Airborne Measurements of CO<sub>2</sub> Column Concentrations made with a Pulsed IPDA Lidar using a Multiple-Wavelength-Locked Laser and HgCdTe APD Detector

Abstract. Here we report on measurements made with an improved CO2 Sounder lidar during the ASCENDS 2014 and 2016 airborne campaigns. The improvements made to the 2011 version of the lidar instrument included incorporating a rapidly wavelength tunable, step-locked seed laser in the transmitter, using a much more sensitive HgCdTe APD detector, and using an analog digitizer with faster readout time in the receiver. We also improved the lidar’s calibration approach and the XCO2 retrieval algorithm. The 2014 and 2016 flights were made over several types of topographic surfaces from 3–12 km aircraft altitudes in the continental US. The results are compared to the XCO2 values from an airborne in situ sensor during spiral-down maneuvers. The 2014 results show significantly improved performance including measurement of horizontal gradients in XCO2 made over the US Midwest that agree with chemistry transport models. The results from the 2016 airborne lidar retrievals show precisions of ~ 0.8 parts per million (ppm) with 1 second averaging over desert surfaces. Measurements in 2016 were also made over fresh snow surfaces with lower surface reflectance at the laser wavelengths. The results from both campaigns showed the mean values of XCO2 retrieved from the lidar consistently agreed with those based on the in situ sensor to within 1 ppm. The precision and accuracies demonstrated should benefit future airborne science campaigns and serve as feasibility demonstrations for a future space-based instrument.