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Acid Rain's Slow Dissolve

May 22, 2012 Posted by: Megan Nortrup, Science Communicator, National Capital Region

Remember acid rain? Ever wondered if it's still around? Acid rain has not gone away, but it has gotten somewhat better.

Acid rain is a threat to both natural areas and to our national monuments and memorials. Many monuments are made from limestone, marble, and bronze-materials that can be altered or slowly dissolved by acid precipitation. "Slowly" is the key word of course. No one expects the Washington Monument to melt into a toothpick, but acid rain damage may slowly add up for our beloved icons.

What Exactly is Acid Rain?

Acid rain is rain that contains nitrogen and sulfur oxides washed out of the air. When these oxides mix with water, they create weak acids that lower the pH of rainwater (and snow, fog, or dew too). Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are alkaline (or basic). "Clean" or unpolluted rain has a slightly acidic pH of 5.6, while acid rain can have a pH as low as 4.

Washington, D.C.'s Rain is Better, But Still Has a Ways to Go

In 1997, the average rain pH around Washington, D.C. was between 4.2 and 4.4.

Now, thanks in part to federal regulations that limit the amount of nitrogen and sulfur oxides that industries produce, the pH of rain in Washington, D.C. has improved. In 2010, the average pH of rain around Washington, D.C. was 4.8 to 4.9. You can see evidence of acid rain's effects in several spots on the National Mall.

The Evidence

When acids in polluted air react with calcite, a calcium-containing mineral in marble and limestone, the calcite dissolves. In exposed areas of buildings and statues, acid rain effects show up as roughened surfaces instead of smooth ones, as pits and pocks where material was removed, and as a loss of carved details. Stone surface material may be lost all over or only in spots that are more reactive.

Sheltered areas on limestone and marble buildings and monuments that rain does not directly touch are at risk too. Sulfur dioxide gas in the air still reacts with calcite in stone, creating black crusts that sometimes peel off, revealing crumbling stone beneath. The black crust is primarily made of the mineral gypsum, which is normally washed away from exposed surfaces by rain. Gypsum is white, but the crystals trap particles of dirt and pollutants as they form, so the crust looks black.

Cleaning jefferson volutes

Cleaning the Jefferson Memorial's volutes

Thomas Jefferson Memorial

One of the striking effects acid precipitation is having on the marble in the Thomas Jefferson Memorial is the loss of silicate mineral inclusions in the marble columns as the calcite matrix holding them together is dissolved. Close examination of the grooves on the columns shows glittery flakes of mica and sometimes grains of pyrite. Loss of material has resulted in a weakening of the stone. In order to prevent stone from falling, ties were placed around the volutes, the scrolls atop the columns, to support them. Before restoration work in 2004, black crusts were visible on the column capitals (tops) that are sheltered from rain and from regular washing of the monument. Black crusts can be removed by intermittent water misting, which softens the crust allowing it to be carefully removed.

The Ulysses S. Grant Memorial

The Ulysses S. Grant Memorial, across the street from the Capitol Building, shows the effects that acid rain has on bronze, a metal alloy consisting of copper and a small amount of tin. The green stains on the statue's marble pedestal come from dissolved and oxidized copper as it runs down from the statue to the ground. The statues show typical deterioration of bronzes in an urban outdoor environment. Similar to stone, areas that do not receive a regular wash from rain trap particles of dirt and pollutants resulting in disfiguring streaks. The NPS's specially trained statue preservation crew periodically washes the bronze statues of the National Mall and Memorial Parks with a conservation detergent and applies a microcrystalline wax to a surface heated with torches. The wax protects the metal for one to two years, depending on exposure, and is easily renewed. (In 2011, the Ulysses S. Grant Memorial was transferred to the care of the Architect of the Capitol.)

Ulysses Grant Memorial Acid Rain Damage

Green streaks are evidence of acid rain's effect on the Ulysses S. Grant Memorial

Beyond the National Mall

Acid rain affects natural areas too, especially lakes, streams, and watersheds. It changes water chemistry in ways that can affect algae, fish, aquatic plants, frogs, salamanders, and other aquatic creatures. For example, acidic pH levels in lake and stream waters cause naturally-occurring aluminum compounds to become more toxic to fish and amphibians.Trees and other plants can exhibit visible death of plant tissue, break down of the waxy covering on leaves, faster leaching of leaf nutrients, and conifers (like pines) can show reduced seed sprouting and seedling growth. Some lichens are also especially sensitive to acid rain.

What You Can Do to Help

It is important to do your part and help limit the creation of the chemicals that cause acid rain. Nitrogen oxides ("nox") and sulfur oxides ("sox") are produced in small amounts naturally but in large amounts by power plants and industries. Nox is also produced by vehicle exhaust, oil and gas production, fertilized crops, livestock production, and municipal and residential activities. To reduce your impact, you can cut down on activities that produce these chemicals.

Thanks for joining us in caring for these special monuments and memorials that make our National Mall great.

Further Reading:

Acid rain effects on the National Mall:
http://pubs.usgs.gov/gip/acidrain/index.html

Acid rain in the Washington, D.C. area:
http://science.nature.nps.gov/im/units/ncrn/products/briefs/NCRN_Wet_Deposition_RB.pdf

Fossils in the Washington, D.C. area:
http://www.nps.gov/cue/publications/paleo_4_24_07_low_res.pdf

Science, Thomas Jefferson Memorial, Ulysses S. Grant Memorial, Acid Rain, Pollution, Nature



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