Sentinel-5p TROPOMI provides two aerosol related data products:
The Sentinel-5p TROPOMI L2_AER_AI (NRTI and OFFL) UV Aerosol Absorbing Index data is in good overall agreement with similar satellite data products from EOS-Aura OMI and Suomi-NPP OMPS. Although compliant with the mission requirement of 1 UVAI unit in 2018, the bias is currently slightly larger than 1 UVAI unit as compared to OMI and OMPS. The reasons for this increasing bias are related to wavelength-dependent degradation and are currently under investigation.
The Sentinel-5p TROPOMI L2_AER_LH (OFFL) data product shows a very good agreement with two other satellite aerosol layer height estimates, from MISR (stereoscopic imagery) and CALIOP (active lidar sensing of the aerosol vertical distribution). TROPOMI TROPOMI AER_LH shows a systematic difference with MISR aerosol plume height of about 600 m (lower for TROPOMI). This is mostly due to the difference in the sensitivity of the instruments and the differences in the algorithms. A difference of about 500 m (lower for CALIOP) is expected from simulations, TROPOMI ALH being sensitive to the centroid aerosol layer height. For very thick plumes the difference between TROPOMI ALH and CALIOP layer height even decreases to only 50 m. This is well within the requirements of 100 hPa for the bias.
The TROPOMI ALH dispersion is large due to cloud contamination and surface effects. With rigorous cloud screening, 50 % of the pixels are already within 1 km of the CALIOP weighted extinction height. Accounting for the expected bias, this is within the requirements of 50 hPa. But this preliminary conclusion needs further investigation and confirmation.
A limitation of the TROPOMI ALH product has become apparent following the severe bushfires in New South Wales during the 2019-2020 fire season, which produced very high altitude smoke plumes (altitude > 20 km). These heights were not anticipated and ALH values are limited to about 13 km altitude. An update to include these very high altitudes is not foreseen for the near future.
Because of the degradation of the UVAI, the applied UVAI filter for the ALH retrievals removes currently a large number of observations, which are in principle well suited for ALH retrievals.
UV Aerosol Index (UVAI)
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).
Aerosol Layer Height (ALH)
The validation of TROPOMI L2_AER_LH data relies on comparisons with other satellite observations:
- the stereoscopic plume height product from MISR, an offline product that can be computed for selected fire plumes, using a freely available code (MINX). It makes use of the nine available viewing directions of MISR, which senses a scene from different directions during an overpass. This provides stereoscopic height information for a scene with enough contrast.
- the weighted extinction height provided by CALIOP, selected where Calipso was closer to Sentinel-5p than 100 km and the sensing time of CALIOP and TROPOMI was less than three hours apart.
These two instruments having a limited swath width, finding suitable co-locations is the main limiting factor for the intended intercomparison.
ALH is retrieved for known aerosol layers which, in the absence of an Aerosol Optical Thickness product, is done by selecting high UV AI values (larger than 0). As a result, mainly desert dust, smoke and volcanic plumes are processed. Therefore, the validation focuses on selected desert dust cases, fires plumes and occasional volcanic eruptions.
Quarterly Validation Report of the Copernicus Sentinel-5 Precursor Operational Data Products #11: April 2018 - June 2021
Previous issues of the quarterly validation report are archived here.