Visualizing Environmental Science

198 CHAPTER 8 Air and Air Pollution

Sam Abell/NG Image Collection

who live and work around chemical factories, incinerators,

or other facilities that produce or use them. To limit the

release of more than 180 HAPs, the Clean Air Act Amend-

ments of 1990 (discussed later in this chapter) regulate

the pollutant emissions of both large and small businesses.

Hazardous air pollutants include:

photochemical smog. Methane is linked to global warm-
ing. Hydrocarbons include:

Ozone (O 3 ) is a form of oxygen considered a pollut-
ant in one part of the atmosphere but an essential compo-
nent of another. In the stratosphere, oxygen reacts with
solar UV radiation to form ozone. Stratospheric ozone
protects Earth’s surface from receiving harmful levels of
solar UV radiation. Unfortunately, certain human-made
pollutants, such as chlorofluorocarbons (CFCs), react
with stratospheric ozone, breaking it down into molecu-
lar oxygen (O 2 ). As a result of this breakdown, more solar
UV reaches Earth’s surface.
Unlike stratospheric ozone, ozone in the
troposphere—the layer of atmosphere closest to Earth’s
surface—is a human-made air pollutant. (Ground-level,
or tropospheric, ozone does not replenish the ozone
depleted from the stratosphere because tropospheric
ozone breaks down to form oxygen long before it drifts
up to the stratosphere.) Ozone in the troposphere is a
secondary air pollutant formed when sunlight triggers
reactions between nitrogen oxides and volatile hydro-
carbons. The most harmful component of photochemi-
cal smog, ozone reduces air visibility and causes health
problems. Ozone also reduces plant vigor, and chronic
ozone exposure (of long duration) lowers crop yields
(Figure 8.7). Chronic exposure to ozone is one possible
contributor to forest decline, and ground-level ozone is
associated with global warming. As discussed:

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A scientist measures the effects of ozone on the growth and

productivity of several plants. Plants exposed to ozone generally

exhibit damaged leaves, reduced root growth, and reduced

productivity. Photographed in Birmensdorf, Switzerland.

Sources of Outdoor Air Pollution

Not all air pollution is human generated. Throughout

Earth’s history, volcanoes have released particulate mat-

ter and sulfur oxides (see What a Scientist Sees). Plants

can also contribute to air pollution, producing a variety

of hydrocarbons in response to heat. The hydrocarbon

isoprene, for example, may protect leaves from high

temperatures. However, isoprene and other hydrocar-

bons are volatile and evaporate into the air, where they

interact with other substances to affect atmospheric

chemistry.

Hazardous air pollutants (HAPs), or air toxics, in-
clude hundreds of other air pollutants—such as chlorine
(Cl 2 ), lead, hydrochloric acid, formaldehyde, radioactive
substances, and fluorides. HAPs are present in very low
concentrations, although it is possible to have high local
concentrations of specific pollutants. They are potentially
harmful and may pose long-term health risks to people

Pollutant Category Characteristics

Methane (CH 4 ) Primary Colorless, odorless gas

Benzene (C 6 H 6 ) Primary Liquid with sweet smell

Pollutant Category Characteristics

Chlorine (Cl 2 ) Primary Yellow-green gas

Formaldehyde Primary Colorless gas with pungent odor

Pollutant Category Characteristics

Ozone (O 3 ) Secondary Pale blue gas with irritating odor