Page 479 - Environment: The Science Behind the Stories

5 These less harmful gases are
expelled as exhaust from
the vehicle’s tailpipe. Carbon dioxide (CO )
2
Water vapor (H O)
2
Nitrogen gas (N )
2

1 Exhaust gases from the Main chemical reactions
engine flow into the 2CO + O 2CO 2
2
catalytic converter. C H + 3O 2CO + 2H O
2 4
2
2
2
CO + NO CO + N 2
2
x
4 These metals help catalyze chemical
reactions, which, with heat and
oxygen, convert the input gases into
Hydrocarbons carbon dioxide, nitrogen, and water
Carbon monoxide (CO) vapor.
Nitrogen oxides (NO ) 2 Inside the stainless steel
x
housing, honeycomb-like
masses maximize surface
area for contact with the
gases. Catalytic metals
(Pd, Rh, Pt)
Substrate
(metal or ceramic) Washcoat (Al O )
2 3
Figure 17.15 Catalytic converters
filter pollutants from vehicle exhaust. 3 The honeycomb structure is
They have improved air quality everywhere covered with aluminum oxide,
they have been introduced. palladium, rhodium, and platinum.
have caused lead emissions to plummet; in the United particulate matter, and lead (Pb)—as well as nitrogen dioxide
States lead emissions fell by 93% in the 1980s alone. (NO ) and tropospheric ozone (O ).
2
3
The reduction of emissions in the United States since Nitrogen dioxide is a highly reactive, foul-smelling red-
1970 has resulted in notably cleaner air throughout the dish brown gas that contributes to smog and acid deposition.
nation. This reduction in outdoor air pollution represents Along with nitric oxide (NO), NO belongs to the family of
2
one of the greatest accomplishments of the United States compounds called nitrogen oxides (NO ). Nitric oxide reacts
X
in safeguarding human health and environmental quality. readily in the atmosphere to form NO , which is both a pri-
2
It demonstrates how seemingly intractable problems can mary and secondary pollutant.
be tackled and addressed within a democratic system when Although ozone in the stratosphere shields us from the
government and industry are informed by science and are dangers of UV radiation, ozone from human activity accumu-
responsive to the public’s demands. The EPA estimates that lates low in the troposphere. tropospheric ozone (also called
between 1970 and 1990 alone, clean air regulations and the ground-level ozone) is a secondary pollutant, created by the
resulting technological advances in pollution control saved interaction of sunlight, heat, nitrogen oxides, and volatile
the lives of 200,000 Americans. Similar advances in pollu- carbon-containing chemicals. A major component of smog,
tion control have been made by most other wealthy nations this colorless gas poses health risks due to the instability of
in recent years. the O molecule. This triplet of oxygen atoms will readily split
3
into a molecule of oxygen gas (O ) and a free oxygen atom.
2
Air quality has improved The oxygen atom may then participate in reactions that can
injure living tissues and cause respiratory problems. Tropo-
As a result of emissions reductions, air quality has improved spheric ozone is the pollutant that most frequently exceeds its
markedly in industrialized nations throughout the world. In the national ambient air quality standard.
United States, the EPA and the states monitor outdoor air qual- Thanks to the actions of scientists, policymakers, industrial
ity by measuring the concentrations of six criteria pollutants, leaders, and everyday citizens, air quality today is far better than
pollutants judged to pose substantial risk to human health. For it was a generation or two ago (Figure 17.17). However, plenty
each of these, the EPA has established national ambient air of room for improvement remains. Concerns over new pollut-
quality standards (NAAQS), which are maximum concentra- ants are emerging, greenhouse gas emissions are altering the cli-
tions allowable in ambient outdoor air. The six criteria pol- mate, and many Americans live in areas where concentrations of
lutants include four of the six pollutants whose emissions are criteria pollutants continue to reach unhealthy levels. Residents
478 monitored—carbon monoxide (CO), sulfur dioxide (SO ), of Los Angeles County, for instance, breathe air that violates
2

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