Validation of ATLID stratospheric aerosol retrievals using SAGE III/ISS and ground-based lidar observations

We present a systematic validation of EarthCARE ATLID stratospheric aerosol retrievals against SAGE III/ISS and ground-based lidars at the Table Mountain Facility (TMF) and the Observatoire de Haute-Provence (OHP), using 895 SAGE III/ISS, 55 TMF, and 26 OHP collocations over August 2024 to April 2026 and spanning background, volcanic, and wildfire smoke conditions. We evaluate a Level-1-based extinction product, derived directly from the ATLID L1 scattering ratio. Against SAGE III/ISS, this L1 product yields a regression slope of 1.04 for the stratospheric aerosol optical depth (sAOD) and an overall median relative extinction difference of −2.2 %; across all matchups, the relative sAOD bias has a global mean of −3.2 % with a standard deviation of 65.7 %. As a function of altitude above the local tropopause, the median relative difference is negative in the first ∼ 3 km above the tropopause (about −10 % to −15 %), close to zero between about 5 and 15 km above the tropopause, and negative again further up; the sAOD comparison also shows a systematic negative bias in the Northern Hemisphere extratropics that increases with latitude. Independent backscatter comparisons against TMF and OHP also show a negative bias just above the tropopause (overall median differences of −5.5 % and −10.6 %, respectively), indicating that this feature originates in the Level-1 scattering-ratio retrieval itself rather than in the extinction-conversion step. Daytime collocations are markedly noisier than nighttime ones (relative-difference standard deviation of ∼ 32 % versus ∼ 17 %), although the median bias remains close to zero in both cases. In contrast, the native L2 product achieves a regression slope of only 0.64 against SAGE III/ISS sAOD, largely driven by frequent null retrievals under low-aerosol conditions. Even when excluding these missed detections, the correlation remains weak ( R  = 0.26), and the L2 product systematically overestimates peak extinction in dense plumes, making it unreliable for stratospheric monitoring in its current processing baseline.