MERLIN: Methane Monitoring Space Mission
MERLIN (MEthane Remote sensing LIdar missioN) is a CNES/DLR space borne Lidar mission initiated late 2009, for global monitoring of atmospheric methane (CH4). The space segment consists in the new platform product line named Myriade Evolutions (range of 400 kg) developed under CNES control, and the first IPDA (Integrated Path Differential Absorption) lidar (Light Detecting And Ranging) instrument under DLR responsibility. The IPDA technique enables measurements in all seasons, at all latitudes. It relies on DIAL (Differential Absorption Lidar) measurements using a pulsed laser emitting at two wavelengths, one wavelength accurately locked on a spectral feature of the methane absorption line, and the other wavelength free from absorption to be used as reference. The Merlin satellite will be operated at an altitude of around 500 km, on a sun-synchronous orbit, either at 06:00 or 18:00 of the local time of the ascending node, depending on launch opportunities. The 1st science objective is to provide a significant improvement on retrieval of CH4 fluxes at synoptic scales, which in turn calls for 1% accuracy on CH4 column averaged air dry-mixing ratio (XCH4) at a 50 km horizontal resolution. A second objective is to contribute to a better understanding of atmospheric chemistry at global scale.
Methane (CH4) is the second most important anthropogenic greenhouse gas (GHG) after carbon dioxide (CO2). It has a global warming potential (GWP) of 25 over 100 years, which makes it more efficient than CO2 at lower concentrations. The main anthropogenic sources of CH4 are emissions from the production of energy, landfills, waste treatment, cattle, rice paddies and the incomplete combustion of biomass. There are also major natural sources of CH4, such as wetlands. The largest sink is the destruction of CH4 by OH in the atmosphere. Due to human activities, the atmospheric mixing ratio of CH4 has increased by over 2.5 since 1750. Furthermore, methane also increases the radiative forcing of ozone and water vapour in the stratosphere. Taking into account these indirect effects, the radiative forcing of CH4 attains about 60% of that of CO2 despite a much lower atmospheric mixing ratio.
The current knowledge of the methane cycle still suffers from large uncertainties. Even on a global scale, the different types of emission have never been accurately quantified. CH4 emissions may be estimated either through a bottom-up approach, by directly measuring fluxes constraining models representing the processes at work and GHG emissions inventories, or through a top-down approach by inverting atmospheric methane observations using a chemical transport model. This second approach relies on surface observation networks which are very accurate but unequally distributed worldwide, and on reconstructions of methane columns by spaceborne passive remote sensing instruments. Even though these passive methods supply many additional data to constrain the methane cycle, the data lack precision, are subject to bias (such as aerosols), and do not offer sufficient coverage of extensive areas of key importance for the methane cycle (upper latitudes and cloud-covered areas, for example).
The Lidar of Merlin is designed to tackle these challenges by providing accurate and well characterized methane total columns on the global scale, and particularly over areas of vital importance for the methane cycle not or insufficiently covered (e.g., Arctic, Eurasia and tropical continents).
An active instrument such as the Merlin Lidar should offer more accurate data than current passive instruments because it is less influenced by the bias associated with the presence of particulate layers in the atmosphere and offers observation of upper latitudes. Its small 100-metre footprint and the absence of bias caused by particulate layers should allow reconstruction of a methane column even in complex meteorological situations.
MERLIN at Laboratoire de Météorologie Dynamique
Benefiting from the strong expertise developed in the lab on the desgining of lidar mission for CO2 (ESA projects FACTS and A-SCOPE), LMD (Pierre Flamant) proposed the Merlin mission to the 2009 Prospective Seminar of CNES. Since then, LMD has been participating at many levels to the preparation of Merlin, which is now under the scientific responsibility of Philippe Bousquet (LSCE). Major activities include :
- Memberships to the CNES/DLR Scientific Advisory Group (SAG).
- Animation of the methane scientific community. This has included the organization of a national workshop on CH4 ( Journées Nationales du Méthane) in 2015.
- Expertise on the IPDA lidar, benefiting from the on-going development of lidars in the team.
- Coordinating the spectroscopic and radiative transfer studies specifically designed for meeting the need in precision of Merlin.
- Development of the processing chain for level2 products (XCH4 and associated weighting function and uncertainties), which includes the development of a mission simulator in order to evaluate the specifications of the instrument, and potential compromises, and to simulate the expected uncertainties on the retrieved CH4 total column.
- Development of a cal/val strategy.
Please, visit https://merlin.cnes.fr/en/MERLIN/index.htm