Visualizing Environmental Science

196 CHAPTER 8 Air and Air Pollution

Types and Sources of Air Pollution

LEARNING OBJECTIVES

1. Define air pollution and distinguish between
   primary and secondary air pollutants.


2. List the seven major classes of air pollutants
   and describe their characteristics and sources.

A

ir pollution can come from natural sources,
such as smoke from a forest fire ignited by
lightning or gases from an erupting volcano.
However, human activities release many kinds
of substances into the atmosphere and contribute greatly
to global air pollution. Some of these substances are harm-
ful when they are inhaled or settle on land and surface
waters, and some substances are
harmful b ecause they alter the
chemistry of the atmosphere.
Although many different air
pollutants exist, we focus on the
seven most important classes from
a regulatory perspective: par-
ticulate matter, nitrogen oxides,
sulfur oxides, carbon oxides, hy-
drocarbons, ozone, and air toxics.
Air pollutants are often di-
vided into two categories, pri -
mary and secondary (Figure 8.6).
Primary air pollutants are released
directly from a source into the at-
mosphere. They include carbon
oxides, nitrogen oxides, sulfur
dioxide, particulate matter, and
hydrocarbons.
Ozone, sulfur trioxide, and
several acids are called secondary
air pollutants because they are
formed from chemical reactions
that take place in the atmosphere.

Major Classes of Air Pollutants

Particulate matter consists of dusts and mists— thousands
of different types of solid and liquid particles suspended
in the atmosphere. Particulate matter includes soil parti-
cles, soot, lead, asbestos, sea salt, and sulfuric acid droplets.
Some particulate matter has toxic or c arcinogenic effects.

air pollution

Various chemicals

(gases, liquids, or

solids) present in

the atmosphere in

high enough levels

to harm humans,

other organisms, or

materials.

primary air

pollutants Harmful

chemicals that enter

directly into the

atmosphere due to

either human activities

or natural processes.

secondary air

pollutants Harmful

chemicals that form in

the atmosphere when

primary air pollutants

react chemically with

one another or with

natural components

of the atmosphere.

Human sources

Natural

sources

Primary air pollutants Secondary air pollutants

Most hydrocarbons

Most particulates

CO

NO NO 2

CO 2 HNO^2

SO 2 HNOH 2 O 23

SO 3

O 3 PANs

H 2 SO 4

salts

MostNO 3 and SO 42

Primary and secondary air pollutants

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Primary air pollutants are emitted, unchanged, from a source

directly into the atmosphere, whereas secondary air pollutants are

produced from chemical reactions involving primary air pollutants.

Particulate matter can scatter and absorb sunlight,

r educing visibility. Urban areas receive less sunlight

than rural areas, partly as a result of greater quantities

of particulate matter in the air. Particulate matter can

corrode metals, erode buildings and sculptures when

the air is humid, and soil clothing and draperies. Lead

particles, which are heavy, tend to travel relatively short

distances before settling on the ground or a water

surface.

All particulate matter eventually settles out of the

atmosphere, but microscopic particles can remain

suspended in the atmosphere for weeks or even years.

Trace amounts of hundreds of different chemicals bind

to these microscopic particles; inhaling the particles

i ntroduces the chemicals, some of which are toxic, into

the human body. Microscopic particles are considered