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Acid Rain and
Our Nation’s Capital
_A Guide to Effects on Buildings and Monuments_
_by_ Elaine McGee
For sale by the U.S. Government Printing Office
Superintendent of Documents, Mail Stop: SSOP, Washington, DC 20402-9328
ISBN 0-16-048068-X
[Illustration: Marble surfaces exposed to rain develop a rough
“sugary” texture because the calcite grains are loosened as the
edges dissolve in the rain water. Column capital volute, Jefferson
Memorial, Washington, D.C.]
[Illustration: A summer rain storm in Washington, D. C. (Memorial
Continental Hall)]
When polluted air mixes with rain, snow, and fog, acid precipitation
forms. This acidity has caused people to worry about the environment;
some reports show that acid rain has affected lakes, trees, and fish
populations in the Northeastern United States and Canada. Another
concern is its effect on historic buildings and monuments.
The booklet focuses on acid rain and its impact on our Nation’s capital.
Rain in Washington, D. C., has an average acidity of 4.2, about as acid
as a carbonated drink and more than ten times as acid as clean,
unpolluted rain. This booklet will define acid rain, explain what
effects it has on marble and limestone buildings, and show, on a walking
tour, some of the places in our Nation’s capital where you can see the
impact of acid precipitation.
[Illustration: The pH scale: pH = 7 is neutral, neither acid or
alkaline; smaller pH values are acid, larger pH values are alkaline.
A liquid with a pH of 3 is ten times as acid as one with a pH of 4.]
1 Battery Acid
2.8 Vinegar
4 Adult fish die
<5.5 ACID RAIN
5.2-6.5 Normal range of precipitation
6-8 Normal range of stream pH
<7 Acid
7 Neutral
>7 Alkaline
8.6 Baking soda and sea water
13 Lye
What is acid rain?
The term “acid rain” is commonly used to mean the deposition of acidic
components in rain, snow, fog, dew, or dry particles. The more accurate
term is “acid precipitation.” Distilled water, which contains no carbon
dioxide, has a neutral pH of 7. Liquids with a pH less than 7 are acid,
and those with a pH greater than 7 are alkaline (or basic). “Clean” or
unpolluted rain has a slightly acidic pH of 5.6, because carbon dioxide
and water in the air react together to form carbonic acid, a weak acid.
Around Washington, D.C., however, the average rain pH is between 4.2 and
4.4.
The extra acidity in rain comes from the reaction of air pollutants,
primarily sulfur oxides and nitrogen oxides, with water in the air to
form strong acids (like sulfuric and nitric acid). The main sources of
these pollutants are vehicles and industrial and power-generating
plants. In Washington, the main local sources are cars, trucks, and
buses.
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.
[Illustration: Wet and dry bucket collector, used to collect samples
for measuring rainfall acidity.]
[Illustration: 1992 annual precipitation-weighted mean hydrogen ion
concentrations as pH
A pH distribution map shows areas in the continental United States
of greatest acidity in the rain.]
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. Because
buildings and monuments made of limestone and marble are more likely to
be damaged by acid precipitation, they are the main focus of this
booklet.
[Illustration: Memorial Bridge in Washington, D.C., is made of
granite, the most widely used stone type.]
[Illustration: Marble used as a trim on the First Bank in
Philadelphia, Pennsylvania.]
How do you recognize limestone and marble?
The main difference between limestone and marble is that limestone is a
sedimentary rock, typically composed of calcium carbonate fossils, and
marble is a metamorphic rock. Limestone forms when shells, sand, and mud
are deposited at the bottom of oceans and lakes and over time solidify
into rock. Marble forms when sedimentary limestone is heated and
squeezed by natural rock-forming processes so that the grains
recrystallize. If you look closely at a limestone, you can usually see
fossil fragments (for example, bits of shell) held together by a calcite
matrix. Limestone is more porous than marble, because there are small
openings between the fossil fragments. Marble is usually light colored
and is composed of crystals of calcite locked together like pieces of a
jigsaw puzzle. Marble may contain colored streaks that are inclusions of
non-calcite minerals.
[Illustration: Limestone is made of fossil fragments, held together
with calcite; the shell near the center is about 1 cm across.
Botanic Gardens building, Washington, D.C.]
[Illustration: Marble is made of calcite crystals (white) and some
colored grains of mica inclusions; the grains in a marble are locked
together like jigsaw puzzle pieces.]
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.
[Illustration: When marble is exposed to acidic rain, sharp edges
and carving details gradually become rounded. Antefixes, roof of the
Philadelphia Merchants’ Exchange (built in 1832).]
[Illustration: Blackened crusts on sheltered portions of the
limestone Chicago Tribune Building, Chicago, Illinois.]
[Illustration: Formed as a result of air pollution, gypsum
alteration crusts have blackened, blistered, and spalled from a
marble baluster at the Organization of American States building,
Washington, D.C.]
[Illustration: Scanning electron microscope photograph of gypsum
crystals with dirt and pollution particles trapped by the network of
crystals. The scale bar is 10 micrometers long.]
[Illustration: A marble column at the Merchants’ Exchange in
Philadelphia shows loss of material where the stone is exposed to
rain and blackening of the stone surface where the stone is
sheltered from rain.]
Where can we see the effects of acid precipitation?
Washington’s buildings and monuments use many different stone types.
Marble and limestone buildings are the most likely to show damage,
because they are more affected by acidic precipitation and urban
pollution. As you follow the tour described in this book, see how
granite and sandstone buildings compare with the marble and limestone in
the same environment.
This guide will help you recognize some geologic features of buildings,
in addition to their historical and architectural aspects, wherever you
travel. However, remember one important point when examining buildings
and monuments for deterioration: stone deterioration has many causes.
Although acid precipitation and urban pollution can accelerate stone
deterioration, people, pigeons, and other organisms may also harm our
stone structures. In addition, the process of weathering has been going
on since the Earth first had an atmosphere. Although we can observe
deterioration of the stone, it is hard to determine how much of the
deterioration is from acid precipitation and how much is from other
causes.
[Illustration: Pigeons sitting on the statue heads have created
distinctive deterioration on this building.]
[Illustration: Flowers and grasses have grown in the cracks between
stones on this church.]
[Illustration: This limestone column in the Lincoln Memorial is
darkened and dirty from people’s hands touching the stone.]
[Illustration: Microorganisms have caused this stain to appear on a
marble column at the Jefferson Memorial.]
What are we doing about acid rain?
Scientists from many disciplines are studying acid precipitation and its
impact. The National Acid Precipitation Assessment Program (NAPAP), a
Federal program involving representatives from more than a dozen Federal
agencies, has sponsored studies on how acid rain forms and how it
affects lakes, crops, forests, and materials. Because buildings and
monuments cannot adapt to changes in the environment, as plants and
animals can, historic structures may be particularly affected by acid
precipitation. Scientists are studying effective control technologies to
limit the emissions from power plants and automobiles that cause acid
rain. The impact and usefulness of regulations that would require limits
on air pollution are also being studied. Finally, scientists are
examining the processes of deterioration to find effective ways to
protect and repair our historic buildings and monuments. Agencies like
the National Park Service, which are charged with protecting and
preserving our national heritage, are particularly concerned not only
about the impact of acid rain but also about making the best choices for
maintaining and preserving our historic buildings and monuments.
[Illustration: Beginning in 1984, the National Acid Precipitation
Assessment Program sponsored exposure site studies of limestone and
marble, to examine the contribution to stone deterioration that
comes from acid precipitation.]
A field guide to buildings in our Nation’s capital
Washington, D.C., has many buildings of historic and cultural
significance, and many of them are made of marble and limestone. This
self-guided tour will point out damage to buildings and monuments in our
Nation’s capital that may have been caused by acid precipitation.
Similar effects may be found in other cities as well.
Places to visit have been divided into several areas, so the trip can be
done either in segments or all in one day. A suggested tour route is
described within each area. A car provides the most efficient transport
between areas, but parking may be hard to find. The Metro subway system
can easily be used to visit all areas except the Jefferson and Lincoln
Memorials. The closest Metro stations in each area are shown on the map.
You will need comfortable walking shoes, and you may want to bring along
a camera, a hand lens (about 10× magnification) for observing details of
minerals and weathering, and a pair of binoculars for closer examination
of inaccessible areas.