County-level emission inventory reveals substantial contributions of small and medium-sized cities to air pollutants and CO2 emissions in China.

China faces the dual challenge of improving ambient air quality while mitigating climate change, yet synergistic control strategies are often formulated at national or provincial scales due to the lack of detailed county-level emission information. This study integrates multi-source data within a unified framework to develop a comprehensive county-level emission inventory of major air pollutants and CO2 for China. The results reveal pronounced spatial heterogeneity in emissions. A total of 929 counties, accounting for approximately 33% of all counties and 24% of China's land area, contribute about 80% of national CO2 emissions and 41 - 67% of air pollutant emissions, indicating the emergence of county-level joint emission hubs across multiple pollutants. Emission intensities are substantially higher in small and medium-sized cities than in large-sized cities, underscoring their underestimated environmental pressures during economic growth. Comparison with conventional GDP-based allocation approaches shows that emissions in small and medium-sized cities are systematically underestimated, whereas those in large-sized cities are overestimated. These findings demonstrate the limitations of economic proxy-based allocation methods and provide a refined evidence base for targeted, synergistic air quality management and carbon mitigation at city and intra-city scales in China.