METEOROLOGY - AIR POLLUTION

METEOROLOGY - AIR POLLUTION

Factors That Affect Air Pollution

If you live in a region that periodically experiences photochemical smog, you may
have noticed that these episodes often occur with clear skies, light winds, and generally
warm sunny weather. Although this may be “typical” air pollution weather, it by no
means represents the only weather conditions necessary to produce high
concentrations of pollutants, as we will see in the following sections.

THE ROLE OF THE WIND

The wind speed plays a role in diluting pollution. When vast quantities of
pollutants are spewed into the air, the wind speed determines how quickly the pollutants
mix with the surrounding air and, of course, how fast they move away from their source.
Strong winds tend to lower the concentration of pollutants by spreading them apart as
they move downstream. Moreover, the stronger the wind, the more turbulent the air.
Turbulent air produces swirling eddies that dilute the pollutants by mixing them with the
cleaner surrounding air. Hence, when the wind dies down, pollutants are not readily
dispersed, and they tend to become more concentrated (see Figure 1).

A. B.

FIGURE 1. If each chimney emits a puff of smoke every second, then where the
wind speed is low (a), the smoke puffs are closer together and more concentrated.
Where the wind speed is greater (b), the smoke puffs are farther apart and more diluted
as turbulent eddies mix the smoke with the surrounding air.
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THE ROLE OF STABILITY AND INVERSIONS

Recall that the atmospheric

stability determines the extent to which
air will rise. Remember also that an
unstable atmosphere favors vertical air
currents, whereas a stable atmosphere
strongly resists upward vertical motions.
Consequently, smoke emitted into a
stable atmosphere tends to spread
horizontally, rather than mix vertically.
The stability of the atmosphere is
determined by the way the air
temperature changes with height (the
lapse rate). When the measured air
temperature decreases rapidly as we
move up into the atmosphere, the
FIGURE 2 (a) During the afternoon, atmosphere tends to be more unstable
when the atmosphere is most unstable, and pollutants tend to be mixed vertically
pollutants rise, mix, and disperse as illustrated in Figure 2a. If, however,
downwind. (b) At night when a radiation the measured air temperature either
inversion exists, pollutants from the decreases quite slowly as we ascend, or
shorter stacks are trapped within the actually increases with height (remember
inversion, while pollutants from the taller that this is called an inversion), the
stack, above the inversion, are able to atmosphere is stable. An inversion
rise and disperse downwind. represents an extremely stable
atmosphere where warm air lies above
cool air (Figure 2b). Any air parcel that attempts to rise into the inversion will, at some
point, be cooler and heavier (more dense) than the warmer air surrounding it. Hence,
the inversion acts like a lid on vertical air motions.
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The inversion depicted in Figure 2b is called a radiation (or surface) inversion.

This type of inversion typically forms during the night and early morning hours when the
sky is clear and the winds are light. As we saw recall, radiation inversions also tend to
be well developed during the long nights of winter.

In Figure 2b, notice that within the stable inversion, the smoke from the shorter
stacks does not rise very high, but spreads out, contaminating the area around it. In the
relatively unstable air above the inversion, smoke from the taller stack is able to rise and
become dispersed. Since radiation inversions are often rather shallow, it should be
apparent why taller chimneys have replaced many of the shorter ones. In fact, taller
chimneys disperse pollutants better than shorter ones even in the absence of a surface
inversion because the taller chimneys are able to mix pollutants throughout a greater
volume of air. Although these taller stacks do improve the air quality in their immediate
area, they may also contribute to the acid rain problem by allowing the pollutants to be
swept great distances downwind.

As the sun rises and the surface warms, the radiation inversion normally
weakens and disappears before noon. By afternoon, the atmosphere is sufficiently
unstable so that, with adequate winds, pollutants are able to disperse vertically (Fig.2b).
The changing atmospheric stability, from stable in the early morning to conditionally
unstable in the afternoon, can have a profound effect on the daily concentrations of
pollution in certain regions. For example, on a busy city street corner, carbon monoxide
levels can be considerably higher in the early morning than in the early afternoon (with
the same flow of traffic). Changes in atmospheric stability can also cause smoke plumes
from chimneys to change during the course of a day.

Radiation inversions normally last just a few hours, while subsidence

inversions may persist for several days or longer. Subsidence inversions, therefore,
are the ones commonly associated with major air pollution episodes. They form as the
air above a deep anticyclone slowly sinks (subsides) and warms.

A typical temperature profile of a subsidence inversion is shown in Figure 3.

Notice that in the relatively unstable air beneath the inversion, the pollutants are able to
mix vertically up to the inversion base. The stable air of the inversion, however, inhibits
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vertical mixing and acts like a lid on the pollution below, preventing it from entering into
the inversion.

FIGURE 3. The inversion layer acts as a lid on the pollutants below. If the
inversion lowers, the mixing depth decreases and the pollutants are concentrated within
a smaller volume.

In Fig. 3, the region of relatively unstable (well mixed) air that extends from the
surface to the base of the inversion is referred to as the mixing layer. The vertical
extent of the mixing layer is called the mixing depth. Observe that if the inversion rises,
the mixing depth increases and the pollutants would be dispersed throughout a greater
volume of air; if the inversion lowers, the mixing depth would de- crease and the
pollutants would become more concentrated, sometimes reaching unhealthy levels.
Since the atmosphere tends to be most unstable in the afternoon and most stable in the
early morning, we typically find the greatest mixing depth in the afternoon and the
shallowest one (if one exists at all) in the early morning. Consequently, during the day,
the top of the mixing layer may clearly be visible (Figure 4). Moreover, during take-off or
landing on day-light flights out of large urban areas, the top of the mixing layer may
sometimes be observed.
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FIGURE 4. A thick layer of polluted air is trapped in the valley. The top of the
polluted air marks the base of a subsidence inversion.

The position of the semipermanent Pacific high off the coast of California
contributes greatly to the air pollution in that region. The Pacific high promotes
subsiding air, which warms the air aloft. Surface winds around the high promote
upwelling of ocean water. Upwelling the rising of cold water from below makes the
surface water cool, which, in turn, cools the air above. Warm air aloft coupled with cool,
surface (marine) air together
produces a strong and persistent
subsidence inversion one that exists
80 to 90 percent of the time over the
city of Los Angeles between June and
October, the smoggy months. The
pollutants trapped within the cool
marine air are occasionally swept
eastward by a sea breeze. This action
FIGURE 5. The leading edge of cool, marine carries smog from the coastal regions
air carries pollutants into Riverside, into the interior valleys producing a
California, as an advancing smog front. smog front (see Figure 5).
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The Smokestack Plumes

A.)

We know that the stability

of the air (especially near the
surface) changes during the

B.) course of a day. These changes

can influence the pollution near
the ground as well as the
behavior of smoke leaving a
chimney. Figure 6 illustrates
different smoke plumes that can
C.) develop with adequate wind, but
different types of stability.

In Fig. 6a, it is early

morning, the winds are light, and
a radiation inversion extends
D.) from the surface to well above

D.) the height of the smokestack. In

this stable environment, there is
little up and down motion, so the
E.) smoke spreads horizontally
rather than vertically. When
E.)
viewed from above, the smoke
plume resembles the shape of a
fan. For this reason, it is referred
to as a fanning smoke plume.

FIGURE 6. As the vertical temperature profile

changes during the course of a day (a through e),
the pattern of smoke emitted from the stack
changes as well.
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Later in the morning, the surface air warms quickly and destabilizes as the
radiation inversion gradually disappears from the surface upward (Fig. 6b). However,
the air above the chimney is still stable, as indicated by the presence of the inversion.
Consequently, vertical motions are confined to the region near the surface. Hence, the
smoke mixes downwind, increasing the concentration of pollution at the surface
sometimes to dangerously high levels. This effect is called fumigation. Here again, we
can see why a taller smokestack is preferred. A taller stack extends upward into the
stable layer, producing a fanning plume that does not mix downward toward the ground.

If daytime heating of the ground continues, the depth of atmospheric instability

increases. Notice in Fig. 6c that the inversion has completely disappeared. Light-to-
moderate winds combine with rising and sinking air to cause the smoke to move up and
down in a wavy pattern, producing a looping smoke plume.

The continued rising of warm air and sinking of cool air can cause the
temperature profile to equal that of the dry adiabatic rate (Fig. 6d). In this neutral
atmosphere, vertical and horizontal motions are about equal, and the smoke from the
stack tends to take on the shape of a cone, forming a coning smoke plume.

After sunset, the ground cools rapidly and the radiation inversion reappears.
When the top of the inversion extends upward to slightly above the stack, stable air is
near the ground with neutral air above (Fig. 4e). Because the stable air in the inversion
prevents the smoke from mixing downward, the smoke is carried upward, producing a
lofting smoke plume. Thus, smoke plumes provide a clue to the stability of the
atmosphere, and knowing the stability yields important information about the dispersion
of pollutants.

Of course, other factors influence the dispersion of pollutants from a chimney,

including the pollutants’ temperature and exit velocity, wind speed and direction, and, as
we saw in an earlier section, the chimney’s height. Overall, taller chimneys, greater wind
speeds, and higher exit velocities result in a lower concentration of pollutants.