Harmonised Validation System for Tropospheric Ozone and Ozone Profile Retrievals from GOME to the Copernicus Sentinels

Research addressing air pollution, the maintenance of the stratospheric ozone layer, and global climate change requires global and long-term monitoring of the vertical distribution of atmospheric ozone at ever-improving resolution and accuracy. Global tropospheric and stratospheric ozone profile measurement capabilities from space have therefore improved substantially over the last two decades, among others with new generation hyperspectral instruments measuring backscattered UV-visible sunlight (GOME, SCIAMACHY, OMI, GOME-2) and thermal emission (TES, IASI) at the nadir of the satellite. Enhanced versions of those instruments are now being developed within EU’s Copernicus Earth Observation programme: the UV-visible-NIR TROPOMI instrument on board of the mid-afternoon satellite Sentinel-5P, to be launched in 2016, and both UV-visible and infrared instruments of the GOME and IASI types on board of the geostationary Sentinel-4 and polar orbiting Sentinel-5, to be launched at the end of the decade. Additionally, stringent climate research user requirements like e.g. the Global Climate Observing System (GCOS) targets call for continuous quality assessment and evolution of ozone data and their associated retrieval algorithms over the whole relevant spatial domain, vertical range, and mission lifetime. The fitness-for-purpose of tropospheric ozone column and ozone profile data products must thus be warily verified by means of in-depth QA/validation studies of the satellite data and associated retrieval algorithms before being used in scientific research and operational applications. To that purpose, an extensive validation system has been developed on the heritage of various validation activities, starting in the 1990s with the first GOME ozone profile validations and progressively extending up to the current ESA Multi-TASTE Phase F data evolution and Ozone_cci production of multi-mission climate data records on ozone. Currently this validation system is being further consolidated with metrological traceability practices and with generic QA guidelines established within the FP7 QA4ECV project. The end-to-end approach of this system combines preliminary QA/QC procedures, data content studies, in-depth information content studies, information-content based co-location procedures, data homogenisation, and the more traditional data comparisons with respect to reference measurements acquired by ground-based networks of ozonesondes and lidars (NDACC, SHADOZ, WMO GAW). An OSSE system with detailed metrology of the remote sensing data is thereby used to assess the propagation of errors associated with differences in smoothing and with mismatches in space and time between the various measurements. In this paper we briefly describe the principles and implementation of this QA/validation system, now in pre-operational phase. Through illustrative evaluation activities from ESA’s Ozone_cci project we demonstrate its broad applicability to virtually all ozone (partial) column and profile datasets. We conclude with a perspective on current developments of this system required to address the specific challenges of the upcoming TROPOMI ozone data validation as envisaged in the S5PVT AO project CHEOPS-5p (Validation of Copernicus HEight-resolved Ozone data Products from Sentinel-5P TROPOMI).