Acid Rain

Acid Rain
From the Ambient Air Quality Series
What is Acid Rain?
Pollutants
The Reaction
Title IV of the Clean Air Act deals with acid
Scientists discovered, and have con-
While being transported by winds, some
rain. The term "acid rain" is used to
firmed, that sulfur dioxide (SO2) and
of these particles get caught up in
describe rain or snow that has a pH lower
nitrogen oxides (NOx) are the primary
clouds. When sulfur dioxide and nitrogen
than what is natural for a given area. Acid
causes of acid rain. Although natural
oxide gases and particles come in con-
rain is measured using a scale called "pH."
sources of sulfur oxides and nitrogen
tact with water droplets in clouds, chem-
The lower a substance's pH, the more
oxides do exist, more than 90% of the
ical reactions can occur where these pol-
acidic it is. Pure water has a pH of 7.0.
sulfur and 95% of the nitrogen emis-
lutants are converted chemically into the
Precipitation with a pH value less than 5 is
sions occurring in eastern North America
secondary pollutants, nitric acid and sul-
considered acid rain. Normal rain is slight-
are of human origin. These primary air
furic acid, both of which dissolve easily in
ly acidic because carbon dioxide dissolves
pollutants arise from the use of coal in
water. The resulting acidic water
into it, so it has a pH of about 5.5. As of
the production of electricity, from base-
droplets can be carried long distances by
the year 2000, the most acidic rain falling
metal smelting, and from fuel combus-
prevailing winds, returning to Earth as
in the US had a pH of about 4.3.
tion in vehicles. The burning of fossil
acid rain, snow, or fog.
fuels like coal and oil products by auto-
Deposition
mobiles and power plants releases large
Our Contribution
Enormous quantities of manmade and
amounts of sulfur dioxide and nitrogen
Because of global winds and mixing in
natural material are added to the air
oxides into the air. In the US, About 2/3
the atmosphere, every country's air pollu-
every day. Most of the materials added to
of all SO2 and 1/4 of all NOx comes
tion contributes to some degree to the
the atmosphere return to the ground
from electric power generation that relies
earth's problem of acid rain. In 1993, the
through a process known as deposition.
on burning fossil fuels like coal.
United States released approximately 90
Deposition occurs when it rains and
snows (wet deposition), but it can also
occur when dust settles out of the
atmosphere during dry periods (dry depo-
sition). Wet deposition refers to acidic
rain, fog, and snow. Dry deposition refers
to acidic gases and particles. About half
of the acidity in the atmosphere falls back
to earth through dry deposition. The
wind blows these acidic particles and
gases onto buildings, cars, homes, and
trees. Dry deposited gases and particles
can also be washed from trees and other
surfaces by rainstorms. When that hap-
pens, the runoff water adds those acids
to the acid rain, making the combination
more acidic than the falling rain alone.

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billion pounds of sulfur dioxide and nitro-
in aquatic systems. Most exposure to
of fish, the bottom is covered with unde-
gen oxides into the air. Rain and snow
mercury comes from eating contaminat-
cayed material, and the areas close to
that falls in the Eastern U.S. typically has
ed fish. Acid rain also reduces how far
shore may be dominated by mosses.
a much lower pH than precipitation in
and how clearly we can see through the
Both natural vegetation and crops can be
other parts of the country. The pH of rain
air, an effect called visibility reduction.
affected. It can alter the protective waxy
and snow in the Eastern U.S. is much
Toxic materials such as copper and lead
surface of leaves, lowering disease resist-
lower than in less populated areas.
can also be leached by acid rain from
ance, and may inhibit plant germination
water pipes into drinking water. The
and reproduction. Acid rain also has
The Effects
interactions between living organisms
been found to speed up the natural
Acid deposition has a variety of effects,
and the chemistry of their aquatic habi-
decay of stone monuments and historical
including damage to forests and soils,
tats are extremely complex. If the num-
buildings. Other materials such as iron,
fish and other living things, materials,
ber of one species or group of species
steel, zinc, and paint can be damaged by
and human health. The nitrogen portion
changes in response to acidification, then
acid rain. The human health effects of
of acid deposition contributes to
the ecosystem of the entire water body
acid rain are also of concern. Although
eutrophication in coastal ecosystems, the
is likely to be affected through the pred-
people aren't directly in danger from
symptoms of which include algal blooms
ator-prey relationships of the food web.
exposure to acid rain, the particles in air
(some of which may be toxic), fish kills,
For example, as the water pH approach-
that lead to acid rain may be a risk to
and loss of plant diversity. Finally, acidifi-
es 6.0, crustaceans, insects, and some
human health.
cation of lakes and streams can increase
plankton species begin to disappear, and
the amount of methyl mercury available
below pH 5.0, the water is largely devoid

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