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Profiling of CH4 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 100 m height resolution is possible for methane.
Agreement between the measured photon-counting rate in the CH4 Raman
channel in the free troposphere and numerical simulations for a typical
CH4 background mixing ratio (2 ppm) confirms that CH4 Raman
scattering is detected. The mixing ratio is calculated from the ratio of
methane (395.7 nm) and nitrogen (386.7 nm) Raman backscatters, and within the
planetary boundary layer, an increase of the CH4 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 CH4 is observed. However, the fluorescence
contribution cannot be completely excluded and, for future measurements, we
plan to install an additional control channel close to 393 nm, where no strong
Raman lines exist and only fluorescence can be observed.
