Heterogeneous reactions of minerals with sulfur and nitrogen oxides

The heterogeneous reactions of SO 2 and NO 2 at sub ppm levels with mineral particles, such as soil and cement, were investigated. These reactions were studied to understand the role of mineral particles in the formation of sulfates and nitrates. Experiments were carried out in a static reaction chamber made of Teflon, where particles are deposited on filters and on electron microscope grids. After exposure to SO 2 or NO 2, individual mineral particles were studied for their sulfate or nitrate content using electron microscopy techniques. In parallel, sulfates and nitrates were also determined by bulk methods applied to the filter samples. Scanning electron microscopy has also been used for the comparison of mineral particles before and after exposure to SO 2. Results obtained both in bulk and individual particle analyses showed clearly that minerals reacted with sulfur and nitrogen oxides to form sulfates and nitrates, respectively. The formation of sulfate on minerals was in the range of 25–150 mg SO 4 2− g −1 mineral ([SO 2] = 0.86 ppm; exposure time 1–5 days; relative humidity (RH) = 85% and various aerosol mass loading). Lower values were obtained for the formation of nitrate, 0.9–4.5 mg NO 3 − g −1 mineral ([NO 2] = 0.18 ppm; exposure time 1–5 days, RH = 60%). Application of electron microscopy and microspot techniques to the same samples showed that minerals reacted with SO 2 or NO 2 to form a layer of sulfate or nitrate on the mineral surfaces. Not all the minerals had reacted. For the conditions given above 20–40% of the minerals (0.5–5.0 μm) reacted with SO 2, and 60–70% of them reacted with NO 2. Microscopy provided direct evidence on the heterogeneous reaction of sulfur and nitrogen oxides with minerals.