Study on the Ground-Based FTS Measurements at Beijing, China and the Colocation Sensitivity of Satellite Data

The Fourier Transform Spectrometer (FTS) at the Beijing Satellite Meteorological Ground Station observed XCO2 (the dry carbon dioxide column) from 2 March 2016 to 4 December 2018. The validation results of ground-based XCO2, as well as GOSAT, OCO-2, and TanSat XCO2, show that the best temporal matching setting for ground-based XCO2 and satellite XCO2 is +/- 1 h, and the best spatial matching setting for GOSAT is 0.5 degrees x 0.5 degrees. Consistent with OCO-2, the best spatial matching setting of TanSat is 5 degrees x 5 degrees or 6 degrees x 6 degrees. Among GOSAT, OCO-2, and TanSat, the satellite observation validation characteristics near 5 degrees x 5 degrees from the ground-based station are obviously different from other spatial matching grids, which may be due to the different observation characteristics of satellites near 5 degrees x 5 degrees. To study the influence of local CO2 sources on the characteristics of satellite observation validation, we classified the daily XCO2 observation sequence into concentrated, dispersive, increasing, and decreasing types, respectively, and then validated the satellite observations. The results showed that the concentrated and decreasing sub-datasets have better validation performance. Our results suggest that it is best to use concentrated and decreasing sub-datasets when using the Beijing Satellite Meteorological Ground Station XCO2 for satellite validation. The temporal matching setting should be +/- 1 h, and the spatial matching setting should consider the satellites observation characteristics of 5 degrees x 5 degrees distance from the ground-based station.