Urban-scale constraints on fossil fuel CO2 emissions from satellite-inferred NO2: implications for air quality–climate co-benefits in Chinese cities (2019–2024)

Cities are major sources of both air pollutants and greenhouse gases due to dense energy consumption, traffic, industry, and residential heating. Fossil fuel carbon dioxide (CO₂) is frequently co-emitted with nitrogen oxides (NO_x), linking urban air quality degradation with climate forcing. However, monitoring urban CO₂ emissions remains challenging because fixed ground stations provide sparse coverage, while bottom-up inventories often lack the temporal responsiveness needed to capture rapid socioeconomic and policy-driven changes. These limitations are particularly critical for cities, where emission patterns are highly heterogeneous in space and time.

Here we present a satellite-based framework to constrain urban and regional fossil fuel CO₂ emissions across China from 2019 to 2024 by exploiting the co-emission relationship between NO_x and CO₂. The approach integrates tropospheric NO₂ vertical column densities observed by the TROPOspheric Monitoring Instrument (TROPOMI) with simulations from the GEOS-Chem chemical transport model. Anthropogenic NO_x emissions are first optimized using a finite-difference mass balance inversion, which links observed NO₂ enhancements to emission perturbations at high spatial resolution. The optimized NO_x fields are then translated into fossil fuel CO₂ emissions using dynamically derived CO₂/NO_x ratios from bottom-up inventories, allowing indirect yet spatially explicit constraints on urban CO₂ emissions.

Our results reveal that China’s fossil fuel CO₂ emissions remained broadly stable over 2019–2024, with pronounced spatial contrasts between urban agglomerations and less developed regions. Persistent emission hotspots are identified over major metropolitan clusters, including the Beijing–Tianjin–Hebei region, the Yangtze River Delta, the Pearl River Delta, and the Fenwei Plain, underscoring the dominant role of cities in national carbon budgets. Despite overall stability at the national scale, many large urban regions exhibit discernible declines in emissions, consistent with strengthened air pollution control policies and structural energy transitions. In contrast, energy-intensive provinces outside the major city clusters continue to show increasing trends, highlighting emerging risks of regional “high-carbon lock-in”. Comparisons with widely used inventories such as EDGAR, MEIC, and Carbon Monitor indicate that the satellite-constrained estimates more effectively capture abrupt emission changes associated with events such as the COVID-19 pandemic and subsequent economic recovery.

Overall, this study demonstrates that satellite-derived NO₂ observations provide a powerful, observation-driven pathway to monitor urban fossil fuel CO₂ emissions at high spatial and temporal resolution. By bridging air quality and greenhouse gas perspectives, the framework offers new opportunities to evaluate the climate co-benefits of urban air pollution policies, support city-scale carbon budgeting, and improve the transparency of emission monitoring in rapidly evolving urban environments.