The role of OCO-3 XCO2 retrievals in estimating global terrestrial net ecosystem exchanges

Abstract. Satellite-based column-averaged dry-air CO2 mole fraction (XCO2) retrievals are frequently used to improve the estimates of terrestrial net ecosystem exchanges (NEEs). The Orbiting Carbon Observatory 3 (OCO-3) satellite, launched in May 2019, was designed to address important questions about the distribution of carbon fluxes on Earth, but its role in estimating global terrestrial NEE remains unclear. Here, using the Global Carbon Assimilation System, version 2, we investigate the impact of OCO-3 XCO2 on the estimation of global NEE by assimilating the OCO-3 XCO2 retrievals alone and in combination with the OCO-2 XCO2 retrievals. The results show that when only the OCO-3 XCO2 is assimilated (Exp_OCO3), the estimated global land sink is significantly lower than that from the OCO-2 experiment (Exp_OCO2). The estimate from the joint assimilation of OCO-3 and OCO-2 (Exp_OCO3&2) is comparable on a global scale to that of Exp_OCO2. However, there are significant regional differences. Compared to the observed global annual CO2 growth rate, Exp_OCO3 has the largest bias and Exp_OCO3&2 shows the best performance. Furthermore, validation with independent CO2 observations shows that the biases of Exp_OCO3 are significantly larger than those of Exp_OCO2 and Exp_OCO3&2 at middle and high latitudes. The reasons for the poor performance of assimilating OCO-3 XCO2 alone include the lack of observations beyond 52° S and 52° N, the large fluctuations in the number of data, and the varied observation time. Our study indicates that assimilating OCO-3 XCO2 retrievals alone leads to an underestimation of land sinks at high latitudes and that a joint assimilation of OCO-2 XCO2 and the OCO-3 XCO2 retrievals observed in the afternoon is required for a better estimation of global terrestrial NEE.