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

Abstract

Developing an anthropogenic carbon dioxides (CO2) 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 CO2 emissions, primarily from fossil fuel (FFCO2), as a viable emission dataset. Despite substantial progress in directly inferring FFCO2 emissions from CO2 observations, substantial challenges remain, particularly in distinguishing local CO2 enhancements from the high background due to the long atmospheric lifetime. Alternatively, using short-lived and co-emitted nitrogen dioxide (NO2) as a proxy in FFCO2 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, NO2 observations act as locators, constraints, and validators for deciphering CO2 plumes downwind of sources, typically at point source and city scales. The ERs-based inversion approach typically consists of two steps: inferring NO2-based nitrogen oxides (NOx) emissions and converting NOx to CO2 emissions using CO2-to-NOx ERs. While integrating NO2 observations into FFCO2 emission inversion offers advantages over the direct CO2-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 NO2-based CO2 emission inversion requires interdisciplinary collaboration across multiple communities of remote sensing, emission inventory, transport model improvement, and atmospheric inversion algorithm development.