Spatiotemporal bias in OCO-2 XCO2 retrievals relative to TCCON observations

Accurate monitoring of atmospheric CO 2 concentrations is essential for understanding carbon dynamics and informing climate-mitigation strategies. This study examines the seasonal variability of satellite-based XCO 2 retrievals from NASA’s OCO-2 mission relative to ground-based TCCON XCO 2 measurements over ten years (2014–2024) across 17 global TCCON sites. The study distinguished between two satellite observation modes, nadir and glint. It assessed how seasonal variations in satellite-derived environmental parameters – normalized difference vegetation index (NDVI), soil surface moisture (SSM), land-surface temperature (LST), and evapotranspiration (ET) – relate to statistical characteristics of the XCO 2 bias. In addition, we evaluated the temporal dynamics of the bias across the dominant land-cover types surrounding each site. Our results identify a latitude-dependent bias in nadir retrievals across Northern Hemisphere sites, with strong associations between nadir bias and land-cover at TCCON sites located > 45°N (R 2 = 0.95) and between 0 and 45°N (R 2 = 0.79). This work provides novel insights into the temporal dynamics of satellite XCO 2 bias relative to ground measurements and demonstrates distinct behaviors across observation modes that have not been reported previously. These findings are crucial for improving the accuracy of global CO 2 mapping and for strengthening the reliability of carbon flux models.