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-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 −2.3% across all matchups, the relative sAOD bias has a global mean of −3.2% with a standard deviation of 65.7%. The altitude-resolved comparison reveals a moderate positive bias near the tropopause (∼+10%), a pronounced negative excursion of up to −15% in the lower-to-mid stratosphere, and convergence toward near-zero bias above ∼25 km, together with a systematic negative sAOD bias in the Northern Hemisphere extratropics that increases with latitude. Independent backscatter comparisons against TMF and OHP reproduce the same negative bias in the lower-to-mid stratosphere (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.