The impact of atmospheric pollution on outdoor cultural heritage : an analytic methodology for the characterization of the carbonaceous fraction in black crusts present on stone surfaces.

COVID-19 has reduced tourism in both museums and historical sites with negative economic effect. The wellbeing and good preservation of monuments is a key factor to encourage again tourism. Historical monuments exposed to outdoor pollution are subjected to well known degradation phenomenon including the formation on their surface of black crusts (BCs) causing blackening and deterioration of the monuments and, as a consequence, a worst fruition by the visitors. The aim of this research is the development and validation of a novel method to characterize and quantify the various components present in the black crusts. SO2 together with the carbonaceous fraction (i.e. OC, organic carbon, and EC, elemental carbon) represent the main atmospheric pollutants involved in the process of BCs formation which consists in the partial transformation of the carbonate substrate into gypsum where black particles are embedded. A new methodology based on the use of TGA/DSC (Thermo Gravimetric Analysis/Differential scanning calorimetry) and CHN (Carbon, Hydrogen, Nitrogen) analysis was set up allowing to determine organic carbon and elemental carbon together with other components such as gypsum. Four standard mixtures simulating BCs composition were prepared and analysed to the set-up the methodology. The new procedure was subsequently applied to study real BCs samples taken from monuments and historical buildings placed in cities heavily affected by atmospheric pollution and by PCA (principal component analysis) their main features, from the point of view of carbonaceous fraction, were highlighted.