Jacobson M.Z. Atmospheric Pollution
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The opposite of an adiabatic expansion is an adiabatic compression, which occurs
when a balloon sinks adiabatically from low to high pressure. The compression is due
to the increased air pressure around the balloon. During an adiabatic compression,
work is converted to kinetic energy (which is proportional to temperature), warming
the air in the balloon.
Whereas dry and wet adiabatic lapse rates describe the extent of cooling of a
balloon rising adiabatically, the environmental lapse rate describes the actual
change in air temperature with altitude in the environment outside a balloon. It is
defined as
e
T
z
(6.1)
where T/z is the actual change in air temperature with altitude. An increasing
temperature with increasing altitude gives a negative environmental lapse rate.
6.6.1.2. Stability
One purpose of examining the dry, wet, and environmental lapse rates is to deter-
mine the stability of the air, where the stability is a measure of whether pollutants
emitted will convectively rise and disperse or build up in concentration near the
surface.
Figure 6.8 illustrates the concept of stability. When an unsaturated parcel of air
(the balloon, in our example) is displaced vertically, it rises, expands, and cools dry
adiabatically (along the dashed line in the figure). If the environmental temperature
profile is stable (right thick line), the rising parcel is cooler and more dense than is the
air in the environment around it at every altitude. As a result of its lack of buoyancy,
the parcel sinks, compresses, and warms until its temperature (and density) equal that
158 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION
EXAMPLE 6.1.
If the observed temperature cools 14 K betw
een the ground and 2 km abo
ve the ground, what is the
environmental lapse rate, and is it larger or smaller than the dry adiabatic lapse rate?
Solution
The environmental lapse rate in this example is
e
7K/km, which is less than the dry adiabatic
lapse rate of
d
9.8 K/km.
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
100 5 15 20 25 30
Altitude (km)
Temperature (°C)
Stable
Unstable
Γ
d
Γ
d
Γ
e
Γ
e
Figure 6.8. Stability and instability in unsaturated air, as described in the text.
of the air around it. In reality, the parcel overshoots its original altitude as it descends,
but eventually obtains that altitude in an oscillatory manner. In sum, a parcel of pollu-
tion at the temperature of the environment that is accelerated vertically in stable air
returns back to its original altitude and temperature. Stable air is associated with near-
surface pollution buildup because pollutants perturbed vertically in stable air cannot
rise and disperse.
In unstable air, an unsaturated parcel that is perturbed vertically continues to accel-
erate in the direction of the perturbation. Unstable air is associated with near-surface
pollutant cleansing. If the environmental temperature profile is unstable (left thick line
in Fig. 6.8), a parcel rising adiabatically (along the dashed line) is warmer and less
dense than is the environment around it at every altitude, and the parcel continues to
accelerate. The parcel stops accelerating only when it encounters air with the same tem-
perature (and density) as the parcel. This occurs when the parcel reaches a layer with a
new environmental lapse rate.
In neutral air (when the dry and environmental lapse rates are equal), an unsatu-
rated parcel that is perturbed v
ertically neither accelerates nor decelerates, but
continues along the direction of its initial perturbation at a constant velocity.
Neutral air results in pollution dilution slower than in unstable air but faster than in
stable air.
Whether unsaturated air is stable or unstable can be determined by comparing the
dry adiabatic lapse rate with the environmental lapse rate. Symbolically, the stability
criteria are
d
dry unstable
e
{
d
dry neutral (6.2)
d
dry stable
If the air is saturated, such as in a cloud, the wet adiabatic lapse rate is used to
determine stability. In such a case, the stability criteria are
w
wet unstable
e
{
w
wet neutral (6.3)
w
wet stable
Although stability at any point in space and time depends on
d
or
w
, but not both,
generalized stability criteria for all temperature profiles are often summarized as
follows:
e
d
absolutely unstable
e
d
dry neutral
{
d
e
w
conditionally unstable (6.4)
e
w
wet neutral
e
w
absolutely stable
These conditions indicate that when
e
d
, the air is absolutely unstable, or unsta-
ble regardless of whether the air is saturated or unsaturated. Conversely, if
e
w
, the
air is absolutely stable, or stable regardless of whether the air is saturated. If the air is
conditional unstable, stability depends on whether the air is saturated. Figure 6.9
illustrates the stability criteria in Equation 6.4.
EFFECTS OF METEOROLOGY ON AIR POLLUTION 159
In thermal low-pressure systems, sunlight warms the surface. The surface energy is
conducted to the air, warming the lower boundary layer, decreasing the stability of the
boundary layer. When the temperature profile near the surface becomes unstable, con-
vective thermals buoyantly rise from the surface, carrying pollution with them.
In thermal high-pressure systems, radiative cooling of the surface stabilizes the
temperature profile, preventing near-surface air and pollutants from rising. In man
y
cases, air near the surface becomes so stable that a temperature inversion forms, fur-
ther inhibiting pollution dispersion. Inversions are discussed next.
6.6.1.3. Temperature Inversions
The stable environmental profile in Fig. 6.8 and Profile 4 in Fig. 6.9 are tempera-
ture inversions, increases in air temperature with increasing height. Inversions are
always stable, but a stable profile is not necessarily an inversion (e.g., Profile 3 in
Fig. 6.9). Inversions are important because they trap pollution near the surface to a
greater extent than does a stable temperature profile that is not an inversion.
An inversion is characterized by its strength, thickness, top/base height, and
top/base temperatures. The inversion strength is the difference between the tempera-
ture at the inversion top and that at its base. The inversion thickness is the difference
between the inversion’s top and base heights. The inversion base height is the height
from the ground to the bottom of the inversion. It is also called the mixing depth
because it is the estimated height to which pollutants released from the surface mix. In
reality, pollutants often mix into the inversion layer itself. Inversion layer characteristics
are illustrated in Fig. 6.10.
160 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
-202468101214
Altitude (km)
Temperature (°C)
Γ
w
Γ
d
Absolutely
stable
Absolutely
unstable
Conditionally
unstable
1
4
3
2
Figure 6.9. Stability criteria for unsaturated and saturated air. If air is saturated, the environ-
mental lapse rate is compared with the wet adiabatic lapse rate to determine stability.
Environmental lapse rates 3 and 4 are stable and 1 and 2 are unstable with respect to satu-
rated air. Environmental lapse rates 2, 3, and 4 are stable and 1 is unstable with respect to
unsaturated air. A rising or sinking air parcel follows the
d
line when the air is unsaturated
and the
w
line when the air is saturated.
EXAMPLE 6.2.
Given the environmental lapse rate from Example 6.1, determine the stability class of the atmosphere.
Solution
The environmental lapse rate in the example was
e
7K/km. Because the wet adiabatic lapse
rate ranges from
w
2 to 6 K/km, the atmosphere in this example is conditionally unstable
(Equation 6.4).
Stable air and inversions, in particular, trap pollutants, preventing them from dis-
persing into the free troposphere and causing pollutant concentrations to b
uild up near
the surface. Figure 6.11 illustrates how such trapping occurs. The figure shows two air
parcels with different initial temperatures released at the surface under an inversion.
Suppose the parcels represent exhaust plumes that are initially warmer than the environ-
ment. Due to their buoyancy, both parcels rise, expand, and cool at the dry adiabatic
lapse rate of near 10 C/km. The parcel released at 20 C rises and cools until its tem-
perature approaches that of the air around it. At that point the parcel decelerates, then
comes to rest after oscillating around its final altitude. The path of this parcel illustrates
how an inversion traps pollutants emitted from the surface. It also illustrates that pollu-
tants often penetrate into the inversion layer. The parcel released at 30 C also rises and
cools, but passes easily through the inversion layer. Because the free troposphere above
the inversion is stable, the parcel ultimately comes to rest above the inversion.
Inversions form whenever a layer of air becomes colder than the layer of air
above it. Common inversion types include the radiation inversion, the large-scale sub-
sidence inversion, the marine inversion, the frontal inversion, and the small-scale
subsidence inversion. These are discussed next.
EFFECTS OF METEOROLOGY ON AIR POLLUTION 161
0
0.5
1.0
1.5
2.0
5 10152025
Altitude (km)
Temperature (°C)
Base
Top
Thickness (km)
Strength (°C)
Mixing depth
0
0.5
1.0
1.5
2.0
2.5
3.0
0 5 10 15 20 25 30
Altitude (km)
Temperature (°C)
Γ
e
Figure 6.10. A temperature inversion.
Figure 6.11. Schematic of pollutants trapped by an inversion and stable air. The parcel of air
released at 20 C rises at the unsaturated adiabatic lapse rate of 10 C/km until its temper-
ature equals that of the environment. This parcel is trapped by the inversion. The parcel of
air released at 30 also rises at 10 C/km. It escapes the inversion, but stops rising in sta-
ble free-tropospheric air, where the environmental lapse rate is 6.5 C/km.
Radiation Inversion
Radiation (nocturnal) inversions occur nightly as land cools by emitting thermal-
IR radiation. During the day, land also emits thermal-IR radiation, but this loss is
exceeded by a gain in solar radiation. At night, thermal-IR emissions cool the ground,
which in turn cools molecular layers of air above the ground, creating an inversion. The
strength of a radiation inversion is maximized during
long, calm, cloud-free nights when the air is dry.
Long nights maximize the time during which thermal-
IR cooling occurs, calm nights minimize downward
turbulent mixing of energy, cloud-free nights mini-
mize absorption of thermal-IR energy by cloud drops,
and dry air minimizes absorption of thermal-IR ener-
gy by water vapor. The morning temperature profile
in Fig. 6.12 shows a radiation inversion. Radiation
inversions also form in the winter during the day in
regions that are not exposed to much sunlight. They
do not form regularly over the ocean because ocean
water cools only slightly at night.
Large-Scale Subsidence In
version
A large-scale subsidence inversion occurs with-
in a surface high-pressure system. In such a system,
air descends, compressing and warming adiabatical-
ly. When a layer of air descends adiabatically, the
entire layer becomes more stable,
often to the point
that an inversion forms. The descension of air and
creation of an inversion in a high-pressure system
both contribute to near-surface pollution buildup.
Figure 6.13 illustrates the formation of a subsidence inversion. The figure shows a
1.37-km thick layer of air based at 3 km. At this altitude, the pressure thickness of the
layer is 114 mb. The initial temperature profile of the layer is stable, but not an inver-
sion. As the layer descends, both its top and bottom compress and warm adiabatically
at the rate of about 10 C/km. Whereas the pressure thickness of the layer remains
constant during descension to conserve mass, the height thickness of the layer
162 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION
260 270 280 290 300 310 320
500
600
700
800
900
1,000
Temperature (K)
Pressure (mb)
Morgan Hill
8/06/90
15:30 PST
03:30 PST
Figure 6.12. Observed temperature profiles in
the early morning and late afternoon at
Morgan Hill, California, on August 6, 1990.
The morning sounding shows a radiation
inversion coupled with a large-scale subsi-
dence inversion. The afternoon sounding
shows a large-scale subsidence inversion.
Figure 6.13. Formation of a subsidence inversion in sinking unsaturated air, as described in
the text.
0
1
2
3
4
5
-30 -20 -10 0 10 20 30
1,013
899
795
701
617
540
Altitude (km)
Temperature (°C)
Γ
d
Initial temperature profile
Final temperature
profile
Γ
d
Pressure (mb)
decreases with decreasing altitude (due to compression) so that at the surface, the pres-
sure thickness of the layer is still 114 mb, but the height thickness is 1 km. Figure 6.13
shows that the final temperature profile in the layer is more stable than is the initial
profile. In fact, the sinking of air created an inversion. Conversely, the lifting of an
unsaturated layer of air decreases the layer’s stability.
Over land at night, air in a large-scale pressure system can descend and warm on
top of near-surface air that has been cooled radiatively, creating a combined radiation/
large-scale subsidence inversion. The morning inversion in Fig. 6.12 shows such a
case. The afternoon profile in the same figure shows the contribution of the large-scale
subsidence inversion. The subsidence inversion in Fig. 6.12 w
as due to the Paci
fic
high, well-known for producing large-scale subsidence inversions. Such inversions are
present 85 to 95 percent of the days of the year in Los Angeles, which is one reason
this city has historically had severe air pollution problems.
Figure 6.12 indicates that between morning and afternoon the radiation inversion
eroded, so that by the afternoon all that remained was the large-scale subsidence inver-
sion. This process is illustrated by Fig. 6.14. The schematic shows that, at 03:00, a
strong radiation/large-scale subsidence inversion exists. At 06:00, after the sun rises, the
ground and near-surface air heat sufficiently to chip away at the bottom of the inversion.
The chipping continues until the late afternoon, when the inversion base height (mixing
depth) reaches a maximum. As the sun goes down and surface temperature cools, the
inversion base height decreases again. If the ground in Fig. 6.14(a) were heated to 35 C
instead of 30 C in the late afternoon, the inversion in the figure would disappear. The
elimination of an inversion due to surface heating is called popping the inversion.
A large-scale subsidence inversion serves as a lid to confine pollution beneath it, as
shown in Fig. 6.15. The mixing depth in which pollution is trapped is generally thick-
est in the late afternoon and thinnest during the night and early morning. Mixing ratios
of primary pollutants, such as CO(g), NO(g), primary ROGs, and primary aerosol par-
ticles usually peak during the morning, when mixing depths are thin and rush hour
emission rates are high. Although emission rates are also high during afternoon rush
hours, afternoon mixing depths are thick, diluting primary pollutants. Afternoon rush
hours are also spread over more hours than are morning rush hours. Despite thick
afternoon mixing depths, some secondary pollutants, such as O
3
(g), PAN(g), and sec-
ondary (aged) aerosol particles, reach their peak mixing ratios in the afternoon, when
their chemical formation rates peak.
EFFECTS OF METEOROLOGY ON AIR POLLUTION 163
0
0.5
1.0
1.5
2.0
5 101520253035
Altitude (km)
Temperature (°C)
06:00 09:00
12:00 16:00
03:00
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
6 8 10 12 14 16 18
Inversion base height (km)
Hour of day
(a) (b)
Figure 6.14. (a) Schematic showing the gradual chipping away of a morning radiation/large-scale subsi-
dence inversion to produce an afternoon large-scale subsidence inversion. (b) Change in the inversion
base height during the day corresponding to the chipping-away process in (a).
Figure 6.16 illustrates the seasonal variation of afternoon inversion profiles in Los
Angeles. During the winter, the Pacific high is further from Los Angeles than it is dur-
ing any other season, and the large-scale subsidence inversion strength is weak. During
the summer, the inversion is strong because the center of the Pacific high is closer to
Los Angeles than it is during any other season.
Marine Inversion
A marine inversion occurs over coastal areas. During the day, land heats faster than
does water. The rising air over land decreases near-surface air pressures, drawing
ocean air inland. The resulting breeze between ocean and land is the sea breeze. As
cool, marine air moves inland during a sea breeze, it forces warm, inland air to rise,
creating warm air over cold air and a marine inversion, which contributes to the
inversion strength dominated by the large-scale subsidence inversion in Los Angeles.
164 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION
0
0.5
1.0
1.5
2.0
2.5
3.0
5 1015202530
Altitude (km)
Temperature (°C)
Summer
Spring
Winter
Figure 6.16. Schematic showing seasonal variation of afternoon inversions in Los Angeles.
Figure 6.15. Photograph of pollution trapped under a large-scale subsidence inversion in Los
Angeles, California, on the afternoon of July 23, 2000.
Small-Scale Subsidence Inversion
As air flows down a mountain slope, it compresses and warms adiabatically, just as
in a large-scale subsidence inversion. When the air compresses and warms on top of
cool air, a small-scale subsidence inversion forms. In Los Angeles, air sometimes
flows from east to west, over the San Bernardino Mountains, into the Los Angeles
Basin. Such air compresses and warms as it descends the mountain onto cool marine
air below it, creating an inversion.
Frontal Inversion
A cold front is the leading edge of a cold air mass and the boundary between a
cold air mass and a warm air mass. An air mass is a large body of air with similar
temperature and moisture characteristics. Cold fronts and warm fronts form around
low-pressure centers, particularly in the cyclones that predominate at 60 N. Cold
fronts rotate counterclockwise around a surface cyclone. At a cold front, cold, dense
air acts as a wedge and forces air in the warm air mass to rise, creating warm air o
ver
cold air and a frontal inversion. Because frontal inversions occur in low-pressure
systems, where air generally rises and clouds form, frontal inversions are not usually
associated with pollution buildup.
6.6.2. Horizontal Pollutant Transport
Large-scale pressure systems affect the direction and speed of winds, which, in turn,
affect pollutant transport. In this subsection, large-scale winds and their effects on
pollution are examined. Small-scale winds are discussed in Section 6.7.4.
6.6.2.1. Effects of Wind Speeds on Pollutants
Winds around a surface low-pressure systems are generally faster than are those
around a surface high-pressure systems, partly because pressure gradients in a low-
pressure system are generally stronger than are those in a high-pressure system.
Fast winds tend to clear out chemically produced pollution faster than do slow
winds, but fast winds resuspend more soil dust and other aerosol particles from the
ground than do slow winds. Most soil-dust resuspension occurs when the soil is
bare, as illustrated in Fig. 6.17.
6.6.2.2. Effects of Wind Direction on Pollutants
Large-scale pressure systems redirect air pollution. When a surface low-pressure
center in the Northern Hemisphere is located to the west of a location, it produces
southerly to southwesterly winds at the location, as seen in Fig. 6.4(a). Similarly, surface
high-pressure centers located to the west of a location produces northwesterly to northerly
winds at the location, as seen in Fig. 6.4(b). In summers, the Pacific high is sometimes
located to the northwest of the San Francisco Bay Area. Under such conditions, air travel-
ing clockwise around the high passes through the Sacramento Valley, where temperatures
are hot and pollutant concentrations are high, into the Bay Area, where temperatures are
usually cooler and pollution also exists. The transport of hot,
polluted air from the
Sacramento Valley increases temperatures and pollution levels in the Bay Area.
6.6.2.3. Santa Ana Winds
In autumn and winter, the Canadian high-pressure system forms over the Great
Basin due to cold surface temperatures. Winds around the high flow clockwise down
EFFECTS OF METEOROLOGY ON AIR POLLUTION 165
the Rocky Mountains, compressing and warming adiabatically. The winds then travel
through Utah, New Mexico, Nevada, and into southern California. As the winds travel
over the Mojave Desert, they heat further and pick up desert soil dust. The winds then
reach the San Gabriel and San Bernardino Mountain Ranges, which enclose the
Los Angeles Basin. Two passes into the basin are Cajon Pass and Banning Pass. As
winds are compressed through these passes, they speed up to conserve momentum
(mass times velocity). The fast winds resuspend soil dust.
The winds, called the
Santa
Ana winds, then enter the Los Angeles Basin, bringing dust with them. If the winds
are strong, they overpower the sea breeze (which blows from ocean to land), clearing
pollution out of the basin to the ocean. Warm, dry, strong Santa Ana Winds are also
responsible for spreading brush fires in the basin. When the Santa Ana Winds are
weak, they are countered by the sea breeze, resulting in stagnation. Some of the w
orst
air pollution events in the Los Angeles Basin occur under weak Santa Ana conditions.
In such cases, pollution builds up over a period of several days. Santa Ana Winds are
generally strongest between October and December.
6.6.2.4. Long-Range Transport of Air Pollutants
Winds carry air pollution, sometimes over great distances. The chimney was devel-
oped centuries ago, not only to lift pollution above the ground, but also to take
advantage of winds aloft that disperse pollution horizontally. Chimneys exacerbate
pollution downwind of the point of emission. Starting in the seventeenth century, for
example, the Besshi copper mine and smelter on Shikoku Island, Japan, was a pollu-
tion source that damaged crops downwind. In the eighteenth century, smoke from soda
ash factory chimneys in France and Great Britain devastated nearby countrysides
166 ATMOSPHERIC POLLUTION: HISTORY, SCIENCE, AND REGULATION
Figure 6.17. Dust storm approaching Spearman, Texas, on April 14, 1935. (Courtesy National
Oceanic and Atmospheric Administration Central Library.)
(Chapter 10). In the 1930s to 1950s, a smelter in Trail, British Columbia, Canada,
released pollution that traveled to Washington State, in the United States. This last case
is an example of transboundary air pollution, which occurs when pollution crosses
political boundaries.
Since the 1950s, it has been recognized that sulfur dioxide, emitted from tall
smokestacks, is often carried long distances before it deposits to the ground as sulfuric
acid. Because most anthropogenic SO
2
(g) is emitted in midlatitudes, where the prevail-
ing near-surface winds are southwesterly and the prevailing elevated winds are
westerly, SO
2
(g) is transported to the northeast or east. If it is emitted high enough,
SO
2
(g) can travel hundreds to thousands of kilometers. The largest smokestack in the
world is located in Sudbury, Ontario, Canada, north of Lake Huron. This nickel-
smelting stack, which is 380 m tall, was designed to carry SO
2
(g) emissions far from
the local region. Not only have emissions from the stack devastated vast areas of land
immediately downwind of it, but the great height of the stack has enabled SO
2
(g) to be
transported long distances, including to the United States.
Long-range transport affects not only pollutants emitted from stacks,
but also pho-
tochemical smog closer to the ground. In 1987, Wisconsin felt that high mixing ratios
of ozone there were exacerbated by ozone transport from Illinois and Indiana.
Wisconsin filed a lawsuit to force Illinois and Indiana to control pollutant emissions
better. The lawsuit led to a settlement mandating a study of ozone transport pathways
(Gerritson, 1993).
Pollutants travel long distances along many other well-documented path-
ways. Pollutants from New York City, for example, travel to Mount Washington,
Vermont. Pollutants travel along the BoWash corridor between Boston and
Washington DC. Pollutants from the northeast United States travel to the clean north
Atlantic Ocean (Liu et al.,
1987; Dickerson et al., 1995; Moody et al., 996; Levy et al.,
1997; Prados et al., 1999). Pollutants from Los Angeles travel northward to Santa
Barbara, northeastward to the San Joaquin Valley, southward to San Diego, and east-
ward to the Mojave Desert. Such pollutants have also been traced to the Grand
Canyon, Arizona (Poulos and Pielke, 1994). Pollutants from the San Francisco Bay
Area spill into the San Joaquin Valley through Altamont Pass.
An example of transboundary pollution is the transport of forest-fire smoke from
Indonesia to six other Asian countries in September 1997. Sulfur dioxide emissions
from China are also suspected of causing a portion of acid deposition problems in
Japan. Aerosol-particles and ozone precursors from Asia tra
vel long distances over the
Pacific Ocean to North America (Prospero and Savoie, 1989; Zhang et al., 1993; Song
and Carmichael, 1999; Jacob et al., 1999). Hydrocarbons, ozone, and PAN travel long
distances across Europe (Derwent and Jenkin, 1991) as do pollutants from Europe to
Africa (Kallos et al., 1998). Pollutants also travel between the United States and
Canada and between the United States and Mexico.
6.6.3. Cloud Cover
Clouds affect pollution in two major ways. First, they reduce the penetration of UV
radiation, therefore decreasing rates of photolysis below them. Second, pollutants
dissolve in cloud water and are either rained out or returned to the air upon cloud evap-
oration. Thus, rain-forming clouds help to cleanse the atmosphere. Because cloud
cover is often greater and mixing depths, higher in surface low-pressure systems than
they are in surface high-pressure systems, photochemical smog concentrations are
EFFECTS OF METEOROLOGY ON AIR POLLUTION 167