Abstract
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.
Highlights
The Orbiting Carbon Observatory-2 (OCO-2), launched on 2 July 2014, is a NASA mission aiming at quantifying the sources and sinks of CO2 at regional scales (100– 1000 km; Crisp, 2015)
We found that fitting an additive offset and the stretch/sharpen fitting gives very similar results, using synthetic solar spectra with realistic OCO-2 instrument line shape (ILS), signal-to-noise ratio (SNR), and varying additive offsets
The 2016 version of the Total Carbon Column Observing Network (TCCON) solar line list shows improvement over the current solar model used in the v7 Level 2 (L2) retrieval algorithm, mainly in the O2A band
Summary
The Orbiting Carbon Observatory-2 (OCO-2), launched on 2 July 2014, is a NASA mission aiming at quantifying the sources and sinks of CO2 at regional scales (100– 1000 km; Crisp, 2015). On-orbit ILS and wavelength registration calibrations for existing grating spectrometers (GOME, GOME-2, SCIAMACHY, OMI, etc.) are typically performed by fitting the measured solar irradiance at the top of the atmosphere with a well-calibrated, high-resolution solar irradiance reference spectrum, assuming analytical function forms of ILS (Chance, 1998; Liu et al, 2005, 2010; Cai et al, 2012; De Smedt et al, 2012; Munro et al, 2016) Compared to these spaceborne instruments that generally targeted the UV– visible bands, OCO-2 resolves rotational vibrational bands of O2 and CO2 in the infrared at much finer resolutions over narrower spectral ranges (e.g., the spectral resolution of OCO-2 is 1 order of magnitude higher than GOME, GOME-2, and SCIAMACHY in the O2A band). The aim of this study is to perform on-orbit OCO-2 ILS calibrations using the instrument’s frequent solar irradiance measurements and evaluate the temporal and inter-footprint variation of ILS functions during the mission
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
