On the night of Sunday to Monday, May 11, the phone kept ringing at Ile-de-France firefighters. A few hours earlier, it had been the same with their colleagues in Hauts-de-France. And, since this Monday morning, it is the inhabitants of the Pays de la Loire – and particularly of Nantes, Sarthe and Le Mans – who are calling the men of fire.
Each time, everyone reports a phenomenon that is both unpleasant and disturbing: a strong smell of sulfur hovering in the air. Each time, the fear of an industrial accident like that of Lubrizol, near Rouen, is mentioned by the people – sometimes panicked – who call 18. Wrongly:
No specific intervention is in progress “, explain the Paris firefighters.
Given the scale of the number of calls, they even cracked an official press release on Twitter Sunday evening, calling on Parisians to calm down.
Smell of sulfur felt in several departments probably linked to recent bad weather. Taken into account, this odor does not correspond to any particular intervention in progress. Please only dial 18-112 in the event of a proven emergency. pic.twitter.com/oIwODSbgzY
– @PompiersParis (@PompiersParis) May 10, 2020
felt in several departments is probably linked to recent bad weather, they explain. If their answer is reassuring, it does not solve this double problem: what is this smell? And what causes it?
What is that smell?
It is a sign of air pollution. To what? To two main elements. To NO2 first – translate: to nitrogen dioxide. This is part of the NOx family, in other words nitrogen oxides, which are formed by the combination of oxygen and nitrogen. Remember that NO2 is a very toxic gas by inhalation. It is acrid and stinging the eyes because, in short, it interacts chemically with water (including therefore with that of our eyes).
However, this odor can also be a sign of sulfur dioxide (SO2) pollution, with the characteristic rotten egg smell. That said, the air quality monitoring organization in Ile-de-France, Airparif, has not noted high levels of SO2, but specifies that this foul odor can also be emitted by a very present component in wastewater, hydrogen sulfide (H2S), a very recognizable gas … but which is not part of its records.
It looks like a mixture between an insect burnt in a halogen flagpole and the old Brussels sprout … ”specifies this Internet user:
But what is this smell in Paris? It looks like a mixture between burnt insect in a halogen flagpole and the old Brussels sprout Several friends also point it out to me at various points in the IDF
– Marie Boscher (@BoscherMarie) May 10, 2020
What is the origin of this smell?
– Sewage mixing
This was suggested by the Paris city hall last night. Emmanuel Grégoire, first deputy, says that
it could be related to the shuffling of the sewerage networks because of the heavy rain which brings up the decomposition gases.
Without being false, on the contrary, the statement does not seem sufficient: certainly, the severe thunderstorms this weekend resulted in heavy precipitation, themselves clogging up and mixing up the rainwater and sanitation network. From there to raise such rotten egg smells, and over such a distance, from Lille to Nantes, the thing seems unlikely.
In a tweet, Emmanuel Grégoire announces that“No fire or industrial incident has been reported”. He adds that “Checks [sont] In progress “.
No fire or industrial incident has been reported. The “sulfur” smell could be linked to heavy precipitation. Checks in progress.
– Emmanuel GREGOIRE (@egregoire) May 10, 2020
Whether sulfur dioxide or hydrogen sulfide has been released into the atmosphere through large city sewers is highly plausible. But these gases are not enough to create a phenomenon of such geographic scope on their own. So what could have amplified the phenomenon?
– The storms themselves
It is not necessarily known, but the stormy phenomena are nitrogen oxide plants (incidentally, the same goes for volcanic eruptions and large forest fires).
More than 1.2 billion lightning strikes occur worldwide each year. Laboratory and field tests have revealed that the core of some lightning reaches 29,700 ° C. Monstrous heat, in any case sufficient to instantly melt the sand and break the oxygen and nitrogen molecules into two individual atoms.
Translate: each of these billions of lightnings produces a breath of nitrogen oxide (Nox) which reacts with sunlight and other gases in the atmosphere to produce ozone, in particular.
However, France experienced a violent electrical activity this weekend. Not billions of lightnings, of course, but enough to produce nitrogen oxides, as Mark Parrington explains in a tweet:
Cold front clearing surface PM2.5 #airpollution across UK & NW Europe on 10 May continue across Europe through coming days. @CopernicusECMWF Atmosphere Monitoring Service @ECMWF regional ensemble forecast visualized by @windyforecast https://t.co/ErIw4r6Fm7 @CopernicusEU pic.twitter.com/LrOWsh5YBO
– Mark Parrington (@m_parrington) May 10, 2020
– A pollution wave
We saw it this weekend, and we can only notice it since this Monday morning: after flows from the south-west which brought powerful and lasting storms over France, an immense cold front, coming from the north -est, quickly repelled this hot and humid air to bring its cool temperatures to the south of the Loire.
What caused besides from Sunday thick mists and fogs above the waters of the English Channel, but also on Cotentin and North Brittany (photo below).
In doing so, this strong wind brought with it, and accumulated along the warm edge, the atmosphere loaded with pollution (NO2 and SO2 in particular) which reigned over Great Britain and Northern Europe (Netherlands, Belgium…), especially above the big cities (Antwerp, for example).
Because, it must be said again, the main source of nitrogen oxide pollution is of human origin: it comes first from the combustion of fossil fuels (coal, fuel oil, natural gas), from vehicle exhausts, but also the fermentation of wet grains stored in silos and some massive agricultural spreading.
This pollution wave, brought by a strong northeast wind, seems in any case here the predominant factor.
As we can see on the map at the head of the article, the NO2 concentration was particularly high on a diagonal between Lille and Nantes, last night – and it should remain so until this evening at least (map above).
Is this phenomenon dangerous?
Yes. And no. As is often the case, it all depends on the quantity of the pollutant considered. In this case, the concentration of NO2 and SO2 in the atmosphere.
The concentrations recorded last night and this morning do not exceed the danger thresholds established by the authorities. Thus, the limit values for Airparif correspond, on an annual average, to 50 µg / m³ for SO2 and 40 µg / m³ for NO2. And the values recorded last night did not seem to have exceeded 10 µg / m³ for NO2.
Anyway, the police headquarters laboratory took samples last night to try to identify the origin of this odor. The results must be known this Monday evening.
The phenomenon has apparently been more spectacular than dangerous. Which does nothing to diminish the reality of this pollution.
To conclude, we have therefore witnessed, for the past 24 hours, a combination of factors – natural and human – and pollution – nitrogen oxides and sulfur dioxide – which have triggered the appearance, on our territory, of this gigantic cloud with a foul odor. Not the conditions dreamed of for a first day of deconfinement.