Page 338 - Fundamentals of Air Pollution 3E

294 19. Transport and Dispersion of Air Pollutants

eddies in a wide range of sizes. The energy cascades through the eddy
sizes, which are described by L. F. Richardson in verse:
Big whirls have little whirls that feed on their velocity
And little whirls have lesser whirls and so on to viscosity.
There are basically two different causes of turbulent eddies. Eddies set in
motion by air moving past objects are the result of mechanical turbulence.
Parcels of superheated air rising from the heated earth's surface, and the
slower descent of a larger portion of the atmosphere surrounding these
more rapidly rising parcels, result in thermal turbulence. The size and,
hence, the scale of the eddies caused by thermal turbulence are larger than
those of the eddies caused by mechanical turbulence.
The manifestation of turbulent eddies is gustiness and is displayed in
the fluctuations seen on a continuous record of wind or temperature. Figure
19-3 displays wind direction traces during (a) mechanical and (b) thermal
turbulence. Fluctuations due to mechanical turbulence tend to be quite
regular; that is, eddies of nearly constant size are generated. The eddies
generated by thermal turbulence are both larger and more variable in size
than those due to mechanical turbulence.
The most important mixing process in the atmosphere which causes the
dispersion of air pollutants is called eddy diffusion. The atmospheric eddies
cause a breaking apart of atmospheric parcels which mixes polluted air
with relatively unpolluted air, causing polluted air at lower and lower
concentrations to occupy successively larger volumes of air. Eddy or turbu-
lent dispersion is most efficient when the scale of the eddy is similar to
that of the pollutant puff or plume being diluted. Smaller eddies are effective
only at tearing at the edges of the pollutant mass. On the other hand,
larger eddies will usually only transport the mass of polluted air as a whole.
The size and influence of eddies on the vertical expansion of continuous
plumes have been related to vertical temperature structure (3). Three ap-

Fig. 19-3. Examples of turbulence on wind direction records: (a) mechanical, (b) thermal.
Source: From Smith (2).