Processes affecting soiling and deterioration of a tall building

摘要

Two of the primary processes affecting soiling and deterioration of stone buildings are deposition of air pollutants to the walls and washing of the walls by rain. Through a combination of experimental and modeling work, these processes were studied at the Cathedral of Learning, a forty-two story limestone building in Pittsburgh, PA.;Quantification of air pollutant deposition was performed using bulk chemical analyses as well as analyses of individual particles. For the case of bulk chemical analyses, airborne concentrations of SO42- particles, carbon particles, SO2 gas, and total NO 3- (particles + HNO3) were measured. Laser particle counts of >0.5 mum and >5 mum particles were obtained on the fifth and sixteenth floors. Results showed that airborne concentrations of the chemical species were invariant with height. Airborne number concentrations of >0.5 mum particles corroborated this result. Although not reflected in the chemical data, measured number concentrations of >5 mum particles on the sixteenth floor were on average 30% greater than those on the fifth floor.;Samples of alteration crust from soiled areas were comprised of gypsum and were littered with anthropogenic particles. Samples from white areas were composed primarily of calcite and were virtually free of such particles. Median values of deposition velocities of small particles to surrogate surfaces positioned in soiled and white areas were similar, although differences were noted for deposition of large particles.;Overall, the results from both the bulk chemical and scanning electron microscopy studies suggest that while deposition of air pollutants varies slightly from one location to the next, soiling patterns at the Cathedral of Learning are not determined solely by these variations. Instead, they are a result of the competing processes of pollutant deposition and washoff by rain. This hypothesis is supported by comparing soiling patterns on the Cathedral from the 1930's with recent patterns: Archival photographs show much greater amounts of soiling, consistent with the greater air pollution levels that existed then.;To elucidate how rain washing affects soiling, the delivery of rain to the walls of a tall building was modeled using a two-equation model for turbulence and Lagrangian trajectory calculations for individual rain drops. Wind speed, wind direction, and rain intensity were measured continuously over a seven-week period. These data served as input parameters for computing fluxes of rain to the Cathedral, modeled as a 30 m x 30 m x 160 m prism.;Preliminary modeling results confirmed the importance of better understanding the delivery of rain. In general, sections of the prism receiving small amounts of rain corresponded to soiled areas of the Cathedral while sections receiving larger amounts corresponded to white areas.;Experimental and modeling results from this study are important to conservators trying to understand better the processes responsible for soiling and deterioration of calcareous stone structures. (Abstract shortened by UMI.).