A Multi-wavelength, Integrated Path Differential Absorption Lidar to Measure XCO2 from Aircraft and Space

James B. Abshire,  NASA GSFC and Univ. of Maryland,  james.b.abshire@nasa.gov (Presenter)
Xiaoli Sun,  NASA GSFC,  xiaoli.sun-1@nasa.gov
Jianping Mao,  NASA GSFC/University of Maryland,  jianping.mao@nasa.gov
Haris Riris,  NASA GSFC,  haris.riris@nasa.gov
Stephan Randolph Kawa,  NASA GSFC,  stephan.r.kawa@nasa.gov
Mark Andrew Stephen,  NASA-GSFC,  mark.a.stephen@nasa.gov

A multi-wavelength CO2 lidar has been developed for measuring XCO2. Its pulsed transmitter steps a single frequency laser across the CO2 absorption line at 1572.33 nm. Typically 30 laser wavelengths are used and the line is scanned at ~300 Hz. The lidar receiver measures the height-resolved backscatter profile and laser energy reflected from the surface at each wavelength. This allows isolating the transmission line shape of CO2 in the nadir path to ground and ocean surfaces. This approach allows consistent XCO2 measurements at all sun angles and in darkness including during the polar winter.
The lidar’s retrieval algorithm uses the lidar-sampled line shape and fits a CO2 line shape based on a layered atmospheric model. It is used to solve for XCO2, Doppler shift, and change of the instrument response with wavelength every second. The backscatter profile allows measurements of thin clouds, aerosols and smoke plumes, and isolating the surface-reflected signal. Together these allow a robust approach designed to minimize biases.
An airborne version of the lidar was used in the 2017 ASCENDS/ABoVE airborne campaign. Eight flights were conducted over California, Canada and Alaska, along with long transit flights. Spiral-down maneuvers allowed comparing the lidar XCO2 retrievals against those computed from in situ measurements. The lidar retrievals show both north-south, east-west, and local gradients in XCO2. They also show better than 1-ppm agreement between lidar and in situ XCO2 in spiral locations. A lidar measurement model has been used to help analyze the results from the airborne campaigns. It includes effects of solar background, photon detection shot noise, detector dark current, laser speckle noise, and the errors from the line fit in the retrieval algorithm.
The space version of the lidar is based on the airborne version but uses more laser amplifier stages and a larger receiver telescope. Engineering models of the space laser components and detector have been built and are nearly at TRL-6. When using the same lidar measurement model with parameters for the space, results for most surfaces show it should achieve <1 ppm error using 1-sec averaging. The presentation will give an overview of the airborne measurements, the measurement model, and the design and performance of the space version.

Poster: Poster_Abshire__138_25.pdf 

Presentation Type: Poster

Session: 2.2c Results expected from future missions

Session Date: Tuesday (6/15) 9:45 AM