An a posteriori method based on photo-acoustic cell information to correct for lidar transmitter spectral shift: Application to atmospheric CO₂ differential absorption lidar measurements

An a posteriori corrective method based on photo-acoustic cell (PAC) information is proposed to correct for laser transmitter spectral shift during atmospheric CO₂ measurements by 2 μm heterodyne differential absorption lidar (HDIAL) technique. The method for using the PAC signal to retrieve the actual atmospheric CO₂ absorption is presented in detail. This issue is tackled using a weighting function. The performance of the proposed corrective method is discussed and the various sources of error associated with the PAC signal are investigated. For 300 shots averaged and a frequency shift (from the CO₂ absorption line center) lower than the CO₂ absorption line half-width, the relative error on HDIAL CO ₂ mixing ratio measurements is lower than 1.3%. The corrective method is validated in absolute value by comparison between HDIAL and in situ sensor measurements of CO₂.