The influence of combustion-derived pollutants on limestone deterioration .2. The wet deposition of pollutant species

The wet deposition of combustion-derived atmospheric pollutant species, on to freshly cut or diesel-smoked Portland and Monks Park limestone and marble samples, was carried out in a laboratory 'wetting and drying' salt spray chamber for 84 days. Along with the effect of CO2-equilibrated de-ionized water at pH 5.5, for clean-rain base-line data, the relative effects of the Cl-, SO42- and NO3- components (5.2, 8.6 and 3.0 mg 1(-1) respectively) of an artificial acid-rain solution were studied. Using spray run-off solution rates of 0.2 mi cm(-2), day(-1) and pH values of 5.5 and 3.8, the work was intended to determine the relative significance of water, anions and hydrogen ions. The effects of exposure were followed by analysis for calcium and anions in the run-off solutions, and by analysis for anions in the stone samples after exposure, weighing and crushing. From the total calcium reacted, i.e. released from limestone to reaction ions and products, a theoretical limestone surface chemical recession rate was calculated. It was found that, at pH 5.5, water and solutions of individual anions gave similar recession rates within the range of 7.8 +/- 1.8 and 11.5 +/- 2.3 mu m yr(-1) for Portland and Monks Park limestones, respectively. At pH 3.8 the individual anion recession rates were increased to 11.1 +/- 0.5 and 13.9 +/- 0.5 mu m yr(-1) respectively, similar to the rates of 12.5 +/- 1.6 and 13.8 +/- 2.2 mu m yr(-1) for the mixed anion artificial acid rain solution on Portland and Monks Park limestones, respectively. Thus the water and hydrogen ion loading effects are significant, as well as the limestone type, but the nature of the anion is not, under the given wetting and drying regime. Further work showed that the increased resistance of marble over that of limestone, to calcium loss, is probably due to its much lower porosity. The results of smoke-coated samples suggested that any surface masking action depends upon particle size distribution.