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.