Monitoring fossil fuel CO2 emissions from co-emitted NO2 observed from space: progress, challenges, and future perspectives

Abstract Developing an anthropogenic carbon dioxides (CO 2 ) emissions monitoring and verification support (MVS) capacity is essential to support the Global Stocktake (GST) and ratchet up Nationally Determined Contributions (NDCs). The 2019 IPCC refinement proposes top-down inversed CO 2 emissions, primarily from fossil fuel (FFCO 2 ), as a viable emission dataset. Despite substantial progress in directly inferring FFCO 2 emissions from CO 2 observations, substantial challenges remain, particularly in distinguishing local CO 2 enhancements from the high background due to the long atmospheric lifetime. Alternatively, using short-lived and co-emitted nitrogen dioxide (NO 2 ) as a proxy in FFCO 2 emission inversion has gained prominence. This methodology is broadly categorized into plume-based and emission ratios (ERs)-based inversion methods. In the plume-based methods, NO 2 observations act as locators, constraints, and validators for deciphering CO 2 plumes downwind of sources, typically at point source and city scales. The ERs-based inversion approach typically consists of two steps: inferring NO 2 -based nitrogen oxides (NO x ) emissions and converting NO x to CO 2 emissions using CO 2 -to-NO x ERs. While integrating NO 2 observations into FFCO 2 emission inversion offers advantages over the direct CO 2 -based methods, uncertainties persist, including both structural and data-related uncertainties. Addressing these uncertainties is a primary focus for future research, which includes deploying next-generation satellites and developing advanced inversion systems. Besides, data caveats are necessary when releasing data to users to prevent potential misuse. Advancing NO 2 -based CO 2 emission inversion requires interdisciplinary collaboration across multiple communities of remote sensing, emission inventory, transport model improvement, and atmospheric inversion algorithm development.