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

Abstract

Abstract. The quality of trace gas products derived from measurements of a space-borne imaging spectrometer is affected by the inhomogeneity of the illumination of the instrument slit and thus by the heterogeneity of the observed scene. This paper aims to quantify this effect and summarise findings on how to mitigate the impact of inhomogeneous slit illumination on tropospheric O 3 , NO 2 , SO 2 and HCHO columns derived from measurements of the Sentinel-4 UVN imaging spectrometer. For this purpose, spectra for inhomogeneous ground scenes have been simulated based on a combination of a radiative transfer model and spatially high resolved MODIS (Moderate Resolution Imaging Spectroradiometer) data. The resulting errors on tropospheric O 3 , NO 2 , SO 2 and HCHO columns derived from these spectra have been determined via an optimal estimation approach. We conclude that inhomogeneous illumination results in significant errors in the data products if the natural inhomogeneity of the observed scenes are not accounted for. O 3 columns are less affected than the other data products; largest errors occur for NO 2 (mean absolute errors about 5 %, maximum error exceeding 50 %, standard deviation of the errors about 8 %). These errors may be significantly reduced (by factors up to about 10) by an appropriate wavelength calibration applied individually to each Earthshine radiance spectrum. With wavelength calibration the estimated mean absolute errors due to inhomogeneity are for all gases well below 1 %; standard deviations of the errors are 1.5 % or lower; maximum errors are about 10 % for NO 2 and around 5 % for the other gases.

The operational methane retrieval algorithm for TROPOMI

“…In space, however, the instrument slit will be illuminated inhomogeneously due to ground scene heterogeneities on scales smaller than the instrument's field of view. Inhomogeneous illumination across the slit leads to a distortion of the ISRF as described by Noël et al (2012), Caron et al (2014) and Landgraf et al (2016), and it can affect the retrieval accuracy of the TROPOMI methane product. Small-scale heterogeneities of the ground scene are generally caused by spatial variations of surface reflection and by broken clouds.…”

Deriving the slit functions from OMI solar observations and its implications for ozone-profile retrieval

“…However, the retrieval using standard Gaussian slit functions shows the smallest variations of biases and variations of residual RMS, which is not fully understood. It is possible that the slit function in radiance measurements cannot be fully represented by the on-orbit slit functions derived from the solar irradiance due to scene inhomogeneity Noël et al, 2012), unaccounted stray lights, or intra-orbit slit function changes (observed in GOME-2 by Beirle et al, 2017) in earthshine spectra.…”

Challenges in Methane Column Retrievals from AVIRIS-NG Imagery over Spectrally Cluttered Surfaces: A Sensitivity Analysis

“…It should also be noted from Noël et al (2012) that the distortion of the slit function for an inhomogeneously illuminated slit could also be another reason for the difference in retrieval performance [38].…”