Remote Sensing of Methane Gas Concentration by Optical Correlation Spectroscopy Lidar

In this contribution, we present a new method to remotely evaluate Methane greenhouse gas concentration in the atmosphere. It is based on combining the Optical Correlation Spectroscopy (OCS) method with laser remote sensing (see fig. 1). The novelty of this work is that an Acoustic Optical Programmable Dispersive Filter (AOPDF) is used and is coupled to ultra-short spectrally broadened femtosecond laser pulses. Hence, this approach avoids using a gas correlation cell, as needed in the standard OCS setup. The accuracy and sensitivity of the OCS-Lidar are evaluated from theoretical and numerical models. They mainly depend on the spectroscopic properties of the atmospheric gas traces, on the laser energy density spectra and the detector sensitivity. All these contributions are evaluated. Possible interference of other atmospheric gas traces, mainly water vapor, are also studied. Moreover, this new approach shows that the CH4-concentration is poorly affected by temperature and pressure of the gas absorption line strength and line-width respectively. This study shows that CH4-remote sensing by OCS-Lidar could efficiently operate in the mid infrared spectral region at 1.65 μm where 1.5 ppm sensitivity and 20% accuracy can be reached within 100 meters spatial resolution and a 1 second duty cycle. Experimental and operational proofs are under progress.