The Aerosol Layer Height (ALH) is a new product with little heritage focusing on the retrieval of vertically localized aerosol layers in the free troposphere. Aerosol vertical profile products exist from other satellite instruments, e.g. Caliop on CALIPSO and MISR on Terra. The Aerosol Index described here is providing the air pressure at the center of the aerosol layer and the height at the center relative to the geoid.
The retrieval of aerosol height is based on absorption by oxygen in the A band, based in near-infrared between 759 and 770 nm. This range is a highly structured line absorption spectrum with the strongest absorption lines occurring between 760 and 761 nm; the bands span a wide range of absorption optical thicknesses.
First validation shows that the ALH is stable and shows good overall agreement with the Caliop retrievals, with TROPOMI generally showing lower ALH compared to Caliop over ocean scenes. The bias of the product can be up to a few hundred meters, but this is still under investigation. The product is very sensitive to cloud contamination and the height will be strongly biased towards the cloud height for partially clouded pixels. Furthermore, the product becomes unreliable for increasing surface albedo such as over deserts, biasing the ALH towards the surface.
Finally, this product also helps to interpret the UV Aerosol Index (UVAI) in terms of aerosol absorption, as the index is strongly height-dependent.