# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 1 Introduction
The inversion systems inferring CH\({}_{4}\) emissions from CH\({}_{4}\) mole fractions are limited by the quality of the underlying atmospheric transport model used (e.g., [22; 23]) and by the uneven distribution of surface observations in space and time (e.g., [24]). Some key regions for the biogeochemical cycles of GHGs remain extremely difficult to access for long-term scientific activities because of their remote location or their political situation (e.g., the Arctic, tropical forests, some African countries). In addition, industrialized regions often have many different sources of CH\({}_{4}\) from, for example, livestock, fuel extraction and waste--some of them concentrated in hotspot regions--which requires high spatial density surface networks to separate and quantify those sources individually [25]. In this context, measuring CH\({}_{4}\) from space becomes important, as it offers a drastic improvement in observational capacity to potentially provide either global coverage of CH\({}_{4}\) concentrations on a regular temporal basis (solar synchronous orbits), or regional coverage with continuous observations during daytime (geostationary orbits). However, the monitoring of a trace gas such as CH\({}_{4}\) from space at a sufficient precision and accuracy to determine the changes in the plume resulting from changes in surface emissions is challenging. This is because the magnitude and variability of the absorption signal measurable at the top of the atmosphere for atmospherically relatively long-lived gases such as CH\({}_{4}\) is relatively small in comparison to the changes in reactive gases such as O\({}_{3}\) and CO, for example. Further, the accurate retrieval of these small changes is limited by the knowledge of spectroscopic parameters and of other atmospheric species which may have overlapping absorptions in the same spectral region. Recent progress in the methane spectroscopy in the near-infrared should reduce uncertainty in retrievals (e.g., [26]). CH\({}_{4}\) fluxes emitted from the surface result in small changes in the spatial distributions of the above CH\({}_{4}\) dry column mole fraction (e.g., typically a few ppb, and up to a few tens of ppb, on a day-to-day basis and at a typical model spatial resolution of 200 \(\times\) 200 km, (see Science Plan of MEthane Remote sensing Lidar missioN (MERLIN) [27] with most of the variation happening in the boundary layer. The level of measurement uncertainty that is required in this context to identify such changes above atmospheric background (\(\sim\)1800 ppb) is typically lower than \(\pm\)2% for the total measurement error, with low systematic errors. Such an already ambitious objective for a space-based measurement of methane columns is still far from the recommendation of the World Meteorological Organisation (WMO) for surface in situ observations of atmospheric methane, which is to be better than \(\pm\)2 ppb (\(<\)0.1%, for point in situ measurements [28]). In the decade beginning in 2000, two passive instruments were launched into sun synchronous orbits and have provided column-averaged dry air mole fractions (XCH\({}_{4}\)) using shortwave infrared absorption spectrometry (SWIR). Between 2002 and 2012, the Scanning Imaging Absorption spectrometer for Atmospheric CHartographY (SCIAMACHY) was operated on board ESA's (European Space Agency) ENVIronmental SATellite [29; 30; 31; 32; 33; 34]. In January 2009, the JAXA (Japan Aerospace Exploration Agency) satellite Greenhouse Gases Observing SATellite (GOSAT) was launched with the TANSO-FTS instrument [35; 36]. Global and regional inverse modelling of CH\({}_{4}\) fluxes made use of SCIAMACHY [37; 38; 39; 40; 41] and GOSAT [22; 42; 43] satellite retrievals to infer CH\({}_{4}\) emissions. Passive SWIR instruments offer a limited coverage in cloud-covered regions and in high latitudes from the autumn to the spring equinox.