Laser Sounder for Measuring Atmospheric CO2 Concentrations for the ASCENDS Mission - Progress
James
B
Abshire, NASA-Goddard,, Code 690, james.b.abshire@nasa.gov
(Presenting)
Haris
Riris, NASA-Goddard,, Code 694, haris.riris@nasa.gov
S.
Randy
Kawa, NASA-Goddard,, Code 613, randy.kawa@gsfc.nasa.gov
Jianping
Mao, RSIS, Inc, jianping.mao@gsfc.nasa.gov
Graham
R.
Allan, Sigma Space, graham.allan@gsfc.nasa.gov
Xiaoli
Sun, NASA-Goddard, Code 694, xiaoli.sun@gsfc.nasa.gov
Mark
A.
Stephen, NASA-Goddard,, Code 554, mark.stephen@gsfc.nasa.gov
Michael
A.
Krainak, NASA-Goddard,, Code 554, michael.krainak@gsfc.nasa.gov
Emily
Wilson, NASA-Goddard,, Code 554, emily.wilson@gsfc.nasa.gov
Accurate global measurements of tropospheric CO2 concentrations are needed with diurnal coverage and monthly temporal resolution to better quantify the processes that exchange atmospheric CO2 with the land and oceans. For this the National Research Council’s 2007 Decadal Survey for Earth Science has recommended following the OCO and GOSAT CO2 measuring missions with a laser-based mission called ASCENDS.
We have been developing a laser technique and technologies to enable the remote measurement of tropospheric CO2 concentrations from space. Our initial goals are to develop and demonstrate the lidar techniques and technologies that permit measurements of the CO2 column abundance over horizontal paths and from aircraft at the few-ppmv level. Our longer-term goal is to demonstrate the needed capabilities of the technique and technologies, and develop a space mission approach and the instrument design for a space mission like ASCENDS. This work in ongoing and has been supported by the NASA ESTO ACT and IIP programs.
Our approach is to use the 1570-nm CO2 band and a pulsed dual channel laser absorption spectrometer. This uses differential lidar absorption measurement in an altimeter mode, and continuously measures at nadir from a near-polar circular orbit. It uses several tunable fiber laser transmitters allowing simultaneous measurement of the absorption from a CO2 absorption line in the 1570 nm band, O2 extinction in the oxygen A-band, as well as surface height and aerosol backscatter in the same measurement path. It directs the narrow co-aligned laser beams toward nadir, and measures the energy of the pulsed laser echoes reflected from land and water surfaces. During the measurement, the lasers are tuned across a selected CO2 line and a region between two O2 lines near 765 nm. The lasers have spectral widths much narrower than the gas absorption lines and are wavelength tuned at kHz rates. The receiver uses a telescope and photon counting detectors, and measures the background light and energies of the laser echoes from the surface. The gas extinction and column densities for the CO2 and O2 gases are estimated from the ratios of the on and off line signals. We use pulsed laser signals and time gating to isolate the laser echo signals from the surface, and to reject photons scattered from thin clouds and aerosols in the path, which can otherwise bias retrievals. High signal-to-noise ratios are required and the gas column absorption estimates need to be quite stable for hours.
We have constructed breadboard versions of the CO2 and O2 sensors, which use fiber lasers and 20 cm diameter telescopes. We have used them to make measurements of CO2 and O2 absorption in the laboratory and over 206, 400-m 1.3 and 2.2 km long horizontal paths. These have been in several sessions extending over multiple days, and have allowed us to compare its estimates to readings from external CO2 sensors. We have also calculated several characteristics of the technique for space, including its expected measurement performance, selected some key technologies for space and have performed a space mission accommodation study. We will show these results in the paper.
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