient.
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From page 140... ...
Intercomparisons have shown that light scattering coefficients measured wig 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.
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From page 141... ...
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.
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