title: Sentinel-4 Level-2 ATBD Total and Tropospheric NO2
title: S4-L2-IUP_KNMI-ATBD-2004
titleType: AlternativeTitle
Creators: name: Sanders, Bram
name: Richter, Andreas
name: Eskes, Henk
Publisher: name: IUP, KNMI
Year: 2024
Subjects: Sentinel-4
S4
ATBD
NO2
Nitrogen Dioxide

description: The present document is the Algorithm Theoretical Basis Document (ATBD) for the Sentinel-4 (S4) Level-2 (L2) algorithm that will deliver total and tropospheric nitrogen dioxide (NO2) column amounts. The purpose of the ATBD is to provide the theoretical basis for the algorithm and to describe the corresponding data product (identifier: S4-L2-NO2). This ATBD is a joint effort of the Institut für Umweltphysik in Bremen (IUP, Universität Bremen, Bremen, Germany; development lead) and the Koninklijk Nederlands Meteorologisch Instituut (KNMI, De Bilt, Netherlands).

Within the Sentinel-4 Level-2 project, development of an operational product proceeds in several steps. The consolidated algorithm is implemented as a science code or bread-board algorithm and is run at the developer team’s host institute (for NO2, this is IUP Bremen). Based on the bread-board algorithm a prototype and an operational processor are developed at the Deutsches Zentrum für Luft- und Raumfahrt (DLR, Oberpfaffenhofen, Germany). The L2 Prototype Processor (L2PP) will be run at the European Space Agency’s European Space Research and Technology Center (ESA-ESTEC, Noordwijk, Netherlands) and the final L2 Operational Processor (L2OP) will be run at the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT, Darmstadt, Germany). The bread-board algorithm and the prototype / operational algorithms are in principle the same although small differences may exist for practical reasons (e.g. implementation issues, computational performance). This document describes the bread-board algorithm.

Sentinel-4 Level-2 development activities are coordinated by DLR and are split up into algorithm bread-boarding activities performed by the various developer teams and headed by the Rutherford Appleton Laboratory (RAL, Didcot, UK), verification activities performed by the various verification teams and headed by the Max-Planck-Institut für Chemie (MPIC, Mainz, Germany) and processor suite activities performed by the various processor teams and headed by DLR. The purpose of the verification activities, which take place in parallel to the bread-boarding activities, is to generate synthetic S4 Level-1b (L1b) test data and to independently verify performance of the Level-2 algorithms. Results from these verification activities directly feed back into bread-board algorithm development.

This document is maintained during the development phase and lifetime of the product, and updates are foreseen when there are changes to the algorithm or data product. 
description: Sentinel-4 is an operational satellite mission providing atmospheric composition data for the European region with a fast, hourly revisit time in support of the Copernicus Atmosphere Monitoring Service (CAMS). The S4 instrument is an Ultraviolet Visible Near-infrared (UVN) spectrometer that will be carried on the geostationary Meteosat Third Generation Sounder (MTG-S) platforms. The satellites’ nominal longitude is 0°. The first MTG-S platform is expected to be put into orbit around 2021 and the second platform around 2029.

The instrument measurement concept combines a scan mirror with a push-broom design. A mirror scanning slowly from east to west captures the light from a narrow strip on the Earth running in the north-to-south direction and reflects it into the telescope. The telescope system in turn images the strip onto the slits of the ultraviolet and visible (UVis) and near-infrared (NIR) spectrometer channels. The across-slit dimension is collapsed into a spectral dimension and spectra are recorded by two-dimensional Charge-Coupled Device (CCD) detectors. The UVN instrument covers the visible wavelength range with a spectral resolution of about 0.5 nm at a sampling ratio of three.

Inhomogeneities in the radiance distribution across the slit may cause small changes to the instrument’s nominal spectral response function. A dedicated S4 L2 support product addresses this issue and the S4 NO2 algorithm optionally uses parameters provided by this support product. Furthermore, because the UVis and the NIR light are treated by two different spectrometer systems, there may be slight mismatches or co-registration errors between ground pixels for the two wavelength bands (in both across-track and along-track directions). These mismatches have to be corrected when information from the two channels is combined. In case of S4 NO2, this mainly concerns the use of cloud parameters retrieved from the O2 A band.

The spatial sampling distance varies across the geographic coverage area due to projection and curvature effects and is about 8 x 8 km at a reference location at 45° N and the satellite’s nominal longitude. There are 551 pixels in the north-to-south direction and about 580 pixels in a single east-to-west scan. A daily Earth observation will consist of about 16 to 20 one-hour east-to-west scans with fast retraces in-between. The total area covered in a single day by Sentinel-4 is the European extended region. The area captured by a single one-hour scan is smaller and varies within the geographic coverage area in a systematic way depending on the time of day and season. Most importantly for S4 NO2, there is a large westward shift in the afternoon bringing the unpolluted North Atlantic region into the field of view only for the second half of the day.  

S4 will be the first mission carrying a UVN imaging spectrometer in a geostationary orbit over Europe. Other, similar spectrometers are the Tropospheric Emissions: Monitoring of Pollution (TEMPO) mission (Zoogman et al., 2016), which covers the North American extended region, and the Geostationary Environment Monitoring Spectrometer (GEMS; Kim, 2012), which covers the East Asian extended region. A clear synergy between geostationary observations and observations by similar spectrometers in polar orbits is expected. The latter typically have global coverage daily but observe a particular location only once a day, at least at low and mid latitudes. For S4 NO2, it is planned to use data from the polar-orbiting Sentinel-5 Precursor (S5P) and Sentinel-5 (S5) instruments, whose global observations of stratospheric NO2 will be used to validate the stratospheric correction step in the S4 tropospheric NO2 retrieval.