Geochemical records of limestone façades exposed to urban atmospheric contamination as monitoring tools?

Magnetic susceptibility, surface rock soiling, elemental composition and lead isotope ratios were measured in surface stone samples collected at different heights of a late 19th century building in Dijon, France. We targeted four limestone façades that differ in orientation and proximity to car traffic. It seems that zinc, copper, sulphur and cadmium are present as diffuse pollutants in urban atmosphere, at least at the scale of the building studied. In contrast, lead and arsenic exhibit point sources: automotive traffic and past coal-burning fly-ash emissions; both coherent with lead isotopic composition measurements. Parameter variations primarily result from exposition to rain washing or micro-scale runoff, and from the closeness, magnitude and origin of anthropogenic sources. Both anthropogenic particles and natural dust tend to be similarly affected by deposition/soiling and rain washing, which act dynamically as competitive processes. Examination of archival photographs suggests that soiling predominated in the past, when the air was rich in black dust, whereas equilibrium or even weathering may occur nowadays due to recent improvements or at least changes in air quality. Using the chemical composition of building façades to provide insights into the magnitude and dispersion of urban atmospheric pollutants may not always be straightforward because of uncertainties related to the period of accumulation.