Deposition of sulfur dioxide to building stones: the influence of the ambient concentration on the deposition velocity

We report on extensive deposition measurements of sulfur dioxide (SO₂) on three types of commonly used building stones (Obernkirchen sandstone, Sand sandstone, Ihrlerstein sandstone). The deposition velocities on the three materials were determined in a number of measuring campaigns at different sites with different levels of air pollution. The measurements clearly imply that there is a strong influence of SO₂ concentration on the deposition velocity. With increasing concentration there is a strong decrease in the deposition velocity. To understand this deposition behavior a simple model with a single rate constant as the only adjustable parameter was established. Rate constants for the three stone materials were determined by least squares analysis and good agreement between experimental and calculated deposition velocities was obtained. According to the model treatment the surface resistance of stone materials increases with increasing SO₂ concentration due to a decrease of the pH in surface films. It follows from the concentration dependence of the deposition velocity that emission control measures for the reduction of ambient SO₂ concentrations do not lead to a proportional reduction of the SO₂ deposition. At low SO₂ concentrations the differences in the deposition behavior of different stone materials diminish and stone surfaces behave like ideal absorbers under such conditions, i.e., the deposition velocity is controlled by the aerodynamic resistance.