A comparison of atmospheric CO2 concentration GOSAT-based observations and model simulations

Satellite observations of atmospheric CO2 are able to truly capture the variation of global and regional CO2 concentration. The model simulations based on atmospheric transport models can also assess variations of atmospheric CO2 concentrations in a continuous space and time, which is one of approaches for qualitatively and quantitatively studying the atmospheric transport mechanism and spatio-temporal variation of atmospheric CO2 in a global scale. Satellite observations and model simulations of CO2 offer us two different approaches to understand the atmospheric CO2. However, the difference between them has not been comprehensively compared and assessed for revealing the global and regional features of atmospheric CO2. In this study, we compared and assessed the spatio-temporal variation of atmospheric CO2 using two datasets of the column-averaged dry air mole fractions of atmospheric CO2 (XCO2) in a year from June 2009 to May 2010, respectively from GOSAT retrievals (V02.xx) and from Goddard Earth Observing System-Chemistry (GEOS-Chem), which is a global 3-D chemistry transport model. In addition to the global comparison, we further compared and analyzed the difference of CO2 between the China land region and the United States (US) land region from two datasets, and demonstrated the reasonability and uncertainty of satellite observations and model simulations. The results show that the XCO2 retrieved from GOSAT is globally lower than GEOS-Chem model simulation by 2 ppm on average, which is close to the validation conclusion for GOSAT by ground measures. This difference of XCO2 between the two datasets, however, changes with the different regions. In China land region, the difference is large, from 0.6 to 5.6 ppm, whereas it is 1.6 to 3.7 ppm in the global land region and 1.4 to 2.7 ppm in the US land region. The goodness of fit test between the two datasets is 0.81 in the US land region, which is higher than that in the global land region (0.67) and China land region (0.68). The analysis results further indicate that the inconsistency of CO2 concentration between satellite observations and model simulations in China is larger than that in the US and the globe. This inconsistency is related to the GOSAT retrieval error of CO2 caused by the interference among input parameters of satellite retrieval algorithm, and the uncertainty of driving parameters in GEOS-Chem model.