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138 � Transportation Statistics Annual Report 1996
short-tons. If included in the transportation sector,
these upstream activities would add 28 percent
more VOC emissions to the mobile source
totals for 1994.
Nitrogen Oxides
Nitrogen dioxide is a criteria pollutant; therefore,
EPA measures NO 2 concentrations in the
ambient atmosphere. NO 2 concentrations across
the United States decreased an average of 9 percent
from 1985 to 1994. From 1993 to 1994,
however, NO 2 concentrations increased by 5 percent.
(USEPA 1995a) Even so, all monitoring
stations in the country met the NO 2 emissions
standard in 1994 for the third year in a row.
(USEPA 1995a) Control of NO 2 emissions are
important for reducing levels of O 3, which exceed
NAAQS in many areas.
Tailpipe and other emissions standards have
been set for all forms of nitrogen oxides, which
include several substances that quickly turn into
NO 2 or can themselves contribute to the formation
of smog. In 1994, according to EPA, electric
utilities and industry accounted for about half of
the NOx emissions; mobile sources accounted
for 45 percent. Highway vehicles accounted for
71 percent of mobile source NO x emissions in
1994 (see table 7-4).
NOx emissions from on-road vehicles declined
by 6.9 percent between 1985 and 1994,
while emissions from off-highway vehicles
increased by 13.2 percent in the same period.
Since 1991, however, NOx emissions from both
highway and nonroad vehicles increased, with
highway vehicle emissions increasing by about
2.1 percent. (USEPA 1995a)
Diesel engines have high compression ratios,
and therefore produce proportionately more
NOx than gasoline engines. Emissions from
diesel-powered vehicles, both road and nonroad
(including railroad and marine diesels), account
for 45 percent of mobile source NOx emissions—a
much higher share than their modest
percentage of the vehicle population and vmt.
Emissions from diesel-powered highway vehicles
decreased by 17 percent between 1985 and
1994. Railroad diesels contributed 9 percent to
the mobile source total in 1994, increasing by 17
percent since 1985. Marine diesels contributed
1.8 percent to the mobile source total in 1994,
increasing by nearly 44 percent since 1985.
Other off-highway diesel emissions increased by
7 percent over the same period. (USEPA 1995a)
Some of the off-highway diesel emissions come
from construction equipment used to build or
maintain transportation infrastructure.
Lead
At one time, transportation vehicles were the
primary source of lead emissions in the United
States, contributing about four-fifths of total
lead emissions as recently as 1985. Air pollution
control programs implemented by EPA, however,
have nearly eliminated lead emissions from
transportation fuels. Unleaded gasoline, introduced
in 1975 to prevent fouling of catalytic
exhaust emissions control devices, accounted
for 99 percent of gasoline sales by 1993.
Although transportation still contributes 32
percent of total lead emissions, the base is much
smaller (see figure 7-4 and table 7-5).
Currently, 10 areas exceed the NAAQS for
lead—mostly due to point sources such as lead
smelters, battery plants, and solid waste disposal.
(USEPA 1995a). Some of these facilities
provide products or disposal services to the
transportation sector.
Particulate Matter
In 1994, more areas were classified as nonattainment
for PM-10 than for any other criteria
pollutant. The number of nonattainment areas
increased from 70 in 1991 to 82 in 1994.
(USEPA 1994, USEPA 1995a) (PM-10 concentrations
have been separately measured since