Up there, at an altitude of 840 kilometres, three piercing eyes cast their analytical gaze on our atmosphere. Forest fires, volcanic eruptions, pollution: nothing escapes them. This set of instruments, consisting of three identical specimens, is the heart of the IASI (Infrared Atmospheric Sounding Interferometer) mission. The initial ambition of this program, supported by the CNES, was to improve weather forecasting while extending its use to atmospheric chemistry researchers. Since the launch into orbit of the first satellite in 2006, the data have been received, processed and analysed at LATMOS at Sorbonne University. Maya George, a research engineer working in this laboratory, explains that “we use the same measurements as meteorologists but we obtain information on the concentrations of gases present in the atmosphere.” Every gas, when emitted in sufficient quantities, acts as a specific marker for a type of phenomenon.

The LATMOS IASI team focuses on carbon monoxide emitted during forest fires, sulphur dioxide from volcanic activity, ammonia from agricultural spraying and ozone, whether natural or man-made. Each of these gases transmits a particular infrared signature by absorbing part of the radiation emitted by the Earth. IASI perceives these variations and transcribes the data into a spectrum. “Each of the three instruments sees the entire Earth twice a day. That’s about 1.3 million spectra per day and per instrument”, tells the research engineer. “The data arrives two and a half hours after the measurement: it’s practically real time.”

Long term snapshots

This global vision allows a close monitoring of the planet even in the most inaccessible of places: “Many volcanoes are closely monitored on the ground but in remote regions or war zones, only satellites can provide rapid information on the evolution of these phenomena.” During the event scientists track gas emissions on a daily basis, such as the carbon monoxide emitted by the fires this summer in California: “Every day we would draw maps and ask ourselves: ‘When is this going to stop?'” recalls Maya. As the fires die down and the volcanoes calm down, the researchers’ work keeps on going: “It is once the event is over that we choose to linger on it to conduct statistical analyses.” Also, the path of certain gases is tracked in the atmosphere, sometimes during several months, acting as a pollution marker.

Although IASI is a powerful tool for understanding extreme events in the short term, only the long term can reveal the extent of its interest for climatologists. “To study trends, we need at least 10 years of data”, adds Maya George. “These large data sets have been measured by the same instruments since 2007, and here lies the strength of the mission. Thanks to them we will be able to carry out studies on climate trends.” As the mission comes to an end, the CNES plans to take over with a new instrument, IASI-NG, scheduled for launch in 2023. Twice as accurate as IASI, this remarkable tool will keep watch on our atmosphere for many years to come.

Translated from Marion Barbé for IPSL

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