Profiling of CH 4 background mixing ratio in the lower troposphere with Raman lidar: A feasibility experiment

Abstract

We present the results of methane profiling in the lower troposphere using LILAS Raman lidar from the Lille University observatory platform (France). The lidar is based on a frequency-tripled Nd:YAG laser, and nighttime profiling up to 4000 with 100m height resolution is possible for methane. Agreement between the measured photon-counting rate in the CH 4 Raman channel in the free troposphere and numerical simulations for a typical CH 4 background mixing ratio (2ppm) confirms that CH 4 Raman scattering is detected. The mixing ratio is calculated from the ratio of methane (395.7 nm) and nitrogen (386.7nm) Raman backscatters, and within the planetary boundary layer, an increase of the CH 4 mixing ratio, up to a factor of 2, is observed. Different possible interfering factors, such as leakage of the elastic signal and aerosol fluorescence, have been taken into consideration. Tests using backscattering from clouds confirmed that the filters in the Raman channel provide sufficient rejection of elastic scattering. The measured methane profiles do not correlate with aerosol backscattering, which corroborates the hypothesis that, in the planetary boundary layer, not aerosol fluorescence but CH 4 is observed. However, the fluorescence contribution cannot be completely excluded and, for future measurements, we plan to install an additional control channel close to 393nm, where no strong Raman lines exist and only fluorescence can be observed.