Effects of particulate matter from gasoline and diesel vehicle exhaust emissions on silicate stones sulfation

The effects of particulate matter (PM) from diesel and leaded gasoline motor vehicles exhaust emissions on sulfation of granites, syenite and gabbro stones have been experimentally studied. Abundant gypsum crystals and corrosion features developed on stones covered with diesel PM (DPM) following 72 h exposure to 100 ppm SO2 at a relative humidity of 100%. In contrast, very small amounts of gypsum were observed on stones covered with gasoline PM (GPM), while no effect was observed on naked control stones. Abundant elemental C and Fe-rich particles in DPM play a critical role in the catalytic oxidation of SO2 and the formation of H2SO4, which is responsible for silicate stone sulfation. Conversely, organic C and Pb-rich particles that are main components of GPM, do not play a significant role in sulfation. The response of each stone type towards sulfation is related to the stability of their constituent silicate minerals towards acid attack. Thus, the stones most susceptible to sulfation are those including nepheline (syenite), olivine, and pyroxene (gabbro), while granites in general, are most resistant to sulfation-related chemical weathering. These results help to explain how black (gypsum) crusts develop on silicate stones, and support limitations for (diesel) vehicular traffic and emission loads in urban centers.