Role of NO2 and SO2 in degradation of limestone

A UNESCO designated World Heritage Site, the city of Bath has many important listed buildings largely constructed of limestone fron local quarries. Among present day atmospheric pollutants, SO2 is particularly damaging to calcareous stone, although its levels are decreasing with the use of low sulphur oil and coal, flue gas cleaning systems, and the switch to natural gas burning for many purposes. Typical urban SO2 concentrations have fallen from 39 to <5 ppb over the past 12 years. In contrast, present day levels of NO2 are enhanced by the increased emissions from automotive exhausts, having risen to 30 ppb from 15 ppb in 1987. In recent years, the effect of NOx species on the deterioration of calcereous stones has also become evident, both exclusively and in the presence of SO2. Bath stone is especially susceptible to attack. To shed new light on the process of gas-stone interaction, specimens of the oolitic limestone have been exposed to gaseous SO2 and NOx under controlled conditions. X-ray photoelectron spectroscopy and weight gain analysis have been used conjointly to determine the chemical state of the reaction products, and to elucidate the mechanism of the process of degradation from its earliest stages.