'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.