Acid rain describes any form of precipitation with high levels of nitric and sulfuric acids. It can
also occur in the form of snow, fog, and tiny bits of dry material that settle to Earth.
Rotting vegetation and erupting volcanoes release some chemicals that can cause acid rain, but
most acid rain falls because of human activities. The biggest culprit is the burning of fossil fuels
by coal-burning power plants, factories, and automobiles.
When humans burn fossil fuels, sulfur dioxide (SO2) and nitrogen oxides (NOx) are released
into the atmosphere. These chemical gases react with water, oxygen, and other substances to
form mild solutions of sulfuric and nitric acid. Winds may spread these acidic solutions across
the atmosphere and over hundreds of miles. When acid rain reaches Earth, it flows across the
surface in runoff water, enters water systems, and sinks into the soil.
Acid rain has many ecological effects, but none is greater than its impact on lakes, streams,
wetlands, and other aquatic environments. Acid rain makes waters acidic and causes them to
absorb the aluminum that makes its way from soil into lakes and streams. This combination
makes waters toxic to crayfish, clams, fish, and other aquatic animals.
Some species can tolerate acidic waters better than others. However, in an interconnected
ecosystem, what impacts some species eventually impacts many more throughout the food
chain—including non-aquatic species such as birds.
Acid rain also damages forests, especially those at higher elevations. It robs the soil of essential
nutrients and releases aluminum in the soil, which makes it hard for trees to take up water. Trees'
leaves and needles are also harmed by acids.
The effects of acid rain, combined with other environmental stressors, leave trees and plants less
able to withstand cold temperatures, insects, and disease. The pollutants may also inhibit trees'
ability to reproduce. Some soils are better able to neutralize acids than others. In areas where the
soil's "buffering capacity" is low, the harmful effects of acid rain are much greater.
The only way to fight acid rain is by curbing the release of the pollutants that cause it. This
means burning fewer fossil fuels. Many governments have tried to curb emissions by cleaning up
industry smokestacks and promoting alternative fuel sources. These efforts have met with mixed
results. But even if acid rain could be stopped today, it would still take many years for its
harmful effects to disappear.
Individuals can also help prevent acid rain by conserving energy. The less electricity people use
in their homes, the fewer chemicals power plants will emit. Vehicles are also major fossil fuel
users, so drivers can reduce emissions by using public transportation, carpooling, biking, or
simply walking wherever possible.
Acid Rain: Do you need to start wearing a rain hat?
Depending on where you live, maybe you've heard of acid rain. Now, acid rain is not pure acid
falling from the sky, but rather it is rainfall or atmospheric moisture that has been mixed with
elements and gases that have caused the moisture to become more acidic than normal. Pure water
has a pH of 7, and, generally, rainfall is somewhat on the acidic side (a bit less than 6). But, acid
rain can have a pH of about 5.0-5.5, and can even be in the 4 range in the northeastern United
States, where there are a lot of industries and cars.
Causes of acid rain
Acidic precipitation can be caused by natural (volcanoes) and man-made activities, such as from
cars and in the generation of electricity. The precursors, or chemical forerunners, of acid rain
formation result from both natural sources, such as volcanoes and decaying vegetation, and manmade sources, primarily emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) resulting
from fossil fuel combustion. The burning of fossil fuels (coal and oil) by power-production
companies and industries releases sulfur into the air that combines with oxygen to form sulfur
dioxide (SO2). Exhausts from cars cause the formation of nitrogen oxides in the air. From these
gases, airborne sulfuric acid (H2SO4) and nitric acid (HNO3) can be formed and be dissolved in
the water vapor in the air. Although acid-rain gases may originate in urban areas, they are often
carried for hundreds of miles in the atmosphere by winds into rural areas. That is why forests and
lakes in the countryside can be harmed by acid rain that originates in cities.
Effects of acid rain
The environment can generally adapt to a certain amount of acid rain. Often soil is slightly basic
(due to naturally occurring limestone, which has a pH of greater than 7). Because bases
counteract acids, these soils tend to balance out some of the acid rain's acidity. But in areas, such
as some of the Rocky Mountains and parts of the northwestern and southeastern United States,
where limestone does not naturally occur in the soil, acid rain can harm the environment.
Some fish and animals, such as frogs, have a hard time adapting to and reproducing in an
acidic environment. Many plants, such as evergreen trees, are damaged by acid rain and acid fog.
I've seen some of the acid-rain damage to the evergreen forests in the Black Forest of Germany.
Much of the Black Forest was indeed black because so much of the green pine needles had been
destroyed, leaving only the black trunks and limbs! You also might notice how acid rain has
eaten away the stone in some cities' buildings and stone artwork.
Geographic distribution of acid rain
Acidity in rain is measured by collecting samples of rain and
measuring its pH. To find the distribution of rain acidity,
weather conditions are monitored and rain samples are
collected at sites all over the country. The areas of greatest
acidity (lowest pH values) are located in the Northeastern
United States. This pattern of high acidity is caused by the
large number of cities, the dense population, and the
concentration of power and industrial plants in the Northeast.
In addition, the prevailing wind direction brings storms and
pollution to the Northeast from the Midwest, and dust from the
soil and rocks in the Northeastern United States is less likely to
neutralize acidity in the rain.
Acid rain and stone
When you hear or read in the media about the effects of acid rain, you are usually told about the lakes,
fish, and trees in New England and Canada. However, we are becoming aware of an additional concern:
many of our historic buildings and monuments are located in the areas of highest acidity. In Europe,
where buildings are much older and pollution levels have been ten times greater than in the United States,
there is a growing awareness that pollution and acid rain are accelerating the deterioration of buildings
and monuments.
Stone weathers (deteriorates) as part of the normal geologic cycle through natural chemical,
physical, and biological processes when it is exposed to the environment. This weathering process, over
hundreds of millions of years, turned the Appalachian Mountains from towering peaks as high as the
Rockies to the rounded knobs we see today. Our concern is that air pollution, particularly in urban areas,
may be accelerating the normal, natural rate of stone deterioration, so that we may prematurely lose
buildings and sculptures of historic or cultural value.
What about buildings?
Many buildings and monuments are made of stone, and many buildings use stone for decorative trim.
Granite is now the most widely used stone for buildings, monuments, and bridges. Limestone is the
second most used building stone. It was widely used before Portland cement became available in the early
19th century because of its uniform color and texture and because it could be easily carved. Sandstone
from local sources was commonly used in the Northeastern United States, especially before 1900.
Nationwide, marble is used much less often than the other stone types, but it has been used for many
buildings and monuments of historical significance. Because of their composition, some stones are more
likely to be damaged by acidic deposition than others. Granite is primarily composed of silicate minerals,
like feldspar and quartz, which are resistant to acid attack. Sandstone is also primarily composed of silica
and is thus resistant. A few sandstones are less resistant because they contain a carbonate cement that
dissolves readily in weak acid. Limestone and marble are primarily composed of the mineral calcite
(calcium carbonate), which dissolves readily in weak acid; in fact, this characteristic is often used to
identify the mineral calcite.
How does acid precipitation affect marble and limestone buildings?
Acid precipitation affects stone primarily in two ways: dissolution and alteration. When sulfurous,
sulfuric, and nitric acids in polluted air react with the calcite in marble and limestone, the calcite
dissolves. In exposed areas of buildings and statues, we see roughened surfaces, removal of material, and
loss of carved details. Stone surface material may be lost all over or only in spots that are more reactive.
You might expect that sheltered areas of stone buildings and monuments would not be affected
by acid precipitation. However, sheltered areas on limestone and marble buildings and monuments show
blackened crusts that have spalled (peeled) off in some places, revealing crumbling stone beneath. This
black crust is primarily composed of gypsum, a mineral that forms from the reaction between calcite,
water, and sulfuric acid. Gypsum is soluble in water; although it can form anywhere on carbonate stone
surfaces that are exposed to sulfur dioxide gas (SO2), it is usually washed away. It remains only on
protected surfaces that are not directly washed by the rain. Gypsum is white, but the crystals form
networks that trap particles of dirt and pollutants, so the crust looks black. Eventually the black crusts
blister and spall off, revealing crumbling stone.
Instructions: In your journal, using Cornell note style (2-column notes)
Define the bold faced words
Answer the following questions:
1. What are 3 causes of acid rain?
2. What are 3 effects of acid rain?
3. Is acid rain a big problem in Colorado?
4. How does acid rain affect limestone buildings?
Acid Rain Reading
Name ______________________
A rain in which the precipitation (rain, sleet snow or hail) is
unusually acidic is called an acid rain. It contains high levels of
hydrogen ions. Acid rain is caused by natural sources like decay of
vegetation, volcanoes, and also from manmade sources like emissions
from factories and various types of pollution in the atmosphere. When
these gases react with water that is present in the air, they can form
acid rain.
(Image source: sxc.hu)
Acid rains are very harmful to the atmosphere. Steel structures like
bridges and towers corrode because of the acid. It also results in the
peeling of paint from the painted surfaces like buildings. Stone
structures get eroded (broken down) because of the acid in the rain.
Define precipitation.
Define acid rain.
Define eroded.
What are 3 causes of acid rain?