Page 22 - Air Pollution Control Engineering

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Air Quality and Pollution Control 3

pollution concentrations were extremely high worldwide, resulting in “episodes” of
high respiratory incidents in London, Rotterdam, Hamburg, Osaka, and New York. During
this period, people in many other cities in the United States experienced serious pollution-
related illnesses, and as a result, efforts to clean up the air were started in the cities of
Chicago, New York, Washington, DC, and Pittsburgh. The substitution of less smoky
fuels, such as natural gas and oil, for coal, for power production and for space heating
accounted for much of the subsequent improvement in air quality.
Air quality in the United States depends on the nature and amount of pollutants
emitted as well as the prevalent meteorological conditions. Air pollution problems in
the highly populated, industrialized cities of the eastern United States result mainly
from the release of sulfur oxides and particulates. In the western United States, air
pollution is related more to photochemical pollution (smog). The latter form of pol-
lution is an end product of the reaction of nitrogen oxides and hydrocarbons from
automobiles and other combustion sources with oxygen and each other, in the pres-
ence of sunlight, to form secondary pollutants such as ozone and PAN (peroxy acetyl
or acyl nitrates).
Temperature inversions effectively “put a lid over” the atmosphere so that emissions
are trapped in relatively small volumes and in correspondingly high concentrations. Los
Angeles, for example, often suffers a very stable temperature inversion and strong solar
input, both ideal conditions for the formation of highly localized smog. Rain and snow
wash out the air and deposit the pollutants on the soil and in water. “Acid rain” is the
result of gaseous sulfur oxides combining with rain water to form dilute sulfuric acid
and it occurs in many cities of eastern United States.

2. CHARACTERISTICS OF AIR POLLUTANTS
Air pollutants are divided into two main groups: particulates and gases. Because
particulates consist of solids and/or liquid material, air pollutants therefore encompass
all three basic forms of matter. Gaseous pollutants include gaseous forms of sulfur
and nitrogen. Gaseous SO is colorless, yet one can point to the bluish smoke leaving
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combustion operation stacks as SO or, more correctly, SO or sulfuric acid mist. Nitric
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oxide (NO) is another colorless gas generated in combustion processes; the brown color
observed a few miles downwind is nitrogen dioxide (NO ), the product of photochem-
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ical oxidation of NO. Although the properties of gases are adequately covered in basic
chemistry, physics, and thermodynamics courses, the physical behavior of particulates
is less likely to be understood. The remainder of this section is thus devoted to the
physical properties of particulate matter, not gaseous pollutants.
Particulates may be subdivided into several groups. Atmospheric particulates consist
of solid or liquid material with diameters smaller than about 50 µm (10 −6 m). Fine par-
ticulates are those with diameters smaller than 3 µm. The term “aerosol” is defined
specifically as particulates with diameters smaller than about 30–50 µm (this does not
refer to the large particulates from aerosol spray cans). Particulates with diameters
larger than 50 µm settle relatively quickly and do not remain in the ambient air.
The movement of small particles in gases can be accounted for by expressions
derived for specific size groups: (1) The smallest group is the molecular kinetic group
and includes particles with diameters much less than the mean free path of the gas