Review of the Literature on the Topic of Acidic Deposition on Stone (1998-09)
It is extremely difficult to determine how much of the deterioration observed on stones is due to “acid rain” since its action is not limited to one mechanism. Rather, it results from the interaction of various mechanisms, many of which also occur in natural weathering. The aim of this survey is to present a summary of the most relevant publications in the field of acid precipitation on stone to help identify the relative importance of the various factors that contribute to the deterioration of this material. It should also contribute in pointing out those areas in which further study and research will prove more fruitful.
The review has been organized on a relatively chronological order so that the readers can understand how trends in the research within this field have developed. It has been divided broadly into four areas discussing thy deposition, wet deposition, deterioration mechanisms and site studies. The articles reviewed have shown that dry deposition, which is mainly influenced by short range transport of pollutants from local sources, is the main source of damage. Wet deposition, responsible for long range transport of pollutants is secondary. However, in areas where buildings and monuments remain wet for a long time, and in rural areas, wet deposition can be the major source of damage. If pollution is very low, this phenomena becomes indistinguishable form the “normal” weathering due to “clean rain”.
Of the most common pollutants, sulfur dioxide from combustion of fossil fuels, or nitrogen oxides from combustion engines, and its oxidation product nitric acid, the former is the most important factor in the deterioration of stone. In part this is due to its capability of reacting with any calcareous component in the stone or atmosphere and producing gypsum. The contribution of sulfates from aerosols, however, are not as important as the contribution of the gaseous sulfur dioxide.
One of the important issues in the susceptibility of stone to attack by sulfur dioxide is its receptivity, i.e., the capability of retaining this compound and reacting with it. This depends on the nature of the stone and the presence of moisture. Since the stone is a porous material, the condensation of moisture on it and within its pores is critical to its reactivity. This is an area, which although it has been studied in many of the laboratory simulation tests, still requires further study. Particularly with regards to defining “time of wetness”.
The deterioration that affects stone, either by crust formation or by dissolution has been studied all over the world. However, much needs to be done in processing the data obtained as a function of the mineralogical composition of the stone, its porosity and pore size distribution, and the finish to which it has been subjected when introduced in the monument.
The present survey should also serve to give a general view of the field to professionals who are interested in the acid rain problem as well as background information for scientists who are planning to continue research in the field.