The role of the gaseous pollutants, HCl and SO2, has been investigated in a laboratory-based atmospheric flow rig. Using an HCl level of 25 ppm at a pollutant bulk presentation rate 10 times that found typically outdoors, a degradation acceleration factor of about 20 times was obtained, together with realistic degradation products. HCl was found to be a more reactive gas than SO2, which was also studied at the 25 ppm level. This may be due either to factors involved in the adsorption of the gases on to the stone surfaces, or the extensive solubility of HCl, or the requirement of an extra step in the conversion of SO2 to sulphate, which would limit the overall reaction rate. The soluble degradation product CaCl2 is readily washed off the stone allowing continuous reaction, while the relatively insoluble CaSO4.2H2O can remain on, or in, the stone surface regions and may inhibit or affect further reaction with SO2. The reaction of limestone with HCl ps is mass-transport limited and greater material losses arise in locations of higher HCl concentration and deposition velocity, i.e. a close-to-source effect is highlighted due to its high solubility and reactivity, but the HCl concentration is, of course, generally about one-fiftieth of that of SO2 outdoors. For SO2, with lower solubility and a necessary oxidation stage to form SO42-, comparatively high deposition velocities are applicable more widely from the source. The consequences include a widespread formation of relatively insoluble CaSO4.2H2O and consequent crust development.
