6 SUMMARY AND CONCLUSIONS

In order to better understand pollutant sources and transport pathways responsible for the soiling of a tall limestone building, this study has investigated whether vertical gradients in airborne pollutant concentrations and deposition fluxes currently exist at the 42-story Cathedral of Learning in Pittsburgh, Pennsylvania. The study has also considered long-term changes in soiling on the building as a means of identifying the roles of pollutant deposition and rain washing in affecting the soiling patterns.

The study involved measurements of airborne concentrations of SO42− particles, elemental carbon particles, SO2 gas, and total NO3− (NO3− particles and HNO3 gas) on the 5th floor, 16th floor, and roof. In addition, SO2 deposition fluxes were measured on the 5th and 16th floors, and laser particle counters were used on these same two floors to measure airborne particle number concentrations in two size ranges: >0.5 μm and >5 μm. The experiments were conducted for 4 weeks during each of the four seasons of the year beginning in fall 1995.

The airborne chemical species measurements indicate that there are no statistically significant vertical gradients for any of the pollutants. The lack of a gradient is attributed to a variety of regional and local sources that are expected to be well mixed by the time they reach the cathedral, vertical winds in the vicinity of the building, and the lack of buildings nearby that might otherwise trap pollutants and prevent vertical mixing. The laser particle counts for >0.5 μm particles likewise show lack of gradient, although the >5 μm particles show slightly greater airborne particle number concentrations on the 16th floor compared with those on the 5th floor.

Deposition fluxes and deposition velocities of SO2 to surrogate surfaces show small but consistent differences among the locations sampled. Values are greatest at locations that are most exposed to the wind such as the outside corners of the patios. The values are greater on the 16th floor than on the 5th floor, partly because two of the three 16th floor sampling locations are situated on the corners of the patio.

Comparison of archival with more recent photographs shows that the soiling on the cathedral has decreased over time. This observation is consistent with decreasing trends in airborne pollutant concentrations over the past several decades. It is thus likely that rain is washing soiled material off the building surface at a greater rate than chemical species are depositing and reacting with the surface. The opposite apparently was true in the 1930s, when air pollutant concentrations were considerably greater than at present.

Overall, these results may be of interest to conservators who must develop strategies for cleaning and restoring building surfaces and for preventing future damage. Although the conclusions reached here are a result of testing at the Cathedral of Learning, it is likely that many of the findings also apply to buildings in other urban areas.

ACKNOWLEDGEMENTS

This research was funded by the National Park Service Cooperative Agreements 042419005 and 00196035. The assistance of the Department of Facilities Management at the University of Pittsburgh and Dominic Fagnelli, the Cathedral of Learning building engineer, are greatly appreciated. The authors wish to acknowledge Susan Sherwood for her valuable suggestions and assistance over the past several years. Thanks are due to Michael Lutz and Timothy Gould for their earlier work on the Cathedral of Learning and to Mitchell Small for help with the statistical analysis. The authors also wish to thank Weiping Dai, Hampden Kuhns, Spyros Pandis, Ross Strader, and Maria Zufall for their frequent and welcome suggestions. ElizaBeth Bede and Chandra Reedy provided comments and insights that were very helpful in the writing of this manuscript. A large fraction of the airborne sampling setup was constructed by Larry Cartwright and his crew. Preshanth Mekala and Karen Pinkston contributed their time to ion chromatography analyses. The historical photograph was furnished by the Carnegie Library of Pittsburgh, Oakland Branch. Justin Parkhurst assisted with the collection of several historical photographs. The computer drawings of the cathedral that appear in figure 2 were created by Judy Lee, David Iorio, and Ivan Locke. Thomas Curry, Sean Jutahkiti, and Anthony Paul are thanked for their work on measuring vertical wind speeds near the walls of the Cathedral of Learning.