Abstract
Abstract. We introduce a method that accounts for errors caused by the slit function 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 in slit function in irradiance, the intra-orbit 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 convolving 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 to 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 fitting coefficients vary with cross-track and along-track pixels and show sensitivity to heterogeneous scenes. The 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 noticeable improvements when using PAs for both standard and super Gaussians, especially for reducing the systematic biases in the tropics and midlatitudes (mean biases of tropospheric column ozone reduced from -1.4∼0.7 to 0.0∼0.4 DU) and reducing the standard deviations of tropospheric ozone column differences at high latitudes (by 1 DU for the super Gaussian). 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.
Highlights
The fitting of measured spectra to simulated spectra is the most basic concept for analysis of the Earth’s atmospheric constituents from satellite measurements
Compared to other trace gases, the retrieval of ozone profiles can be more susceptible to the accuracy of instrumental spectral response function (ISRF) due to the large spectral range, where the radiance spans a few orders of magnitude, and to the fact that the spectral fingerprint for the tropospheric ozone is primarily provided by the 310–330 nm absorption features residing in the temperature-dependent Huggins bands
Knowledge of the instrument spectral response functions (ISRFs) or slit functions is important for ozone profile retrievals from the Hartley and Huggins bands
Summary
The fitting of measured spectra to simulated spectra is the most basic concept for analysis of the Earth’s atmospheric constituents from satellite measurements. Accurate calibration and simulation of measurements are essential for the successful retrieval of atmospheric constituents. The knowledge of the instrumental spectral response function (ISRF) or slit function could affect the accuracies of both calibration and simulation, as it is required for the convolution of a high-resolution reference spectrum to the instrument’s spectral resolution in the wavelength calibration and for the convolution of high-resolution absorption cross. Compared to other trace gases, the retrieval of ozone profiles can be more susceptible to the accuracy of ISRFs due to the large spectral range, where the radiance spans a few orders of magnitude, and to the fact that the spectral fingerprint for the tropospheric ozone is primarily provided by the 310–330 nm absorption features residing in the temperature-dependent Huggins bands. The efforts to characterize and verify the ISRFs have preceded the analyses of ozone profiles from satellite and aircraft measurements (X. Liu et al, 2005, 2010; Cai et al, 2012; C. Liu et al, 2015; Sun et al, 2017; Bak et al, 2017)
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