Monuments as sampling surfaces of recent traffic pollution

Background, aim and scope A new approach towards monuments, considering them as a passive sampler of pollution, is presented. Cultural Heritage objects suffer daily the damages of environmental pollution, especially in those areas interested by heavy traffic. Since monuments undergo only periodic conservation or maintenance works, surfaces are able to accumulate atmospheric deposit and to record changes in its composition. An optimised analytical protocol was developed in order to quantify platinum and rhodium at trace level on surfaces. The two elements have become tracers of automobile emissions in recent years, since the introduction of catalytic converters, and could have catalytic effects on the decay reactions of natural and artificial stone materials. As a first case study, the cement mortar surfaces of a twentieth century monument, the Camerlata Fountain, in Como (Italy) were investigated. Materials and methods The surfaces of the monument were scraped in areas both exposed to atmosphere and sheltered by the architectural elements of the building. The powders were dissolved by microwave-assisted mineralisation with a solution of HCl and HNO₃. The solution was filtered, irradiated and analysed by adsorptive cathodic stripping voltammetry. The powders were also analysed by infrared spectroscopy and X-ray diffraction in order to determine the chemical and mineralogical composition. Results and discussion An analysis protocol was set up considering the matrix effect and the expected low concentrations of the two metals. The results enlightened variable concentration values and distribution areas of platinum (0.013-45 μg/kg) and rhodium (0.55-274.4 μg/kg), suggesting the ability of artificial stone surfaces to accumulate the two elements. The sample chemical and mineralogical composition was consistent with a typical cement plaster interested by decay phenomena. Conclusions This work investigated the relation between Cultural Heritage and pollution by another point of view. The analytical protocol presented in this paper was effective in determining platinum and rhodium in traces on the investigated stone surfaces with negligible matrix effects. Recommendation and perspectives The presence of platinum and rhodium on monument surfaces should be of significant interest when planning Cultural Heritage conservation. A better knowledge of the role of the two metals in decay phenomena could impact in a positive way artwork conservation.