Pollution writ in stone

Once a treasure trove of pollutants

By Hamish Johnston

Earlier today I caught a few talks in a session called “The Greening of Pittsburgh”.

One talk could have been called “The Cleaning of the Cathedral of Learning” because it focused on how that building’s limestone facade was first blackened by smoke and then blasted clean by the rain.

The study of the Pittsburgh landmark was done by Cliff Davidson and colleagues at Carnergie Mellon University, who looked at historical photos of the building; took samples of the material staining the building; studied how the building is affected by driving rain; and did computer simulations of the wind patterns around the building.

They found that just a few years after the cream-coloured building was completed in 1930 it was completely blackened by smoke. But around 1945 the city began to enforce anti-smoke rules and by 1950 erosion caused by driving rain was beginning to clean the 42-storey building.

The team using wind and rain measurements and simulations the team were able to understand why some sides of the building were getting cleaner, while others remained relatively sooty — because they were exposed to pollutants from a nearby steel works. This was confirmed by studying the chemical composition of the staining, some of which was iron-based.

Sadly, the study came to an abrupt end a few years when the University of Pittsburgh had the entire building sand-blasted clean.

In a different study, Davidson and colleagues discovered that about 75% of particulate-matter pollution in Pittsburgh today comes from outside the city — mostly from coal-fired generators in the Midwest as far away as Iowa.

So if you have a bad air day it Pittsburgh — or anywhere else in Eastern North America — it’s probably because millions of people to the west of you are turning up their air conditioners.