9 Abstract 10 We introduce a method that reduces the spectral fit residuals caused by the slit function errors in an 11 optimal estimation based spectral fitting process to improve ozone profile retrievals from the Ozone 12 Monitoring Instrument (OMI) ultraviolet measurements (270-330 nm). Previously, a slit function was 13 parameterized as a standard Gaussian by fitting the Full Width at Half Maximum (FWHM) of the slit 14 function from climatological OMI solar irradiances. This cannot account for the temporal variation of slit 15 function in irradiance, the intra-orbit slit function changes due to thermally-induced change and scene 16 inhomogeneity, and potential differences in the slit functions of irradiance and radiance measurements. As 17 a result, radiance simulation errors may be induced due to using the convolved reference spectra with 18 incorrect slit functions. To better represent the shape of the slit functions, we implement a more generic 19 super Gaussian slit function with two free parameters (slit width and shape factor); it becomes standard 20 Gaussian when the shape factor is fixed to be 2. The effects of errors in slit function parameters on radiance 21 spectra, referred as “Pseudo Absorbers (PAs)”, are linearized by convolving high-resolution cross sections 22 or simulated radiances with the partial derivatives of the slit function with respect to the slit parameters. 23 The PAs are included in the spectral fitting scaled by fitting coefficients that are iteratively adjusted as 24 elements of the state vector along with ozone and other fitting parameters. The fit coefficients vary with 25 cross-track and along-track pixels and show sensitivity to heterogeneous scenes. The total PA spectrum is 26 quite similar in the Hartley band below 310 nm for both standard and super Gaussians, but is more distinctly 27 structured in the Huggins band above 310 nm with the use of super Gaussian slit functions. Finally, we 28 demonstrate that some spikes of fitting residuals are slightly smoothed by accounting for the slit function

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