Measurements of Atmospheric CO2 Column in Cloudy Weather Conditions using An IM-CW Lidar at 1.57 Micron

摘要

1. Introduction Changes in atmospheric carbon dioxide (CO2) concentrations are considered as one of the key factors in the net radiative heating to the Earth's climate system and in determining future global climate change [1, 2]. The U.S. National Research Council has identified the need for an Active Sensing of CO2 Emission over Nights, Days and Seasons (ASCENDS) space mission [3] to reduce uncertainties in and to gain a better understanding of CO2 sources and sinks. In preparing for the ASCENDS mission, NASA Langley Research Center and Exelis Inc. have jointly developed and demonstrated the capability of CO2 column measurements with an intensity-modulated continuous-wave (IM-CW) lidar. Results of CO2 column measurements from aircraft flight campaigns using the prototype IMCW lidar, Multifunctional Fiber Laser Lidar (MFLL), have been very encouraging [4, 5]. MFLL operates in the 1.57-□m CO2 absorption band with one laser wavelength positioned on the CO2 absorption line center (online) at 1571.112 nm, and two other laser wavelengths (offlines) positioned +50 pm on either side of the absorption line. CO2 column differential absorption optical depth (DAOD) values are derived from combined online and offline measurements using the Integrated Path Differential Absorption (IPDA) approach. The signal-to-noise ratio (SNR) for clear sky IPDA measurements of CO2 DAOD for a 10-s average was found to be as high as 1300, resulting in an error 0.077% or equivalent XCO2 column precision of -0.3 ppm [4, 6].