Encyclopedia of Environmental Science and Engineering, Volume I and II

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AIR POLLUTION SOURCES

Air pollution may be defined as the presence in the atmosphere

of any substance (or combination of substances) that is detri-

mental to human health and welfare; offensive or objectionable

to man, either externally or internally; or which by its presence

will directly or indirectly adversely affect the welfare of man.

(“Air Pollution,” Homer W. Parker, 1977.) The substances

present in the atmosphere which cause this detriment to health

and welfare are the air pollutants.

A considerable quantity of air pollution occurs naturally

as a consequence of such processes as soil erosion and volca-

nic eruptions. However, those pollutants which pose a threat

to human health and cause extensive damage to property are

primarily derived from activities associated with the devel-

opment of community living, as well as with the growth of

affluence and living standards in industrial societies. These

activities include the burning of fuel for heat and power, the

processing of materials for food and goods, and the disposal

of wastes.

Much of the materials which pollute our atmosphere rep-

resent valuable resources which are being wasted. We have

available today the technological means of controlling most

sources of air pollution. The cost of control however has been

estimated on the order of 10 to 20 percent of the world’s gross

national product. Moreover, full implementation of the con-

trol measures that would be necessary to achieve healthful

air quality in many of our large centers of population would

require significant changes in lifestyle in those areas.

POLLUTANT CLASSIFICATIONS

Air pollutants are numerous, each with its own peculiar charac-

teristics. Therefore it is usual to have these pollutants classified

by some design. Classification allows for the study of pollut-

ants in subgroups on the basis of some characteristic of interest

or concern and also provides an ordering which makes it easier

to formulate air pollution control programs. Accordingly, the

classification of air pollutants may be based on:

1. How the pollutants are borne into the atmosphere.


2. The physical state of the pollutant.


3. The molecular composition of the pollutants.


4. The nature of the problem or health threat associ-
   ated with the pollutants.

Classification According to the Method of Entry into

the Atmosphere

This classification contains two categories: (1) Primary and

(2) secondary.

Primary Pollutants Primary air pollutants are emitted into

the atmosphere directly from identifiable sources whether

from mechanical or chemical reaction processes. Examples

of such direct discharge from an identifiable source into the

atmosphere include the complete and incomplete combustion

of carbonaceous fuels from industrial processes and automo-

bile engines yielding carbon monoxide and carbon dioxide.

Secondary Pollutants These pollutants are those which are

formed as a result of some reaction in the atmosphere. This

reaction may occur between any combination of air pollut-

ants (including primary pollutants) and natural components

of the atmosphere. Some of these reactions require the pres-

ence of sunlight and are called photo-chemical reactions. An

example of such a reaction is the formation of ozone from the

interaction of organic and nitrous compounds in the presence

of sunlight.

Classification According to the Physical State of the

Pollutant

According to their state of matter, pollutants may be classi-

fied as: (1) gaseous and (2) particulate.

Gaseous Pollutants Most air pollutants exhibit gaseous prop-

erties in that they tend to obey gas laws, for example, there is a

predictable interrelationship between their pressure, volume and

temperature. In many ways these pollutants behave like air itself

and do not tend to settle out or condense over long periods.

However, they almost always undergo some form of

chemical transformation while resident in the atmosphere.

Approximately 90% of air pollutants are gaseous.

Particulate Pollutants Any pollutant that is not gaseous is

defined as a particulate pollutant or particulate whether they

exist in the form of finely divided solids or liquids. The larger

particulates after having been introduced into the air tend

to settle out quickly and affect lives and property near the

source. The smaller and lighter particles travel further away,

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