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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 10, issue 3
Atmos. Meas. Tech., 10, 939-953, 2017
https://doi.org/10.5194/amt-10-939-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Atmos. Meas. Tech., 10, 939-953, 2017
https://doi.org/10.5194/amt-10-939-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 10 Mar 2017

Research article | 10 Mar 2017

Characterization of the OCO-2 instrument line shape functions using on-orbit solar measurements

Kang Sun1, Xiong Liu1, Caroline R. Nowlan1, Zhaonan Cai1, Kelly Chance1, Christian Frankenberg2,3, Richard A. M. Lee3, Randy Pollock3, Robert Rosenberg3, and David Crisp3 Kang Sun et al.
  • 1Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, USA
  • 2Division of Geological and Planetary Science, California Institute of Technology, Pasadena, CA, USA
  • 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA

Abstract. Accurately characterizing the instrument line shape (ILS) of the Orbiting Carbon Observatory-2 (OCO-2) is challenging and highly important due to its high spectral resolution and requirement for retrieval accuracy (0. 25 %) compared to previous spaceborne grating spectrometers. On-orbit ILS functions for all three bands of the OCO-2 instrument have been derived using its frequent solar measurements and high-resolution solar reference spectra. The solar reference spectrum generated from the 2016 version of the Total Carbon Column Observing Network (TCCON) solar line list shows significant improvements in the fitting residual compared to the solar reference spectrum currently used in the version 7 Level 2 algorithm in the O2 A band. The analytical functions used to represent the ILS of previous grating spectrometers are found to be inadequate for the OCO-2 ILS. Particularly, the hybrid Gaussian and super-Gaussian functions may introduce spurious variations, up to 5 % of the ILS width, depending on the spectral sampling position, when there is a spectral undersampling. Fitting a homogeneous stretch of the preflight ILS together with the relative widening of the wings of the ILS is insensitive to the sampling grid position and accurately captures the variation of ILS in the O2 A band between decontamination events. These temporal changes of ILS may explain the spurious signals observed in the solar-induced fluorescence retrieval in barren areas.

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Accurately characterizing the instrument line shape (ILS) of the Orbiting Carbon Observatory-2 (OCO-2) is challenging and highly important due to its high spectral resolution and requirement for retrieval accuracy. Measured ILS during preflight experiments has been used in the OCO-2 CO2 retrieval. This study derives the on-orbit ILS of OCO-2 using its solar measurements and answers the questions whether on-orbit ILS has changed compared to preflight and whether it varies during the mission.
Accurately characterizing the instrument line shape (ILS) of the Orbiting Carbon Observatory-2...
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