Assessing the consistency of X CO ₂ products from DQ-1 ACDL and passive satellites using a unified comparison framework

Da Qi-1 (DQ-1) carries the Aerosol and Carbon Dioxide Lidar (ACDL), a spaceborne integrated path differential absorption (IPDA) lidar that retrieves the column-averaged dry-air mole fraction of carbon dioxide ( X CO ₂ ) using active laser measurements. Unlike passive near-infrared/shortwave-infrared sensors that rely on reflected sunlight, ACDL can observe at night and high latitudes, but its sampling and effective column definition differ from passive X CO ₂ products. Evaluation is therefore needed before active and passive observations can be interpreted jointly. We compare DQ-1 ACDL with the Orbiting Carbon Observatory (OCO-2 and OCO-3) and Greenhouse gases Observing SATellite (GOSAT and GOSAT-2) missions from June 2022–December 2024 using a unified framework that combines Copernicus Atmosphere Monitoring Service (CAMS)-based coherence assessment, sampling availability, daily 2° aggregation, spatiotemporal collocation, and correction for X CO ₂ column-definition differences. Validation against the Total Carbon Column Observing Network (TCCON) gives R =0.92, a mean bias of 0.21 ppm, and a root-mean-square error (RMSE) of 1.49 ppm for daytime DQ-1, comparable to GOSAT. Over coherent and well-sampled regions, DQ-1 is broadly consistent with OCO-2 and OCO-3, with mean differences generally below 0.5 ppm and similar spatial and meridional structures. Over ocean, the DQ-1–OCO differences vary with surface reflectance and wind speed, with different reflectance relationships for OCO-2 and OCO-3. Nighttime DQ-1 is also broadly consistent with CAMS-derived X CO ₂ . Regional analyses show complementary DQ-1 coverage where passive observations are limited. These results support DQ-1 ACDL as a quantitatively comparable and complementary component of the satellite X CO ₂ observing system.