Environmental Engineering Reference
In-Depth Information
to be stunted or perhaps killed. The acid attacks the microorganisms that
release nutrients to the soil and removes nutrients already present. Plant
germination and reproduction are inhibited. The cumulative effect is that
even if the plant survives, it will be very weak and less able to survive
strong winds, heavy rainfall or even a brief dry period.
Excess acidity causes mercury and aluminum to be leached from the
soil, and when these elements are carried into lakes, aquatic life is
affected. Natural mechanisms that neutralize the acidity of normal rain-
water cannot cope with the excess acidity of acid rain, and many organ-
isms in the lake cannot survive. As acidity increases, organisms die in
sequence. First the plankton and crustaceans die off, and then the bass,
crayfi sh, and mayfl ies. By the time the acidity has reached about 50 times
normal, all the fi sh have died. At 200 times normal, frogs die. Many
streams in northeastern United States have become so acidic that they
can no longer support life.
Nitrogen Compounds
Most of the nitrogenous emissions are made at ground level from motor
vehicles, in contrast to sulfurous emissions, which are released high into
the air from smokestacks. Because nitrogen compounds are a major
ingredient in the formation of ground-level smog, they are removed from
the air very quickly, form little nitric acid (HNO
3
), and contribute little
to acid rain falling from thousands of feet above.
One-third of nitrogen emissions comes from motor vehicles. Before
the advent of modern catalytic converters in cars in the mid-1990s,
nitrogen emissions were much greater than they are today. The catalytic
converter converts the nitrogenous compounds into nitrogen (the main
gas in the air) and water. The converter is capable of reducing nitroge-
nous emissions by 95 percent but is effective only if the engine is warmed
up. Many trips taken by walk-avoiding Americans are either too short
or involve lots of stopping and starting, so the engine does not get hot
enough for the catalytic converter to be effective.
Lead
Airborne lead is of diminished importance in the United States. Leaded
paint has been banned since the 1970s, and the sale of leaded gasoline
was totally banned in 1986. Lead in the air decreased by nearly 97
percent between 1980 and 2005, lakes and reservoir waters have seen
decreases of as much as 70 percent, and human blood in the United States
