On the use of EPIC/OCRA to support the Geo-Ring constellation for air quality

Abstract

The Geo-Ring constellation is a set of three geostationary instruments with the aim of monitoring the air quality and the atmospheric composition over the Northern Hemisphere with daytime hourly resolution. It consists of the South Korean GEMS (Geostationary Environment Monitoring Spectrometer, launched in 2020), the US-American TEMPO (Tropospheric Emissions: Monitoring of POllution, launched in 2023) and the European S4/UVN (Sentinel-4 Ultraviolet-Visible-Near-infrared, to be launched in 2025).

The precise retrieval of trace- and greenhouse gases requires the a priori knowledge about the presence and properties of clouds in the remote sensing scenes. In particular, the determination of the cloud fraction by means of OCRA (Optical Cloud Recognition Algorithm) makes use of monthly cloud-free maps which need to be computed with a representative set of instrument measurements over its geographic coverage area (at least one year). During the first year of mission, for example, Sentinel-4 will need the support from other instruments to generate the monthly cloud-free maps on an hourly basis. In this regard, the US-American EPIC (Earth Polychromatic Imaging Camera, launched in 2015) is a good candidate to support the Geo-Ring constellation for air quality. Its location at the Lagrangian point L1 offers daytime global coverage of the Earth, with an acquisition cadence between one and two hours, and its 10 channels overlap with the spectral range of GEMS, TEMPO and S4/UVN.

In this study, we analyse the synergies between the EPIC mission and the Geo-Ring constellation to generate OCRA cloud fraction products from geostationary missions. We use GEMS and TEMPO as test cases to assess the feasibility and performance of EPIC-based cloud-free maps for the determination of the OCRA cloud fraction for these missions, with the final aim of providing EPIC-based cloud-free maps for S4/UVN since the beginning of Sentinel-4 operations.