Air quality indicators
Unexpected lichens can be seen forming the small circular yellowish orange plaques of common orange lichen Xanthoria parietina and the greenish white ones of lecanora lichen (Lecanora muralis). They grow on the ends of the bridge’s wooden planks, safe from being trampled. Lichen is an association between a minute alga and a mushroom. They live in symbiosis. The alga, due to its chlorophyll, manufactures carbonaceous food vital to the fungus, which supplies sustenance and retains rainwater, a source of minerals. Thus, they are able to settle where there is no soil. Depending on the air quality, they are valuable bio-indicators.
The presence of common orange lichen, Xanthoria parietina, a nitrophilous lichen, and its abundant growth in an atmosphere rich in nitrogen oxides, is indicative of pollution from nitrogen dioxide (N02). This pollutant originating in traffic is one of the main air pollutants in Paris. In contrast, the presence of lecanora, lichen sensitive to sulfur dioxide (SO2), indicates a decrease of the pollutant in the air over many years.
The Airparif association is approved by the state to conduct observation of air quality in Ile-de-France. It assesses the level of pollutant concentrations, establishes forecasts and circulates bulletins on air quality accompanied by health recommendations.