The TROPOMI Ozone Profile retrieval algorithm and its use for stratospheric and tropospheric atmospheric research

The status and the most recent developments of the algorithm of the operational TROPOMI (Tropospheric Monitoring Instrument) Ozone Profile product will be presented in this contribution. TROPOMI is the payload onboard of the single-satellite Sentinel-5 Precursor (S5P), an atmospheric composition mission that is part of the EU Copernicus program. The aim of the TROPOMI ozone profile product is to continue the record of the stratospheric ozone, monitor changes, improve the accuracy of the retrieved stratospheric profiles and focus on the tropospheric ozone, which is also important for climate studies. The monitoring of the evolution of stratospheric and tropospheric ozone is important as the ozone plays an important role in atmospheric chemistry and radiative balance throughout the atmosphere. The stratospheric ozone layer from 15 to 50 km absorbs the harmful UV radiation, protecting life at the surface. Tropospheric ozone is a greenhouse gas affecting the climate. In addition, ozone in the lower troposphere is a toxic component of air pollution with significant public health and agricultural impacts.

The operational ozone profile product of TROPOMI provides the ozone profile at 33 pressure levels in the atmosphere and as 6 sub-columns with a vertical sampling depending on the altitude. The ozone profile is derived from the UV spectral range (270-330 nm), with an horizontal spatial resolution of approximately 28x28 km². The derived ozone profile contains 6-7 independent pieces of information, providing a vertical resolution in the range 7 – 15 km. The retrieval is based on the Optimal Estimation method, which combines the information from the measured spectra with the a-priori information. In addition to the a-priori profile, the retrieved profiles and their errors, the algorithm also provides diagnostic information, such as the averaging kernel matrix of the ozone profile elements, an essential parameter also for the product validation operated by the ESA/Copernicus Atmospheric Mission Performance Cluster/Validation Data Analysis Facility (ATM-MPC/VDAF).

The algorithm also relies on accurate Level 1B radiometric calibration of both radiance and irradiance data, which addresses in particular the spectral region below 310 nm. From comparison to other satellites sensors as well as to forward models, it is known that the TROPOMI UV spectral bands show systematic radiometric deviations. To address this issue, a radiometric correction based on a comparison with forward models has been developed. The derived correction parameters are updated regularly in the operational product in order to follow the changes of the instrument over time due to its optical degradation.

The improvements in the operational algorithm, regarding the radiometric calibration and the choice of a new climatology for the ozone profile a-priori information and its uncertainty, are crucial aspects for the ozone retrieval and they will be shown during this talk.

Finally, with the availability of the reprocessed ozone profile data from the beginning of the mission, this contribution will also show the results of the algorithm performances throughout the TROPOMI mission, in ozone hole conditions and for tropospheric enhancements.