'We must face reality. Even if we have sharply reduced urban industrial pollution caused, this has been largely offset by the polluting gas and particle emissions of cars. And it is these that are the major cause of the dirt that becomes encrusted on the facades of historical buildings and monuments,' stresses Hélène Cachier, a researcher at the France’s Centre National de la Recherche Scientifique (CNRS). Cachier, a chemist specialising in climate and environmental sciences, is coordinator of the Caramel project. The project aims to correct our present lack of knowledge of the quantitative mechanisms by which particles of soot present in the atmosphere interact with stone materials.

'Although we understand the chemical nature of the damage, we still have to look at the causes. The mix of carbon soot found in the black crust which coats the stone is very complex. It contains carbon particles from motor vehicles, traces of interactions with atmospheric gases, and the effects of the wearing away of the material itself. Therefore, the carbon needs to be separated out from the other components and then really come under the microscope.'

The most difficult problem is to understand the various ways in which a monument reacts to a given concentration of pollutants. Launched in 2001, the Caramel project first focused on four test sites: the church of St. Eustache in Paris, Seville Cathedral, the Duomo in Milan and Florence Cathedral. Samples were taken of the air and of the patina blackened by carbon particles.

A complex cocktail

A whole battery of instruments is used to capture and analyse the atmospheric pollutants and understand the variability factors. At specific moments, the mass, number and size of the carbon particles are determined precisely. Samples are collected by filters which are replaced weekly to assess seasonal variations.

Initial findings have confirmed the importance of the presence of carbon particles in aerosols in urban areas. These are generated mainly by motor vehicles and vary according to peak traffic times, while also showing high sensitivity to meteorological conditions (air acidity or dampness). One of the most revealing findings was the degree to which readings taken at a single site can vary. The same measuring device can record, in real time, concentrations ranging from 0.5 to 35 µg of carbon per m3, depending on traffic levels.

One example is Seville Cathedral, which is situated in a predominantly pedestrian area. However, the Avenida de la Constitution, which is open to diesel buses, runs alongside it. This has meant that concentrations between the two facades of the cathedral vary by a factor of 10. Widely reported in the Spanish press, this attack on the city’s heritage has prompted residents' committees and shopkeepers' associations to act. The transport authorities are now promising to phase out the diesel buses and replace them with electric ones.