Active and passive satellite observations coupled with carbon–nitrogen synergy for urban fossil fuel CO ₂ emissions monitoring

Abstract. Accurate estimation of fossil fuel CO2 (ffCO2) emissions is essential for climate prediction and the development of mitigation policies. Top-down carbon–nitrogen joint observations offer the potential for more reliable ffCO2 estimates. Here, we establish an inversion framework for urban ffCO2 emissions based on combined active–passive satellite observations. Urban ffCO2 distributions were first constructed using satellite NO2 data and CO2–NOx emission ratios, and monthly ffCO2 emissions for selected global cities were then estimated by integrating the total column dry-air carbon dioxide (XCO2) from the DQ-1 ACDL instrument. Our results show that satellite-derived NOx emissions provide strong constraints on urban anthropogenic CO2 estimates. Validation against TCCON ground-based observations indicates that, compared with conventional top-down inversion approaches, our method more accurately reproduces urban ffXCO2 plume distributions. We further evaluated the influence of different CO2–NOx ratio calculation methods on ffCO2 estimates and found variations exceeding 150, exerting a substantial impact on emission inversions. Under observational constraints, the uncertainty in CO2–NOx ratios derived from different methods decreased by 9.79 %–38.78 %, and the variation range was reduced by more than 100 %, converging toward a consistent magnitude. This study advances understanding of the spatiotemporal patterns of urban ffCO2 emissions and provides a unified perspective for future CO2–NOx-based anthropogenic carbon emission estimation.