Vertical gradients of pollutant concentrations and deposition fluxes on a tall limestone building

The role of air pollutants in the soiling of a limestone building was investigated by measuring pollutant airborne concentrations and deposition at different heights at the Cathedral of Learning in Pittsburgh, Pennsylvania. Airborne concentrations of sulfate particles (SO₄²⁻), carbon particles, sulfur dioxide gas (SO₂), and total NO₃⁻(particles + HNO₃) were measured simultaneously on the 5th floor, 16th floor, and roof (42d floor), while laser particle counts of >0.5 μm and >5 μm particles were obtained on the 5th and 16th floors. SO₂deposition fluxes to wall-mounted surrogate surfaces were measured at a total of nine locations on the 5th and 16th floors. Measurements were conducted during four 4- week time periods over one year. Results showed that airborne concentrations of the chemical species were invariant with height. Airborne number concentrations of >0.5 μm particles corroborated this result. Although not reflected in the chemical data, measured number concentrations of >5 μm particles on the 16th floor were on average 30% greater than those on the 5th floor. The spatially averaged highest and lowest deposition velocities of SO₂(1.0 cm/s and 0.6 cm/s) never differed by more than a factor of two for the different time periods. The relative differences in deposition velocities from one location to another were consistent throughout all of the sampling experiments. The 16th floor deposition velocities were greater than those on the 5th floor due, at least in part, to the fact that sampling locations on the 16th floor were more exposed to wind. The absence of gradients suggests that soiling patterns on the cathedral are determined by the competing processes of pollutant deposition ahd rain washing. This hypothesis is supported by comparing soiling patterns on the cathedral from the 1930s with recent patterns: Archival photographs show much greater amounts of soiling, consistent with the greater air pollution levels that existed then. Results of this study can assist in designing cleaning and treatment protocols for other buildings with similar geometry in similar environments.