Understanding Our Planet Through Chemistry
In addition to affecting people, plants, and wildlife, air pollution also affects rocks and soils. One of the problems it causes is the degradation of buildings and monuments, especially those built out of limestone or marble. These rock types, both almost pure calcite (calcium carbonate), are commonly used throughout the world as a building stone.
Studies to determine damage caused by air pollution have pointed to changes in the acidity of the air and rain. 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. The gas reacts with calcium-carbonate building stone to form calcium sulfate (gypsum). 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.
Current research on acid rain is directed at defining the degree of stone damage due to both dry and wet deposition. Scientists are measuring the effects of acid rain on historic stone buildings and monuments across the country. They are exposing samples of marble and limestone to weathering at specific field sites and simulating depositional processes under highly controlled laboratory conditions.
The effects of both dry and wet deposition are evaluated by the chemical analyses of the stone surfaces before and after exposure and of rain run-off solutions collected from test slabs.
Recent research by the USGS and other agencies conducted under the National Acid Precipitation Assessment Program has shown that test samples of marble erode 15 to 30 micrometers per year, while limestone (which is less compact than marble) erodes from 25 to 45 micrometers per year. (These measurements are slightly less than those of the diameter of a human hair). Approximately 20 percent of this erosion is caused by acid rain. The remaining 80 percent is the result of the natural solubility of the stone in rain water. Because the effects of acid rain only develop over an extended period of time, high-precision analytical chemistry plays a central role in measuring these effects.