Acid Rain Definition
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.
Air Pollution Creates Acid Rain
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. Cars, trucks, and airplanes use gasoline, another fossil fuel.
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.
Acid Precipitation
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 gives rain a pH of 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.
Dry Deposition
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.
Acid Rain is a Problem That Can Travel
The chemical reactions that change air pollution to acid rain can take from several hours to several days.
Years ago, when smokestacks were only a few stories high, pollution from smokestacks usually stayed near
the ground and settled on land nearby. This caused unhealthy conditions for plants and animals near the
smokestacks. To reduce this pollution, the government passed a law permitting the construction of very
tall smokestacks. At that time, people thought that if the pollution were sent high into the air it would no
longer be a problem. Scientists now know that this is incorrect. Sending pollution high into the sky
increases the time that the pollution stays in the air. The longer the pollution is in the air, the greater
are the chances that the pollutants will form acid rain. In addition, the wind can carry these pollutants
for hundreds of miles before they become joined with water droplets to form acid rain. For that reason,
acid rain can also be a problem in areas far from the polluting smokestacks. Dry deposition is usually more
abundant near the cities and industrial areas where the pollutants are released.
Natural Acids
There are also natural sources of acids such as volcanoes, natural geysers and hot springs. Nature has
developed ways of recycling these acids by absorbing and breaking them down. These natural
acids contribute to only a small portion of the acidic rainfall in the world today. In small amounts,
these acids actually help dissolve nutrients and minerals from the soil so that trees and other plants can
use them for food. The large amounts of acids produced by human activities overload this natural acidity.
EPA page: aciddef
