Monday, October 29, 2012

Atmospheric Pollution and Stone Decay: St Paul’s Cathedral

I have recently published a paper with colleagues from Oxford, Cambridge, Sussex and York on a 30-year measurement of erosion rates on St Paul’s Cathedral, London (  - you need to have an account with the journal to access the paper). Pleasingly, the academic work did get some press ( , , ) and I even did a very short radio interview on local radio (distracted during it because they just started their afternoon quiz to which I knew the answer!)

The paper outlines how the rates of erosion (and the rates of surface change) of five micro-erosion meter sites around the cathedral have changed over the decades since 1980 and how this has mirrored a dramatic fall in pollution levels in central London.

Figure 1 Micro-erosion meter site with protective caps being removed for remeasurement

Figure 2 Erosion rates, rates of surface change and environmetnal variables for central London 1980-2010

The erosion rates have dropped since the closure of Bankside power station in the early 1980s with atmospheric pollution, as indicated by sulphur dioxide levels dropping from 80ppb in 1980 to about 3ppb in 2010. Erosion rates fell from 49 microns per year in the decade 1980-1990 to 35 microns per year in the decade 2000-2010. Erosion rates in the 1980-1990 were statistically significantly higher than erosion rates in both the decades 1990-2000 and 2000-2010. Erosion rate in the decade 1990-2000 and 2000-2010 were statistically similar. Although the decline in erosion rates was not as steep as the fall in pollution levels, erosion rates are now at a level that could be explained by the action of the acidity of ‘normal’ or ‘natural’ rainfall alone. ‘Normal’ rainfall is a weak carbonic acid, produced by the reaction of carbon dioxide and water in the atmosphere, meaning that ‘normal’ rainfall has an acidity of about pH5.6.

Erosion rates represent the loss of material from a surface but not all points’ measured lost material, some gained height over the measurement periods – this is surface change. Points can gain height for a number of reasons; slat in the stone could distort and push the surface up, lichens and bacteria could form crusts that raise the surface and eroded material might be deposited in depressions in the surface causing an apparent raising of the surface. The rates of surface change were 44 microns per year in the decade 1980-1990 but fell to 26 microns per year by the decade 2000-2010 (and were only 25 microns per year in the decade 2000-2010). This suggests that rats of surface change fell in a similar manner to rates of erosion and match the drop in sulphur dioxide levels as well.

Back in the 1980s the long-term rates of erosion, since the early 1700s were also determined using lead find. These lead fins were produced when the holes used to raise the stone blocks into the balustrade were filled with lead. Over time the fins became proud of the surface as the stone around them eroded. By measuring the height difference between the fins and the stone (and then dividing by the time of exposure) the long-term rates of erosion can be calculated. The long-term rates from 1690/1700 to 1980 were about 78 microns per year. This suggests that the cathedral has experienced much higher erosion in the decades before 1980. This does suggest that the erosion rates we have managed to measure from 1980 onwards, and the associated pollution levels, were not as damaging to the cathedral as those experienced in the years up to 1980.

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