Page 56 - Fundamentals of Air Pollution

32 2. The Natural versus Polluted Atmosphere

Fig. 2-5. Transport and diffusion from source to receptor.

straight line between the source and the receptor, but may meander some-
what about that line as wind direction fluctuates from its mean value over
the time of transit between the two points)—tend to make the concentration
of the plume as it arrives at the receptor less than its concentration on
release from the stack. The sum of all these processes is called diffusion.
The process of diffusion becomes increasingly complex as the number of
sources and receptors increases; as sources and receptors begin to group
together into towns and cities; as some of the sources and receptors move,
e.g., vehicles; and, finally, as the weather and topography become more
complex than a wind blowing in one direction over a flat desert for a
prolonged period of time.
If the plume being transported is above the height where ground-based
convective and turbulent processes will bring it down to the ground reason-
ably close to its origin, it may travel for hundreds of miles at that height
before being brought to earth, by these processes, in a remote community.
This is known as long-range or long-distance transport.

D. Significant Deterioration of Air Quality
It may be desirable to curtail transport of pollution to areas whose air is
presently quite clean, even though, after such transport, the air quality of
the area would be considerably cleaner than would be required by air quality
standards. This concept is called prevention of significant deterioration of
the air quality in such areas. It requires definition of how much deterioration
can be considered insignificant.

E. Polluted Atmosphere
Sections II and III describe the types and form of gases, vapors, and
particles in the atmosphere. Definitions of terms are offered at the beginning