Visualizing Environmental Science

Types and Sources of Air Pollution 197

and damage stone and other materials. Sulfuric acid

and other sulfur oxides damage plants and irritate the

respiratory tracts of humans and other animals. Sulfur

oxides include:

more dangerous than larger particles because they are
inhaled more deeply into the lungs.
The Environmental Protection Agency (EPA)
samples microscopic particulate matter at 1,000 loca-
tions around the United States because its composi-
tion varies with location and season. Particulate matter
includes:

Pollutant Category Characteristics

Dust particles Primary Solid particles

Lead (Pb) Primary Solid particles

Sulfuric acid (H 2 SO 4 ) Secondary Liquid droplets

Pollutant Category Characteristics

Nitrogen dioxide (NO 2 ) Primary Reddish-brown gas

Nitric oxide (NO) Primary Colorless gas

Nitrogen oxides are gases produced by chemical
interactions between nitrogen and oxygen when a
source of energy, such as fuel combustion, produces
high temperatures. Collectively known as NOx,
nitrogen oxides consist mainly of nitric oxide (NO),
nitrogen dioxide (NO 2 ), and nitrous oxide (N 2 O).
Nitrogen oxides inhibit plant growth and, when in-
haled, aggravate health problems such as asthma.
They are involved in the production of photochemical
smog (discussed later in the chapter) and acid deposi-
tion (see Chapter 9). Nitrous oxide is associated with
global warming, and it depletes ozone in the strato-
sphere (again, see Chapter 9). Nitrogen oxides cause
metals to corrode and textiles to fade and deteriorate.
Nitrogen oxides include:

Sulfur oxide gases result from chemical interac-
tions between sulfur and oxygen. Sulfur dioxide (SO 2 ),
a colorless, nonflammable gas with a strong, irritating
odor, is emitted as a primary air pollutant. Sulfur triox-
ide (SO 3 ) is a secondary air pollutant that forms when
sulfur dioxide reacts with oxygen in the air. Sulfur tri-
oxide, in turn, reacts with water to form another sec-
ondary air pollutant, sulfuric acid. Sulfur oxides play a
major role in acid deposition, and they corrode metals

Carbon oxides are the gases carbon monoxide (CO)

and carbon dioxide (CO 2 ). Carbon monoxide is a color-

less, odorless, and tasteless gas produced in larger quanti-

ties than any other atmospheric pollutant except carbon

dioxide. Carbon monoxide is poisonous. Because hemo-

globin in blood has a stronger affinity for carbon mon-

oxide than for oxygen, carbon monoxide reduces the

blood’s ability to transport oxygen. Carbon dioxide, also

colorless, odorless, and tasteless, is associated with global

climate change. Carbon oxides are:

Hydrocarbons are a diverse group of organic com-

pounds that contain only the elements hydrogen and

carbon. Small hydrocarbon molecules, such as methane

(CH 4 ), are gaseous at room temperature. Methane is

colorless and odorless and is the principal component

of natural gas. (The odor of natural gas comes from

sulfur compounds deliberately added so that humans

can detect the gas’s presence.) Medium-sized hydrocar-

bons, such as benzene (C 6 H 6 ), are liquids at room tem-

perature, although many are volatile and may evaporate

easily. The largest hydrocarbons, such as the waxy fuel

paraffin, are solids at room temperature. The many dif-

ferent hydrocarbons have a variety of effects on human

and animal health. Some cause no adverse effects, some

injure the respiratory tract, and others cause cancer.

All except methane contribute to the production of

Pollutant Category Characteristics

Sulfur dioxide (SO 2 ) Primary Colorless gas with

strong odor

Sulfur trioxide (SO 3 ) Secondary Reactive colorless

gas

Pollutant Category Characteristics

Carbon monoxide (CO) Primary Colorless, odorless

gas

Carbon dioxide (CO 2 ) Primary Colorless, odorless

gas