Page 55 - Fundamentals of Air Pollution

IV. Concepts 31

Oxidation of SO 2 is slow in a mixture of pure gases, but the rate is increased
by light, NO 2/ oxidants, and metallic oxides which act as catalysts for the
reaction. The formed acids can react with particulate matter or ammonia
to form salts.

B. Receptors

A receptor is something which is adversely affected by polluted air. A
receptor may be a person or animal that breathes the air and whose health
may be adversely affected thereby, or whose eyes may be irritated or whose
skin made dirty. It may be a tree or plant that dies, or the growth yield or
appearance of which is adversely affected. It may be some material such
as paper, leather, cloth, metal, stone, or paint that is affected. Some proper-
ties of the atmosphere itself, such as its ability to transmit radiant energy,
may be affected. Aquatic life in lakes and some soils are adversely affected
by acidification via acidic deposition.

C. Transport and Diffusion
Transport is the mechanism that moves the pollution from a source to a
receptor. The simplest source-receptor combination is that of an isolated
point source and an isolated receptor. A point source may best be visualized
as a chimney or stack emitting a pollutant into the air; the isolated point
source might be the stack of a smelter standing by itself in the middle of
a flat desert next to the body of ore it is smelting. The isolated receptor
might be a resort hotel 5 miles distant on the edge of the desert. The effluent
from the stack will flow directly from it to the receptor when the wind is
along the line connecting them (Fig. 2-5). The wind is the means by which
the pollution is transported from the source to the receptor. However,
during its transit over the 5 miles between the source and the receptor,
the plume does not remain a cylindrical tube of pollution of the same
diameter as the interior of the stack from which it was emitted. Instead,
as it travels over the 5-mile distance, turbulent eddies in the air and in the
plume move parcels from the edges of the plume into the surrounding air
and move parcels of surrounding air into the plume. If the wind speed is
greater than the speed of ejection from the stack, the wind will stretch out
the plume until the plume speed equals wind speed. These two pro-
cesses—mixing by turbulence and stretch-out of the plume, plus a third
one—meandering (which means that the plume may not follow a true