Composed mainly of dinitrogen N2 (78% by volume), dioxygen O2 (21% by volume) and argon (0,95% by volume), the air is more or less contaminated by gaseous, liquid or solid pollutants of natural origin (emissions from vegetation, oceans, volcanoes, etc.) or produced by human activities (factory chimneys, exhaust pipes, etc.).
The polluting species emitted or transformed into the atmosphere are numerous. Even if their concentrations are very low (generally measured in micrograms per cubic meter), they can have effects particularly on health.
Pollutants, which, once in the atmosphere, can react with each other and form new pollutants, two large families of pollutants are distinguished : primary pollutants (emitted directly) and secondary pollutants (which form in the air).
Primary pollutants
Primary pollutants are mainly emitted into the atmosphere. They come from sources of pollution such as road traffic, industries, heating, agriculture, etc.
These are for example:
- Carbon oxides
- Sulfur oxides
- Nitrogen oxides
- Light hydrocarbons
- Volatile organic compounds (VOCs)
- Particles (PM10 and PM2.5)
- Metals (lead, mercury, cadmium, etc.)
Secondary pollutants
Secondary pollutants are not directly released into the atmosphere, but arise from chemical reactions of gases with each other. This is particularly the case for:
- Secondary particles
- Ozone (O3)
- Nitrogen dioxide...
Some pollutants like nitrogen dioxide and particulate matter are both primary and secondary pollutants.
Focus on Ozone
Ozone results from the chemical transformation of oxygen in contact with nitrogen oxides and hydrocarbons, in the presence of solar ultraviolet radiation and high temperature.. Ozone as well as other photochemical pollutants (PAN or peroxyacetyl nitrates, aldehydes, ketones, etc.) constitute smog, this brownish cloud which sometimes stagnates above large cities like Paris.
The formation of ozone requires a certain time during which the air masses move. This explains why ozone levels are higher in rural areas around the Paris region than in the Paris metropolitan area where their precursors were produced.
Ozone is a pollutant that travels and therefore presents a regional rather than a local problem.. Pollution from the urban area impacts surrounding rural areas and the urban area itself can be subject to significant ozone import phenomena from other regions or even other countries, particularly from Northern Europe and from the east.
Pollutants and various sources
Pollutants in the atmosphere can be of natural origin (they are then emitted by vegetation, soil erosion, volcanoes, oceans, etc.) or of anthropogenic origin, that is to say that they are “emitted” by human activities.
All sectors of human activity are likely to emit atmospheric pollutants: industrial activities, waste treatment and construction sites, transport (road, air, river), domestic activities (heating in particular), agriculture or forestry.
In the atmosphere, the pollutants observed are those emitted directly by these sources but also those which result from physico-chemical reactions between chemical components (primary pollutants and other constituents of the atmosphere) governed by meteorological conditions.
Census of pollutants source by source
The sources of pollutants, their geographic distribution, and the quantities of pollutants emitted can vary significantly depending on the time of year, or even the time of day. To identify them and better understand them, Airparif carries out an emissions inventory, for atmospheric pollutants and for the main greenhouse gases. This inventory makes it possible to identify:
- The responsibility of the different sources,
- The geographical distribution of emissions,
- Their evolution over time.
The emissions inventory is a valuable tool for identifying the sources of pollutants on which actions would be most effective and for testing reduction scenarios that take these issues into account.
The main sources of air pollution in Île-de-France
The major sources of air emissions in Ile-de-France are:
- the residential sector (mainly heating),
- road transport,
- depending on the local specificities of certain territories, activities such as industry, energy production, airport platforms, construction sites, agriculture, etc.
The density of pollutant emissions in the conurbation per km² is extremely high, compared to other regions, but the quantity emitted per inhabitant is, on the other hand, rather lower.
