Jacobson M.Z. Atmospheric Pollution
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resulted in the evolution of aerobic respiration, which led to a more efficient means of
producing molecular nitrogen, the major constituent in today’s atmosphere.
2.5. PROBLEMS
2.1. Explain why the Earth’s core consists primarily of iron whereas its crust con-
sists primarily of oxygen and silicon.
2.2. Why does the Earth receive radiation from the sun as if the sun is an emitter
with an effective temperature of near 6,000 K, when in fact, the hottest temper-
atures of the sun are more than 15 million K?
2.3. Even though the moon is effectively the same distance from the sun as the
Earth, the moon has no effective atmosphere. Why?
2.4. What is the intensity of radiation emitted by a hot desert (330 K) relati
ve to
that emitted by the stratosphere over the South Pole during July (190 K)?
2.5. What prevents the Earth from having magma oceans on its surface today, even
though temperatures in the interior of the Earth exceed 4,000 K?
2.6. What peak wavelength of radiation is emitted in the center of the Earth, where
the temperature is near 4,000 K?
2.7. What elements do you expect to be most abundant in soil-dust particles lifted
by the wind? Why?
2.8. Describe the nitrogen cycle in today’s atmosphere. What would happen to
molecular nitrogen production if nitrification were eliminated from the cycle?
(Hint: Consider the processes occurring in the preoxygen atmosphere.)
2.9. Identify at least three ways in which oxygen improved the opportunity for
higher life forms to develop on the Earth.
2.10. What is the advantage of aerobic respiration over fermentation?
2.11. What is the source of oxygen during photosynthesis in green plants? Explain.
48 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION
STRUCTURE AND
COMPOSITION OF THE
PRESENT-DAY
ATMOSPHERE
3
50
I
n this chapter, the structure and composition of the present-day atmosphere
are described. The structure is defined in terms of the variation of pressure,
density, and temperature with height. Pressure and density are controlled by
gases in the air, the most abundant of which are molecular nitrogen and oxygen. The
temperature structure is controlled by the distribution of gases that absorb ultraviolet
(UV) and thermal-infrared (IR) radiation. Pressure, density, and temperature are inter-
related by the equation of state. Gases in the air include fixed and variable gases. In the
following, the structure of the atmosphere in terms of pressure, density, and tempera-
ture variations with height is discussed. The main constituents of the air are then
examined in terms of their sources and sinks, abundances, health effects, and impor-
tance with respect to different air pollution issues.
3.1. AIR PRESSURE AND DENSITY STRUCTURE
Air consists of gases and particles, but the mass of
air is dominated by gases. Of all gas molecules in
the air, more than 99 percent are molecular nitrogen
or oxygen. Thus, oxygen and nitrogen are responsi-
ble for the current pressure, temperature, and
density structure of the Earth’s atmosphere. Air
pressure, the force of air per unit area, can be cal-
culated as the summed weight of all gas molecules
between a horizontal plane and the top of the
atmosphere and divided by the area of the plane.
Thus, the more oxygen and nitrogen present, the
greater the air pressure. Because the weight of air
per unit area above a given altitude is always
greater than that above any higher altitude, air pres-
sure decreases with increasing altitude. In fact,
pressure decreases exponentially with increasing
altitude. Standard sea-level pressure is 1013 mil-
libars [mb, in which 1 mb 100 N m
2
100 kg
m
1
s
2
100 Pascal (Pa) 1 hectaPascal (hPa)].
The sea-level pressure at a given location and time
typically differs by 10 to 20 mb from standard
sea-level pressure. In a strong low-pressure system,
such as at the center of a hurricane, the actual sea-
level pressure may be more than 50 mb lower than
standard sea-level pressure.
Atmospheric pressure was first measured in
1643 by Italian physicist Evangelista Torricelli
(1608–1647; Fig. 3.1), an associate of Galileo
Galilei (1564–1642). Torricelli filled a glass tube
1.2 m long with mercury and inverted it onto a dish.
He found that only a portion of the mercury flowed
from the tube into the dish, and the resulting space above the mercury in the tube was
devoid of air (a vacuum). Torricelli was the first person to record a sustained vacuum.
After further observations, he suggested that the change in height of the mercury in the
Figure 3.1. Experiment with mercury barome-
ter, conducted by Evangelista Torricelli
(1608–1647) and directed by Blaise Pascal
(1623–1662). The third person is the artist,
Ernest Board. Courtesy of the Edgar Fahs
Smith Collection, University of Pennsylvania
Library.
tube each day was caused by a change in atmospheric pressure. Air pressure balanced
the pressure exerted by the column of mercury in the tube, preventing the mercury from
flowing freely from the tube. Decreases in air pressure caused more mercury to flow out
of the tube and the mercury level to drop. Increases in air pressure had the opposite
effect. The inverted tube Torricelli used to derive this conclusion was called a mercury
barometer, where a barometer is a device for measuring atmospheric pressure.
Soon after Torricelli’s discovery, French mathematician Blaise Pascal (1623
–
1662)
confirmed Torricelli’s theory. He and his brother-in-law, Florin Périer, each carried a
glass tube of mercury, inverted in a bath of mercury, up the hill of Puy-de-Dôme,
France. Each recorded the level of mercury at the same instant at different altitudes
on the hill, confirming that atmospheric pressure decreases with increasing altitude.
STRUCTURE AND COMPOSITION OF THE PRESENT-DAY ATMOSPHERE 51
0
20
40
60
80
100
0 200 400 600 800 1000
Altitude (km)
Air pressure (mb)
1 mb (above 99.9%)
10 mb (above 99%)
100 mb (above 90%)
500 mb (above
50%)
(a)
0
20
40
60
80
100
0 0.4 0.8 1.2
Altitude (km)
Air density (kg m
-3
)
(b)
Figure 3.2. (a) Pressure and (b) density versus altitude in the Earth’s lower atmosphere. The
pressure diagram shows that 99.9 percent of the atmosphere lies below an altitude of about
48 km (1 mb), and 50 percent lies below about 5.5 km (500 mb).
EXAMPLE 3.1.
To what height must mercury in a barometer rise to balance an atmospheric pressure of 1,000 mb,
assuming the density of mercury is 13,558 kg m
3
?
Solution
The pressure (kg m
1
s
2
) exerted by mercur y equals the product of its density (kg m
3
), gravity
(9.81 m s
2
), and the height (h, in meters) of the column of mercur y. Equating this pressure with the
pressure exer ted by air and solving for the height gives
indicating that a column of mercur
y 29.6 inches (75.2 cm) high exerts as much pressure at the Ear
th
’s
surface as a 1,000-mb column of air extending between the surface and the top of the atmosphere.
h
1,000 mb
13,558
kg
m
3
9.81
m
s
2
100 kg m
1
s
2
1 mb
0.752 m 29.6 inches
In 1663, the Royal Society of London built its own mercury barometer based on
Torricelli’s model. A more advanced aneroid barometer was developed in 1843. The
aneroid barometer measured pressure by gauging the expansion and contraction of a
metal tightly sealed in a case containing no air.
Air density is the mass of air per unit volume of air. Because oxygen and nitrogen
are concentrated near the Earth’s surface, air density peaks near the surface. Air densi-
ty decreases exponentially with increasing altitude.
Figure 3.2 shows standard profiles of air pressure and density. Figure 3.2(a) shows
that 50 percent of atmospheric mass lies between sea level and 5.5 km. About
99.9 percent of mass lies below about 48 km. The Earth’s radius is about 6,378 km.
Thus, almost all of the Earth’s atmosphere lies in a layer thinner than 1 percent of the
radius of the Earth.
3.2. PROCESSES AFFECTING TEMPERATURE
Temperature is proportional to the average kinetic energy of an air molecule. From
gas kinetic theory, the absolute temperature (K) of air is obtained from
(3.1)
where k
B
is Boltzmann’s constant (1.3807 10
23
kg m
2
s
2
K
1
molecule
1
), is
the average mass of one air molecule (4.8096 10
26
kg molecule
1
), and is the
average thermal speed of an air molecule (m s
1
). The right side of Equation 3.1 is
the kinetic energy of an air molecule at its average thermal speed.
Measurements of relative air temperature changes were first attempted in 1593 by
Galileo, who devised a thermoscope to measure the expansion and contraction of air
upon its heating and cooling, respectively. However, the instrument did not have a
scale and was unreliable. In the mid-seventeenth century, the thermoscope was
replaced by the liquid-in-glass thermometer developed in Florence, Italy. In the early
eighteenth century, useful thermometer scales were developed by Gabriel Daniel
Fahrenheit (1686–1736) of Germany and Anders Celsius (1701–1744) of Sweden.
The temperature at a given location and time is affected by energy transfer
processes, including conduction, convection, advection, and radiation. These processes
are discussed briefly in the following subsections.
3.2.1. Conduction
Conduction is the transfer of energy in a medium (the conductor) from one molecule to
the next in the presence of a temperature gradient. The medium, as a whole, experiences
no molecular movement. Conduction occurs through soil, air, and particles. Conduction
v
a
M
4
k
B
T
1
2
M v
a
2
52 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION
EXAMPLE 3.2.
What is the thermal speed of an air molecule at 200 K? At 300 K?
Solution
From Equation 3.1, the thermal speed of an air molecule is 382 m s
1
at 200 K and 468 m s
1
at 300 K.
affects air temperature by transferring energy between the soil surface and the bottom
molecular layers of the air. The rate of a material’s conduction is determined by its
thermal conductivity (,J m
1
s
1
K
1
), which quantifies the rate of flow of thermal
energy through a material in the presence of a temperature gradient.
Table 3.1 gives
thermal conductivities of a few substances. It shows that liquid water, clay, and dry
sand are more conductive than is dry air. Thus, energy passes through air slower than it
passes through other materials given the same temperature gradient. Clay is more con-
ductive and dry sand is less conductive than is liquid water.
The flux of energy due to conduction (
W m
2
) can be approximated with the
conductive heat flux equation,
(3.2)
where T (K) is the change in temperature over a distance z (m). At the ground, mol-
ecules of soil and water transfer energy by conduction to overlying molecules of air.
Because the temperature gradient (Tz) between the surface and a thin (e.g., 1 mm)
layer of air just abov
e the surface is large, the conductive heat
flux at the ground is
large. Above the ground, temperature gradients are smaller and conductive heat fluxes
through the air are smaller than they are at the ground.
3.2.2. Convection
Convection is the transfer of energy, gases, and particles by the mass movement of air,
predominantly in the vertical direction. It differs from conduction in that during con-
duction, energy is transferred from one molecule to another, whereas during
convection, energy is transferred as the molecules themselves move. Two important
types of convection are forced and free.
Forced convection is an upward or downward vertical movement of air caused
by mechanical means. Forced convection occurs, for example, when (1) horizontal
near-surface winds converge (diverge), forcing air to rise (sink); (2) horizontal winds
H
c
ΔT
Δz
STRUCTURE AND COMPOSITION OF THE PRESENT-DAY ATMOSPHERE 53
Table 3.1. Thermal Conductivities of Four Media
Dry air at constant pressure 0.0256
Liquid water 0.6
Clay 0.920
Dry sand 0.298
Substance Thermal Conductivity () at 298.15 K (J m
1
s
1
K
1
)
EXAMPLE 3.3.
Compare the conductive heat flux through a 1-mm thin layer of air touching the surface if T 298 K
and T 12 K with that through the free troposphere, where T 273 K and T/z 6.5 K km
1
.
Solution
For air, 0.0256 J m
1
s
1
K
1
; thus, the conductive heat flux at the surface is H
c
307 W m
2
. In
the free troposphere, H
c
1.5 x 10
4
W m
2
, which is much smaller than is the value at the surface.
Thus, heat conduction thr
ough the air is important only adjacent to the ground.
encounter a topographic barrier, forcing air to rise; or (3) winds blow over objects
protruding from the ground, creating swirling motions of air, or eddies, which mix air
vertically and horizontally. Objects of different size create eddies of different size.
Turbulence is the effect of groups of eddies of different size. Turbulence from wind-
generated eddies is called mechanical turbulence.
Free convection (thermal turbulence) is a predominantly vertical motion produced
by buoyancy, which occurs when the sun heats different areas of the ground differential-
ly. Differential heating occurs because clouds or hills block the sun in some areas but not
in others, or different surfaces lie at different angles relative to the sun. Over a warm,
sunlit surface, conduction transfers energy from the ground to molecules of air adjacent
to the ground. The warmed air above the ground rises buoyantly, producing a thermal.
Cool air from nearby is drawn down to replace the rising air. Near the surface, the cool
air heats by conduction then rises, feeding the thermal. Free convection occurs most
readily over land when the sky is cloud-free and winds are light.
3.2.3. Advection
Advection is the horizontal movement of energy, gases, and particles by the wind.
Like convection, advection results in the mass movement of molecules.
3.2.4. Radiation
Radiation, first defined in Section 2.2, is the transfer of energy by electromagnetic
waves or photons, which do not require a medium, such as air, for their transmission.
Thus, radiative energy transfer can occur even when no atmosphere exists, such as above
the moon’s surface. Conduction cannot occur above the moon’s surface because too few
molecules are present in the moon’s atmosphere to transfer energy away from its surface.
Similarly, neither convection nor advection can occur in the moon’s atmosphere.
3.3. TEMPERATURE STRUCTURE OF THE ATMOSPHERE
The bottom 100 km of the Earth’s atmosphere is called the homosphere, which is a
region in which major gases are well mixed. The homosphere is divided into four lay-
ers in which temperatures change with altitude. These are, from bottom to top, the
troposphere, stratosphere, mesosphere, and thermosphere. Figure 3.3 shows an
average profile of the temperature structure of the homosphere.
3.3.1. Troposphere
The troposphere is divided into the boundary layer (ignored in Fig. 3.3) and the free
troposphere. These regions are briefly discussed as follows.
3.3.1.1. Boundary Layer
The boundary layer extends from the surface to between 500 and 3,000 m alti-
tude. All humans live in the boundary layer, so it is this region of the atmosphere in
which pollution buildup is of the most concern. Pollutants emitted near the ground
accumulate in the boundary layer. When pollutants escape the boundary layer, they can
travel horizontally long distances before they are removed from the air. The boundary
54 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION
layer differs from the free troposphere in that the temperature profile in the boundary
layer responds to changes in ground temperatures over a period of less than an hour,
whereas the temperature profile in the free troposphere responds to changes in ground
temperatures over a longer period (Stull, 1988).
Figure 3.4 shows a typical temperature variation in the boundary layer over
land during the day and night. During the day [Fig. 3.4(a)], the boundary layer is
characterized by a surface layer, a convective mixed layer, and an entrainment zone.
The surface layer, which comprises the bottom 10 percent of the boundary layer, is
a region of strong change of wind speed with height (wind shear). Because the
boundary-layer depth ranges from 500 to 3000 m, the surface layer is about 50 to
STRUCTURE AND COMPOSITION OF THE PRESENT-DAY ATMOSPHERE 55
0
10
20
30
40
50
60
70
80
90
100
180 200 220 240 260 280 300
1,013
265
55
12
2.9
0.8
0.22
0.052
0.011
0.0018
0.00032
Altitude (km)
Temperature (K)
Tropopause
Stratopause
Mesopause
Stratosphere
Troposphere
Mesosphere
Thermosphere
Ozone
layer
Pressure (mb)
Figure 3.3. Temperature structure of the Earth’s lower atmosphere, ignoring the boundary
layer.
Cloud layer
Entrainment zone/
Inversion layer
Free troposphere
Convective mixed layer
Surface layer
Subcloud layer
Daytime temperature
Altitude
Boundary layer
Entrainment zone/
Inversion layer
Free troposphere
Surface layer
Nighttime temperature
Residual layer
Nocturnal
boundary
layer
Altitude
Boundary layer
(a) (b)
Figure 3.4. Variation of temperature with height during the (a) day and (b) night in the atmos-
pheric boundary layer over land under a high-pressure system. Adapted from Stull (1988).
300 m thick. Wind shear occurs in the surface layer simply because wind speeds at the
ground are zero and those above the ground are not.
The convective mixed layer is the region of air just above the surface layer.
When sunlight warms the ground during the day, some of the energy is transferred
from the ground to the air just above the ground by conduction. Because the air above
the ground is now warm, it rises buoyantly as a thermal. Thermals originating from the
surface layer rise and gain their maximum acceleration in the convective mixed layer.
As thermals rise, they displace cooler air aloft downward; thus, upward and downward
motions occur, allowing air and pollutants to mix in this layer.
The top of the mixed layer is often bounded by a temperature inversion,which is
an increase in temperature with increasing height. The inversion inhibits the rise of ther-
mals originating from the surface layer or the mixed layer. Some mixing (entrainment)
between the inversion and mixed layer does occur; thus, the inversion layer is called an
entrainment zone. Pollutants are generally trapped beneath or within an inversion; thus,
the closer the inversion is to the ground, the higher pollutant concentrations become.
Other features of the daytime boundary layer are the cloud and subcloud layers.
A
region in which clouds appear in the boundary layer is the cloud layer, and the region
underneath is the subcloud layer.
During the night [Fig. 3.4(b)], the ground cools radiatively, causing air tempera-
tures to increase with increasing height from the ground, creating a surface inversion.
Once the nighttime surface inversion forms, pollutants, when emitted, are confined to
the surface layer.
Cooling at the top of the surface layer at night cools the bottom of the mixed layer,
reducing the buoyancy and associated mixing at the base of the mixed layer. The portion
of the daytime mixed layer that loses its buoyancy at night is the nocturnal boundary
layer. The remaining portion of the mixed layer is the residual layer. Because thermals
do not form at night, the residual layer does not undergo much change at night, except at
its base. At night, the nocturnal boundary layer thickens, eroding the residual-layer base.
Above the residual layer, the inversion remains.
3.3.1.2. Free Troposphere
The free troposphere lies between the boundary layer and the tropopause. It is a
region in which, on average, the temperature decreases with increasing altitude. The
average rate of temperature decrease in the free troposphere is about 6.5 K km
1
. The
temperature decreases with increasing altitude in the free troposphere for the following
reason: The ground surface receives energy from the sun daily, heating the ground, but
the top of the troposphere continuously radiates energy upward, cooling the upper
troposphere. The troposphere, itself, has relatively little capacity to absorb solar ener-
gy; thus, it relies on energy-transfer processes from the ground to maintain its
temperature. Convective thermals from the surface transfer energy upward, but as
these thermals rise into regions of lower pressure, they expand and cool, resulting in a
decrease of temperature with increasing height in the troposphere.
The tropopause is the upper boundary of the troposphere. Above the tropopause
base, temperatures are relatively constant with increasing height before increasing
with increasing height in the stratosphere.
Figure 3.5(a) and (b) show zonally averaged temperatures for a generic January and
July, respectively. A zonally averaged temperature is found by averaging temperatures
over all longitudes at a given latitude and altitude. The figure indicates that tropopause
heights are higher (15 to 18 km) over the equator than over the poles (8 to 10 km). Strong
56 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION
vertical motions over the equator raise the base of the ozone layer there. Because ozone is
responsible for warming above the tropopause, pushing ozone to greater heights over the
equator increases the altitude at which warming begins. Near the poles, downward motions
push stratospheric ozone downward, lowering the tropopause height over the poles.
Temperatures at the tropopause over the equator are colder than they are over the
poles. One reason is that the higher base of the ozone layer over the equator allows
tropospheric temperatures to cool to a greater altitude over the equator than over the
poles. A second reason is that lower- and mid-tropospheric water vapor contents are
much higher over the equator than they are over the poles. Water vapor absorbs
thermal-IR radiation emitted from the Earth’s surface, preventing that radiation from
reaching the upper troposphere.
3.3.2. Stratosphere
The stratosphere is a large temperature inversion. The inversion is caused by ozone, which
absorbs the sun’s UV radiation and reemits thermal-IR radiation, heating the stratosphere.
Peak stratospheric temperatures occur at the top of the stratosphere because this is the alti-
tude at which ozone absorbs the shortest UV wavelengths reaching the stratosphere (about
0.175 m). Although the ozone content at the top of the stratosphere is low, each ozone
molecule can absorb short wavelengths, increasing the average kinetic energy and, thus,
temperature (through Equation 3.1) of all molecules. Short UV wavelengths do not pene-
trate to the lower stratosphere. Ozone densities in the stratosphere peak at 25 to 32 km.
3.3.3. Mesosphere
Temperatures decrease with increasing altitude in the mesosphere just as they do in
the free troposphere. Ozone densities are too low, in comparison with those of oxygen
and nitrogen, for ozone absorption of UV radiation to affect the average temperature
of all molecules in the mesosphere.
STRUCTURE AND COMPOSITION OF THE PRESENT-DAY ATMOSPHERE 57
200
280
180
160
290
190
240
220
210
220
200
240
260
220
210
-80 -60 -40 -20 0 20 40 60
100
80
60
40
20
0
Latitude (deg)
Altitude (km)
270
180
160
210
290
140200
220
240
220
260
260
240
220
200
210
-80 -60-40 -20 0 20 40 60 80
100
80
60
40
20
0
Latitude (deg)
Altitude (km)
(a) (b)
80
Figure 3.5. Zonally and monthly averaged temperatures for (a) January and (b) July. The thick
solid lines are, from bottom to top, the tropopause, stratopause, and mesopause, respectively.
Data for the plots were compiled by Fleming et al. (1988).