Field Evaluation of Column CO₂ Retrievals From Intensity‐Modulated Continuous‐Wave Differential Absorption Lidar Measurements During the ACT‐America Campaign

Abstract

We present an evaluation of airborne Intensity-Modulated Continuous-Wave (IM-CW) lidar measurements of atmospheric column CO2 mole fractions during the ACT-America project. This lidar system transmits online and offline wavelengths simultaneously on the 1.57111-µm CO2 absorption line, with each modulated wavelength using orthogonal swept frequency waveforms. After the spectral characteristics of this system were calibrated through short-path measurements, we used the HITRAN spectroscopic database to calculate the average-column CO2 mole fraction (XCO2) from the lidar measured optical depths. Using in situ measurements of meteorological parameters and CO2 concentrations for calibration data, we demonstrate that our lidar CO2 measurements were consistent from season to season and had an absolute calibration error (standard deviation) of 0.80 ppm when compared to XCO2 values calculated from in situ measurements. By using a 10-second or longer moving average, a precision of 1 ppm or better was obtained. The estimated CO2 measurement precision for 0.1-s, 1-s, 10-s, and 60-s averages were determined to be 3.4 ppm 1.2 ppm, 0.43 ppm, and 0.26 ppm, respectively. These correspond to measurement signal-to-noise ratios of 120, 330, 950, and 1600, respectively. The drift in XCO2 over one-hour of flight time was found to be below 0.1 ppm. These analyses demonstrate that the measurement stability, precision and accuracy are all well below the thresholds needed to study synoptic-scale variations in atmospheric XCO2. This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as

Evaluation of OCO‐2 X Variability at Local and Synoptic Scales using Lidar and In Situ Observations from the ACT‐America Campaigns

“…The MFLL, developed by Harris Corp., with support and evaluation by NASA's LaRC (Dobbs et al, ; ), was designed specifically to address the goals set by the Active Sensing of CO 2 over Nights, Days, and Seasons (ASCENDS) mission and is the first intensity‐modulated continuous‐wave (IM‐CW) laser absorption spectrometer (LAS). This section provides only a brief overview of the MFLL and its operation during ACT‐America; further details of the MFLL instrument for ACT‐America can be found in Campbell et al ().…”

“…We use HITRAN2012 tables for (Rothman et al, ) and MERRA2 reanalysis data for meteorological inputs (Gelaro et al, ). For the full details of our spectroscopy models and associated assumptions, please see Campbell et al ().…”

“…MFLL optical depths throughout the mission demonstrated some range‐dependent bias, the cause of which is currently unknown. As discussed more thoroughly in Campbell et al (), the value of this bias varied between individual flights, but by comparing MFLL optical depth measurements to in situ spiral data from the aircraft, a line was fit to the data to determine the difference in lidar optical depth versus in situ optical depth. An average bias correction for each campaign was empirically developed by the LaRC data processing team and applied to all flights in each campaign, effectively removing this range dependence.…”

Atmospheric Carbon and Transport – America (ACT‐America) Data Sets: Description, Management, and Delivery

“…Vertical profiles were included within all flight plans, and were used to document local vertical structure including ABL depths (Pal, 2019 ). Spiral profiles were used for direct comparison to remotely sensed column CO 2 and CH 4 (e.g., Campbell et al., 2020 ; Bell et al., 2020 ). During the five campaigns, a total of 121 RFs, more than 1,140 h of observations, 570 level legs, and 1,363 vertical profiles were conducted using the two aircraft (Table 2 ; Figure 2 ).…”

Retrieval algorithm for the column CO<sub>2</sub> mixing ratio from pulsed multi-wavelength lidar measurements

“…Several types of dual-wavelength (online and offline) IPDA lidar have been demonstrated previously for measuring XCO 2 from aircraft (Spiers et al, 2011;Menzies et al, 2014;Jacob et al, 2019;Dobler et al, 2013;Campbell et al, 2020;Refaat et al, 2016Refaat et al, , 2020Refaat et al, , 2021Amediek et al, 2017;Zhu et al, 2019Zhu et al, , 2020. A multi-wavelength IPDA lidar has also been reported.…”