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| Cleaning up Europe’s cultural and historic landmarks

Take a walk in any big European city and what do you see? If you look closely you’ll discover cultural landmarks, elaborate architecture and impressive historic monuments – that is if you can see through the layers of dust and dirt. The EU-funded CARAMEL project is studying the ugly patinas that coat our buildings, finding out how they form and how to stop them. This important work has been funded under the EC’s 5th Framework Programme for Research – Key Action: 'City of Tomorrow and Cultural Heritage'.

City traffic is blackening Europe's landmarks © Peter Gutierrez
City traffic is blackening Europe's landmarks
© Peter Gutierrez
It is now widely agreed that modern atmospheric pollution is the main culprit in the blackening of European cities. Moreover, the soiling phenomenon appears to be accelerating, making it necessary to continually and repetitively clean and restore monuments and historic buildings. The challenge for the European CARAMEL project (‘Carbon content and origin of damage layers in European monuments’) lies in understanding the development of patinas in atmospheres influenced by multiple pollutants.

Down to basics

“Any comprehensive study of atmospheric pollutants and the composition of urban patinas must focus on soot particles,” explains project coordinator Hélène Cachier of France’s CNRS. “Also referred to as black carbon or elemental carbon (EC), these particles are known to be vectors for the transport and chemical evolution of pollutants. They are major constituents of aerosols (a system of solid particles dispersed in a gas) in the European atmosphere and in the black crusts that form on monuments, but their role has never been accurately quantified and the relationship between atmospheric concentrations and soiling rate has never been investigated.”

One of the CARAMEL project’s overall objectives has been to provide ‘user-friendly’ guidelines, including thresholds for atmospheric soot concentrations compatible with an acceptable soiling rate. These guidelines would be of use to policy makers in soiling prediction and, ultimately, in the protection of Europe’s cultural heritage against atmospheric hazards.

CARAMEL investigators have travelled across Europe, looking for the dirtiest and most disgusting landmarks to study. Samples have been taken at the Saint Eustache Cathedral in Paris, at the Cathedrals of Seville and Milan, at the Vittoriano in Rome, the Corner Palace in Venice and at the Mechelen Cathedral in Belgium. Sites were selected to encompass the diversity of European air pollution sources and meteorological conditions.

Meanwhile, atmospheric measurements were taken in the same areas during week-long intensive campaigns, followed by periodic monitoring over the course of a year. The study focused on both coarse and fine atmospheric particles, their distribution and thorough chemical composition. The characterisation of particles and their temporal and seasonal profiles allowed the identification of sources. All of this required reliable sampling and analytical techniques, which were developed, modified and improved during the project.

New methods for studying patina-related damage were also developed. Black patinas were analysed for carbon content of embedded particles. Their composition was then compared to that of atmospheric particles. Here too, new analytical procedures were developed, adapted and optimised.

An aesthetic index was created combining patina soot concentration results and levels of aesthetic appreciation, determined through interviews with both building conservation and maintenance professionals and members of the public. In addition, computer-based studies of aesthetic appreciation involved the exploration of virtual environments.

Solid results

Maintaining heritage sites makes sense © Peter Gutierrez
Maintaining heritage sites makes sense
© Peter Gutierrez
Elemental carbon was found to be prevalent both in the urban atmosphere and in black crusts that form on buildings. Significant variations in concentration, related both to source intensity and meteorological conditions, were observed for soot particles from one site to another. “We have shown,” says Cachier, “that, unlike in past eras, transport is now the overwhelming contributor of atmospheric gaseous and particulate pollutants in most European urban zones.” The nature of the ubiquitous black patina has also changed dramatically from once-prevalent thick coatings to thinner caramel-like soiling layers.

More findings:

Carbonaceous particles form the main fraction of European urban atmospheric aerosols – the CARAMEL assessment resulted in the establishment of a unique database on urban aerosols, valid for Western European urban sites where drastic sulphur abatement policies, applied over the past three decades, have successfully resulted in low atmospheric sulphur concentrations.

Carbonaceous soot particles form, with sulphur, a significant portion of the historical crusts seen on buildings, but may present different habits, structure and distribution, depending on their source and the nature of the building materials.

Carbonaceous particles in the atmosphere and in black crusts are predominantly of anthropogenic transport-related origin.

Mathematics and aesthetics of soiling – the rate of darkening of material surfaces was shown to follow a properly bounded exponential function. Results of questionnaires on aesthetic appreciation have revealed strong positive or negative reactions generally related to the obscuring of architectural forms.

Atmospheric soot thresholds – a web-based ‘user friendly’ model has been developed, setting specific recommended values for soot content in EC atmospheric aerosols. The model comprises a series of arguments aimed at helping those responsible for cultural heritage to assess relevant factors and to make better-informed decisions.

Europe’s unique identity is closely related to its cultural heritage. The CARAMEL consortium is offering robust arguments to help decision makers improve the urban atmospheric environment and to protect buildings and monuments that are a part of Europe’s history and charm.

“We are a small partnership,” explains Cachier, “involving just four members, but we have been very active and I think we have to say the project has been a success. The preservation of monuments in European cities relates directly to our quality of life and is also likely to have very important economic consequences in terms of tourism and leisure activities.”

Related research themes


graphic element

Dr Hélène Cachier
Laboratoire des Sciences du Climat et de l’Environnement (LSCE)