Role of particulate matter from vehicle exhaust on porous building stones (limestone) sulfation

This work, for the first time, experimentally demonstrates the relationship between motor vehicle emissions and the decay of ornamental calcareous stone, by means of sulfation processes (the well-known phenomenon of Black-crust formation). The critical catalytic effects of carbon (soot) and metal-rich particles from vehicle exhaust result in the acceleration of the rate of fixation of atmospheric SO2 to form gypsum on the limestones (calcarenites) used to build Granada Cathedral (Spain). The analysis of particulate matter deposited on the building (carbonaceous and metal-rich particles), as well as of emissions from both leaded-gasoline and diesel motor vehicles confirms that the origin of the particulate matter found in the surface of decayed building stones from Granada Cathedral is consistent with having been contributed by motor vehicle exhaust. Experimental data indicate the role played by this particulate matter in the fixation of atmospheric SO2 as sulfates (gypsum) on calcareous materials in the presence of humidity. We have also experimentally demonstrated that there is a close relationship between the composition of the particulate matter and the fixation rates of the SO2 in the form of sulfate: (a) diesel engine exhaust, which is primarily composed of soot and metallic particles bearing Fe and Fe-S as major elements and of Cr, Ni, Cu, and Mn as trace elements, plays the largest part in the catalytic oxidation rates of SO2; (b) the emissions from gasoline engines, composed of minor quantities of soot and high concentrations of Pb- and Br-bearing particles, cause a lower rate of SO2 fixation as gypsum on limestones. From these experimental findings, a new hypothesis is proposed concerning the sulfation of the limestones.