China's Fossil Fuel CO₂ Emissions Estimated Using Surface Observations of Coemitted NO₂

Accurate estimates of fossil fuel CO₂ (FFCO₂) emissions are of great importance for climate prediction and mitigation regulations but remain a significant challenge for accounting methods relying on economic statistics and emission factors. In this study, we employed a regional data assimilation framework to assimilate in situ NO₂ observations, allowing us to combine observation-constrained NO_x emissions coemitted with FFCO₂ and grid-specific CO₂-to-NO_x emission ratios to infer the daily FFCO₂ emissions over China. The estimated national total for 2016 was 11.4 PgCO₂·yr^-1, with an uncertainty (1σ) of 1.5 PgCO₂·yr^-1 that accounted for errors associated with atmospheric transport, inversion framework parameters, and CO₂-to-NO_x emission ratios. Our findings indicated that widely used "bottom-up" emission inventories generally ignore numerous activity level statistics of FFCO₂ related to energy industries and power plants in western China, whereas the inventories are significantly overestimated in developed regions and key urban areas owing to exaggerated emission factors and inexact spatial disaggregation. The optimized FFCO₂ estimate exhibited more distinct seasonality with a significant increase in emissions in winter. These findings advance our understanding of the spatiotemporal regime of FFCO₂ emissions in China.