Effects of Meteorology on Air Pollution - Air Pollution and Global Warming: History, Science, and Solutions - page 138

of the perturbation. Unstable air is associated with near-
surface pollutant cleansing. If the environmental tem-
perature profile is unstable (left thick line in Figure 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 temperature (and density) as the par-
cel. 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 unsaturated parcel that is perturbed
vertically neither accelerates nor decelerates, but con-
tinues 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

Example 6.2
Given the environmental lapse rate from Exam-
ple 6.1, determine the stability class of the atmo-
sphere.

Solution
The environmental lapse rate in the example was
e 7 ◦ Ckm −1 .Becausethewet adiabatic

lapse rate ranges from

=+

6 ◦ Ckm −1 ,
the atmosphere in this example is conditionally
unstable (Equation 6.4).

w =+ 2to +

In thermal low-pressure systems, sunlight warms the
surface. The surface energy is conducted to the air,
warming the lower boundary layer and decreasing the
stability of the boundary layer. When the temperature
profile near the surface becomes unstable, convective
thermals rise buoyantly from the surface, carrying pol-
lution with them.
In thermal high-pressure systems, radiative cooling
of the surface stabilizes the temperature profile, pre-
venting near-surface air and pollutants from rising. In
many cases, air near the surface becomes so stable that a
temperature inversion forms, further inhibiting vertical
pollution dispersion. Inversions are discussed next.

> d dry unstable

= d

e dry neutral (6.2)

< d

dry stable

If the air is saturated, such as in a cloud, the wet adia-
batic lapse rate is used to determine stability. In such a
case, the stability criteria are

6.6.1.3. Temperature Inversions
The stable environmental profile in Figure 6.8 and Pro-
file 4 in Figure 6.9 are both temperature inversions ,

> w wet unstable

e = w wet neutral (6.3)

2.2

< w wet stable

Conditionally
unstable

2

Although stability at any point in space and time
depends on

1.8

d w

w ,but not both, generalized stability
criteria for all temperature profiles are often summa-
rized as follows:

d or Absolutely

unstable

Absolutel y
stable

1.6

3

2

1.4

1.2

4

1

e > d absolutely unstable

1

e = d

dry neutral

0.8

d > e > w conditionally unstable (6.4)

-2 0 2 468 0 2 4

Temperature ( o C)

e = w wet neutral

e < w absolutely stable

Figure 6.9. Stability criteria for unsaturated and
saturated air. If air is saturated, the environmental
lapse rate is compared with the wet adiabatic lapse
rate to determine stability. Environmental lapse rates
3and4arestable, and 1 and 2 are unstable, with
respect to saturated 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

These conditions indicate that when e > d ,theair is

absolutely unstable or unstable, regardless of whether
the air is saturated or unsaturated. Conversely, if

e <
w ,theair is absolutely stable or stable, regardless
of whether the air is saturated. If the air is condition-
ally unstable ,stability depends on whether the air is
saturated. Figure 6.9 illustrates the stability criteria in
Equation 6.4.

d line when the air is unsaturated and the w line

when the air is saturated.