Visualizing Environmental Science

Controlling Air Pollutants 207

The Clean Air Act of 1970 and its amendments in 1977

and 1990 required progressively stricter controls of mo-

tor vehicle emissions. The provisions of the Clean Air Act

Amendments of 1990 include the development of “super-

clean” cars, which emit lower amounts of nitrogen oxides

and hydrocarbons, and the use of cleaner-burning gaso-

line in the most polluted cities in the United States. More

recent automobile models do not produce as many pollut-

ants as older models. Yet despite the increasing percentage

of newer automobile models on the road, air quality has

not improved in some areas of the United States because

of the large increase in the number of cars being driven.

The Clean Air Act Amendments of 1990 focus on

industrial airborne toxic chemicals in addition to mo-

tor vehicle emissions. The Clean Air Act Amendments of

1990 required a 90 percent reduction in the atmospheric

emissions of 189 toxic chemicals. To comply with these

requirements, both small businesses (such as dry clean-

ers) and large manufacturers (such as chemical compa-

nies) installed pollution-control equipment if they had

not already done so.

nitrogen oxide emissions. Nitrogen oxides produced
during high-temperature combustion processes in indus-
try can be removed from smokestack exhausts.
Modification of furnaces and engines to provide more
complete combustion helps control the production of both
carbon monoxide and hydrocarbons. Catalytic afterburn-
ers, used immediately following combustion, oxidize most
unburned gases. The use of catalytic converters to treat
auto exhaust reduces carbon monoxide and volatile hydro-
carbon emissions about 85 percent over the life of the car.
Careful handling of petroleum and hydrocarbons, such as
benzene, reduces air pollution from spills and evaporation.

The Clean Air Act

There is good news and bad news about air pollution in the
United States. The bad news is that many locations through-
out the country still have unacceptably high levels of one or
more air pollutants. Moreover, health experts estimate that
air pollution causes the premature deaths of thousands of
people in the United States each year. The good news is
that overall air quality has improved since 1970.
This improvement is largely due to the U.S. Clean Air
Act (CAA), first passed in 1970 and updated and amended
in 1977 and 1990. The CAA authorizes the Environmen-
tal Protection Agency (EPA) to apply and enforce the
CAA by establishing limits on the amount of specific air
pollutants permitted everywhere in the United States. In-
dividual states must meet deadlines to reduce air pollu-
tion to acceptable levels. States may pass more stringent
pollution controls than the EPA authorizes, but they can’t
mandate weaker limits than those stipulated in the CAA.
The EPA has focused on six air pollutants—lead,
particulate matter, sulfur dioxide, carbon monoxide, ni-
trogen oxides, and ozone—and established maximum
acceptable concentrations for each. The most dramatic
improvement so far has been in the amount of lead in
the atmosphere, which showed a 98 percent decrease
between 1970 and 2000, primarily because of the switch
from leaded to unleaded gasoline.
Atmospheric levels of the other pollutants, with
the exception of particulate matter, have also declined
(Figure 8.15). For example, between 1980 and 2010, sul-
fur dioxide emissions declined 83 percent. During this
same time, U.S. gross domestic product increased more
than 120 percent, energy consumption increased 23 per-
cent, and vehicle miles increased about 95 percent. As
will be discussed in Chapter 9, emissions of carbon diox-
ide have increased substantially since 1970.

200

180

160

140

120

100

80

60

40

20

Million metric tons per year

Carbon

monoxide

Sulfur

dioxide

Nitrogen

oxides

Particulate

matter

(PM=10)

Volatile

organic

compounds

1970

2011

Courtesy of Air Quality Planning and Standards,Office of Air and Radiation, EPA

Emissions in the United States, 1970 and 2011

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Carbon monoxide, sulfur dioxide, volatile organic compounds

(many of which are hydrocarbons), and nitrogen oxides showed

decreases; only particulate matter did not decline. “PM = 10”

applies to particles less than or equal to 10 Mm (10 micrometers).

Since 1990 the EPA has also monitored PM = 2.5, which are very

small particles less than or equal to 2.5 Mm.

Why have emissions of particulate matter

increased since 1970, when all other emissions have gone down?

Think Critically