Thirty years of erosion and declining atmospheric pollution at St Paul's Cathedral, London

The relationship between limestone deterioration (in the form of recession or erosion rates) and changing air quality has been the subject of much debate. A 30-year record of limestone erosion rates from the balustrade of St Paul's Cathedral, London, UK has been obtained from five micro-erosion meter (MEM) sites (last remeasured in September 2010) providing a unique, long-term dataset with which to examine the nature and causes of changing deterioration, particularly the existence of a memory effect. Whilst atmospheric sulphur dioxide concentrations fell from a daily average of 80 ppb in the early 1980s to less than 3 ppb by the late 2000s, erosion rates (measured as surface lowering or recession) only declined from 0.049 mm a−1 to 0.035 mm a−1 over the same period. A more conservative measure of the rate of surface change (which includes both lowering and raising of the surface) fell from 0.044 mm a−1 to 0.026 mm a−1 over the same period. The rates of surface change and erosion were significantly higher in the 1980–1990 measurement period compared to the 1990–2000 measurement period, whilst the average rates for the 1990–2000 measurement period were approximately the same as those for the 2000–2010 measurement period. There is no clear evidence for a memory effect, and rates of erosion and surface change now approach those found on natural karst surfaces.