# MERLIN: A French-German Space Lidar Mission Dedicated to Atmospheric Methane
## 1 Introduction  
Monitoring methane (CH\({}_{4}\)) atmospheric concentrations from space is an important but challenging scientific problem. It is important because human-induced emissions and land use changes make the global concentration of CH\({}_{4}\) increase and contribute to additional radiative forcing with other increasing greenhouse gases, ultimately leading to the rise of the global mean Earth surface temperature [1]. Methane emissions have various sources, mostly anthropogenic (\(\sim\)2/3), which can be grouped around three processes [2]: the biogenic anaerobic degradation of organic matter by archaea (natural wetlands and inland waters, enteric fermentation and manure, rice cultivation, waste management, termites); the thermogenic formation in the Earth's crust under high temperatures and pressure (natural degassing of the Earth's crust, exploitation of fossil fuels); and the pyrogenic combustion of biomass under low-O\({}_{2}\) condition (biomass and biofuel burning). Even if total methane emissions are estimated with a global uncertainty of only 5-6% by atmospheric-based studies, this uncertainty increases to 20-30% for process-based estimates, and individual sources have uncertainties on the order of 30-40% for anthropogenic sources and larger than 100% for some natural sources (e.g., freshwater emissions, e.g., [2]). Methane dry-air mole fractions have been directly measured with high accuracy (better than 0.1% in the last years, [3]) at reference ground-based stations since the late 1970s [4]. The surface networks have developed throughout the years and, today, provide more than 150 regularly sampled or continuous point measurements at the surface or along tall towers [3]. They are complemented by regular vertical profiles and transects by small airplanes in the planetary boundary layer (e.g., [5]) or by commercial aircraft [6; 7] and by some regional aircraft campaigns (e.g., [8; 9; 10]). The concept of AirCore [11] has also offered the possibility to measure vertical profiles of greenhouse gases (GHG) from the surface to the stratosphere (up to 30 km). On the remote-sensing side, the Total Carbon Column Observing Network (TCCON) uses ground-based Fourier transform spectrometers (FTS) to measure atmospheric column abundances of CH\({}_{4}\) and other molecules using solar absorption spectroscopy in the near-infrared spectral region [12]. This surface-based remote-sensing technique makes use of sunlight, and can only be performed throughout the day during clear sky conditions, with the sun typically at least 10\({}^{\circ}\) above the horizon. During the last years solar absorption spectrometry, using backscattered sunlight detected with a spectrometer on an aircraft [13], was used to estimate CHemissions from strong and localized point sources like coal mine [14] ventilation shafts, or landfills [15], for instance. Surface-based CH\({}_{4}\) observations have been used for decades to estimate CH\({}_{4}\) sources using data-driven methods and atmospheric inversion techniques, which optimally combine observed mole fractions ratios, some prior knowledge about emissions and sinks, and an atmospheric transport and chemistry model linking emissions and sinks to atmospheric observations (e.g., [2; 16; 17]). Converting atmospheric observations to surface emissions has been done mostly from global scale to regional scale (e.g., [2]). However, recently, various multi-institution collaborations have developed methodologies for the quantification of methane emissions in urban environments such as Indianapolis (Indiana) or Los Angeles (California) in the United States (e.g., [18; 19; 20; 21]). The importance of the monitoring of GHG surface fluxes based on atmospheric observations has grown in the context of climate change mitigation after the Paris climate agreement, because it in principle allows emissions and their changes to be tracked with independent data from country-based declarations to the United Nations Framework Convention on Climate Change (UNFCCC).