Protecting Visibility in National Parks and Wilderness Areas (1993) / Chapter Skim
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2 Visibility Conditions in the United States
2 Visibility Conditions in the United States
Pages 29-56
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From page 29... ...
Studies that have related historical visibility trends to historical emission trends are summarized, and major emission source types (natural and anthropogenic) , that affect visibility are discussed.
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From page 30... ...
The spatial patterns in this map are based on airport observations of visual range which differ from measurements of standard visual range.) Airport visual range measurements are based on the identification by human observers of targets at known distances from the observation point, whereas standard visual range is calculated from light extinction measurements.
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31 £ ,~ o ~ N , 1 o I ~ an -~ ~ ~72: ~ I- ~ .- ~ ~ ~ ~ ~ O ·- Us Cn 5 _ ~ I, ~ O 0< _ - .
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However, visual range decreases sharply to the east and west of this area. Median visual range falls to less Man 50 km in a narrow band along the northern Pacific coast, less than 30 On in the central valley of California, and to less than 15 km in the Los Angeles basin ~rijonis, 1982a)
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from the National Park Service network, 1983-1986. Source: Eldred et al., 1987 (From Trijonis et al., 1990~.
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from the National Park Service network, 1983-1986. Fine elemental carbon concentrations in ~g/m3 can be estimated by dividing the fine-particle absorption coefficient (in Mm~1)
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from the National Park Service network, 1983-1986. The nonsulfate hydrogen is calculated from the total hydrogen concentration by subtracting the hydrogen that is associated with sulfates.
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Seasonal Patterns Two data sets show the seasonal visibility patterns for North America. The first data set is quarterly median light extinction data obtained at NPS automated camera sites for 1986-1988 (see Table 2-!
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VISIBILITY IN THE UNITED STATES 37 T~L,E.,.,.22.~.'..'..Q=.""r.',.d2y"'M~' '''Vis' t"R' ges"'~r"'N' 'ion4'' - ""Se' ' e''"'''"" ,:.' 2 A,."'t'' "' '' "'S' ''"' 9'' '''.'''"""'""'"'""''"""'";'"'''" """""''';';'''"';'''';" ~::.June-.,..: .,':' ' ,:~, ,..:Sept.
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From page 38... ...
In the West, the airport results reported by Husar are problematic because of the lack of suff~ciently distant targets at some sites and also because of the limited statistical resolution in the reported results; interpretation also is diflF~cult because the results are based on data from a mixture of urban and rural locations (Trijonis et al., 1990~. Def~nitive results must await a revised analysis of the NPS camera data and airport visibility data or, better yet, an analysis of the transmissometer data from the new NPS monitoring system (see Chapter 4~.
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From page 39... ...
7 km · ~.7 km 1 FIGURE 2-8a Median airport visual range for January, averaged over 1979-1983. Larger circles correspond to lower visual range (greater light extinction)
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Statistical Patterns The statistical distribution of visibility, especially with respect to worst-case values (greatest light extinction levels) is of interest.
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41 The numbers in Table 2-2 are measures of the episodicity of light extinction levels; a high number indicates that extreme haze episodes are severe relative to the median. The four regions in Table 2-2 show considerable variation: the 99th percentile values exceed median extinction levels by a factor of about 3 to 8; the 95th percentile values by a factor of 2.5 to 4; and the 90th percentile values by a factor of 2 to 3.
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By the 1980s, howev PROTECTING VISIBILITY IN NATIONAL PARKS 30 E 07 oo 0 .o > 6 at, 10 /\ Extinction ~/ Total Fine-Particle Mass A A Fine Sulfate Mass (as NH4HSO41 \ 1 1 1 1 We< ~e' 33~' pi 0~\ Se? 0~ We of FIGURE 2-10 Seasonal patterns in SOA2_, 200 E o 1 00 ·~ , , fine-particle mass, and light extinction for rural areas of the eastern United States.
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From page 43... ...
The most significant historical visibility declines occurred in the East for the Bird quarter, a phenomenon also reported by others (Munn, 1973; Trijonis et al., 1978; Husar, 1988; Husar et al., 1981; Sloane, 1982, 1983, 1984; Trijonis, 1982b)
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so em ~ · 44 ·.
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Regional and seasonal trends are well correlated with historical light-extinction and SO2 emission trends. Since the late 1940's, light extinction has declined moderately in the North during the winter and increased moderately during the summer; these trends are matched closely by the trends in SO2 emissions.
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From page 48... ...
and He types of materials present in the unpolluted atmosphere for Me eastern and western United States. The evaluation incorporates data on the concentrations of the major components of airborne particles: SO42-, organic matter, elemental carbon, nitrates, soil dust, and water.
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For the arid West, the average background visual range is estimated as 230 i 35 km, a value equivalent to a light extinction level of 17 i Ah Mm~i, which is about I.5 times Me level caused by Rayleigh scattering alone. The error bounds are estimates of uncertainties in the spatial and annual averages.)
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(1990) estimated that the major contributors to natural extinction levels in the East are Rayleigh scattering (46%)
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Anthropogenic Sources Fine particles are the primary cause of anthropogenic haze. Coarse particles (predominantly soil dust)
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53 o ID 1 - ,1~ 4,: onto Pro _ ~ ~ is_ _ ~ ~ O 3 ~ ~ ·.
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SUMMARY Visibility impairment episodes range in scale from local plumes to widespread regional haze. The most intense regional haze in the United States occurs in the East, where the median standard visual range (calculated from airport data)
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55 At 3 An o ._ Ct ILL C' ._ o Ct ._ rem - as: I......
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The major contributors to natural extinction are Rayleigh scattering, organics, water, and suspended dust. The main constituents of anthropogenic haze are sulfates, organics, elemental carbon, soil dust, nitrates, and water.
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