Damage processes on stones in urban environment: field exposure tests and laboratory analyses contributing to pollution impact evaluation

Stone conservation of historical built heritage in polluted atmosphere is an outstanding topic. In particular, the deposition, accumulation and interaction with the substrate of complex mixture of pollutant particles and gases can cause the formation of damage layers and, in consequence, entail chemical modification, physicomechanical damages as well as aesthetic change of stone. Furthermore, environmental policies have recently restricted the concentration of atmospheric sulphur dioxide, considered as the most detrimental pollutant for carbonate materials, while an increase of nitrogen compounds, ozone and organic compounds due to higher vehicular traffic may lead to a chromatic variation of the damage layer and a probably growth in biological degradation. Until now, the impact of pollution on cultural heritage was studied by analysing samples collected from historic buildings or performing tests in simulation chamber and/or in field, but gaps still remain in developing damage and dose-response functions useful for the long-term management of cultural heritage sites and in measuring the deposition fluxes on materials. The Institute of Atmospheric Sciences and Climate, ISAC-CNR (Bologna), in collaboration with the Department of Physics and Earth Sciences of the University of Ferrara, started in November 2015 a research work aimed at assessing the effect of urban atmospheric pollution on mainly carbonate sedimentary and metamorphic stones by performing field exposure tests in Italian cities characterised by different environmental conditions. The methodological approach selected for setting up the field tests and study the impact of pollution is presented. As a first step the selection of the proper stone substrates, time and sites for the exposure and placement of the samples is carrying out. Specifically, the choice of the appropriate litotypes is factoring in marble and limestone widely employed in historic Italian architecture and characterised by an almost totally carbonate composition. Preference for samples exposure will be given to sites located outdoor, partially sheltered from the rain wash-out, in areas strongly affected by pollution due to vehicular traffic. The exposed samples will undergo mineralogical, petrographic and geochemical analyses (such as Optical Microscopy, Inductively Coupled Plasma Mass Spectrometry, Ion Chromatography analysis and Thermal-chemical methodology proposed by Ghedini et al., (2006) using a CHNSO combustion analyser), carried out in predefined time intervals, to characterise the deterioration products coming from the interaction between pollutants and stone substrate in terms of typology, origin and damage caused to stone. Moreover, the integration with colorimetric analysis will hopefully allow to identify a correlation between the deposited soluble and carbon fractions and changes of colorimetric parameters, for setting up damage functions. Finally, environmental monitoring of atmospheric particulate matter will be organised in order to acquire useful information from the comparison between the concentrations of pollutants deposited on stone substrate with those present in the atmosphere