Does car-free day in Paris reduce pollution?

Last Sunday, the city of Paris renewed the “Paris breathes” operation, which aims to ban cars from driving in the center between 11 am and 6 pm, to the delight of some Parisians happy to enjoy a sunny day. During that time, it was strictly forbidden to drive around the city behind the wheel of a car, even an electric one. Only buses, taxis, VTCs and other emergency vehicles were tolerated on the streets of the capital, as were Parisians returning from vacation with proof of residence. The opportunity to verify once again whether this sharp reduction in car traffic in Paris has a direct effect on local pollution and, if so, to quantify it precisely.

Because, as fans of “all cars” remind us, the long period of confinement between March and May 2020 did not prevent the city of Paris from being crossed by an episode of air pollution. Likewise, measurements of the concentrations of the main pollutants recorded in the atmosphere in Paris on Sunday (ozone, nitrogen dioxide, PM10 particles and PM2.5 particles) did not drop in large proportions. But according to data collected by Airparif, there is indeed a direct effect of road traffic disruption on air pollution in Paris. Thus, last Sunday, emissions of nitrogen dioxide (a pollutant emitted mainly by the exhausts of thermal cars in the region) fell by 20% on average in the city between 11 am and 6 pm compared to a normal Sunday. As for the fine PM2.5 particles, they dropped by only 5% in the same time period.

Multiple sources of pollution

An estimated 62% of nitrogen dioxide emissions in Paris come from road traffic. That’s why it’s these harmful emissions that fall the most in Paris when cars are stationary. During the major lockdown in early 2020, Airparif had thus measured a drop of around 25% in these nitrogen dioxide emissions (to a drop of around 15% in fine particulate concentrations). Nitrogen dioxide emissions have dropped by up to 50% near major highways. But, as Airparif expert Antoine Trouche reminds us, the sources of pollution in Paris are multiple and their analysis remains complex, especially for particles. “For PM10 particles, 17% of their total amount in Paris is produced by road transport, 18% by agricultural activities, 17% by construction sites and 35% by the residential sector. In this last 35%, 86% of PM10 particles come from heating wood, while this represents only 6% of the heat used for heating in the Ile-de-France region. But there are also other pollution factors to consider: the nitrogen dioxide emitted by cars can, in unfavorable weather conditions, recombine in the atmosphere with ammonia from agricultural propagation and form additional harmful particles.The Airparif specialist also states that these secondary particles thus formed are at the origin of the pollution peak that occurred during the first confinement, in parallel with agricultural expansion. Although car traffic was greatly reduced, “a small amount of nitrogen dioxide is enough to cause this phenomenon“. However, it is difficult to accurately distinguish the sources of nitrogen dioxide in this case, knowing that the residential and tertiary sector also produces it (31%).

It would be tempting, especially when defending the car, to emphasize that episodes of limited car traffic do not prevent the air in Paris from being loaded with harmful substances at times. According to Antoine Trouche, the concentrations measured in the air vary in large proportions depending on the weather, road traffic, wind and any polluting activities (farming, construction, industry, etc.) carried out outside the city at certain times. But even if stopping car traffic doesn’t eliminate all the pollution from the region, its beneficial impact is quite quantifiable in Paris, according to research by Airparif. Still, to improve the air in Paris, fighting car pollution alone will not be enough.

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