Atmospheric CO_2 column measurements with an airborne intensity-modulated continuous wave 1.57 μm fiber laser lidar

摘要

The 2007 National Research Council (NRC) Decadal Survey on Earth Science and Applications from Space recommended Active Sensing of CO_2 Emissions over Nights, Days, and Seasons (ASCENDS) as a midterm, Tier II, NASA space mission. ITT Exelis, formerly ITT Corp., and NASA Langley Research Center have been working together since 2004 to develop and demonstrate a prototype laser absorption spectrometer for making high-precision, column CO_2 mixing ratio measurements needed for the ASCENDS mission. This instrument, called the multifunctional fiber laser lidar (MFLL), operates in an intensity-modulated, continuous wave mode in the 1.57 μm CO_2 absorption band. Flight experiments have been conducted with the MFLL on a Lear-25, UC-12, and DC-8 aircraft over a variety of different surfaces and under a wide range of atmospheric conditions. Very high-precision CO_2 column measurements resulting from high signal-to-noise ratio (>1300) column optical depth (OD) measurements for a 10 s (～1 km) averaging interval have been achieved. In situ measurements of atmospheric CO_2 profiles were used to derive the expected CO_2 column values, and when compared to the MFLL measurements over desert and vegetated surfaces, the MFLL measurements were found to agree with the in situ-derived CO_2 columns to within an average of 0.17% or ～0.65 ppmv with a standard deviation of 0.44% or ～1.7 ppmv. Initial results demonstrating ranging capability using a swept modulation technique are also presented.