The influence of relative humidity on the SO2 deposition velocity to building stones: a chamber study at very low SO2 concentration

We report on a chamber study to determine the influence of SO₂ concentration and relative humidity (RH) on the deposition velocity of SO₂ to a carbonaceous sandstone (Ihrlerstein sandstone) and to two silicate sandstones (Sand sandstone and Obernkirchen sandstone). Two different chambers were used, one operated at constant inlet SO₂ concentration, the other one at constant outlet concentration. Measurements were carried out at realistically low SO₂ concentrations ranging from 15 ppb to 200 ppb. The relative humidity was varied from 45 to 95 %. The measurements confirm a strong influence of concentration on the deposition velocity for all stones investigated. This concentration dependence can be represented using a very simple model with only one rate constant as the only adjustable parameter. A significant influence of the relative humidity on the deposition velocity to the carbonaceous stone is also observed. In contrast, the deposition velocity to the silicate stones is independent of RH. It is demonstrated that the combined influences of concentration and RH on the deposition velocity can be only correctly determined in chambers with constant outlet concentration.