Pulsed Lidar Measurements of CO2 Column Concentrations in the 2017 ASCENDS Airborne Campaign and Comparison with In Situ Measurements

The CO2 Sounder lidar is a pulsed, multiple-wavelength integrated path differential absorption lidar for measuring CO2 column concentrations. The lidar measures the range resolved shape of the 1572.33 nm CO2 absorption line to scattering surfaces, including the ground and the tops of clouds. Airborne measurements are currently using 30 fixed-wavelength samples distributed across the line. Analysis estimates the lidar range and pulse energies at each wavelength 10 times per second. For each second the retrievals solve for the CO2 absorption line shape and the column average CO2 concentrations by using radiative transfer calculations, the aircraft altitude and range to the scattering surface, and the atmospheric conditions.

The lidar was flown on the NASA DC-8 in late July and early August as part of the 2017 ASCENDS airborne campaign. The campaign objectives were to assess the accuracy of airborne IPDA lidar measurements of CO2 column concentrations (XCO2) and to extend these lidar measurements, for the first time, to the Arctic. To date six science flights have been conducted with simultaneous XCO2 measurements from the lidar and in-situ CO2 measurements made at aircraft altitude with the AVOCET and Picarro in-situ sensors. Over 30 spiral-down maneuvers have been conducted in the campaign over locations in California, the US Midwest, in Northwest Territories (NWT) Canada and in mid and western Alaska. Since each spiral maneuver allows comparing the retrievals of XCO2 from the lidar against those computed from in-situ measured CO2, this campaign allows an unprecedented opportunity to assess the lidar’s performance over a diverse set of conditions, including those in the Arctic.

The paper will describe the flight paths, and will present the results from analyzing the retrieved XCO2 from the lidar in the campaign over the different regions. It will also present the results of comparing the lidar-retrieved XCO2 to those computed from the in-situ sensors at the numerous spiral-down locations.