In the laboratory of the Across project (Atmospheric chemistry of suburban forests), researchers are put to the test. They can be shaken, their machines are hot and noisy. But above all, they have to work at an altitude of 300 meters. They operate inside an airplane, a flying laboratory.
The machine is an ATR42 completely modified to carry a range of scientific instruments. It belongs to the French Instrumented Aircraft Service for Environmental Research (Safire), a public research infrastructure that brings together Météo France, CNRS and CNES. As part of the Across project, around fifteen flights have been planned around Paris during the months of June and July. They aim to better understand local pollution: the composition of pollutants, the way they are formed, their transformations when they disperse, etc.
« Large urban agglomerations like Paris are often surrounded by green areas remarks Paola Formenti (CNRS), one of the project’s scientific managers. A plume of pollution then surrounds the cities and diffuses into nature. Urban pollutants, in the form of gases or particles, mix with compounds emitted by forests. However, explains the researcher, we still know very little about the result of these interactions. ” We want to know how these mixtures change the quantities and properties of pollutants, for example modify their effects on the climate says Vincent Michoud, teacher-researcher at the Interuniversity Laboratory of Atmospheric Systems (LISA) and member of the project.
Sophisticated instruments
The researchers defined five flight paths, each following the pollution plume from its formation to its dilution in forest areas on the outskirts of Paris. These journeys can go up to 200 kilometers around the city, from Cergy-Pontoise aerodrome. The day before each departure, one of the routes is chosen according to the weather forecast.
In flight, a sampling vein under the aircraft routes the air inside continuously to the various analysis instruments. Eight engineers and scientists on board evaluate the data. A mass spectrometer, for example, allows them to distinguish compounds according to their molecular family. Other instruments all around the aircraft provide information on particle size and weather conditions, among other things.
Among the compounds thus observed: carbon soot emitted by vehicles, sulphates from thermal power stations, desert dust transported from northern Africa and volatile organic compounds from vegetation. But, before drawing a conclusion on the composition of the pollution, it will require in-depth analyzes of the data.
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