Validation Summary
The S5P L2_AER_AI (NRTI and OFFL) UV Aerosol Absorbing Index is in good overall agreement with similar satellite data products from EOS-Aura OMI and Suomi-NPP OMPS. The aerosol index data set was reprocessed and released in 2023 using processor version 02.04.00. The reprocessed data and is fully compatible with AER_AI OFFL versions up to and including version 02.08.00. The previously observed strongly negative bias and downward trend was resolved with new reprocessed L1b data including a correction for observed degradation in both the irradiance and radiance. After the L1 update an offset value was applied. Starting mid-2021, a slight downward trend of the global mean aerosol index is found, which is may be partly related to remaining L1 degradation and is under investigation. Nevertheless, the amplitude of this trend is very small (about -0.1). Starting with version 2.9.1 (late-Nov 2025), a positive offset has been introduced due to a change in the lookup table (LUT) where the NO2 absorption has been included. This change is being evaluated and will be updated in a future version of the data processor (likely Q4 2026) and prior to any reprocessing activity to ensure a consistent data record for the entire mission. The absolute magnitude of the jump introduced by the change of the LUT is small, and the spatial patterns hardly affected. The UVAI remains within mission requirements. With start of version 2.9.1, also a new, additional aerosol index for 354/388 calculated using a scattering cloud model (Henyey-Greenstein) was introduced.
Validation methodology
The TROPOMI L2_AER_AI UV aerosol index is not a geophysical quantity that can be directly compared to independent measurements from ground or to model results. The way to validate this index is to compare it to coincident satellite measurements from different sensors. For the validation of Sentinel-5p TROPOMI UVAI, measurements from EOS-Aura OMI and Suomi-NPP OMPS are well suited for that purpose. Focus is placed on several case studies for different known aerosol sources: biomass burning smoke, desert dust, and volcanic aerosol sources.
In addition to the validation using satellite observations, the TROPOMI UVAI data products can also be checked for internal consistency. For example, the following tests are performed:
- the dependence of the UVAI on the observation geometry (in particular on the SZA and the VZA of the measurement) can be investigated;
- the UVAI values for clear sky and low aerosol amount should be close to zero;
- the geographical patterns of the UVAI can be compared to those of other measurements, e.g., trace gas distributions of large biomass burning plumes or volcanic plumes.
It should be noted that for TROPOMI the UVAI is calculated for two wavelength pairs, 388 / 354 nm and 380 / 340 nm, the first one allowing a direct comparison to the UVAI from OMI (which is also calculated for 388 / 354 nm).
