Effects of Pollution on Visibility, Ultraviolet Radiation, and Colors in the Sky - Air Pollution and Global Warming: History, Science, and Solutions - page 155

thermal-IR radiation, heats the stratosphere, causing the
stratospheric temperature inversion (Figure 3.3). Ozone
absorption also protects the surface of the Earth by pre-
venting nearly all UV wavelengths 0.245 to 0.29

intercept it. If the molecules absorb the radiation, the
intensity of the radiation diminishes. The ability of a gas
molecule to absorb radiation is embodied in the absorp-
tion cross section of the gas, b a,g,q (cm 2 per molecule),
where the subscripts a , g , and q mean “absorption,” “by
agas,”and “by gas q ,” respectively. Wavelength sub-
scripts 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
gasmolecule.
The product of the number concentration and absorp-
tion cross section of a gas is called an absorption extinc-
tion coefficient. An extinction coefficient measures the
loss of electromagnetic radiation due to a specific pro-
cess, per unit distance. Extinction coefficients, symbol-
ized with

m

and most wavelengths 0.29 to 0.32 mfrom reaching

the troposphere.
Although the gases in Table 7.1 absorb UV radia-
tion, the mixing ratios of all except O 3 (g), O 2 (g), and
N 2 (g) are too low to have much effect on UV penetra-
tion to the surface. For instance, stratospheric mixing
ratios of water vapor (

6 ppmv) are much lower than
are stratospheric mixing ratios of O 2 (g), which absorbs
many of the same UV wavelengths as does water vapor.
Thus, water vapor has little effect on UV attenuation in
the stratosphere. Similarly, stratospheric mixing ratios
of carbon dioxide (393 ppmv in 2011) are much lower
than are those of O 2 (g) or N 2 (g), both of which absorb
the same wavelengths as does carbon dioxide.
The only gas that absorbs visible radiation suffi-
ciently to affect visibility is nitrogen dioxide [NO 2 (g)],
but its effect is important only in polluted air, where
its mixing ratios are sufficiently high. Mixing ratios
of the nitrate radical [NO 3 (g)], which absorbs even
further into the visible spectrum than does nitrogen
dioxide, are low, except at night or in the early morning.
Thus, NO 3 (g) does not affect visibility. Although ozone
mixing ratios can be high, ozone is a relatively weak
absorber of visible light. However, ozone absorption of
green light after a volcano can cause beautiful colors in
the sky (Section 7.3.5).

<

,have units of inverse distance (cm − 1 ,m − 1 ,
or km − 1 ) and vary with wavelength. The absorption
extinction coefficient (cm − 1 )ofgas q is

σ

σ a , g , q =

N q b a , g , q (7.1)

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

σ a,g )isthesumofEquation 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 absorp-
tion extinction coefficients of both NO 2 (g) and O 3 (g)
at two mixing ratios. It indicates that nitrogen diox-
ide affects extinction (and therefore visibility) primarily
at high mixing ratios and at wavelengths below about
0.5 0.1 km − 1 ). In polluted air, such

as in Los Angeles, NO 2 (g) mixing ratios typically range
from 0.01 to 0.1 ppmv and peak near 0.15 ppmv dur-
ing the morning. A typical value is 0.05 ppmv. O 3 (g)
has a larger effect on extinction than does NO 2 (g) at
wavelengths below about 0.32

m(when

σ a,g,q >

7.1.1.2. Gas Absorption Extinction Coefficient
Visibility is affected by 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
agas is reduced by absorption. Suppose incident radia-
tion of intensity I 0 travels a distance d x

m. O 3 (g) mixing ratios

x 0 through
a uniformly mixed absorbing gas q of number concen-
tration N q (molecules per cubic centimeter of air). As
the radiation passes through the gas, gas molecules

= x −

10 1

10 0

NO 2 (g) (0.25 ppmv)

10 -1

NO 2 (g) (0.01 ppmv)

10 -2

O 3 (g) (0.25 ppmv)

10 -3

10 -4

I 0 I

10 -5

O 3 (g) (0.01 ppmv)

10 -6

d x

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7

x 0 x

Wavelength (µm)

Figure 7.2. Attenuation of incident radiance I 0 due to
absorption in a column of gas.

Figure 7.3. Extinction coefficients due to NO 2 (g) and
O 3 (g) absorption when T = 298 K and p a = 1,013 hPa.