The evolution and forecast of the consequences of the current global warming depends largely on our ability to predict the evolution of biosphere-atmosphere exchanges of CO 2 and CH 4 . To locate and quantify CO 2 / CH 4 fluxes at the surface on a global scale, an inverse method called "top-down" can be used. This method is based on the use of concentration measurements, if possible, carried out on the surface and transport models to estimate surface fluxes. Transport modeling is therefore as fundamental as the ability to perform precise and exact concentration measurements from a satellite.
Part of the LIDAR instrumental developments mentioned on this site are motivated by the objective of performing a LIDAR DIAL / IPDA measurement of atmospheric CO 2 from space in line with the A-SCOPE (Advanced Space Carbon and climate) proposal. Observation of Planet Earth) submitted to ESA in 2007 ( ESA PHLAC project : Pulsed Holmium Laser for Atmospheric CO 2 monitoring - H2020 HOLDON project : HgCdTe APD Optimization for Lidar Detection Of greeNhouse gases).
The principle of such a mission is indicated below. The payload consists of a single mode pulsed laser sequentially delivering On (absorbed by CO 2 ) and Off (non-absorbed as reference) pulses into the atmosphere. As part of an IPDA (Integrated Path Differential Absorption) measurement, the signals reflected by the surface are analyzed to determine the differential optical thickness due to CO 2 . The XCO 2 mixing ratio is then estimated as a function of spectroscopic parameters and meteorological variables.
Since 2011, the LIDAR group has also been working on the Franco-German mission CNES-DLR MERLIN (Methane Remote Sensing Lidar Mission) [Ehret, RS, 2017] which is currently in phase C for a launch planned for 2024. MERLIN is based on the same principle that for A-SCOPE and will be the first differential absorption lidar mission in IPDA mode to be launched in space. MERLIN will deliver as main product the CH 4 mixing ratio integrated in the air column. The characteristics of the MERLIN mission are available here ( link ... ).
The LIDAR group's involvement is multiple:
- Member of SAG (Science Advisory Group)
- LIDAR expertise for CNES , monitoring of payload development by AIRBUS and performance analysis
- Development of a LIDAR signal simulator from different measurement configurations: scene, instrumental data, weather variables, spectroscopic data [Cassé, Atmosphere, 2019]
- Development of a LIDAR signal processor and prototypes of algorithms for restoring the mixing ratio in CH 4 [Tellier, AMT, 2018]
- Preparation of the validation of MERLIN products
The final objective of the MERLIN mission is the estimation of CH 4 fluxes on the surface using an optimization method. An estimate of the details and bias is presented in [Bousquet, JGR, 2018] .