Remote sensing of methane with broadband laser and optical correlation spectroscopy on the Q-branch of the 2ν₃ band

With a global warming ability 72 times higher than carbon dioxide on short-term climatology, methane gas is one of the most important greenhouse gases in the Earth's atmosphere. After one decade of global constant value, the global methane concentration in the Earth's atmosphere is now increasing, so that methane monitoring is becoming a challenge, both at global and local scales. In this contribution, we propose a study on the ability to achieve accurate range-resolved methane concentration measurements in the Earth's atmosphere by combining broadband optical correlation spectroscopy (OCS) with lidar. Our study focuses on the 2ν₃ methane absorption band, centered at the 1665 nm wavelength, which lies in an absorption infrared window of the Earth's atmosphere. A numerical study has been achieved to assess the influence of the methane absorption spectroscopic line parameters on the retrieved methane-mixing ratio when applying the OCS-lidar methodology. It is shown that an atmospheric temperature gradient induces a systematic error on the retrieved methane-mixing ratio, and this error is here accurately evaluated. Moreover, we show that methane-mixing ratios are weakly sensitive to the pressure broadening of the absorption line and to the statistical variation of the absorption line strength. The finding is that, thanks to accurate methane line parameters in the 2ν₃ methane absorption band, an accurate remote sensing of methane-mixing ratio, in the tens of ppm range, can be achieved with OCS-lidar with only a weak constraint on the value of the temperature of the atmosphere.