“Sulfur smell”
in Ile-de-France:
several hypotheses
for a mysterious scent
An unpleasant odor was widely felt in the Paris region during the night from Sunday to Monday. The analyzes carried out by the Paris prefecture found no answer.
The laboratory of the Prefecture of Police made samples last night to try to identify the origin of the smell of sulfur which annoyed the Ile-de-France residents this Sunday evening .LP/Fred Dugit
The inhabitants of the north of Île-de-France spent time with their noses out the window this Sunday evening. A sulfur scent emanated everywhere in the north of the capital, causing many worried messages on social networks.
Ile-de-France firefighters have received numerous calls to report an unusual smell of sulfur in the air, without being able to give an explanation on Monday for this phenomenon, possibly linked to bad weather. The smell was reported as far as Melun (50 km south of Paris) and Meaux (50 km east of Paris). But also since this Monday morning, in Hauts-de-France or in the Pays de la Loire.
"The smell of sulfur was recognizable but the concentration very, very low," said Monday morning Emmanuel Grégoire, first deputy to the mayor of Paris, who commented on the "incident" as early as Sunday evening on social networks. The police headquarters laboratory took samples last night to try to identify the origin of this odor.
No compound identified
"These analyzes did not identify any particular compound, in particular sulfur, which could have been the cause of these odors," announced the Prefecture of Police in the evening. She also recalls that she "was not aware of any event of the industrial incident type in the region which could have been the source".
Several hypotheses are envisaged: "the sanitation source" of waste water or "a cloud of sulfur dioxide arriving from abroad", said Emmanuel Grégoire on behalf of the Paris city hall. On this last point, "the experts tell us that it is possible," he added without giving further details. Sulfur dioxide (SO2) can be recognized by a characteristic rotten egg smell.
As for Airparif, the regional body that monitors air pollution in Île-de-France, we remain cautious about this second scenario. "There is little risk of import and there would have been other reports in other regions" said Pierre Pernot, Airparif engineer. "On our measurement network, there were no significant increases in air pollution this Sunday," he continues. "It is quite logical because the very dispersive conditions, significant precipitation and strong wind, do not favor such pollution", adds Pierre Pernot. “They are more likely to be local sources of odor, even if it affects a good part of the region. This goes in the direction of the sanitation track ”, specifies the Airparif executive.
"Buried gases" that come out with precipitation ?
"There are different compounds that can come out of sanitation, including sulfur compounds, and in this case the human nose is the best measure of characterization to characterize the smell", analyzes Pierre Pernot again. The foul odor could have been emitted by a component present in wastewater, hydrogen sulfide (H2S), but which is not part of Airparif's surveys.
At Cedre (Center for Documentation of Research and Experimentation on Accidental Water Pollution) the hypothesis of a flow of rainwater causing this olfactory pollution seems plausible. "Following very heavy precipitation, the water navigates to recovery basins, then creates unusual currents which create abnormal mixing," explains Nicolas Tamic, Deputy Director of Cedre. Gases that are in the mud buried at the bottom then rise to the surface and then emerge at the level of the drainage basins, "adds the scientist. He specifies that this remains only a theory in the present case, since its establishment was not requested for more precise analyzes, but that this type of scenario has already happened.
Contacted, SIAAP, the main Ile-de-France water purification structure, for its part ensures that "the checks, measurements and monitoring carried out since yesterday and again this morning indicate that the origin of the odors felt by the population does not come from the sanitation system managed by SIAAP ”.
"No abnormal value designating the presence of a gas having the odor of sulfur has been measured in particular on the sensors installed in its networks", writes the organization in a press release, specifying that it had noted "no technical malfunction on its installations (networks and stations) ”.
Nitrogen dioxide from thunderstorms or from abroad ?
Another hypothesis: the possibility of nitrogen oxide pollution. If it can be caused by thunderstorms, and there were many this weekend in the Paris region, it could also result from nitrogen dioxide (NO2) pollution from northern Europe.
According to Vincent-Henri Peuch, director of Copernicus, a European Union monitoring program, the air content changed during the evening, becoming charged with nitrate, ammonium and sulfate, under the influence of the masses d’air from northern Europe. But "we think that it is mainly pollution by fine particles (PM2.5 / PM10) rather than NO2, even if the flow of North probably brought back quickly a whole set of pollutants and which remained in low layers ", He specifies to the Parisian.
Airparif therefore assured that it had not observed very high levels of NO2 on Sunday: 40 micrograms per m3. A rather low threshold, the information threshold being fixed at 200 and the alert threshold at 400.
The results of the prefecture's investigation will no doubt allow us to know a little more.
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