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Discussion papers | Copyright
https://doi.org/10.5194/amt-2018-62
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Apr 2018

Research article | 03 Apr 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

Improving the Retrieval of XCO2 from Total Carbon Column Network Solar Spectra

Joseph Mendonca1, Kimberly Strong1, Debra Wunch1, Geoffrey C. Toon2, David A. Long3, Joseph T. Hodges3, Vincent T. Sironneau3, and Jonathan E. Franklin4 Joseph Mendonca et al.
  • 1Department of Physics, University of Toronto, Toronto, ON, Canada
  • 2Jet Propulsion Laboratory, Pasadena, CA, USA
  • 3National Institute of Standards and Technology, Gaithersburg, MD, USA
  • 4Harvard John A. Paulson School of Engineering and Applied Sciences, Cambridge, MA, USA

Abstract. High-resolution absorption spectra of the a1g←X3Σg O2 band measured using cavity ring-down spectroscopy were fitted using the Voigt and speed-dependent Voigt line shapes. We found that the speed-dependent Voigt line shape was better able to model the measured absorption coefficients than the Voigt line shape. Total columns of O2 were retrieved from ground-based high-resolution absorption spectra from four Total Carbon Column Observing Network (TCCON) sites using both Voigt and speed-dependent Voigt line shapes to calculate absorption coefficients. A lower O2 concentration was retrieved with the speed-dependent Voigt line shape, with the difference increasing as a function of solar zenith angle. CO2 total columns were also retrieved from the same spectra using a Voigt line shape and speed-dependent Voigt with line mixing. The column-averaged dry-air mole fraction of CO2 (XCO2) was calculated using the CO2 and O2 columns retrieved with both line shapes from measurements made over a one-year period at the four sites and compared. The inclusion of speed dependence reduces the airmass dependence of XCO2. The TCCON empirical airmass correction factor for XCO2 derived from a year of measurements from TCCON sites at Darwin, Lamont, and Park Falls for XCO2 improved from −0.0071±0.0057 to −0.0012±0.0054 when speed dependence was included. XCO2 retrieved with the Voigt and speed-dependent Voigt line shapes was compared to aircraft profiles measured at 13 TCCON sites. The bias between the TCCON measurements and the integrated aircraft profile measurements was reduced from 0.9897±0.0005 to 1.0041±0.0005 for XCO2 retrieved with the Voigt and speed-dependent Voigt line shapes respectively. These results suggest that speed dependence should be included in the forward model when fitting near-infrared CO2 and O2 spectra to improve the accuracy of XCO2 measurements.

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Short summary
In order to study the carbon cycle, accurate remote sensing measurements of XCO2 are required. Which means that accurate absorption coefficients of CO2 and O2 in the retrieval algorithm are required. We use high-resolution laboratory spectra of O2 to derive accurate absorption coefficients of O2. By applying the O2 absorption coefficients in the retrieval of XCO2 from ground-based solar absorption spectra we show that the error on retrieved XCO2 is decreased.
In order to study the carbon cycle, accurate remote sensing measurements of XCO2 are required....
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