Jacobson M.Z. Atmospheric Pollution

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A less subjective and, now, a regulatory definition of visibility is the meteorolog-

ical range. Meteorological range can be explained in terms of the following

example. Suppose a perfectly absorbing dark object lies against a white background

at a point x

, as shown in Fig. 7.21. Because the object is perfectly absorbing, it

reflects and emits no visible radiation; thus, its visible radiation intensity (I) at point

is zero, and it appears black. As a viewer backs away from the object, background

white light of intensity I

scatters into the field of view, increasing the intensity of

light in the viewer’s line of site. Although some of the added background light is

scattered out of or absorbed along the field of view by gases and aerosol-particles, at

some distance away from the object, so much background light has entered the path

between the viewer and the object that the viewer can barely discern the black object

against the background light.

The meteorological range is a function of the contrast ratio, deﬁned as

(7.8)

The contrast ratio gives the difference between the background intensity and the inten-

sity in the viewer’s line of site, all relative to the background intensity. If the contrast

ratio is unity, then an object is perfectly visible. If it is zero,

then the object cannot be

differentiated from background light.

The meteorological range is the distance from an object at which the contrast ratio

equals the liminal contrast ratio of 0.02 (2 percent). The liminal or threshold contrast

ratio is the lowest visually perceptible brightness contrast a person can see. It varies

from individual to individual. Koschmieder (1924) selected a value of 0.02. Middleton

(1952) tested 1,000 people and found a threshold contrast range of between 0.01 and

0.20, with the mode of the sample between 0.02 and 0.03. Campbell and Maffel (1974)

found a liminal contrast of 0.003 in laboratory studies of monocular vision.

Nevertheless, 0.02 has become an accepted liminal contrast value for meteorological

range calculations. In sum, the meteorological range is the distance from an ideal dark

object at which the object has a 0.02 liminal contrast ratio against a white background.

The meteorological range can be derived from the equation for the change in

object intensity along the path described in Fig. 7.21. This equation is

ratio



 I

198 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Scattering out of path

Scattering into path

= 0

Figure 7.21. Change of radiation intensity along a beam. A radiation beam originating from a

dark object has intensity I  0 at point x

. Over a distance dx, the beam’s intensity increas-

es due to scattering of background light into the beam. This added intensity is diminished

somewhat by absorption along the beam and scattering out of the beam. At point x, the net

intensity of the beam has increased close to that of the background intensity.

(7.9)

where all wavelength subscripts have been removed, 

is the total extinction coefﬁcient,



accounts for the scattering of background light radiation into the path, and 

accounts for the attenuation of radiation along the path due to scattering out of the path

and absorption along the path. A total extinction coefﬁcient is the sum of extinction

coefﬁcients due to scattering and absorption by gases and particles. Thus,





a,g



s,g



a,p



s,p

(7.10)

Integrating Equation 7.9 from I  0 at point x

 0 to I at point x with constant 

yields the equation for the contrast ratio,

(7.11)

When C

ratio

 0.02 at a wavelength of 0.55 m, the resulting distance x is the meteor-

ological range (also called the Koschmieder equation).

In polluted tropospheric air, the only important gas-phase visible light attenuation

processes are Rayleigh scattering and absorption by NO

(g). Table 7.3 shows the

meteorological ranges derived from calculated extinction coefﬁcients, resulting from

these two processes in isolation. For NO

(g), the table shows values at two mixing

ratios, representing clean and polluted air respectively. For Rayleigh scattering, one

meteorological range is shown because Rayleigh scattering is dominated by molecu-

lar nitrogen and oxygen, whose mixing ratios do not change much between clean and

polluted air.

At a wavelength of 0.55 m, the meteorological range due to Rayleigh scattering

alone was 334 km, indicating that, in the absence of all pollutants, the furthest one can

see along the horizon, assuming a liminal contrast ratio of 2 percent, is near 350 km.

Waggoner et al. (1981) reported a total extinction coefﬁcient at Bryce Canyon, Utah,

corresponding to a meteorological range of less than 400 km, suggesting that visibility

was limited by gas scattering.

ratio



 I

 e





 I)

EFFECTS OF POLLUTION ON VISIBILITY, ULTRAVIOLET RADIATION, AND ATMOSPHERIC OPTICS 199

(7.12)x 

3.912



Table 7.3. Meteorological Ranges (km) Resulting from Rayleigh Scattering and

(g) Absorption at Selected Wavelengths

(g) Absorption

 (m) Rayleigh Scattering 0.01 ppmv NO

(g) 0.25 ppmv NO

(g)

0.42 112 296 11.8

0.50 227 641 25.6

0.55 334 1,590 63.6

0.65 664 13,000 520

200 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

In Table 7.3, the meteorological range due to NO

(g) absorption decreased from

1,590 to 63.6 km when the NO

(g) mixing ratio increased from 0.01 to 0.25 ppmv.

Thus, NO

(g) absorption reduced visibility more than did Rayleigh scattering when

the NO

(g) mixing ratio was high. Results from a project studying Denver’s brown

cloud showed that NO

(g) accounted for about 7.6 percent of the total reduction in

visibility, averaged over all sampling periods, and 37 percent of the total reduction

during periods of maximum NO

(g). Scattering and absorption by aerosol particles

caused most remaining extinction (Groblicki et al. 1981). In sum, NO

(g) attenuates

visibility in urban air when its mixing ratios are high.

Although the effects of Rayleigh scattering and NO

(g) absorption on visibility

are nonnegligible in polluted air, they are less important than are scattering and

absorption by aerosol particles. Scattering by aerosol particles causes between 60 and

95 percent of visibility reductions. Absorption by aerosol particles causes between 5

and 40 percent of reductions (Cass, 1979; Tang et al., 1981; Waggoner et al., 1981).

Table 7.4 shows meteorological ranges derived from extinction coefﬁcient mea-

surements for a polluted and less polluted day in Los

Angeles. Particle scattering

dominated light extinction on both days. On the less-polluted day, gas absorption, par-

ticle absorption, and gas scattering all had similar small effects. On the polluted day,

the most important visibility reducing processes were particle scattering, particle

absorption, gas absorption, and gas scattering, in that order.

Equation 7.12 relates a theoretical quantity, meteorological range, to a measured

extinction coefﬁcient. When prevailing visibility, a subjective quantity, and the extinc-

tion coefﬁcient are measured simultaneously, they usually do not satisfy Equation

7.12. Instead, the relationship between prevailing visibility and the measured extinc-

tion coefﬁcient must be obtained empirically. Grifﬁng (1980) studied measurements of

prevailing visibility (V, in km) and extinction coefﬁcients (km

1

) over a ﬁve-year peri-

od and derived the empirical relationship

(7.13)

Figure 7.22 shows a map of estimated extinction coefﬁcients in the United States

between 1991 and 1995 derived from prevailing visibility measurements substituted

into Equation 7.13. During winters, visibility was poorest (extinction coefﬁcients were

highest) in southern and central California, Illinois, Indiana, Iowa, Kentucky, and

Michigan. During summers, visibility was poorest in Los Angeles and much of the

Midwest and southern United States. Winter visibility loss in central California

V 

1.9



Table 7.4. Meteorological Ranges (km) Resulting from Gas Scattering, Gas

Absorption, Particle Scattering, Particle Absorption, and All Processes at a

Wavelength of 0.55 m on a Polluted and less Polluted Day in Los Angeles

Gas Gas Particle Particle

Day Scattering Absorption Scattering Absorption All

Polluted (8/25/83) 366 130 9.6 49.7 7.42

Less Polluted (4/7/83) 352 326 151 421 67.1

Meteorological ranges derived from extinction coefﬁcients of Larson et al. (1984).

EFFECTS OF POLLUTION ON VISIBILITY, ULTRAVIOLET RADIATION, AND ATMOSPHERIC OPTICS 201

1991–95 Quarter 1

0.2

0.15

0.1

0.05

1/km

Figure 7.22. United States maps of corrected light extinction coefﬁcient (

), derived from

prevailing visibility measurements (V ) with the equation 

 1.9/V for winter (Quar ter 1) and

summer (Quarter 3) 1991–1995 (Schichtel et al., 2001). Large extinction coefﬁcients corre-

spond to poor visibility

Data obtained during rain, snow, and fog were eliminated. Corrected extinction coefﬁcients

were obtained by applying relative humidity correction factors to adjust all extinction coefﬁ-

cients to a “dry” relative humidity of 60 percent, then selecting the “dry” extinction

coefﬁcient at each monitoring site and for each season corresponding to the 75th percentile

of all extinction coefﬁcients at the site and for the season.

0.2

0.15

0.1

0.05

1/km

1991–95 Quarter 3

was likely caused by a combination of low-lying winter inversions, high relative

humidities, and heavy particle loadings. Visibility loss in Los Angeles was due to

smog present all year. Poor visibility in the winter in the Midwest was due to power-

plant emissions combined with high relative humidities. Summer visibility degradation

in the Midwest and southern United States was due to a combination of power-plant

emissions, organic particles from photochemical smog and vegetation, and high rela-

tive humidities.

7.3. COLORS IN THE ATMOSPHERE

Red sunsets and blue skies arise from preferential scattering of light by gas molecules.

Rainbows arise from interactions of light with raindrops. Additional optical phenome-

na are discussed next.

7.3.1. White Hazes and Clouds

When the relative humidity is high (but still less

than 100 percent), aerosol particles increase in size

by absorbing liquid water. When their diameters

approach the wavelength of light, the particles enters

the Mie regime (Section 7.1.5) and scatter all wave-

lengths with equal intensity, producing a whitish

haze. Hazes resulting from water growth on pollu-

tion particles, are common in urban areas under

sunny skies (Fig. 7.23).

Cloud and fog drops, which contain more liquid

water than do aerosol particles, appear white because

they scatter all wavelengths of visible light with equal

intensity. When pollution is particularly heavy and the

relative humidity is high, clouds and fogs are difﬁcult

to distinguish from hazes, as shown in Fig. 7.24.

7.3.2. Reddish and Brown Colors in Smog

Reddish and brown colors in smog, such as those

seen in Fig. 7.25, are due to three factors. The ﬁrst is

preferential absorption of blue and some green light

by NO

(g), which allows most green and red to be

transmitted, giving smog with NO

(g) a yellow,

brown, or reddish-green color. Brown layers result-

ing from NO

(g) can be seen most frequently from 9

to 11

A.M. in polluted air, because these are the hours

that NO

(g) mixing ratios are highest. The second is

preferential absorption of blue light by nitrated aromatics and polycyclic aromatic

hydrocarbons (PAHs) in aerosol particles. Figure 7.10 shows that a variety of nitrated

aromatic compounds preferentially absorb not only UV, but also short visible wave-

lengths. PAHs are also absorbers of short visible wavelengths. The blue-light absorption

and brown transmission by these compounds enhances the brownish color of smog.

202 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Figure 7.23. Haze over Los Angeles, May

1972. Photo by Gene Daniels, U.S. EPA, avail-

able from Still Pictures Branch, U.S. National

Archives.

EFFECTS OF POLLUTION ON VISIBILITY, ULTRAVIOLET RADIATION, AND ATMOSPHERIC OPTICS 203

Figure 7.24. Combined haze and fog over Los Angeles, near the San Gabriel Mountains, May

1972. Photo by Gene Daniels, U.S. EPA, available from Still Pictures Branch, U.S. National

Archives.

Figure 7.25. Reddish and brown colors in Los Angeles smog on December 19, 2000.

Third, when soil dust concentrations are high, air can appear reddish or brown because

soil particles also preferentially absorb blue and green light over red light. In the

absence of a dust storm and away from desert regions, the concentration of soil-dust

particles is generally low and the effect of soil dust on atmospheric optics is limited.

7.3.3. Black Colors in Smog

Black colors in smog are due primarily to absorption by black carbon, a component of

soot. Soot is emitted primarily during coal, diesel fuel, jet fuel, and biomass burning.

Black carbon absorbs all wavelengths of white light, transmitting none, thereby

appearing black against the background sky.

7.3.4. Red Skies and Brilliant Horizons in Smog

Whereas the sun itself appears red at sunset and sunrise, and the horizon appears red

during sunset and sunrise when aerosol particles are present (Fig. 7.8),

the sky can also

appear red in the afternoon and red horizons can become more brilliant in the presence

of air pollution. As discussed in Section 7.1.2.2, red horizons occur because aerosol

particles scatter the sun’s blue, green, and red light, providing a source of white light

to a viewer from all points along the horizon. Gas molecules scatter the blue and green

wavelengths out of the viewer’s line of site, allowing only the red to be transmitted

from the horizon to the viewer. When heavy smog is present, particle concentrations

near the surface and aloft increase sufﬁciently to allow this phenomenon to occur in

the afternoon, as seen in Fig. 7.26. Smog particles similarly enhance the brilliance of

the red horizon at sunset, as seen in Fig. 6.21.

204 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Figure 7.26. Red sky in the late afternoon over transmission lines near Salton Sea,

California, May 1972. The smog causing the optical effect originated in Los Angeles. Photo

by Charles O’Rear, U.S. EPA, available from Still Pictures Branch, U.S. National Archives.

7.3.5. Purple Glow in the Stratosphere

After a strong volcanic eruption, a purple glow may appear in the stratosphere, as

shown in Fig. 7.27. This glow results because volcanos emit sulfur dioxide gas,

which converts to sulfuric acid–water particles, many of which reach the strato-

sphere. Such particles scatter light through the stratospheric ozone layer. Ozone

weakly absorbs green and some red wavelengths (Fig. 7.3), transmitting blue and

some red, which combine to form purple. The purple light is scattered back to a view-

er’s eye by the enhanced stratospheric particle layer, causing the stratosphere to

appear purple.

7.4. SUMMARY

In this chapter, processes that affect radiation were discussed with an emphasis on

the effects of gases and aerosol particles on visibility degradation and UV light

reduction. Processes affecting radiation through the air include absorption and

scattering. Gas absorption of UV radiation is responsible for the stratospheric

inversion and most photolysis reactions. Gas absorption of visible light is important

only when NO

(g) mixing ratios are high. Gas (Rayleigh) scattering affects only

UV and visible wavelengths. The most important gas scatterers are molecular oxy-

gen and molecular nitrogen. All aerosol particles absorb near-and thermal-IR

radiation, but only a few, including black carbon and certain soil components,

absorb visible (including UV) radiation. Several organic aerosol components,

particularly nitrated and aromatic compounds, preferentially absorb UV radiation,

causing UV and ozone reductions in polluted air. All aerosol components scatter

EFFECTS OF POLLUTION ON VISIBILITY, ULTRAVIOLET RADIATION, AND ATMOSPHERIC OPTICS 205

Figure 7.27. Purple sunset taken from Palos Verdes, California, following the 1982 El Chichon

volcanic eruption. Photo by Jeffrey Lew, UCLA.

radiation. The most important visibility degrading processes in polluted air are

particle scattering, particle absorption, gas absorption, and gas scattering, in that

order. In clean air, visibility is limited in the horizontal to a few hundred kilometers

by gas scattering.

7.5. PROBLEMS

7.1. Calculate the fractional transmission of radiation through a swimming pool 2.0

m deep at a wavelength of 0.5 m when only absorption of light by liquid

water is considered. At what depth in the water do you expect the incident light

to be reduced by 99 percent due to the absorption?

7.2. If the angle of incidence, relative to the surface normal, of 0.5 m wavelength

light entering water from the air is 30, determine the angle of refraction in the

water.

7.3. If 0.5 m wavelength light travels from water to air, what angle of incidence

(with respect to the surface normal in water) is necessary for the angle of

refraction to be 90? This angle of incidence is referred to as the critical

angle. What happens if the angle of incidence in water exceeds the critical

angle?

7.4. If gas molecules in the atmosphere preferentially scattered longer w

avelengths

over shorter wavelengths, what colors would you expect the sky to be and the

sun to be at noon, afternoon, and sunset?

7.5. Explain why nitrogen dioxide gas causes more visibility reduction than does

ozone gas.

7.6. Calculate the meteorological range at 0.55 m resulting from aerosol particle

(a) scattering alone

(b) absorption alone

assuming the atmosphere contains 10

particles cm

3

of 0.1 m diameter

spherical particles made of black carbon. Use Fig. 7.19 to obtain single-particle

absorption and scattering efﬁciencies and assume efﬁciencies at 0.5 m are

similar to those at 0.55 m.

7.7. If a person’s liminal contrast ratio is 0.01 instead of 0.02, what would be the

expression for their meteorological range? For a given total extinction coefﬁ-

cient, how much further (in percent) can the person see when their liminal

contrast ratio is 0.01 instead of 0.02?

7.8. Assume that the atmosphere contains 10,000 g m

3

of liquid water, and the

density of liquid water is 1.0 g cm

3

. For which size spherical particles is visi-

bility reduced the most at a wavelength of 0.55 m:

206 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

(a) 0.5 m diameter particles

(b) 10.0 m diameter particles

Assume single-particle scattering efﬁciencies in Fig. 7.20 can be used for

wavelengths of 0.55 m. Show calculations.

7.9. Explain why soil often appears brown. What is the difference between soil that

appears brown and soil that appears red?

7.6. PROJECT

Find an outside, elevated location where, on a clear day, it is possible to see far

in at least some directions without obstruction by buildings or mountains. Pre-

determine the distances to a set of landmarks that co

ver at least 180

 along the

horizon circle, but not necessarily in continuous sectors around the circle. On

three different days and at two times during each day (pick a morning and an

afternoon time, and use the same two times on each day),

(a) estimate the prevailing visibility

(b) make a list of the colors you see in the sky along the horizon and where

you see them

After all data are gathered, discuss your ﬁndings in terms of differences among

days and between times of day. Comment on how the relative humidity, wind,

and large-scale pressure system may have affected results if that information

is available. Which of these factors do you think had the greatest effect on

visibility?

EFFECTS OF POLLUTION ON VISIBILITY, ULTRAVIOLET RADIATION, AND ATMOSPHERIC OPTICS 207