Total ozone from LEO and GEO: ground-based validation and mutual consistency

The recent launches of Sentinel-4 on MTG-S and Sentinel-5 on EPS-SG-A1 significantly strengthen the Copernicus contribution to the international constellation of low-Earth orbiting (LEO) and geostationary (GEO) satellites dedicated to atmospheric ozone and air quality monitoring. Together with the already operational Sentinel-5P LEO and GEMS (East Asia) and TEMPO (North America) GEO missions, these nadir sounders form an unprecedented observing system coordinated to ensure complementary spatial and temporal sampling and long-term commitment. As a prerequisite to a truly integrated exploitation of this constellation, the present contribution reports on two complementary ozone data validation activities undertaken to build confidence in their traceability to community-agreed standards and in the mutual consistency across missions.As a first step, we summarize the global, in-depth, and recurrent validation of Sentinel-5P TROPOMI total ozone column data, conducted by the ESA/Copernicus Atmospheric Mission Performance Cluster (ATM-MPC). The ATM-MPC validation service assesses TROPOMI’s traceability to the well-established ground-based Brewer, Dobson, and ZSL-DOAS measurements contributing to WMO’s Global Atmosphere Watch (GAW) and the Network for the Detection of Atmospheric Composition Change (NDACC), and to measurements from the more recent Pandonia Global Network (PGN). The permanent QA/QC of TROPOMI demonstrates the value of this mission as the initial reference sounder for the constellation. As a second step, we present pioneering regional validation and evaluation results for the GEO missions GEMS and TEMPO performed within ESA’s PEGASOS project. This latter activity integrates independent ground-based validation and the use of TROPOMI data as a LEO transfer standard for cross-mission consistency assessment.The ATM-MPC results confirm the high quality of TROPOMI total ozone observations, characterized by low bias, small random uncertainty, and temporal stability meeting community requirements, both with respect to ground-based reference measurements and relative to the historical benchmark provided by the Aura OMI mission. Building on these resources – both the ground-based validation infrastructure and TROPOMI as well characterized and accurate measurement system-  we assess the quality of the total ozone products from GEMS and TEMPO. The analysis demonstrates overall good data quality across both GEO missions, while also identifying dependences of bias and dispersion on measurement parameters and influence quantities. These findings highlight specific areas where further algorithm refinements may enhance consistency and performance within the emerging global GEO–LEO ozone observing system.We would like to acknowledge explicitly the long-term dedication of all the ground-based instrument teams (WMO-GAW, NDACC, PGN) to acquire high quality data and make them available to the satellite community.