| Summary: | In this study we present the retrieval of the column-averaged dry air mole fraction of carbon dioxide (<i>XCO<sub>2</sub></i>) from the TanSat observations using the ACOS (Atmospheric CO<sub>2</sub> Observations from Space) algorithm. The <i>XCO<sub>2</sub></i> product has been validated with collocated ground-based measurements from the Total Carbon Column Observing Network (TCCON) for 2 years of TanSat data from 2017 to 2018. Based on the correlation of the <i>XCO<sub>2</sub></i> error over land with goodness of fit in three spectral bands at 0.76, 1.61 and 2.06 μm, we applied an a posteriori bias correction to TanSat retrievals. For overpass averaged results, <i>XCO<sub>2</sub></i> retrievals show a standard deviation (SD) of ~2.45 ppm and a positive bias of ~0.27 ppm compared to collocated TCCON sites. The validation also shows a relatively higher positive bias and variance against TCCON over high-latitude regions. Three cases to evaluate TanSat target mode retrievals are investigated, including one field campaign at Dunhuang with measurements by a greenhouse gas analyzer deployed on an unmanned aerial vehicle and two cases with measurements by a ground-based Fourier-transform spectrometer in Beijing. The results show the retrievals of all footprints, except footprint-6, have relatively low bias (within ~2 ppm). In addition, the orbital <i>XCO<sub>2</sub></i> distributions over Australia and Northeast China between TanSat and the second Orbiting Carbon Observatory (OCO-2) on 20 April 2017 are compared. It shows that the mean <i>XCO<sub>2</sub></i> from TanSat is slightly lower than that of OCO-2 with an average difference of ~0.85 ppm. A reasonable agreement in <i>XCO<sub>2</sub></i> distribution is found over Australia and Northeast China between TanSat and OCO-2.
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