Latest developments of the Aerosol Layer Height retrieval from S5P/TROPOMI

The height of aerosols is important for many applications, such as the Earth’s radiation budget, transport of aerosols, aviation, retrieval of aerosol optical thickness and the atmospheric correction. Active instruments, such as the lidars on Caliop and Aeolus have provided important insight in the vertical distribution of aerosols, but these missions had a small spatial coverage and have now ended. EarthCare will provide an important replacements for these instruments, but the daily, global coverage provided by passive instruments, such as TROPOMI, remains essential.

In 2019, the first operational, global Aerosol Layer Height (ALH) product was released, retrieved from near-infrared measurements by TROPOMI on Sentinel-5P.  The operational algorithm uses a machine learning technique in the forward model, to quickly and accurately simulate the around 4000 spectral absorption lines in the O₂-A band around 760 nm, and the inversion problem is solved iteratively using an optimal estimation routine, in order to have a proper error estimation.

Since its release, many important improvement have been implemented. First focused on single, selected layers of absorbing aerosols, the processor now provides the ALH for all cloud-free scenes, including scattering aerosol layers. The algorithm performs well over oceans (within the 1 km accuracy requirement) but not over land surfaces. The surface albedo is an important error source, especially over bright surfaces and for thin aerosol layers. In order to improve the retrieval over land, the surface albedo can be fitted, yielding highly improved results. However, for the operational processor this required the retraining of the neural network, since the derivatives to the fit parameters are needed in the optimal estimation routine. Therefore, the derivatives to surface albedo at two wavelengths in the continuum (outside the  O₂-A band) were added to the algorithm forward model. This yielded improved accuracy over land and a large increase in the number of successful retrievals. The latest version of the S5P/TROPOMI ALH (version 2.6.0, including all these improvements and the surface fit) was released in November 2023. We will present the algorithm and the latest validation results, including ALH estimates from various instruments in space and from ground-based lidar networks. 

The TROPOMI ALH algorithm is developed and maintained within the EU Copernicus program. Its developments are important for Sentinel-3 OLCI, for which a similar O2-A band retrieval is being developed, and the upcoming geostationary mission Sentinel-4 and the successor missions for S5-precursor (Sentinel-5), which will also have a similar ALH product.