September 19, 2021

Between carbon emissions and fine particle concentration, the confusion of 40 million motorists

Since Tuesday 1is June, the Greater Paris metropolis banned the circulation of Crit’Air 4 and 5 vehicles during the week, between 8 a.m. and 8 p.m., to reduce air pollution. The measure is criticized by the general delegate of the association 40 Millions of motorists. For the association’s general delegate, Pierre Chasseray, the measure, in addition to being unfair, is ineffective. As proof, he argues, the spring 2020 containment had only a minor impact on pollution.

What he said

“During the first confinement, 87% of automobile traffic was suppressed in Ile-de-France, for a reduction in carbon emissions of 7%”, asserted, Tuesday 1is June, Pierre Chasseray on the air at BFM-TV.

Why is it very imprecise

  • A real drop automobile traffic

Pierre Chasseray is right on one point: road traffic dropped drastically during the first confinement – then to a lesser extent during the second and third confinements.

However, the evolution of automobile traffic in a city is very difficult to objectify and the traffic data remains fairly approximate. In Paris, “driving assistance” platforms, such as TomTom, Waze or Coyote, transmit their users’ GPS geolocation information. Available data shows a decrease of 89% compared to the same period in 2019, according to TomTom Traffic Index and 72% for Coyote. Contacted, Mr Chasseray did not “Remembers” where the 87% figure comes from.

  • Confusion between “carbon emission” and “concentration of fine particles »

If the general delegate of 40 Million motorists is right about the reduction in automobile traffic, he is very imprecise by evoking a ” 7% reduction in carbon emissions ”. In reality, during confinement, it is the concentration of fine particles which decreased by 7% in Ile-de-France, according to figures from Airparif, an association approved by the Ministry of Ecology and responsible for measuring daily air quality in the Ile-de-France region.

Mr. Chasseray confuses two types of emissions linked to automobile traffic: carbon dioxide (CO2), which is a greenhouse gas, and fine particles, microscopic dust, released, among other things, during fuel combustion. Highly toxic to the respiratory tract, these particles are responsible for 48,000 deaths per year in France, according to a benchmark survey by Public Health France.

It also creates confusion between two different concepts: concentration (the proportion of a substance in the air) and emissions, which are the quantity of pollutants directly released into the atmosphere.

In the first days of the first containment, carbon dioxide emissions had fallen by 70%.

  • The concentration of fine particles has actually fallen by only 7%

During the first confinement, the concentration of fine particles in the air fell much less sharply, despite the drastic decrease in automobile traffic. First, road transport is not the only responsible for the emission of fine particles. This sector is the source of only a quarter of emissions, ahead of the construction and agriculture sector (15%), but behind residential heating (50%). These activities did not stop during confinement, which explains a moderate drop in fine particle concentrations in Ile-de-France.

In addition, the exceptional meteorological conditions of the first confinement (particularly hot and dry) did not make it possible to further lower the concentration of fine particles, as already recalled by Atmo France, the national network of approved associations for monitoring the quality of air in a statement in 2020.

  • Other indicators show effects on air quality

There is indeed a link between a drop in automobile traffic and an improvement in air quality, as associations and scientists point out. Pierre Chasseray does not mention it, but the first confinement resulted in a 25% decrease in nitrogen dioxide concentrations (NO2). And unlike fine particles, NO2 mainly comes from road traffic. According to Atmo France, a reduction of 50% has even been observed along the main roads.

According to the review The Lancet Planetary Health, Paris is the 4e European city with the highest mortality from exposure to nitrogen dioxide. This irritant gas is also responsible for the formation of ozone in the lower layers of the atmosphere as well as acid rain and the eutrophication of ecosystems.