Regional CO₂ fluxes from 2010 to 2015 inferred from GOSAT XCO₂ retrievals using a new version of the Global Carbon Assimilation System

• Main Authors: F. Jiang, H. Wang, J. M. Chen, W. Ju, X. Tian, S. Feng, G. Li, Z. Chen, S. Zhang, X. Lu, J. Liu, J. Wang, W. He, M. Wu
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-02-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/1963/2021/acp-21-1963-2021.pdf
• ISSN: 1680-7316; 1680-7324

<p>Satellite retrievals of the column-averaged dry air mole fractions of CO<span class="inline-formula">₂</span> (XCO<span class="inline-formula">₂</span>) could help to improve carbon flux estimation due to their good spatial coverage. In this study, in order to assimilate the GOSAT (Greenhouse Gases Observing Satellite) XCO<span class="inline-formula">₂</span> retrievals, the Global Carbon Assimilation System (GCAS) is upgraded with new assimilation algorithms, procedures, a localization scheme, and a higher assimilation parameter resolution. This upgraded system is referred to as GCASv2. Based on this new system, the global terrestrial ecosystem (BIO) and ocean (OCN) carbon fluxes from 1 May 2009 to 31 December 2015 are constrained using the GOSAT ACOS (Atmospheric CO<span class="inline-formula">₂</span> Observations from Space) XCO<span class="inline-formula">₂</span> retrievals (Version 7.3). The posterior carbon fluxes from 2010 to 2015 are independently evaluated using CO<span class="inline-formula">₂</span> observations from 52 surface flask sites. The results show that the posterior carbon fluxes could significantly improve the modeling of atmospheric CO<span class="inline-formula">₂</span> concentrations, with global mean bias decreases from a prior value of 1.6 <span class="inline-formula">±</span> 1.8 ppm to <span class="inline-formula">−</span>0.5 <span class="inline-formula">±</span> 1.8 ppm. The uncertainty reduction (UR) of the global BIO flux is 17 %, and the highest monthly regional UR could reach 51 %. Globally, the mean annual BIO and OCN carbon sinks and their interannual variations inferred in this study are very close to the estimates of CarbonTracker 2017 (CT2017) during the study period, and the inferred mean atmospheric CO<span class="inline-formula">₂</span> growth rate and its interannual changes are also very close to the observations. Regionally, over the northern lands, the strongest carbon sinks are seen in temperate North America, followed by Europe, boreal Asia, and temperate Asia; in the tropics, there are strong sinks in tropical South America and tropical Asia, but a very weak sink in Africa. This pattern is significantly different from the estimates of CT2017, but the estimated carbon sinks for each continent and some key regions like boreal Asia and the Amazon are comparable or within the range of previous bottom-up estimates. The inversion also changes the interannual variations in carbon fluxes in most TransCom land regions, which have a better relationship with the changes in severe drought area (SDA) or leaf area index (LAI), or are more consistent with previous estimates for the impact of drought. These results suggest that the GCASv2 system works well with the GOSAT XCO<span class="inline-formula">₂</span> retrievals and shows good performance with respect to estimating the<span id="page1964"/> surface carbon fluxes; meanwhile, our results also indicate that the GOSAT XCO<span class="inline-formula">₂</span> retrievals could help to better understand the interannual variations in regional carbon fluxes.</p>