Surface albedo fitting in the optimal estimation routine of the TROPOMI oxygen A-band aerosol layer height product.

The Sentinel-5P/TROPOMI  Aerosol Layer Height (ALH) is based on an optimal estimation (OE) approach, fitting cloud-free measurements to synthetic reflectances in the strongest oxygen absorption (O₂-A) band, provided by a neural network trained with high resolution simulated reflectances. The ALH has been continuously improved since its release in 2019, focusing especially on (bright) land surfaces, over which the ALH product showed underestimated aerosol layer heights (biased towards the surface). In this presentation the latest updates of the ALH product will be discussed, including the introduction of the Directional Lambertian-Equivalent Reflectance (DLER) climatology to improve the surface albedo characterisation over land. Secondly, by adding the surface albedo in the feature vector of the OE inversion, using the DLER as prior information, retrievals improved considerably over land, especially in the case of bright surfaces. New retrievals over land now largely match the retrievals over ocean, which have shown a good comparison with validation data since its release, most notably with CALIOP weighted extinction heights. The albedo is fitted for both land and ocean surfaces, but the implementation is different over land and ocean because of the large range of land surface albedos. Over ocean, the retrievals are optimised by tuning the a priori error settings, while over land the a priori surface albedo values are relaxed so the fitting procedure can incorporate the albedo effects in the retrieval.  About 1.5 times more converged results were obtained with the current implementation, with low land-ocean contrasts in the aerosol layer height retrievals. The average difference with CALIOP weighted extinction height decreased for selected cases from about −1.9 km to −0.9 km over land and from around −0.8 km to +0.1 km over ocean.

We will show the latest results of the TROPOMI ALH products, by comparisons with CALIOP and GEMS Aerosol layer height retrievals, and show some preliminary results with EarthCARE data. Implementing of this algorithm for Sentinel-3/OLCI O₂-A band measurements and the algorithm developments for the upcoming Sentinel-4 and Sentinel-5 mission will also be highlighted.