For more than two centuries, the great development of industries, transport and heating has led to significant emissions of sulfur, nitrogen and carbon compounds into the atmosphere. These compounds are either gaseous (SO2, I have notx, CO, CO2 ...), or particulate (flying centers et soot). Subjected over the years to their action, the materials of the facades, mainly stone, cement and glass, deteriorate.
Did you know that facades darken especially in their lower parts and sheltered from the rain?
Observation of a building or statue reveals the extent of this physical and aesthetic degradation attributed to the deposition and attachment of blackish dust. Thus, on the same facade, dark areas and light areas coexist. The first, sheltered from the rain, are covered with a thin film of soot associated with a small quantity of sulfates and carbonates. Conversely, light areas, hit by rain or covered by running water, offer the appearance of bare, washed or even eroded material: the particles deposited between two rains have been evacuated, as well as the sulfates and carbonates that could have formed. If the dark areas are old and have not been cleaned for several decades, they contain not thin films but thick black crusts that are highly sulfated and contain fly ash. These thick crusts were formed at a time when sulfur dioxide pollution was significant, which is no longer the case today in Paris.
The distribution of these dark and light zones on the same facade responds to a simple logic: the upper parts of the building, more frequently affected by rain, have a majority of light zones, while its lower parts, subject more directly to emissions from the car traffic, contain a majority of dark areas. Towards the base of the walls, the interplay of atmospheric pollution, rain, rising salt-laden water from the ground and the greater or lesser fragility of the stone linked to its composition and its porosity, leads to the formation of a puzzle of small black, gray and white spots due to the periodic detachment of small scales with sinuous contours.
The surface of all materials can become covered in black soot: stone, plaster, cement, concrete, glass, stained glass, brick, ceramic, wood, plastic, metals... but only those which contain carbonates can become sulphated at depth because the O2 transforms them easily: this is the case of limestones and calcareous sandstone.
Does air pollution also damage windows and stained glass?
The glass in the windows and facades of many large contemporary buildings is chemically stable due to its composition (silicon, calcium and sodium): rain, even acidic, alters it very little. On the other hand, on the areas that it washes, it leaves whitish or grayish traces which make it blurry; on areas that it does not reach, deposits of black soot would quickly develop if regular cleaning did not prevent them from forming.
The case of old stained glass windows is more worrying: of different composition from that of modern windows (silicon, calcium and potassium), they are easily chemically attacked by rain, to the point of being deeply corroded, or even holes. In areas sheltered from the rain, deposits of black soot form and remain in place, because the stained glass windows are not regularly cleaned, except during major restoration campaigns, which are rare and very expensive.
In the past, did we also see the effect of pollution on buildings in cities?
The pollution of cities is not a recent phenomenon: the Parisian monuments bear witness to this. When burning wood, coal, heavy or light fuel oil, gasoline, kerosene, natural gas, blackish carbonaceous particles are emitted which will settle and become embedded in the facades, thus causing them to blacken. . It is therefore not forbidden to think that before the industrial revolution of the end of the 18nd and the beginning of 19nd centuries, the air of cities was already polluted by the residues of the combustion of wood, which was the only fuel for heating, cooking and crafts, and by that of the oil and tallow with which we illuminated, particularly in churches. In addition, we must not forget that these cities were overpopulated, especially in the Middle Ages, and that combustion sites were numerous and dense.
Tangible proof of this pollution of Parisian air before the industrial revolution exists on the original heads of the statues of the Kings of Judah which adorned the facade of Notre-Dame, above the three main portals, from the Gothic era. (11nd century) to the French Revolution. They were shot down in 1792 and buried in an unknown location in 1796. Found by chance in 1977, they are currently on display at the National Museum of the Middle Ages at the Hôtel de Cluny. They show gray crusts containing wood debris: the pollution from the fumes of this fuel was dense enough to encrust the stone of the cathedral.
Other interesting testimonies are reported by the Parisian painter Demachy on several of his paintings painted before the industrial revolution and exhibited at the Carnavalet Museum, for example: "The clearance of the colonnade of the Louvre" (1763) or "The demolition of the Church of Saint-Barthélemy in the City" (1770). Marks of atmospheric pollution at that time, due to the burning of wood, are painted exactly in the places where we expect to find them because they are the same locations as in the current era: the upper parts of the columns and walls, sheltered from the rain, are black although in full light.
Can we assess in advance the effects of air pollution on building materials?
Predicting and evaluating the effects of atmospheric pollution on materials means being able to say in advance what the degree of sulfation or carbonation, the loss or gain of mass, the blackening of the stone, or even the loss of transparency of the glass and stained glass windows when they are placed, for a given period, in a polluted environment of which the concentration in gases and particles, in addition to the average temperature, air humidity, height and acidity of rain. This forecast consists of establishing the relationship which links the doses of pollution received by the material to the effects, or responses, which they induce on it, what is called a "dose-response function".
To establish this function, it is necessary to have a large number of doses and responses, in order to carry out statistical processing. This large number is established either during exposure experiments to atmospheric pollution in real sites with measured and varied conditions, or by experiments in an atmospheric simulation chamber. The first have the advantage of making all the atmospheric parameters act simultaneously and in synergy, the second have the advantage of being able, on the contrary, to fix all the parameters except one, the one that we will vary in the room and whose different doses will induce different responses.
A simulation experience on a real site has been taking place for several years in the center of Paris, at the top of the north tower of the Saint-Eustache church, in the heart of the pedestrian Halles district. Air pollution there is that of background pollution Parisian. Samples of Parisian limestone and glass like that used for windows and stained glass windows are exposed there, sheltered or not from the rain. Regularly recorded, they are analyzed and their responses (sulphation and carbonation, even weak; gain or loss of mass; blackening, loss of transparency) are linked to the different doses of gas and particles they received on the site. . These doses are measured in Saint-Eustache by Airparif in the Les Halles station, the meteorological data being provided by Météo-France.
To date, three dose-response functions concerning built heritage materials have been established during major international research programs. A first links the loss of mass of limestone exposed to rain to the quantity and acidity of the rain, as well as to the SO content of the air.2 and nitric acid; a second links the loss of transparency of the glass to the soot and SO content of the air2 and in NO2 ; a last one links the superficial loss of old stained glass windows in potassium and calcium to the relative humidity of the air and its SO content2 and No2. Thus, thanks to these functions, the risk incurred by a material placed in a given location whose atmospheric pollution characteristics are known can be assessed.
And tomorrow ? Many questions that remain unanswered
What is the current action on materials with pollutants other than SO2, including those produced by transport, which are at the center of concerns since sulfur pollution, largely of industrial origin, has considerably decreased: nitrogen oxides (NOx), carbon dioxide (CO2), ozone (O3), Volatile Organic Compounds (VOC), very fine carbon particles...?
Furthermore, among the foreseeable developments in the environment, those linked to global climate change are currently at the forefront of the scientific and media scene. A projection for the end of the 21st century , based on the combination of the dose-response function of recession of the surfaces of limestone facades exposed to rain with the British Hadley model of climate evolution, shows that the dissolution of these facades by water loaded with CO2 should increase to become greater than that due to SO2 and acid rain, both in urban and rural areas. This recession, which was around 15 to 20 µm/year at the end of the 20nd century in Ile de France, should increase by around 2 µm/year at the end of the 21nd century. Thus, atmospheric concentrations of CO2 would become the main factor in erosion of limestone building facades. The same type of application of climate change models combined with dose-response functions is underway for facades sheltered from rain (blackening) and for glass and stained glass windows, by comparing, moreover, the forecasts from the British Hadley model with those from Météo-France's Arpège model.