Retrieval algorithm for the column CO2 mixing ratio from pulsed multi-wavelength lidar measurements

Abstract

The retrieval algorithm for CO₂ column mixing ratio from 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 multiple wavelengths. The algorithm uses a least-squares fit between the CO₂ line shape computed from a layered atmosphere model and 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 at the received laser signal wavelength, 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 produces a secondary absorption in the wings of the CO₂ absorption line under humid 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 the covariance matrix of the fit. The averaging kernel is also provided similarly to that used for passive trace-gas column measurements. Examples are presented of using the algorithm to retrieve XCO₂ from measurements of the NASA Goddard airborne CO₂ Sounder lidar that were made at constant altitude and during spiral-down profile maneuvers.