Construction Of A Global Carbon Assimilation Inversion Scheme Based On Ground-based Atmospheric CO₂ Concentration Observation

Carbon dioxide（CO₂）is an important greenhouse gas in the atmosphere,and clarifying the spatial and temporal characteristics of atmospheric CO₂ sources and sinks can help to understand the global carbon cycle process in depth and contribute to the carbon emission inventory of countries’compliance with the Paris Agreement on Climate Change every five years after 2023.The atmospheric carbon flux inversion method is one of the effective methods to estimate global surface carbon fluxes,however,the algorithm to assimilate CO₂ and invert carbon sources and sinks based on the global Earth system model is not publicly available to China,and there are large uncertainties in the current carbon flux inversion estimation,especially in the sparsely observed regions such as China due to the uneven distribution of ground-based observation data.In addition,with the national goal of"dual carbon",there is a need for the global model to provide not only autonomous and more accurate information on carbon sources and sinks to understand the global situation,but also to provide more accurate boundary data for the regional carbon sources and sinks assimilation inversion system in China.This thesis develops a global carbon assimilation inversion scheme for China based on the improved GEOS-Chem 3D global chemical transport model combined with the ensemble root-mean-square filtered data assimilation algorithm,and additionally assimilates continuous atmospheric CO₂ concentration observations from five atmospheric background stations of China’s Greenhouse Gas Observation Network（CGHGNET）on top of the Obs Pack global ground-based atmospheric CO₂concentration observations to increase the Chinese regional constraints,and analyze the spatial and temporal distribution characteristics of global surface natural carbon fluxes in 2019,and give new data and understanding especially for the terrestrial carbon sink in China.The global terrestrial carbon fluxes and ocean-atmosphere carbon exchange in2019 from inversion are about-2.12±1.09 and-2.53±1.01 Pg C,which absorbs 21.1%and 25.1%of global fossil fuel emissions in 2019,respectively,and the uncertainties are reduced by 61%and 25%,respectively,compared with those before assimilation.The global residual fossil fuel emission is 5.41 Pg C,and the uncertainty is smaller than the range of atmospheric inversion deviation reported by Global Carbon Budget 2021,and is consistent with the global average atmospheric CO₂ growth rate of 5.39 Pg C in2019 obtained by the National Oceanic and Atmospheric Administration based on global average sea surface data,indicating that the global model carbon in China constructed in this paper assimilation inversion scheme constructed in this paper can reasonably constrain the net exchange of global atmospheric CO₂.In terms of spatial distribution,the northern hemisphere is the main contributor to global terrestrial carbon sinks,among which eastern North America,mid-and high-latitudes of Asia and Europe,and most of China are the main carbon sink areas,while terrestrial ecosystems as carbon sources are mainly distributed in southeastern North America,south of the equator in Africa,northern Russia,and parts of tropical Asia.In addition,terrestrial carbon sinks at mid-and high-latitudes in the Northern Hemisphere exhibit seasonal variations closely linked to vegetation growth cycles,with the strongest carbon sinks in summer and weaker sinks in winter and spring.The inverted terrestrial carbon sink of China is in 2019 is about 0.37±0.05 Pg C,offsetting 13%of China’s fossil fuel emissions in the same year,with an 81%reduction in uncertainty compared to the a priori flux without assimilation.Among them,grassland,forest,and cropland contribute the most to the terrestrial carbon sink in China,with 0.15,0.13,and 0.08 Pg C.Sensitivity tests through the Chinese regional sites show that the newly introduced CGHGNET observations can provide more constrained information for the Chinese region.The terrestrial carbon flux uncertainty is further reduced by about 68.8%compared to the unintroduced ones,and the spatial pattern of terrestrial carbon sinks in China is more reasonable after the introduction of the new observations,which also indicates that additional observations in China are essential to improve the ability to invert the spatial characteristics of terrestrial carbon sinks in China.The construction and practical application of our own global carbon assimilation inversion scheme will not only help to clarify the spatial distribution of global carbon sources and sinks and understand the temporal variation of carbon fluxes but also be valuable for monitoring and accounting for the effect of national"dual carbon"actions.