Soiling of limestone caused by air pollution has been studied at the Cathedral of Learning on the University of Pittsburgh campus. The Cathedral was constructed in the 1930s during a period of heavy pollution in Pittsburgh, PA. Archival photographs show that the building became soiled while it was still under construction. Reductions in air pollutant concentrations began in the late 1940s and 1950s and have continued to the present day. Concurrent with decreasing pollution, soiled areas of the stone have been slowly washed by rain, leaving a white, eroded surface. The patterns of white areas in archival photographs of the building are consistent with computer modeling of rain impingement showing greater wash off rates at higher elevations and on the corners of the building.
The Cathedral of Learning, 1930 (From University Archives, University of Pittsburgh)
Winds during the rainstorms are predominantly from the quadrant SW to NW at this location, and wind speeds as well as rain intensities are greater when winds are from this quadrant as compared with other quadrants; the sides of the building facing these directions are much less soiled than the opposing sides. Overall, these results suggest that rain washing of soiled areas on buildings occurs over a period of decades, in contrast to the process of soiling that occurs much more rapidly.