Soiling patterns on a tall limestone building: Changes over 60 years

Soiling of limestone caused by air pollution has been studied at the Cathed ral of Learning on the University of Pittsburgh campus. The Cathedral was c onstructed in the 1930s during a period of heavy pollution in Pittsburgh, P A. Archival photographs show that the building became soiled while it was s till under construction. Reductions in air pollutant concentrations began i n the late 1940s and 1950s and have continued to the present day. Concurren t with decreasing pollution, soiled areas of the stone have been slowly was hed by rain, leaving a white, eroded surface. The patterns of white areas i n archival photographs of the building are consistent with computer modelin g of rain impingement showing greater wash off rates at higher elevations a nd on the corners of the building. Winds during the rainstorms are predomin antly from the quadrant SW to NW at this location, and wind speeds as well as rain intensities are greater wh en winds a re from this quadrant as comp ared with other quadrants; the sides of the building facing these direction s are much less soiled than the opposing sides. Overall, these results sugg est 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 rapidl y.