International Regulation of Urban Smog Since the 1940s - Air Pollution and Global Warming: History, Science, and Solutions - page 181

The first converters were single-bed converters that
used Pt and Pd in the ratio 2:1 as the catalyst. In this
converter, called an oxidation converter ,CO(g) and
unreacted THCs were burned (oxidized) over the cat-
alyst at temperatures of 250 ◦ Cto600 ◦ Ctoproduce
CO 2 (g) and H 2 O(g). Single-bed converters did not con-
trol NO x (g).
Asecond bed ( reduction converter )was developed
to reduce NO x (g) to N 2 (g) and O 2 (g). The catalysts in
this bed are typically Rh, Ru, Pt, and/or Pd. When NO(g)
or NO 2 (g) impinge upon the metal catalyst, the catalyst
briefly bonds to the nitrogen atom and shears off the
oxygen atom(s), allowing the oxygen to form O 2 (g).
When a second nitrogen atom bonds to the catalyst
adjacent to the first, the two combine to form N 2 (g),
which is released back to the exhaust stream.
The three-way catalyst, developed in 1979, allowed
for the simultaneous oxidation of unreacted THCs and
CO(g) and reduction of NO x (g) in a single bed. The use
of this catalyst requires a specific input air-to-fuel ratio
of 14.8:1 to 14.9:1 and a temperature range of 350 ◦ C
to 600 ◦ Cforitto remain effective in converting all
three groups of compounds. At high air-to-fuel ratios,
CO(g) and THCs are converted efficiently, but NO x (g)
is not. At low ratios, the reverse is true. At temperatures
less than 350 ◦ C, conversion efficiency falls off fast. At
250 ◦ C, it is near zero. The catalysts in the three-way
structures are usually platinum and rhodium at a ratio
of Pt:Rh

was 17.2 mpg, set in 1979. It gradually increased to
20.7 mpg, where it stayed until 2004 before gradually
increasing again to 24.1 mpg in 2011.
CAFE standards do not directly limit the emissions of
smog-forming pollutants, but they do indirectly reduce
such emissions. CAFE standards set a lower limit for
average vehicle mileage, reducing fuel use. Because
carbon dioxide and smog-forming pollutant emissions
are proportional to fuel use, the standards indirectly
reduce emissions of such pollutants.

8.1.9. Clean Air Act Amendments of 1977
On August 7,1977, Congress passed the Clean Air Act
Amendments of 1977 (CAAA77, PL 95-95), which
extended the date for mandated automobile emission
reductions of THCs and CO(g) to 1980. The NO x (g)
emission standard was relaxed from 0.4 g/mi to 1.0
g/mi, and the deadline for compliance extended to 1981.
A0.8 grams per gallon (g/gal) standard was also intro-
duced for lead. The lead standard was tightened in 1980
to 0.5 g/gal. In 1980, the U.S. EPA also set limits on
diesel fuel particulate emissions and required CO(g)
emissions from heavy-duty trucks to be reduced by 90
percent by 1984.
In 1977, most U.S. states had nonattainment areas
where at least one NAAQS had not been achieved.
CAAA77 required states that had at least one nonat-
tainment area to describe, in a revised SIP, how they
would achieve attainment by December 31, 1982.
CAAA77 also formalized a permitting program, ini-
tiated by the U.S. EPA in 1974, to prevent significant
deterioration (PSD) of air quality in regions that were
already in attainment of NAAQS. Under the program,
Class I, II, and III regions were designated. Class I
regions included pristine areas, such as national and
international parks and national wilderness areas, where
no new sources of pollution were allowed. Class II
regions comprised areas where moderate changes in air
quality were allowed, but where stringent regulations
were desired. Class III regions involved areas where
major growth and industrialization were allowed as long
as pollutant levels did not exceed NAAQS. Before a
new pollution source can be built or an existing source
can be modified to increase pollution in a PSD region
that allows growth, a PSD permit must be obtained. To
obtain a permit, the polluter proposing the new source
or change must ensure that the best available control
technology (BACT) will be installed and the resulting
pollution will not lead to a violation of an NAAQS. A
BACT is a pollution control technology that results in

= 5:1.

8.1.8. Corporate Average Fuel Economy
Standards
In response to the 1973/1974 oil embargo, which
reduced the supply and increased the cost of oil-derived
fuels, the U.S. Congress passed the Energy Policy Con-
servation Act of 1975. This act gave power to the
Administrator of the National Highway Transportation
and Safety Administration to set Corporate Average
Fuel Economy (CAFE) standards for cars and light
trucks (trucks and vans lighter than 3,900 kg and sport
utility vehicles lighter than 4,500 kg). Under this law,
automobile manufacturers pay fines if the average fuel
economy of the fleet of vehicles they sell is less than
the CAFE standard set. The U.S. EPA measures vehicle
fuel economy to determine compliance under the act.
In 1978, the CAFE standard was first set for pas-
senger vehicles at 18 miles per gallon (mpg). It fluc-
tuated but gradually increased to 27.5 mpg by 1990,
where it stayed through 2010. In 2011, the standard was
increased to 30.2 mpg. The first light truck standard