Bridging the Gap: A comprehensive review and cross-check analysis for China's Non-CO2 greenhouse gas emission estimates.

Non-CO2 greenhouse gases (GHGs), mainly including CH4, N2O, SF6, HFCs, PFCs, and NF3, contribute significantly to climate change, yet their emission estimates exhibit substantial uncertainties. This study provides a comprehensive assessment of China's non-CO2 GHG emission estimates by systematically analyzing over 100 research articles encompassing both bottom-up and top-down studies. Our analysis reveals that multi-year average uncertainties in non-CO2 GHG estimates (±21-38 %) substantially exceed those calculated based on China's National Greenhouse Gas Inventories, and are approximately 4-7 times greater than CO2 emission uncertainties (±5.6-5.9 %). These wide estimation ranges have persisted despite substantial increases in research studies, indicating fundamental methodological challenges, which may hinder the mitigation policymaking. Through systematic cross-check between bottom-up and top-down approaches, we identify three primary sources of uncertainty: (1) inaccessible or inconsistent activity data, (2) incomplete emission source coverage, and (3) outdated emission factors in bottom-up studies; while top-down estimates are affected by limitations in inversion systems and scarce observational data. This work addresses critical knowledge gaps and provides specific pathways for improving the accuracy of China's non-CO2 GHG emissions quantification, supporting more effective climate policy development and emission reduction strategies. The findings directly inform China's climate mitigation policies and contribute to global efforts in achieving the Paris Agreement targets.