With containment, automobile pollution has decreased significantly near the main roads. Dijon is one of the main decreases according to Atmo France.
COVID-19: focus on the exposure of local residents to motor vehicle pollution near major roads before / during containment
THE ESSENTIAL
• In March 2020, the average daily concentrations of nitrogen oxides (NOx, pollutants mainly emitted by traffic) fell sharply near the main highways since the introduction of containment, thereby reducing the exposure of residents of major highways at levels generally lower than those of a usual Sunday;
• On stations representative of the exposure of the population to pollution due to traffic, the difference before / during confinement is 30 to 75% depending on the city;
• The comparison between the months of March 2019 and March 2020 also illustrates this drop in concentrations, while keeping in mind that the variations observed result partly from the impact of confinement on traffic but also from weather conditions which may have been very different locally between 2019 and 2020.
Local measures in addition to national models
Atmo France and the approved air quality monitoring associations (AASQA) which it federates have organized to maintain the essential regulatory missions of measurement, monitoring and public information on air quality during this period of covid-19 pandemic.
In addition to, among other publications by Ineris on the evolution of air quality in France, satellite images of space agencies, press releases from Atmo France and locally from the various AASQAs on the impact of confinement on air quality, Atmo France carried out a synthetic focus on the evolution of nitrogen oxides (NOx) near high-traffic highways thanks to historical data from the measurement stations of the approved associations in charge of monitoring and information on air quality in France. NOx largely comes from road transport; However, the drastic drop in the latter is one of the most striking elements of confinement for the whole of metropolitan and overseas territory. For people whose place of life is close to major highways, their exposure to NOx is particularly important.
Thanks to AASQA surveillance and field data, it is possible to see, throughout the territory, the history of air pollution from various pollutants, in particular NOx, as well as the evolution of this pollution.
Why choose nitrogen oxides (NOx) as an indicator?
NOx includes nitrogen monoxide (NO) and nitrogen dioxide (NO2) expressed in µg / m3 NO2 equivalent. The main source of NOx emissions in cities is road traffic for more than 2/3. Indicators of road traffic pollution, these pollutants are regulated by European legislation. Their monitoring on the territory is compulsory and NO2 is a particular species monitored because of its toxicity. France is, moreover, in litigation with the European Commission for non-compliance with the directives concerning the concentrations of this pollutant in the ambient air of many agglomerations. Since the NO and NO2 balance is very sensitive to temperature, the NOx indicator is more stable than simple NO2 during periods of significant temperature variation.
NOx are also precursors of other pollutants: under certain climatic and sunshine conditions, they react with certain pollutants according to complex physicochemical processes occurring in the atmosphere. They react in particular with volatile organic compounds (VOCs) to lead to the formation of tropospheric ozone or with ammonia (NH3) to form secondary particles (the levels of which have remained high for some days from France, despite the containment measures).
Significant reduction in exposure to nitrogen oxides (NOx) along the traffic arteries
Atmo France with the support of AASQA analyzed the evolution of average daily NOx concentrations from March 1 to 31, 2020 at stations representative of the exposure to pollution due to traffic in the main French agglomerations. Since NOx emissions are globally linked to road traffic, the data from the traffic exposure reference station have been used for all regional prefectures as well as for agglomerations of more than 500,000 inhabitants.
Evolution of the nitrogen oxide (NOx) levels measured on major roads in March 2020 before and during containment
The drop in road traffic linked to confinement had the impact of an overall drop in NOx concentrations throughout metropolitan France and overseas. In March 2020, the implementation of containment led to a decrease in average daily NOx concentrations near road axes by 50% in Bordeaux; 70% in Toulouse; 67% in Fort-de-France; 69% in Dijon; 62% in Rennes and 69% in Marseille for example (see infographic above).
The evolution of NOx levels was also analyzed for several agglomerations with more than 500,000 inhabitants. The impact of the decrease in road traffic is also visible there on NOx concentrations. For example, the agglomeration of Paris observed a decrease of 73%, the agglomeration of Grenoble by 72%, the agglomeration of Toulon by 49% and the agglomeration of Nice by 69%.
Methodology
In order to show the impact of confinement on NOx pollution, Atmo France carried out at a few representative traffic stations in each agglomeration a ratio between the average daily concentrations of NOx for the month of March before the establishment of confinement (1st to March 17, 2020) and at the start of containment (March 18 to 31, 2020). Expressed as a percentage, this ratio indicates the evolution of the NOx levels measured on major highways in March 2020 before and during containment.
The daily NOx concentrations for the month of March 2019 are also shown in the graphs in Part III in order to illustrate the state of the NOx concentrations in 2019 at the same period * The comparison between the months of March 2019 and March 2020 illustrates also this drop in concentrations, while keeping in mind that the variations observed result on the one hand from the impact of confinement on traffic but also from weather conditions which may have been very different locally between 2019 and 2020.
* Following technical incidents, in certain regions, some daily data are unavailable. Note that the regulations require the AASQA to have at least 90% data over the year.
A net impact compared to the same period in 2019
Communicated
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