Pollution erosion at St Paul's Cathedral in record 300-year low


Industrial decline and cleaner energy production have led to pollution erosion at St Paul's Cathedral being at a 300-year low.

A study of the iconic building's Portland Stone facade found sulphur dioxide levels - responsible for acid rain - have fallen by 95% over 30 years.

The results from the building's balustrade mean the Christopher Wren masterpiece is the safest it has ever been from erosion, the study from the universities of Portsmouth, Oxford, Sussex and Cambridge claimed.

The drop is largely due to a decrease in industry and power generation in central London and the capital increasingly moving to cleaner energy.

Acid rain is now responsible for a fraction of 1% of the damage to St Paul's and the rate of erosion at the cathedral is now dominated by natural rainfall, which is a weak carbonic acid with a pH of about 5.6.

The researchers concluded that the building is now safe having survived the ravages of a growing metropolis, the Industrial Revolution, the plumes of sulphuric dioxide gas and smoke from the nearby Bankside Power Station - now the Tate Modern - and Londoners' love of coal fires.

Scientists co-ordinated by Robert Inkpen from the University of Portsmouth monitored the rate of erosion on the building's balustrade between 1980 and 2010.

They found atmospheric sulphur dioxide concentrations fell from a daily average of 80 parts per billion in the early 1980s to less than three parts per billion by the late 2000s.

The average rate of erosion over the 30-year period subsequently fell by 22% from 0.049mm - equivalent to about half the thickness of a human hair - in 1980 to 0.035mm - about a third of the thickness of a human hair - in 2010.

Dr Inkpen said: "We were surprised that the results were so compelling - the drop in erosion over 30 years is quite dramatic and the data clearly illustrates erosion rates have now fallen to levels you would expect with just natural rainfall."

The research team began their study a year before the closure of Bankside Power Station in 1981. It had been generating electricity for 29 years emitting carbon dioxide, sulphur dioxide and nitrous oxide.

The year it opened, in 1952, pollution across the city was commonplace and London's 'great smog' of December in that year saw thousands die due to pollution.

Dr Inkpen explained: "The cocktail of gases emitted by the power station and the fact it was directly across the river from St Paul's means it is highly likely it was responsible for the more modern erosion of the cathedral.

"But it is by no means the only problem. London's growth as a major international city since Wren's day and the pollution produced by the industrial and commercial activity of a thriving city as well as domestic smoke have all contributed to producing high erosion rates in the past."

Previous research by the team suggested that between 1710 and 1980 the yearly erosion rate was at least double the current erosion rate.

The research team measured five sites inside the balustrade, including those facing north, south, east and west.

The west-facing site, directly exposed to the pollution plume from the now defunct power station, showed the most erosion.

Dr Inkpen added: "Reduction of atmospheric pollution to virtually zero for sulphur dioxide is an important factor in explaining the high rates 1980-1990. In addition, it could be argued that as air quality has continued to improve, microflora, such as algae and lichen as well as microbial activity, has continued apace between 2000-2010, resulting in a more even colonisation of the surface and potentially lower erosion rates into the future."

St Paul's was designed by Sir Christopher Wren after the Great Fire of London in 1666 destroyed its predecessor. Work started in 1675 and was completed in 1710. The balustrade was added - against Wren's wishes - in 1718.

It was the first cathedral to be built after the English Reformation in the 16th century.

The researchers have continued studying erosion and surface change rates at the cathedral and will report their next findings in 2020.

Their latest research is published in the journal Atmospheric Environment.


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