Atmospheric CO2 Column Measurements Under Clear and Cloudy Conditions Using an Airborne Intensity-Modulated Continuous-Wave Lidar

Abstract:

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.