Quantification and mitigation of the impact of scene inhomogeneity on Sentinel-4 UVN UV-VIS retrievals

摘要

The quality of trace gas products derived from measurements of a space-borneimaging spectrometer is affected by the inhomogeneity of the illumination ofthe instrument slit and thus by the heterogeneity of the observed scene. Thispaper aims to quantify this effect and summarise findings on how to mitigatethe impact of inhomogeneous slit illumination on tropospheric O₃,NO₂, SO₂ and HCHO columns derived from measurements of theSentinel-4 UVN imaging spectrometer. For this purpose, spectra forinhomogeneous ground scenes have been simulated based on a combination of aradiative transfer model and spatially high resolved MODIS (ModerateResolution Imaging Spectroradiometer) data. The resulting errors ontropospheric O₃, NO₂, SO₂ and HCHO columns derived from thesespectra have been determined via an optimal estimation approach. We concludethat inhomogeneous illumination results in significant errors in the dataproducts if the natural inhomogeneity of the observed scenes are not accountedfor. O₃ columns are less affected than the other data products; largesterrors occur for NO₂ (mean absolute errors about 5%, maximum errorexceeding 50%, standard deviation of the errors about 8%). These errorsmay be significantly reduced (by factors up to about 10) by an appropriatewavelength calibration applied individually to each Earthshine radiancespectrum. With wavelength calibration the estimated mean absolute errors dueto inhomogeneity are for all gases well below 1%; standard deviations ofthe errors are 1.5% or lower; maximum errors are about 10% for NO₂and around 5% for the other gases.