ABSTRACT
The concomitant deposition of soot onto the surfaces led to thick disfiguring black gypsum crusts. Declining concentrations of corrosive components in air has made the accumulation of black particles more important and raised the relevance of aesthetic considerations. Particle deposition gradually darkens the surfaces of buildings over time. Although the darkening of buildings appears as an exponential of the process, slight difficulties arise with the boundary conditions. Public perception of darkening can be investigated using in situ questionnaires at historic buildings, which can exhibit varying degrees of blackening. This technique can also be used to assess the success of treatments and choice of replacement stone. Despite the overall improvements in air quality, diesel-derived particulate matter was increasing in concentration in heavily trafficked areas. The changes under way have both aesthetic and economic implications for the cleaning and maintenance of historic buildings.
