Impact of air pollution in deterioration of carbonate building materials in Italian urban environments

• Geochemical study of black crusts developed on three monuments located in Milan, Florence and Rome.
• Trace element composition of the black crusts is greatly influenced by anthropogenic pollution.
• Geochemical characterization of black crusts can be used as a reliable indicator of environmental pollution.

This work presents results from a petrographic, morphological and chemical study of the black crusts developing on monuments in three Italian cities, the Cathedral of Milan, the Cathedral of St. Maria del Fiore in Florence, and the Vittoriano Monument in Rome.Black crusts (BCs) were studied with traditional techniques such as optical microscopy (OM), scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDS) and infrared spectroscopic techniques (FT-IR), in combination with laser ablation inductively coupled mass spectrometry (LA-ICP-MS), which has shown itself to be particularly useful in determining concentrations of heavy metals in BCs.Although the BCs of the three monuments show general enrichment in heavy metals with respect to the substrate (S), interesting differences were observed among them. The BCs from Milan are the richest in heavy metals, particularly Pb and Zn, reflecting the severe air pollution of this very large city, which, in addition to its intense traffic, is located in the most highly industrialized area of Northern Italy. The BCs from the south-eastern side of the Cathedral of St. Maria del Fiore in Florence, facing a pedestrian area, show little enrichment in heavy metals, and those from the Vittoriano Monument in Rome, which is exposed to intense road traffic, display variable enrichment, attributable to mobile emission sources.Results show that the various enrichment trends in heavy metals observed in the BCs of these three monuments are due to many factors: various sources of anthropogenic pollution, sampling height, exposure, orientation, and the shape of the deposition surface.