Role of NO 2 and SO 2 in degradation of limestone

Abstract

A UNESCO designated World Heritage Site, the city of Bath has many important listed buildings largely constructed of limestone from 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 NO_x 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 NO_x 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.