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Discussion papers
https://doi.org/10.5194/amt-2019-47
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-47
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Feb 2019

Research article | 12 Feb 2019

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

Linearization of the effect of slit function changes for improving OMI ozone profile retrievals

Juseon Bak1, Xiong Liu1, Kang Sun2, Kelly Chance1, and Jae-Hwan Kim3 Juseon Bak et al.
  • 1Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
  • 2Research and Education in eNergy, Environment and Water Institute, University at Buffalo, Buffalo, NY, USA
  • 3Atmospheric Science Department, Pusan National University, Busan, Korea

Abstract. We introduce a method that reduces the spectral fit residuals caused by the slit function errors in an optimal estimation based spectral fitting process to improve ozone profile retrievals from the Ozone Monitoring Instrument (OMI) ultraviolet measurements (270–330 nm). Previously, a slit function was parameterized as a standard Gaussian by fitting the Full Width at Half Maximum (FWHM) of the slit function from climatological OMI solar irradiances. This cannot account for the temporal variation of slit function in irradiance, the intra-orbit slit function changes due to thermally-induced change and scene inhomogeneity, and potential differences in the slit functions of irradiance and radiance measurements. As a result, radiance simulation errors may be induced due to using the convolved reference spectra with incorrect slit functions. To better represent the shape of the slit functions, we implement a more generic super Gaussian slit function with two free parameters (slit width and shape factor); it becomes standard Gaussian when the shape factor is fixed to be 2. The effects of errors in slit function parameters on radiance spectra, referred as Pseudo Absorbers (PAs), are linearized by convolving high-resolution cross sections or simulated radiances with the partial derivatives of the slit function with respect to the slit parameters. The PAs are included in the spectral fitting scaled by fitting coefficients that are iteratively adjusted as elements of the state vector along with ozone and other fitting parameters. The fit coefficients vary with cross-track and along-track pixels and show sensitivity to heterogeneous scenes. The total PA spectrum is quite similar in the Hartley band below 310 nm for both standard and super Gaussians, but is more distinctly structured in the Huggins band above 310 nm with the use of super Gaussian slit functions. Finally, we demonstrate that some spikes of fitting residuals are slightly smoothed by accounting for the slit function errors. Comparisons with ozonesondes demonstrate substantial improvements with the use of PAs for both standard and super Gaussians, especially for reducing the systematic biases in the tropics and mid-latitudes and reducing the standard deviations at high-latitudes. Including PAs also makes the retrievals consistent between standard and super Gaussians. This study corroborates the slit function differences between radiance and irradiance demonstrating that it is important to account for such differences in the ozone profile retrievals.

Juseon Bak et al.
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Short summary
This work improves OMI ozone profile retrievals by accounting for spectral fit residuals caused by the slit function errors as a pseudo absorbers in optimal estimation based spectral fitting process.
This work improves OMI ozone profile retrievals by accounting for spectral fit residuals caused...
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