Validation Summary

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.