THE GROWTH OF ‘‘BLACK CRUSTS’’ ON ALCAREOUS BUILDING STONES IN PALERMO (SICILY):A FIRST APPRAISAL OF ANTHROPOGENIC AND NATURAL SULPHUR SOURCES

Chemical, mineralogical and isotopic composition (34S/32S) analyses were carried out on “black crusts” developed on the old buildings of Palermo (Sicily) with the aim to distinguish between natural and anthropogenic sulphur sources. Furthermore individual samples of total suspended particulate (TSP) were collected and analysed in order to estimate the influence of air pollution on the decay process of calcareous stones in a typical southern Mediterranean coastal environment. SEM/EDS analysis, X-ray diffractometry, IR spectroscopy and liquid chromatography led to the compositional characterization of “black crusts” and airborne particulate matter in the city. Sulphur isotopic compositions of gypsum-bearing crusts showed a prevailing contribution of anthropogenic sources (vehicle exhaust and other combustion processes). Natural sulphur and/or sulphate sources (biogenic and/or sea-spray) were shown to play a secondary role. These data can be considered a starting point for the determination of the rate of growth of “black crusts” in a coastal Mediterranean urban environment like Palermo. In the future, they might be useful to local authorities for planning more efficient maintenance action in order to slow as much as possible the rate of decay of the monumental heritage of the city.