Sensitivity studies for a space-based methane lidar mission

Abstract

Methane is the third most important greenhouse gas in the atmosphere after water vapour and carbon dioxide. A major handicap to quantify the emissions at the Earth's surface in order to better understand biosphere-atmosphere exchange processes and potential climate feedbacks is the lack of accurate and global observations of methane. Space-based integrated path differential absorption (IPDA) lidar has potential to fill this gap, and a Methane Remote Lidar Mission (MERLIN) on a small satellite in Polar orbit was proposed by DLR and CNES in the frame of a German-French climate monitoring initiative. System simulations are used to identify key performance parameters and to find an advantageous instrument configuration, given the environmental, technological, and budget constraints. The sensitivity studies use representative averages of the atmospheric and surface state to estimate the measurement precision, i.e. the random uncertainty due to instrument noise. Key performance parameters for MERLIN are average laser power, telescope size, orbit height, surface reflectance, and detector noise. A modest-size lidar instrument with 0.45 W average laser power and 0.55 m telescope diameter on a 506 km orbit could provide 50-km averaged methane column measurement along the sub-satellite track with a precision of about 1 % over vegetation. The use of a methane absorption trough at 1.65 μm improves the near-surface measurement sensitivity and vastly relaxes the wavelength stability requirement that was identified as one of the major technological risks in the pre-phase A studies for A-SCOPE, a space-based IPDA lidar for carbon dioxide at the European Space Agency. Minimal humidity and temperature sensitivity at this wavelength position will enable accurate measurements in tropical wetlands, key regions with largely uncertain methane emissions. In contrast to actual passive remote sensors, measurements in Polar Regions will be possible and biases due to aerosol layers and thin ice clouds will be minimised

Self- and air-broadened line shapes in the 2ν3 P and R branches of 12CH4

“…We believe that the results from this study should be useful for applications such as in the French-German space mission MERLIN to be launched in 2019 [11] and the Lidar measurements from an airborne Integrated Path Differential Absorption (IPDA) Lidar system currently under development at NASA [12]. As in all our previous studies, we recommend that all the present line parameters be applied for consistent results and not mixed with other measurements without detailed validations.…”

“…The Franco-German space mission MERLIN (Methane Remote Sensing Lidar Mission) scheduled to launch in 2019 will monitor the global distributions of column CH 4 using methane absorption features at 6077 cm -1 [11,12]. Accurate knowledge and the variation of absorption at specific wavelengths are required in the retrieval and analyses of these observations.…”

Increased efficiency of Er:YAG lasers at 1645 nm using narrow bandwidth diode lasers and dual-wavelength resonant pumping

“…1. Sensitivity studies suggest the optimum wavelength of the laser transmitter to be at 1645.55 nm [2]. Currently, OPO/OPA systems are primarily considered as methane DIAL transmitters [3].…”

Laser energy monitor for double-pulsed 2-μm IPDA lidar application

“…The loss of the range-resolved profiling sensitivity that is provided through conventional DIAL is a limitation of IPDA technique. On the other hand, relying on hard target and laser energy monitor strong signals provides high signal-to-noise ratio (SNR) which contributes significantly to the measurement sensitivity and accuracy of the IPDA technique [1][2][3][4].…”

“…Remote sensing using the integrated path differential absorption (IPDA) lidar is suitable for monitoring different atmospheric trace gases [1][2][3][4]. Similar to the differential absorption lidar (DIAL), the technique relies on the differentiation between strong and weak absorbing features of the gas to be monitored with respect to wavelength.…”