These balusters, on the Pan American Union Building, Washington, D.C., were made from Georgia marble, and were installed in 1910. They demonstrate the effects of dry deposition of sulfur dioxide, which causes the formation of gypsum. Gypsum traps particulate matter to form heavy, black incrustation. In some areas, the gypsum crust has flaked off the balusters exposing a fresh but very rough surface. In fact, the term acid rain is now commonly used in the media as well as scientific studies.
Acid rain affects carbonate stone buildings and monuments in two ways.
The first is by dry deposition of sulfur dioxide gas, increasingly contributed to the atmosphere by the combustion of fossil fuels. As gypsum forms on the surfaces of the stone, it traps particulate matter, forming a blackened crust. The second effect of acid rain is wet deposition. Natural rain water is a weak carbonic acid solution and all carbonate-stone surfaces that are washed by rainwater are subject to gradual erosion. This erosion is accelerated, however, by the increased acidity of rain in the eastern United States, which is often 10 times greater than in areas where acidic pollutants are absent.