TROPOMI tropospheric ozone data: Quality assessment and application

Contributing to the European Union’s Copernicus Earth Observation programme since October 2017, the Sentinel-5 Precursor (S5P) satellite mission is dedicated to global atmospheric composition measurements for the monitoring and study of air quality and climate. On board of the S5P early afternoon polar satellite, the imaging spectrometer TROPOMI (TROPOspheric Monitoring Instrument) performs nadir measurements of the Earth’s radiance from the UV-visible to the short-wave-infrared spectral ranges at a much finer spatial resolution than its predecessors do, and from which the global distribution of several atmospheric trace gases is retrieved daily, including ozone.

Ozone in the troposphere is the third most important anthropogenic contributor to greenhouse radiative forcing. The distribution of tropospheric ozone is highly variable over a wide range of spatial and temporal scales due to a complex interplay between dynamical, chemical, and radiative processes. Global measurement systems are faced with the challenge of accurately capturing this variability at the scale of interest. In this contribution, we therefore present a comprehensive quality assessment of the recently reprocessed and hence homogenous TROPOMI tropospheric ozone data records, and demonstrate their application. A distinction is made between the tropospheric column and profile products.

The Convective Cloud Differential technique (CCD) is applied to derive three-day running mean ozone columns between surface and 270 hPa over the tropical belt, covering nearly five years of TROPOMI data. These data are characterized primarily by analysing comparisons to SHADOZ ozonesonde and other, currently operating GOME-type sounders (EOS-Aura OMI, Metop-B GOME-2). We will show that the TROPOMI bias varies somewhat with reference instrument, but generally remains below about four Dobson Units. We find signs of a weak latitudinal pattern and a moderate seasonal pattern in the mean differences, again, depending on the reference instrument.

TROPOMI’s operational ozone profile retrieval algorithm is based on the optimal estimation method and was implemented in November 2021, now covering over one year of data. Validation results are collected from both the ESA/Copernicus Atmospheric Mission Performance Cluster/Validation Data Analysis Facility (ATM-MPC/VDAF) and from the S5P Validation Team (S5PVT) AO project CHEOPS-5p. The quality assessment relies on the analysis of retrieval diagnostics in terms of data and information content studies, and on the comparison of TROPOMI data with ground-based measurements. The latter are acquired by ozonesondes and lidars taking part in WMO's Global Atmosphere Watch and its contributing networks NDACC and SHADOZ, and are collected in a harmonized formatting from the ESA Atmospheric Validation Data Centre (EVDC). With a mean comparison bias below 10 % throughout the entire profile and a dispersion of the order of 20 % in the troposphere, the mission requirements are mostly met.

Finally, our verification of the presence of known geophysical structures and cycles confirms that TROPOMI tropospheric ozone data meet the needs of the atmospheric research community. Ultimately, this work paves the way for a more comprehensive evaluation of tropospheric ozone data records contributing to the ongoing tropospheric ozone assessment report (TOAR).