Air pollution often rhymes with global warming. To the point that some sometimes confuse them. Especially in summer, when ozone pollution peaks multiply. But it is a mistake. Because heat is only one of the factors involved in the chemical process at the origin of this pollutant. And the two phenomena are far from always evolving in the same direction, especially in cities.
To have a more precise vision of air pollution, its origins, its consequences and the solutions to curb it, Mediacités spoke with Isabelle Coll, professor of chemistry and specialist in atmospheric sciences at the Paris East Creteil University. Since 2014, this teacher-researcher has been developing “integrated and multidisciplinary urban modeling, taking into account urban expansion, the economy, individual and collective practices, traffic flows and transport networks, in order to simulate the impact of urban activity on the flow of pollutant emissions, air quality and the exposure of city dwellers”.
What is air pollution?
Isabelle Coll : When we talk about air pollution, we are talking about the concentration of chemical species in the atmosphere. This of course depends on the emissions, that is to say on what is directly rejected, but also on the way in which the pollutants disperse or are transformed into new pollutants under the effect of the light intensity or the heat. The concentrations – which are generally expressed in micrograms per cubic meter – are therefore dependent on the intensity and nature of the polluting activities but also on meteorological dynamics. Air pollution is the result of all these phenomena, which is why this notion is complex.
What are the main pollutants in urban areas?
In cities, the main focus is on two types of pollutants: nitrogen dioxide and fine particles. Nitrogen dioxide (NO2) is a gas resulting from all combustion. The annual regulatory limit is 40 micrograms per cubic meter of air. This value is often exceeded near road traffic, which has been the subject of disputes between France and the European Union. NO2 is a nitrogenous compound (N) continuously emitted with nitric oxide (NO). They transform one and the other permanently and take part in the production of ozone but it is the NO2 which is supervised for its health danger.
Fine particles are species of grains, liquid or solid, suspended in the air and excessively small (their size ranges from nanometer to micrometer). The older they get, the bigger they get. They can aggregate solid particles emitted when the brakes of a vehicle rub; soot particles emitted during combustion; dust torn from the ground; or even gaseous, organic chemical compounds, which have been oxidized and transformed. Two classes of particles are regulated: PM10 (less than 10 microns) and PM2.5 (less than 2.5 microns). The latter are the most dangerous because they can penetrate quite deeply into our respiratory system up to the alveoli of the lungs. From there, they can release certain compounds into the blood and thus reach all the organs.
What other potential pollutants are there? . .