1.65 µm CH4 ground-based differential absorption lidar measurements in the atmosphere

Methane (CH₄) is the second anthropogenic greenhouse gas (GHG) in the atmosphere that contributes to the global warming after CO₂. If the methane emissions have a unique sink by OH oxidation, the various different sources, both anthropogenic (around 2/3) and natural, make complex the understanding of its atmospheric concentration. On the anthropogenic side (mainly gas exploitation and burning) it is fundamental to have a tool to verify inventories at different scales (from local methanizer to megacity) and prevent production network leakage in the atmosphere. As for surface-atmosphere exchanges of CO₂, it is fundamental to study at different scales the spatial pattern and magnitude of the natural CH₄ sources (biogenic anaerobic degradation of organic matter in wetlands, landfill and waste, livestock, rice cultivation, thermite, geological sources) and to understand their evolution with the global warming.

Lidar has an important role to play in such topic as it can make: (i) a 3D mapping of CH₄ concentration in anthropogenic plumes, (ii) vertical profiles to study transport processes in the atmosphere, (iii) even measure direct flux and (iv) provide CH₄ Earth global measurements from a space platform as it will be for MERLIN CH₄ integrated path differential absorption lidar CNES/DLR ongoing mission.

A new ground-based Differential Absorption Lidar (DIAL) for atmospheric methane (CH₄) profiling has been developed at LMD. The lidar emitter relies on a new hybrid fibered/bulk Er:YAG laser that delivers dual On/Off 8 mJ/ 300 ns pulses at a repetition frequency of 1 kHz in the methane line triplet at 1645.55 nm and out of at 1645.3 nm. It is associated with a direct detection receiver with a 50cm diameter telescope, a 2-nm linewidth interference optical filter, a near infrared photomultiplier (PMT) and a data acquisition and real time signal processing system working both in analogic and photon counting mode depending the application. First horizontal and vertical measurements in the atmosphere have been achieved and compared with in situ sensor and will be presented at the conference.