MERLIN: a Franco-German IPDA Mission dedicated to Atmospheric Methane

A new Lidar mission: MERLIN, focusing on methane (CH4), the second anthropogenic greenhouse gas with an average mixing ratio of 1.7 ppm, has been initiated late 2009 in cooperation between France and Germany for an expected launch in 2016 for a 3 years operation. MERLIN is a micro-sat, it is challenging wrt science and Lidar technique. Atmospheric methane, as well as CO2, is a key player in climate change issue retaining full attention of the scientific community for better modelling and observations, especially from space because ground-based networks are sparse and unevenly distributed. The direct effect for CH4 is associated to a Global Warming Potential that is 24 times larger than CO2. Including all the effects the radiation forcing of CH4 amounts up to 50% of that of CO2. The issue is more on sources for CH4 for atmospheric radical OH is the overwhelming sink. Now, 65 % of total CH4 emissions are due to human activities, and all CH4 sources show large uncertainties. Moreover, changing high latitude landscapes, melting permafrost, potential sources in artic ocean and methane hydrate in the shallow geosphere are additional issues and worries. The science objective calls for global observations in all seasons. It is driven primarily by CH4 surface fluxes at regional scale (500 x 500 km2) with an accuracy of 1 % or better. In this respect, the Lidar technique provides undisputable advantages i) no bias due to aerosols contribution in usefull Lidar signals, ii) all seasons coverage (including high latitudes in Winter!), iii) night and day soundings, iv) effective sounding in partly cloudy conditions and above dense clouds. A joint Science Advisory Group (SAG) has been established by CNES and DLR to elaborate the mission Users Requirements Document to be used by the Project Team to draw the mission specifications wrt orbit, altitude, sampling, and instrument specifications wrt accuracy and bias. The Lidar instrument payload is a pulsed dual wavelengths OPO laser operating sequentially at 1.645 μm to provide CH4 column content using surface returns. The OPO laser is pumped by a high performance master Nd-YAG laser. The OPO laser is injection seeded and cavity controlled to provide stable single longitudinal mode emissions at the on-line wavelength. The Lidar is developed by Germany under DLR responsibility. It will be operated from a new MYRIADE evolutions platform developed by France under CNES responsibility. Given the resources in terms of orbit, mass and electric power, the measurements will be performed using the Integrated Path Differential Absorption (IPDA) technique relying on surface reflections. The main mission product is the column weighted dry-air mixing ratio (i.e. XCH4) along the satellite sub-track (no scanning) with measurements accumulation of 50 km. The level 2: XCH4, products are computed from level 1: Differential Absorption Optical Depth (DOAD) and CH4 weighting function (WF). The WF is computed using the absorption cross-section at the on-line wavelength on one hand and meteorological information: surface pressure, temperature and humidity (to be provided by NWP analysis) on the other hand. Mid 2012, the MERLIN project has proceeded through a preliminary phase 0 and successfully completed a phase A review.