Page 346 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

324 11 Air Dispersion

Fig. 11.5 Diurnal cycle of air
temperature above ground

may extend to as high as thousands of meters, where the atmosphere above is very
stable. Now the ground level starts to cool down before sunset. And another cycle
begins.
When the meteorological conditions are unknown, Pasquill classes A–F can be
determined from the weather conditions and the wind speed measured at 10 m
above the ground, u 10 [18]. They are shown in Table 11.1.

11.2.6 Wind Speed

Wind speed and direction affect the dispersion of air pollutants. Air pollutants are
better dispersed in strong winds owing to the strong mixing effect, both horizontally
and vertically. Typical wind speed at ground level is no less than 1 m/s. Air
movement below this speed is referred to as calm air. Usually wind speed is lower
at the ground level than that at a higher elevation. In an unstable planetary boundary
layer vertical motion of air is significant, and it increases the ground level wind in
the early afternoon as a result of self-limiting instability.
Let’s start our analysis with a simple case, where wind is developed in an open
area smooth surface in an adiabatic atmosphere. Similar to the boundary layer
concept (see Chap. 2), there is also a boundary layer above the ground, which can
be as high as 500 m. The friction is negligible at higher elevation. In this case, the
region where ground friction plays a significant role is within the planetary
boundary layer. The ground level (surface) shear stress s 0 is defined as
2
s 0 ¼ q u ð11:15Þ
0
where u is called friction speed and q is the ground-level air density. Although the
0
term “friction velocity” is widely used in air dispersion modeling, rigorously