Jacobson M.Z. Atmospheric Pollution

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EFFECTS OF POLLUTION

ON VISIBILITY,

ULTRAVIOLET

RADIATION, AND

ATMOSPHERIC OPTICS

isibility, ultraviolet (UV) radiation intensity, and optical phenomena are

affected by gases, aerosol particles, and hydrometeor particles interacting

with solar radiation. In clean air, gases and particles affect how far we can

see along the horizon and the colors of the sky, clouds, and rainbows. In polluted air,

gases and aerosol particles affect visibility, optical phenomena, and UV radiation

intensity. In this chapter, visibility, optics, and UV transmission in clean and polluted

atmospheres are discussed. An understanding of these phenomena requires a study of

the interaction of solar radiation with gases, aerosol particles, and hydrometeor parti-

cles through several optical processes, including reﬂection, refraction, diffraction,

dispersion, scattering, absorption, and transmission. These processes are described

next.

7.1 PROCESSES AFFECTING SOLAR RADIATION

IN THE ATMOSPHERE

The solar spectrum is divided into UV (0.01 to 0.38 m), visible (0.38 to 0.75 m),

and near-infrared (IR) (0.75 to 4.0 m) wavelength ranges (Section 2.2).

In 1666, Sir Isaac Newton (1642–1727;

Fig. 7.1), an English physicist and mathemati-

cian, showed that when white, visible light passed

through a glass prism, each wavelength of the light

bent to a different degree, resulting in the separation

of the white light into a variety of colors that he

called the light spectrum. Although the spectrum is

continuous (the eye can distinguish 10 million col-

ors), Newton discretized the spectrum into seven

colors: red, orange, yellow, green, blue, indigo, and

violet, to correspond to the seven notes on a musical

scale. When the colors of the spectrum were recom-

bined, they reproduced white light.

Newton also deﬁned primary colors as blue,

green, and red. When pairs of primary colors are

added together, they produce new colors. For exam-

ple, equal amounts of red plus green produce

yellow; equal amounts of red plus blue produce

magenta; and equal amounts of green plus blue pro-

duce cyan. For simplicity, the visible spectrum here

is divided into wavelengths of the primary colors

blue (0.38 to 0.5 m), green (0.5 to 0.6 m), and red (0.6 to 0.75 m).

The visible spectrum provides most of the energy that keeps the Earth warm. The

visible spectrum also affects the distance we can see and colors in the atmosphere. It is

no coincidence that the acuity (keenness) of our vision peaks at 0.55 m, in the green

part of the visible spectrum, which is near the wavelength of the sun’s peak radiation

intensity. Our eyes have evolved to take advantage of the peak intensity in this part of

the visible spectrum.

When radiation passes through the Earth’s atmosphere, it is attenuated or redirect-

ed by absorption and scattering by gases, aerosol particles, and hydrometeor particles.

These processes are discussed next.

180

Figure 7.1. Sir Isaac Newton (1642–1727).

7.1.1. Gas Absorption

Absorption occurs when radiative energy (such as from the sun or Earth) enters a sub-

stance and is converted to internal energy, increasing the temperature of the substance.

Absorption removes radiation from an incident beam, reducing the amount of radiation

received past the point of absorption. Next, absorption by gases is discussed with

respect to the solar spectrum.

7.1.1.1. Gas Absorption at Ultraviolet and Visible Wavelengths

Gases selectively absorb radiation in different portions of the electromagnetic

spectrum. Table 7.1 lists selected gases that absorb visible or UV radiation. Of the

gases, all absorb UV, but only a few absorb visible radiation. Gases that absorb visible

or UV radiation are often, but not always, photolyzed by this radiation into simpler

products.

The gases that affect UV radiation the most are molecular oxygen [O

(g)], molec-

ular nitrogen [N

(g)], and ozone [O

(g)]. Oxygen absorbs wavelengths shorter than

0.245 m and nitrogen absorbs wavelengths shorter than 0.1 m. Oxygen and nitro-

gen prevent nearly all solar wavelengths shorter than 0.245 m from reaching the

troposphere.

In 1880, M. J. Chappuis found that ozone absorbed visible radiation between the

wavelengths of 0.45 and 0.75 m. The absorption bands in this wavelength region are

now called Chappuis bands. In 1881, John Hartley ﬁrst suggested that ozone was

present in the upper atmosphere, and hypothesized that the reason that the Earth’s sur-

face received little radiation shorter than 0.31 m was because ozone absorbed these

wavelength. The absorption bands of ozone below 0.31 m are now called the Hartley

bands. In 1916, English physicists Alfred Fowler (1868–1940) and Robert John Strutt

(1875–1947, the son of Lord Rayleigh) showed that ozone also weakly absorbed

between 0.31 and 0.35 m wavelengths. The bands of absorption in this region are

now called the Huggins bands. Ozone absorption of UV and solar radiation, followed

by reemission of thermal-IR radiation, heats the stratosphere, causing the stratospheric

EFFECTS OF POLLUTION ON VISIBILITY, ULTRAVIOLET RADIATION, AND ATMOSPHERIC OPTICS 181

Table 7.1. Wavelengths of Absor

ption in the V

isible and UV Spectra b

y Some

Atmospheric Gases

Gas Name Chemical Formula Absorption Wavelengths (m)

Visible/Near-UV/Far-UV absorbers

Molecular oxygen O

(g) <0.245

Carbon dioxide CO

(g) <0.21

Water H

O(g) <0.21

Molecular nitrogen N

(g) <0.1

Far-UV absorbers

Formaldehyde HCHO(g) <0.36

Nitric acid HNO

(g) <0.33

Near-UV/Far-UV absorbers

Ozone O

(g) <0.35,0.45–0.75

Nitrate radical NO

(g) <0.67

Nitrogen dioxide NO

(g) <0.71

temperature inversion (Fig. 3.3). Ozone absorption also protects the surface of the

Earth by preventing nearly all UV wavelengths 0.245 to 0.29 m and most wave-

lengths 0.29 to 0.32 m from reaching the troposphere.

Although the gases in Table 7.1, aside from O

(g), O

(g), and N

(g), absorb UV radi-

ation, their mixing ratios are too low to have much effect on such radiation. For instance,

stratospheric mixing ratios of water vapor (0 to 6 ppmv) are much lower than are those

of O

(g), which absorbs many of the same wavelengths as does water vapor. Thus, water

vapor has little effect on UV attenuation in the stratosphere. Similarly, stratospheric mix-

ing ratios of carbon dioxide (370 ppmv) are much lower than are those of O

(g) or

(g), both of which absorb the same wavelengths as does carbon dioxide.

The only gas that absorbs visible radiation sufﬁciently to af

fect visibility is

nitro-

gen dioxide [NO

(g)], but its effect is important only in polluted air where its mixing

ratios are sufﬁciently high. Mixing ratios of the nitrate radical [NO

(g)], which also

absorbs visible radiation, are low, except at night or in the early morning. Thus,

(g) does not affect visibility. Although ozone mixing ratios can be high, ozone is a

relatively weak absorber of visible light.

7.1.1.2. Gas Absorption Extinction Coefﬁcient

The determination of visibility depends on all processes that attenuate or enhance

radiation. In this subsection, attenuation by gas absorption is brieﬂy discussed.

Figure 7.2 illustrates how radiation passing through a gas is reduced by absorption.

Suppose incident radiation of intensity I

travels a

distance dx  x  x

through a uniformly mixed

absorbing gas q of number concentration N

(mole-

cules per cubic centimeter of air). As the radiation

passes through the gas, gas molecules intercept it. If

the molecules absorb the radiation, the intensity of

the radiation diminishes. The ability of a gas mole-

cule to absorb radiation is embodied in the

absorption cross section of the gas, b

a,g,q

(cm

per

molecule), where the subscripts a, g, and q mean

absorption, by a gas, and by gas q, respectively.

Wavelength subscripts were omitted. An absorption

cross section of a gas is an effective cross section that results in radiation reduction by

absorption. Its size is on the order of, but usually not equal to, the real cross-sectional

area of a gas molecule.

The product of the number concentration and absorption cross section of a gas is

called an absorption extinction coefﬁcient. An extinction coefﬁcient measures the loss

of electromagnetic radiation due to a speciﬁc process per unit distance. Extinction

coefﬁcients, symbolized with , have units of inverse distance (cm

1

, or km

1

)

and vary with wavelength. The absorption extinction coefﬁcient (cm

1

) of gas q is



a,g,q

 N

a,g,q

(7.1)

The gas absorption extinction coefﬁcient due to the sum of all gases (

a,g

) is the sum of

Equation 7.1 over all absorbing gases. The greater the absorption extinction coefﬁcient

of a gas in the visible spectrum, the more the gas reduces visibility. Figure 7.3 shows

absorption extinction coefﬁcients of both NO

(g) and O

(g), at two mixing ratios.

The ﬁgure indicates that nitrogen dioxide affects extinction (and therefore visibility)

182 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Figure 7.2. Attenuation of incident radiance I

due to absorption in a column of gas.

primarily at high mixing ratios and at wavelengths below about 0.5 m. In polluted air,

such as in Los Angeles, NO

(g) mixing ratios typically range from 0.01 to 0.1 ppmv

and peak near 0.15 ppmv during the morning. A typical value is 0.05 ppmv. O

(g) has a

larger effect on extinction than does NO

(g) at wavelengths below about 0.32 m.

(g) mixing ratios in polluted air usually peak between 0.05 and 0.25

ppmv.

The reduction in radiation intensity (where I denotes intensity) at a given wave-

length with distance through an absorbing gas can now be deﬁned as

I  I



a,g,q

(xx

)

 I

N

a,g,q

(xx

)

(7.2)

This equation states that incident radiation I

is reduced by a factor e



a,g,q

(xx

)

between points x

and x by gas absorption. From this equation, it is evident that the

absorption cross section of a gas can be determined experimentally by measuring the

attenuation of radiation through a homogenous gas column, such as the one shown in

Fig. 7.2, of known path length N

(x  x

) (molecules cm

2

), then extracting the cross

section from Equation 7.2.

7.1.2. Gas Scattering

Gas scattering is the redirection of radiation by a gas molecule without a net transfer

of energy to the molecule. When a gas molecule scatters, incident radiation is redirected

symmetrically in the forward and backward direction and somewhat off to the side, as

shown in Fig. 7.4. The ﬁgure shows the probability distribution of incident energy

scattered by a gas molecule.

EFFECTS OF POLLUTION ON VISIBILITY, ULTRAVIOLET RADIATION, AND ATMOSPHERIC OPTICS 183

-6

-5

-4

-3

-2

-1

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7

Extinction coefficient (km

-1

)

Wavelength (μm)

(g) (0.25 ppmv)

(g) (0.01 ppmv)

Figure 7.3. Extinction coefﬁcients due to NO

(g) and O

(g) absorption when T  298 K and

 1013 mb.

EXAMPLE 7.1.

Find the fraction of incident radiation intensity (the transmission) passing through a uniform gas col-

umn of length 1 km when the number concentration of the gas is N

 10

molecules cm

3

and the

absorption cross section of a gas molecule is b

a,g,q

 10

19

per molecule.

Solution

From Equation 7.1, the extinction coefﬁcient through the gas is 

a,g,q

 N

a,g,q

 10

7

1

. From

Equation 7.2, the transmission is I/I

 e

10



x 10

 0.99. Thus 99 percent of incident radiation trav-

eling through this column reaches the other side.

The scattering of radiation by gas molecules or by aerosol particles much smaller

than the wavelength of light is called Rayleigh scattering. Because gas molecules are

on the order of 0.0005 m in diameter and a typical wavelength of visible light is 0.5

m, all gas molecules are Rayleigh scatterers. Because molecular nitrogen [N

(g)],

molecular oxygen [O

(g)], argon [Ar(g)], and water

vapor [H

O(g)] are the most abundant gases in the

air, these are the most important Rayleigh scatterers.

Rayleigh scattering is named after Lord Baron

Rayleigh, born John William Strutt (1842–1919;

Fig. 7.5), who also discovered argon gas with Sir

William Ramsay in 1894 (Chapter 1). Rayleigh’s

theoretical work on gas scattering was published in

1871, (Rayleigh, 1871) 23 years before his discov-

ery of argon.

7.1.2.1. Gas-Scattering Extinction Coefﬁcient

The gas scattering extinction coefﬁcient

(cm

1

) of total air (Rayleigh scattering extinction

coefﬁcient) is analogous to that of the gas absorption

extinction coefﬁcient, and is deﬁned as



s,g

 N

s,g

(7.3)

where N

is the number concentration of air mole-

cules (molecules cm

3

) at a given altitude and b

s,g

the scattering cross section (cm

molecule

1

) of a

typical air molecule. The scattering cross section of

an air molecule is an effective cross section that

results in radiation reduction by scattering, and is

proportional to the inverse of the fourth power of the

wavelength. This means that gas molecules in the air

scatter short (blue) wavelengths preferentially over

long (red) wavelengths.

7.1.2.2. Colors of the Sky and Sun

The variation of the Rayleigh scattering cross

section with wavelength explains why the sun

appears white at noon, yellow in the afternoon, and

red at sunset and why the sky is blue. White sunlight

that enters the Earth’s atmosphere travels a shorter

distance before reaching a viewer’s eye at noon than at any other time during the day,

as illustrated in Fig. 7.6. During white light’s travel through the atmosphere, blue

wavelengths are preferentially scattered out of the direct beam by air molecules, but

not enough blue light is scattered for a person looking at the sun to notice that the inci-

dent beam has changed its color from white. Thus, a person looking at the sun at noon

often sees a white sun. The blue light that scatters out of the direct beam is scattered

by gas molecules multiple times, and some of it eventually enters the viewer’s eye

when the viewer looks away from the sun. As such, a viewer looking away from the

sun sees a blue sky.

184 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Incoming energy

Figure 7.4. Probability distribution of where

incident energy is scattered by a gas mole-

cule (located at the center of the diagram).

Figure 7.5. Lord Baron Rayleigh (John William

Strutt) (1842–1919).

Although Lord Rayleigh formalized the theory of scattering by gases and aerosol

particles much smaller than the wavelength of light, it may have been Leonardo

DaVinci (1452–1519) who ﬁrst suggested that blue colors in the sky were due to inter-

actions of sunlight with atmospheric constituents. He wrote (c. 1500),

I say that the blueness we see in the atmosphere is not intrinsic color, but is caused

by warm vapor evaporated in minute and insensible atoms on which the solar rays

fall, rendering them luminous against the inﬁnite darkness of the ﬁery sphere

which lies beyond and includes it.... If you produce a small quantity of smoke

from dry wood and the rays of the sun fall on this smoke and if you place (behind

it) a piece of black velvet on which the sun does not fall, you will see that the

black stuff will appear of a beautiful blue color.... Water violently ejected in a

ﬁne spray and in a dark chamber where the sunbeams are admitted produces then

blue rays.... Thus it follows, as I say, that the atmosphere assumes this azure hue

by reason of the particles of moisture which catch the rays of the sun.

What DaVinci saw, however, was not scattering by gas molecules but scattering by

small aerosol particles, namely, dry wood-smoke particles and small liquid-water

drops. Because such particles are smaller than the wavelength of light, they preferen-

tially scatter blue light as do gas molecules. Small aerosol particles are well known to

scatter blue light preferentially. Preferential scattering of blue light by small aerosol

particles in the air is responsible for the blue appearance of the Blue Ridge

Mountains in Virginia and the Blue Mountains in Australia. Occasionally after a for-

est ﬁre, a blue moon appears due to the scattering of blue light by small organic

aerosol particles. Similarly, after a heavy rain, the relative humidity is low, and aerosol

particles lose much of their water, shrinking their size enough to preferentially scatter

blue light and giving the sky a deep blue appearance.

EFFECTS OF POLLUTION ON VISIBILITY, ULTRAVIOLET RADIATION, AND ATMOSPHERIC OPTICS 185

Blue

Red

Earth Atmosphere

Space

White

Noon

Yellow

Afternoon

Sunset/

Twilight

Red

Green

Blue

Green

Red

Figure 7.6. Colors of the sun. At noon, the sun appears white because red, green, and some

blue light transmit to a viewer’s eye. In the afternoon, sunlight traverses a longer path in the

atmosphere, removing more blue. At sunset, most green is removed from the line of sight,

leaving a red sun. After sunset, the sky appears red due to refraction between space and the

atmosphere.

In the afternoon, light takes a longer path through the air than it does at noon; thus,

more blue and some green light is scattered out of the direct solar beam in the after-

noon than at noon. Although a single gas molecule is less likely to scatter a green than

a blue wavelength, the number of gas molecules along a vie

wer

’s line of sight is so

large in the afternoon that the probability of green light scattering is sizable. Nearly all

red and some green are still transmitted to the viewer’s eye in the afternoon, causing

the sun to appear yellow. In clean air, the sun can remain yellow until just before it

reaches the horizon, as shown in Fig. 7.7.

When the sun reaches the horizon at sunset, sunlight traverses its longest distance

through the atmosphere, and all blue and green and some red wavelengths are scat-

tered out of the sun’s direct beam. Only some direct red light transmits, and a viewer

sees a red sun. Sunlight can be seen after sunset because sunlight refracts as it enters

Earth’s atmosphere, as illustrated in Fig. 7.6. Refraction is the bending of light as it

passes from a medium of one density to a medium of a different density, and is dis-

cussed in Section 7.1.4.2.

During and after sunset, the whole horizon often appears red (Fig. 7.8) due to the

presence of aerosol particles along the horizon. Aerosol particles scatter blue, green,

and red wavelengths of direct sunlight, sending such light in all directions along

the horizon. At all points along the horizon, additional particles re-scatter some

of the scattered blue, green, and red light toward the viewer’s eye. As these wave-

lengths pass through the air, blue and green are scattered preferentially out of the

viewer’s line of site by gas molecules, whereas most red wavelengths are transmitted,

causing the horizon to appear red. Red horizons are common over the ocean, where

sea-spray particles are present, as well as over land, where soil and pollution parti-

cles are present.

The time after sunset during which the sky is still illuminated due to refraction is

called twilight. Twilight also occurs before sunrise. The length of twilight increases

186 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION

Figure 7.7. Yellow sun in the early evening off the island of Maui in the South Paciﬁc Ocean.

with increasing latitude. At midlatitudes in the summers, the length of twilight is about

one-half hour. At higher latitudes in summers, twilight in the morning may merge with

that in the evening, creating a twilight that lasts all night, called a white night.

7.1.3. Aerosol and Hydrometeor Particle Absorption

All aerosol and hydrometeor particles absorb thermal-IR and near-IR radiation, but

only a few absorb visible and UV radiation. Next, visible-and UV-absorption proper-

ties of aerosol particles and the effect of aerosol particle absorption on UV radiation

and pollution are discussed.

7.1.3.1. Important Absorbers of Visible and UV Radiation

The strongest aerosol particle absorber in the visible spectrum is black carbon.

Other visible absorbers include hematite [Fe

(s)] and aluminum oxide [alumina-

(s)]. Hematite is found in many soil-dust particles. Some forms of aluminum

oxide are found in soil-dust particles and others are found in combustion particles.

Aerosol particle absorbers in the UV spectrum include black carbon, hematite, alu-

minum oxide, and certain organic compounds. The organics absorb UV radiation but

little visible radiation. The strongest near-UV absorbing organics include certain nitrat-

ed aromatics, polycyclic aromatic hydrocarbons (PAHs), benzaldehydes, benzoic

acids, aromatic polycarboxylic acids, and phenols (Jacobson, 1999c).

Most particulate components are weak absorbers of visible and UV radiation.

Silicon dioxide [SiO

(s), silica], which is the white, colorless, crystalline compound

found in quartz, sand, and other minerals, is an example. Sodium chloride [NaCl(s)],

ammonium sulfate [(NH

)

(s)], and sulfuric acid [H

(aq)] are also weak

absorbers of visible and UV radiation. Soil-dust particles, which contain SiO

(s),

(s), Fe

(s), CaCO

(s), MgCO

(s), and other substances, are moderate absorbers

EFFECTS OF POLLUTION ON VISIBILITY, ULTRAVIOLET RADIATION, AND ATMOSPHERIC OPTICS 187

Figure 7.8. Red horizon after the sun dips below a deck of stratus clouds over the Paciﬁc

Ocean, signifying the beginning of twilight.