Atmospheric CO2 Column Measurements Under Clear and Cloudy Conditions Using an Airborne Intensity-Modulated Continuous-Wave Lidar
Thursday, 18 December 2014
This study focuses on the atmospheric CO2 measurements using the Exelis’ airborne Intensity-Modulated Continuous-Wave (IM-CW) Laser Absorption Spectrometer (LAS) system operating in the 1.57-mm CO2 absorption band. The atmospheric CO2 estimates above clouds and for entire columns are retrieved from the data obtained during the summer 2011 and spring 2013 ASCENDS flight campaigns. The lidar returns from clouds and surfaces are discriminated by the range-encoded IM signals transmitted by the airborne LAS system. Under thin cloud conditions, lidar systems generally have strong enough return signals from the surface for CO2 retrieval. For optically thick clouds, CO2 columns above the clouds are estimated with lidar returns from the cloud tops, and neighboring clear sky areas are used to measure the total CO2 columns to the surface. Case studies show that the full-column atmospheric CO2 measurements are very similar in adjacent clear and thin-cloud regions, while the signal-to-noise ratio (SNR) values of the CO2 columns are very different due to differences in the transmissions from aircraft to surface over cloudy and clear regions. For example, the measured CO2 columns to land surfaces were found to be about 398 ppm with an average SNR values for 0.1-s averages of about 140 and 38 in clear and thin-cloud conditions, respectively. Under thick clouds conditions, considerable variations in lidar returns for the extended targets are found. Also, their CO2 differential absorption optical depth values are normally smaller than those to the surface. These effects significantly reduce the precisions of CO2 column measurements both above clouds and to the surface under cloudy conditions compared to those to the surface in clear skies. Still, column-averaged CO2 mixing ratio (XCO2) estimates above clouds for daytime observations are expected to be slightly higher than those for the entire atmospheric column due to CO2 uptake by vegetation at the surface.