AEROPRO: Aerosol Profile Retrieval Concept Development and Validation for Sentinel-4

Sentinel-4 (S-4) will be an operational satellite mission in geostationary orbit aiming at continuous monitoring of atmospheric composition, in particular with respect to air quality, on a European basis with an hourly revisit time. The Sentinel-4 mission is part of the European initiative Copernicus/Global Monitoring for Environment and Security (GMES), which is dedicated to the implementation of a sustained operational capacity for Earth observation. The main data products of the Sentinel-4/UVN (Ultraviolet-Visible NearIR) sounder instrument are O3, NO2, SO2, HCHO and aerosol. The Sentinel-4 UVN instruments will be embarked on the EUMETSAT MTG-Sounder platforms1,the first one to be launched in 2021.
Trace gas retrievals from space-based nadir-view observations of reflected sunlight require an accurate scattering correction, which relies on auxiliary information on surface, cloud, and aerosol characteristics. Information on aerosol properties and their vertical distribution is an important input to this scattering correction and to air quality estimates, especially of particulate matter (PM). In the context of the Sentinel-4 mission, the aerosol profile shall be retrieved from the high spectral resolution observations in the O2 A-band (see table below for the characteristics of the relevant UVN channel). The aim of this study is the development and validation of an algorithm for the retrieval of aerosol profile information from the O2 A-band of Sentinel 4/UVN. The O2 A-band is an absorption band of oxygen at wavelengths around 760 nm. The shape and depth of the band is sensitive to the unknown vertical profile of atmospheric scatterers (clouds and aerosols), whereas the vertical profile of oxygen itself is accurately known.
This final report consists of all Technotes (TNs) written in this study. First, a literature overview of existing algorithms for aerosol profile retrieval from the O2 A-band is given in TN1. Since the algorithm has to be tested on real satellite data, and to be validated with reference data, we use selected aerosol events with collocated satellite data and validation data, especially aerosol lidar data. This approach is called “reverse validation”. For the satellite data we use data from the Fourier Transform Spectrometer (FTS) on the GOSAT satellite and from GOME-2 on the Metop-A satellite. In TN2ab the validation data and the GOSAT satellite data for selected aerosol events (cases) are described. For the validation, data from the CARBONEXP aircraft campaign (Aug-Sept 2011) as well as from groundbasedlidar stations are used. For the aerosol profile algorithm we make use of and extend beyond the algorithm developments for the TROPOMI instrument on ESA’s Sentinel-5 Precursor mission. The description of the retrieval algorithm (ATBD) for S-4 is given in TN3. Application of the algorithm to GOSAT data for th selected aerosol cases and validation of the retrieval results is described in TN5. Further retrieval performance analysis is given in TN6. The selection of aerosol cases for GOME-2 is described in TN8a. Application of the algorithm to GOME-2 data is described in TN8b, as well as the retrieval performance analysis and validation. Finally, conclusions and recommendations for aerosol profile retrieval from S-4 and S-5 are given in TN7.