Acid Rain and Our Nation's Capital A Guide to Effects on Buildings and Monuments
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
Marble surfaces exposed to rain develop a rough “sugary” texture because the calcite grains areloosened as the edges dissolve in the rain water. Column capital volute, Jefferson Memorial,Washington, D.C.
A summer rain storm in Washington, D. C. (Memorial Continental Hall)
When polluted air mixes with rain, snow, and fog, acidprecipitation forms. This acidity has caused people to worry about the environment; some reports showthat acid rain has affected lakes, trees, and fish populations in the Northeastern United Statesand 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 asclean, unpolluted rain. This booklet will define acid rain, explain what effects it has on marble andlimestone buildings, and show, on a walking tour, some of the places in our Nation’s capitalwhere you can see the impact of acid precipitation.
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 acidiccomponents in rain, snow, fog, dew, or dry particles. The more accurateterm 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 witha pH greater than 7 are alkaline (or basic). “Clean” or unpolluted rain has aslightly acidic pH of 5.6, because carbon dioxide and water in the air react togetherto 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, primarilysulfur 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 vehiclesand industrial and power-generating plants. In Washington, the main localsources are cars, trucks, and buses.
Acidity in rain is measured by collecting samples of rain and measuring itspH. To find the distribution of rain acidity, weather conditions are monitoredand rain samples are collected at sites all over the country. The areas ofgreatest acidity (lowest pH values) are located in the Northeastern UnitedStates. This pattern of high acidity is caused by the large number of cities,the dense population, and the concentration of power and industrial plants inthe Northeast. In addition, the prevailing wind direction brings storms andpollution to the Northeast from the Midwest, and dust from the soil and rocksin the Northeastern United States is less likely to neutralize acidity in therain.
Wet and dry bucket collector, used to collect samples for measuring rainfall acidity.
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 therain.
When you hear or read in the media about the effects of acid rain, you areusually told about the lakes, fish, and trees in New England and Canada.However, we are becoming aware of an additional concern: many of our historicbuildings and monuments are located in the areas of highest acidity. InEurope, where buildings are much older and pollution levels have been tentimes greater than in the United States, there is a growing awareness thatpollution and acid rain are accelerating the deterioration of buildings andmonuments.
Stone weathers (deteriorates) as part of the normal geologic cycle throughnatural chemical, physical, and biological processes when it is exposed tothe environment. This weathering process, over hundreds of millions ofyears, turned the Appalachian Mountains from towering peaks as high as theRockies to the rounded knobs we see today. Our concern is that air pollution,particularly in urban areas, may be accelerating the normal, natural rate ofstone deterioration, so that we may prematurely lose buildings and sculpturesof historic or cultural value.
What about buildings?
Many buildings and monuments are made of stone, and many buildingsuse stone for decorative trim. Granite is now the most widely used stone forbuildings, monuments, and bridges. Limestone is the second most usedbuilding stone. It was widely used before Portland cement became availablein the early 19th century because of its uniform color and texture and becauseit could be easily carved. Sandstone from local sources was commonlyused in the Northeastern United States, especially before 1900.Nationwide, marble is used much less often than the other stone types, but ithas been used for many buildings and monuments of historical significance.Because of their composition, some stones are more likely to be damaged byacidic deposition than others. Granite is primarily composed of silicate minerals,like feldspar and quartz, which are resistant to acid attack. Sandstone isalso primarily composed of silica and is thus resistant. A few sandstones areless resistant because they contain a carbonate cement that dissolves readilyin weak acid. Limestone and marble are primarily composed of the mineralcalcite (calcium carbonate), which dissolves readily in weak acid; in fact,this characteristic is often used to identify the mineral calcite. Because buildingsand monuments made of limestone and marble are more likely to bedamaged by acid precipitation, they are the main focus of this booklet.
Memorial Bridge in Washington, D.C., is made ofgranite, the most widely used stone type.
Marble used as a trim on the FirstBank in Philadelphia, Pennsylvania.
How do you recognize limestone and marble?
The main difference between limestone and marble is that limestone is asedimentary rock, typically composed of calcium carbonate fossils, and marbleis a metamorphic rock. Limestone forms when shells, sand, and mud aredeposited at the bottom of oceans and lakes and over time solidify into rock.Marble forms when sedimentary limestone is heated and squeezed by naturalrock-forming processes so that the grains recrystallize. If you lookclosely at a limestone, you can usually see fossil fragments (for example,bits of shell) held together by a calcite matrix. Limestone is more porous thanmarble, because there are small openings between the fossil fragments.Marble is usually light colored and is composed of crystals of calcite lockedtogether like pieces of a jigsaw puzzle. Marble may contain colored streaksthat are inclusions of non-calcite minerals.
Limestone is made of fossil fragments, held togetherwith calcite; the shell near the center isabout 1 cm across. Botanic Gardens building,Washington, D.C.
Marble is made of calcite crystals (white) andsome colored grains of mica inclusions; thegrains in a marble are locked together like jigsawpuzzle 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 withthe calcite in marble and limestone, the calcite dissolves. In exposed areasof buildings and statues, we see roughened surfaces, removal of material,and loss of carved details. Stone surface material may be lost all over oronly in spots that are more reactive.
You might expect that sheltered areas of stone buildings and monumentswould not be affected by acid precipitation. However, sheltered areas onlimestone and marble buildings and monuments show blackened crusts thathave spalled (peeled) off in some places, revealing crumbling stone beneath.This black crust is primarily composed of gypsum, a mineral thatforms from the reaction between calcite, water, and sulfuric acid. Gypsum issoluble in water; although it can form anywhere on carbonate stone surfacesthat are exposed to sulfur dioxide gas (SO₂), it is usually washed away. It remainsonly on protected surfaces that are not directly washed by the rain.Gypsum is white, but the crystals form networks that trap particles of dirtand pollutants, so the crust looks black. Eventually the black crusts blisterand spall off, revealing crumbling stone.
When marble is exposed to acidic rain, sharpedges and carving details gradually becomerounded. Antefixes, roof of the PhiladelphiaMerchants’ Exchange (built in 1832).
Blackened crusts on sheltered portions ofthe limestone Chicago Tribune Building,Chicago, Illinois.
Formed as a result of air pollution, gypsum alterationcrusts have blackened, blistered, andspalled from a marble baluster at theOrganization of American States building,Washington, D.C.
Scanning electron microscope photograph ofgypsum crystals with dirt and pollution particlestrapped by the network of crystals. Thescale bar is 10 micrometers long.
A marble column at the Merchants’ Exchange in Philadelphia shows loss of material where thestone is exposed to rain and blackening of the stone surface where the stone is sheltered fromrain.
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, becausethey are more affected by acidic precipitation and urban pollution. As you followthe tour described in this book, see how granite and sandstone buildingscompare with the marble and limestone in the same environment.
This guide will help you recognize some geologic features of buildings, inaddition to their historical and architectural aspects, wherever you travel.However, remember one important point when examining buildings and monumentsfor deterioration: stone deterioration has many causes. Although acidprecipitation 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 hadan atmosphere. Although we can observe deterioration of the stone, it is hardto determine how much of the deterioration is from acid precipitation and howmuch is from other causes.
Pigeons sitting on the statue heads have createddistinctive deterioration on this building.
Flowers and grasses have grown in thecracks between stones on this church.
This limestone column in the LincolnMemorial is darkened and dirty from people’shands touching the stone.
Microorganisms have caused this stain to appearon a marble column at the JeffersonMemorial.
What are we doing about acid rain?
Scientists from many disciplines are studying acid precipitation and itsimpact. The National Acid Precipitation Assessment Program (NAPAP), aFederal program involving representatives from more than a dozen Federalagencies, has sponsored studies on how acid rain forms and how it affectslakes, crops, forests, and materials. Because buildings and monuments cannotadapt to changes in the environment, as plants and animals can, historicstructures may be particularly affected by acid precipitation. Scientists arestudying effective control technologies to limit the emissions from powerplants and automobiles that cause acid rain. The impact and usefulness ofregulations that would require limits on air pollution are also being studied.Finally, scientists are examining the processes of deterioration to find effectiveways to protect and repair our historic buildings and monuments.Agencies like the National Park Service, which are charged with protectingand preserving our national heritage, are particularly concerned not onlyabout the impact of acid rain but also about making the best choices formaintaining and preserving our historic buildings and monuments.
Beginning in 1984, the