Retrieval Algorithm for the Column CO<sub>2</sub> Mixing Ratio from Pulsed Multi-Wavelength Lidar Measurements

Abstract

Abstract. The retrieval algorithm for the column mixing ratio of CO2 from the measurements of a pulsed multi-wavelength integrated path differential absorption (IPDA) lidar is described. The lidar samples the shape of the 1572.33 nm CO2 absorption line at 15 or 30 wavelengths. The algorithm uses a least-squares fit between the CO2 line shape computed from a layered atmosphere model to that sampled by the lidar. In addition to the column average CO2 dry air mole fraction (XCO2), several other parameters are also solved simultaneously from the fit. These include the Doppler shift in the received laser signal wavelengths, the product of the surface reflectivity and atmospheric transmission and a linear trend in the lidar receiver's spectral response. The algorithm can also be used to solve for the average water vapor mixing ratio, which causes a secondary absorption in the wings of the CO2 absorption line under high humidity conditions. The least-squares fit is linearized about the expected XCO2 value which allows the use of a standard linear least-squares fitting method and software tools. The standard deviation of the retrieved XCO2 is obtained from covariance matrix of the fit. An averaging kernel is defined similarly to that used for passive trace-gas sounding. Examples are presented of using the algorithm to retrieve XCO2 from the measurements from NASA Goddard's airborne CO2 Sounder lidar made at a constant altitude and during spiral-down maneuvers.

Attenuated atmospheric backscatter profiles measured by the CO2 Sounder lidar in the 2017 ASCENDS/ABoVE airborne campaign

“…The XCO 2 value is retrieved via an algorithm using a linear least-squares fit of the modeled CO 2 line shape to the lidar sampled absorption line shape. Details about the signal processing for the XCO 2 retrieval can be found in Sun et al (2021).…”

“…The lidar receiver uses a single photon sensitive HgCdTe avalanche photodiode (APD) and an analog-to-digital convertor (ADC) that records the entire atmospheric backscatter profile, as well as the surface reflected signal. The surface returns are used to retrieve the XCO 2 of the atmosphere column traveled by the laser pulse (Sun et al, 2021). The atmospheric backscatter signals were not directly used for XCO 2 retrieval, except for cloud identifications.…”

“…They can be used for retrieving partial column XCO 2 to cloud tops (Mao et al, 2018). They also enable identifying and profiling smoke plumes from wildfires to help assess their impact on XCO 2 (Mao et al, 2021).…”

Measuring Atmospheric CO2 Enhancements From the 2017 British Columbia Wildfires Using a Lidar

“…The lidar retrieval algorithm uses a least‐squares fit between the 30 wavelengths of the lidar measurements and the calculated CO 2 absorption line shape to retrieve XCO 2 (Ramanathan et al., 2018; Sun et al., 2021). The approach allows use of a standard linear least squares method to simultaneously solve for Doppler frequency shift, surface reflectance at off‐line wavelengths, and a linear non‐uniformity (slope) in the receiver spectral response as well.…”