Using continuous data to estimate clear‐sky errors in inversions of satellite CO 2 measurements

We used continuous measurements of atmospheric CO 2 at two stations to investigate potential errors in inversions of temporal averages of satellite clear‐sky column retrievals. Compared to the complete data sets, the mid‐day CO 2 on clear days was systematically lower with a larger winter difference. Net ecosystem exchange (NEE) of CO 2 was enhanced on clear vs. all days, the summer boundary layer was deeper, and the CO concentration was systematically lower. During winter these differences cannot account for the CO 2 bias, which must be caused by advection. Summertime errors reflect a tradeoff between deeper mixing and enhanced NEE on clear days. If these sites represent mid‐latitude forests and if the CO 2 difference is confined to the bottom 15% column mass, then inversions of temporally‐averaged satellite column data products will incur a −0.2 to −0.4 ppm bias. CO 2 concentrations must therefore be assimilated at the place and time observed.