Vehicle exhaust contains nitrogen oxides and sulfur dioxide, which combine with water to form strong acids. Many exposed areas on buildings and statues show roughened surfaces and loss of detail in carvings. The gypsum crystals grow into a network of thin blades that traps particles of dirt, resulting in a dark crust on the surface. This gypsum crust dissolves in water, so it accumulates only in sheltered areas protected from rainfall. The marble balustrade on the west side of the Capitol building shows damage from acid rain dissolving the mineral calcite.
Questions still remain about acid rain and building stone in the Washington area.
For example, the process of weathering has turned the hard rocks of the Piedmont Plateau into the soft sediments washed downriver to form the Coastal Plain. Similar weathering processes affect the stones in buildings and monuments. Geologists at the U.S. Geological Survey are working with scientists at other Federal agencies, including the National Park Service, to learn more about the connections between acid rain and the deterioration of our buildings and monuments through laboratory experiments and studies of the building stones themselves. In order to protect our historically and culturally significant buildings, we need to understand how the various processes of weathering and deterioration can affect the stones from which they were built.