Abstract
Abstract. The Ozone Monitoring Instrument (OMI) has been successfully measuring the Earth's atmospheric composition since 2004, but the on-orbit behavior of its slit functions has not been thoroughly characterized. Preflight measurements of slit functions have been used as a static input in many OMI retrieval algorithms. This study derives on-orbit slit functions from the OMI irradiance spectra assuming various function forms, including standard and super-Gaussian functions and a stretch to the preflight slit functions. The on-orbit slit functions in the UV bands show U-shaped cross-track dependences that cannot be fully represented by the preflight ones. The full widths at half maximum (FWHM) of the stretched preflight slit functions for detector pixels at large viewing angles are up to 30 % larger than the nadir pixels for the UV1 band, 5 % larger for the UV2 band, and practically flat in the VIS band. Nonetheless, the on-orbit changes of OMI slit functions are found to be insignificant over time after accounting for the solar activity, despite of the decaying of detectors and the occurrence of OMI row anomaly. Applying the derived on-orbit slit functions to ozone-profile retrieval shows substantial improvements over the preflight slit functions based on comparisons with ozonesonde validations.
Highlights
The Dutch–Finnish Ozone Monitoring Instrument (OMI) on board the NASA Aura satellite has been measuring the direct sunlight and backscattered sunlight from the Earth since 2004
The UV band is optically separated into the UV1 band (264–311 nm, ∼ 0.6 nm resolution) and the UV2 band (307–383 nm, ∼ 0.4 nm resolution), and the VIS band covers 349–504 nm at ∼ 0.6 nm resolution (Dirksen et al, 2006)
The OMI instrument has a wide, 115◦ field of view, which is divided into 30 cross-track positions in the UV1 band and 60 cross-track positions in the UV2 and VIS bands
Summary
The Dutch–Finnish Ozone Monitoring Instrument (OMI) on board the NASA Aura satellite has been measuring the direct sunlight and backscattered sunlight from the Earth since 2004. In both the spectral dimension (columns) and the spatial dimension (rows, or cross-track direction) of the 2-D detectors and have been measured accurately in a preflight experiment (Dirksen et al, 2006) These preflight slit functions have been adopted in a wide range of operational OMI retrieval algorithms (Kurosu et al, 2004; Veefkind et al, 2006; Chan Miller et al, 2014; Wang et al, 2014; González Abad et al, 2015; van Geffen et al, 2015; Li et al, 2017). The Smithsonian Astrophysical Observatory (SAO) ozone-profile retrieval algorithm derives on-orbit slit functions from averaged OMI solar observations in 2005– 2007, assuming standard Gaussian slit function form, which showed better performance than using the complex preflight slit functions (Liu et al, 2010). We investigate the temporal variation of the slit function using more than 10 years of OMI irradiance data, compare the on-orbit slit functions derived from irradiance with the preflight ones, and evaluate multiple on-orbit slit function forms by validating the ozone profiles retrieved using different slit functions with ozonesonde observations
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