Detection and trends of atmospheric long-lived halogenated gases from space with IASI

Since the 1900s, emissions of CFCs and other long-lived halogenated species have profoundly affected the chemical and radiative equilibrium of our atmosphere. These halogenated compounds are known to be powerful greenhouse gases and contribute, for chlorinated and brominated compounds, to the depletion of the ozone layer and to the development of the ozone hole. With the aim of mitigating their deleterious impact on the environment, most long-lived halogenated species are controlled by international treaties, the most famous of which is the Montreal Protocol of 1987. Their monitoring is essential to evaluate the effectiveness of international regulations with regard to emissions, and ultimately to assess the environmental impacts of the changes at play. Sustained global networks located on ground have provided highly accurate measurements of the atmospheric concentrations of long-lived halogenated species for 50 years. The aim of this thesis is to assess the potential of infrared satellite sounders operating in the nadir geometry, to contribute to their monitoring and thereby complement existing surface measurement networks. The research work is centered on the exploitation of the measurements from the IASI sounder. Its stability and the consistency between the diff erent instruments on the successive Metop (A,B and C) is remarkable and makes it a reference for climate monitoring. Among other things, IASI offers the potential to investigate trends in the concentration of various species better than with any other previous or current hyperspectral infrared sounder. The low noise of IASI radiances is also such that even weakly absorbing atmospheric constituents can be tracked, on single or at least on averaged spectra. As a first step, we sought to determine which long-lived halogenated species can be detected in IASI spectra. To achieve this, we have developed an innovative methodology, based on spectral whitening, which allows to isolate the weak spectral signatures of several long-lived halogenated species by removing most of the largest and most variable spectral features from IASI spectra. The major outcome is the unambiguous identifi cation of eight long-lived halogenated species, namely CFC-11, CFC-12, CCl4 ,HCFC-22, HCFC-142b, HFC-134a, CF4 and SF6. Three of these (HCFC-142b, HFC-134a, SF6 ) were never detected before. In a second stage, we sought to retrieve the atmospheric concentrations of the species detected in the first part of the work. For that, we rely on an unconstrained generalized least squares estimation retrieval methodology. Focussing on two oceanic regions over northern and southern mid-latitudes, we derive the monthly concentrations between 2008 and 2022. The trend evolution in the diff erent chemical species, either negative or positive, is validated against what is observed with ground-based measurement networks and other satellite data. The results demonstrate the potential of IASI and follow-up missions for establishing a unique long-term time series of themost important long-lived halogenated species.

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