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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/amt-2020-429
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2020-429
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  13 Nov 2020

13 Nov 2020

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This preprint is currently under review for the journal AMT.

Retrieval of atmospheric CO2 vertical profiles from ground-based near-infrared spectra

Sébastien Roche1, Kimberly Strong1, Debra Wunch1, Joseph Mendonca2, Colm Sweeney4, Bianca Baier3,4, Sébastien C. Biraud5, Joshua L. Laughner6, Geoffrey C. Toon7, and Brian J. Connor8 Sébastien Roche et al.
  • 1Department of Physics, University of Toronto, Toronto, ON, Canada
  • 2Climate Research Division, Environment and Climate Change Canada, Toronto, ON, Canada
  • 3Cooperative Institute for Research in Environmental Sciences, University of Colo rado, Boulder, CO, USA
  • 4NOAA Global Monitoring Laboratory, Boulder, CO, USA
  • 5Lawrence Berkeley National Laboratory, Berkeley, CA, USA
  • 6California Institute of Technology, Pasadena, CA, USA
  • 7Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
  • 8BC Consulting Limited, Martinborough, New Zealand

Abstract. We evaluate vertical profile retrievals of CO2 from 0.02 cm−1 resolution ground-based near-infrared solar absorption spectra with the GFIT2 algorithm, using improved spectroscopic linelists and line shapes. With these improvements, CO2 profiles were obtained from sequential retrievals in five spectral windows with different vertical sensitivities. A sensitivity study using synthetic spectra shows that the leading source of uncertainty in the retrieved CO2 profiles is the error in the a priori temperature profile, even with 3-hourly reanalysis a priori profiles. A 2 °C error in the temperature profile in the lower troposphere between 0.6 and 0.85 atm causes deviations in the retrieved CO2 profiles that are larger than the typical vertical variations of CO2. To distinguish the effect of errors in the a priori meteorology and trace gas concentration profiles from those in the instrument alignment and spectroscopic parameters, we retrieve CO2 profiles from atmospheric spectra while using an a priori built from coincident AirCore, radiosonde, and surface in situ measurements at the Lamont, Oklahoma (USA) Total Carbon Column Observing Network station. In those cases, the deviations in retrieved CO2 profiles are also larger than typical vertical variations of CO2, suggesting that remaining errors in the forward model limit the accuracy of the retrieved profiles. Implementing a temperature retrieval or correction, and quantifying and modeling an imperfect instrument alignment, are critical to improve CO2 profile retrievals. Without significant advances in modeling imperfect instrument alignment, and improvements in the accuracy of the temperature profile, the CO2 profile retrieval with GFIT2 presents no clear advantage over scaling retrievals for the purpose of ascertaining the total column.

Sébastien Roche et al.

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Short summary
We evaluate CO2 profile retrievals from ground-based near-infrared solar absorption spectra after implementing several improvements to the GFIT2 retrieval algorithm. Realistic errors in the a priori temperature profile (~2 °C in the lower troposphere) are found to be the leading source of differences between the retrieved and true CO2 profiles, differences that are larger than typical CO2 variability. A temperature retrieval or correction is critical to improve CO2 profile retrieval results.
We evaluate CO2 profile retrievals from ground-based near-infrared solar absorption spectra...
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