Geoscience Reference
In-Depth Information
one-fourth of all nitrogen oxides in the atmosphere come from
electric power generation plants that rely on burning fossil fuels
such as coal.
After these acids are emitted from factories in the industrial
Midwest, they are carried aloft and eastward by the prevailing
westerly winds. Subsequently, they are deposited on the surface
of Earth through precipitation or within fog. In these downwind
areas the pH of rainfall is about 4, whereas it is generally
above 5 in the western half of the country. Similar patterns
have been identified in other parts of the world, particularly
eastern Europe.
The impact of acid rain on the landscape and as a weathering
agent has been significant. With respect to chemical weather-
ing, acid rain dramatically accelerates the weathering process.
Although some of this weathering would have naturally oc-
curred by carbonation, it has been enhanced by acid rain. In
addition to the acceleration of chemical weathering, acid rain
is also associated with several negative environmental impacts
especially on forests and lakes. Forests that lie within the belt of
acid precipitation are often stressed (Figure 14.13). This stress
occurs in part because supporting soils are acidified, which ac-
celerates the leaching of nutrient cations before the trees absorb
them. Evidence also exists that acid deposition on the needle
leaves of conifers reduces their tolerance to cold. Similarly, the
pH of lakes and ponds can be lowered due to acid rain, causing
stress to the plants and animals that live within those water bod-
ies. Many lakes in New England, for example, do not support
fish because they contain toxic levels of inorganic acids.
Although acid rain is a serious environmental problem,
recent studies indicate that environmental regulations imposed
in association with the
Clean Air Act
are having a positive effect,
Figure 14.12 Effects of carbonation in limestone.
When
carbon dioxide in the air dissolves in rainwater, it forms a weak
carbonic acid that can erode limestone. This limestone bed in
Switzerland was once a solid rock layer that has been extensively
worn by acidic rainwater flowing over it.
precipitation contains this carbonic acid, which is sufficiently
acidic to dissolve many minerals, especially limestone (cal-
cium carbonate). When rainfall collects on the rock surface,
it dissolves the limestone and forms pits and rounded surfaces
(Figure 14.12). If you are interested in your local weather-
ing rates, visit a cemetery and compare the decomposition in
limestone versus marble or slate tombstones. Since marble and
slate are much more resistant than limestone to weathering, the
difference can be dramatic.
Human Interactions: Acid Rain
As just described, carbonation is the natural process through
which rocks are chemically weathered when they come into
contact with acidic water. In a similar way, by-products of hu-
man industrial activity cause chemical weathering of rocks
through the process of acid rain.
Acid rain
is the broad term
applied to the way in which industrial acids fall out of the atmo-
sphere in association with precipitation.
In North America, acid rain is a particular problem in the
northeastern part of the United States and eastern Canada. This
area lies downwind of large industrial cities, such as Chicago,
Detroit, Cleveland, and Pittsburgh, where large amounts of
chemical pollutants are emitted into the atmosphere. In par-
ticular, sulfur dioxide and nitrogen oxides are the primary acids
linked to acid rain. According to the United States Environmen-
tal Protection Agency, about two-thirds of all sulfur dioxide and
Figure 14.13 Forest stress due to acid rain.
Acidic rainfall is
concentrated in the eastern part of the United States. Forests in
these areas, such as this stand of Fraser firs in the Appalachian
Mountains, are affected because soils are acidified, which accel-
erates leaching of nutrient cations before the trees can absorb
them. In addition, trees become less tolerant to cold weather.
Acid rain
The precipitation by rain, fog, or snow of strong
mineral acids, primarily sulfur dioxide and nitrogen oxides,
that originated in factories.
