Acid rain is rain that is more acidic than
normal. Acid rain is a complicated problem.
Caused by air pollution, acid rain's spread
and damage involves weather, chemistry,
soil, and the life cycles of plants and animals
on the land and from acid rain in the water.
Scientists have discovered that air pollution
from the burning of fossil fuels is the major
cause of acid rain. Power plants and
factories burn coal and oil. Power plants use
that coal and oil to produce the electricity we
need to heat and light our homes and to run
our electric appliances. We also burn
natural gas, coal, and oil to heat our homes.
The smoke and fumes from burning fossil
fuels rise into the atmosphere and combine
with the moisture in the air to form acid rain.
The main chemicals in air pollution that
create acid rain are sulfur dioxide and
nitrogen oxides. Acid rain usually forms high
in the clouds where sulfur dioxide and
nitrogen oxides react with water, oxygen,
and oxidants. This forms a mild solution of
sulfuric acid and nitric acid. Sunlight
increases the rate of most of these
reactions. Rainwater, snow, fog, and other
forms of precipitation containing those mild
solutions of sulfuric and nitric acids fall to the
earth as acid rain.
Water moves through every living plant and
animal, streams, lakes, and oceans in the
hydrologic cycle. In that cycle, water
evaporates from the land and sea into the
atmosphere. Water in the atmosphere then
condenses to form clouds. Clouds release
the water back to the earth as rain, snow, or
fog. When water droplets form and fall to the
earth they pick up particles and chemicals
that float in the air. Even clean, unpolluted
air has some particles such as dust or
pollen. Clean air also contains naturally
occurring gases such as carbon dioxide.
The interaction between the water droplets
and the carbon dioxide in the atmosphere,
and to a lesser extent, from chlorine which is
derived from the salt in the sea, gives rain
an average pH of about 5.6, making even
clean rain slightly acidic. Other natural
sources of acids and bases in the
atmosphere may lower or raise the pH of
unpolluted rain. However, when rain
contains pollutants, especially sulfur dioxide
and nitrogen oxides, the rain water can
become very acidic.
Acid rain does not account for all of the
acidity that falls back to earth from
pollutants. About half the acidity in the
atmosphere falls back to the earth through
dry deposition as gases and dry particles.
The wind blows these acidic particles and
gases onto buildings, cars, homes and
trees. In some instances, these gases and
particles can eat away the things on which
they settle. Dry deposited gases and
particles are sometimes washed from trees
and other surfaces by rainstorms. When that
happens, the runoff water adds those acids
to the acid rain, making the combination
more acidic than the falling rain alone. The
combination of acid rain plus dry deposited
acid is called acid deposition.
The chemical reactions that...
