The importance of digital elevation model accuracy in XCO2 retrievals: improving the Orbiting Carbon Observatory 2 Atmospheric Carbon Observations from Space version 11 retrieval product

Knowledge of surface pressure is essential for calculating column-averaged dry-air mole fractions of trace gases, such as CO2(X-CO2). In the NASA Orbiting Carbon Observatory 2 (OCO-2) Atmospheric Carbon Observations from Space (ACOS) retrieval algorithm, the retrieved surface pressures have been found to have unacceptable errors, war-ranting a parametric bias correction. This correction depends on the difference between retrieved and a priori surface pressures, which are derived from a meteorological model that is hypsometrically adjusted to the surface elevation using a digital elevation model (DEM). As a result, the effectiveness of the OCO-2 bias correction is contingent upon the accuracy of the referenced DEM. Here, we investigate several different DEM datasets for use in the OCO-2 ACOS retrieval algorithm: the OCODEM used in ACOS v10 and previous versions, the NASADEM+ (a composite of SRTMv4, ASTERGDEMv3, GIMP, and RAMPv2 DEMs) used in ACOS v11,the Copernicus GLO-90 DEM (GLO-90 DEM), and two po-lar regional DEMs (Arctic DEM and REMA). We find that the NASADEM+ (ASTER GDEMv3) has a persistent negative bias on the order of 10 to 20 m across most regions north of 60 degrees N latitude, relative to all the other DEMs considered(OCODEM, Arctic DEM, and GLO-90 DEM). Variations of10 m in DEM elevations lead to variations in XCO2 of approximately 0.4 ppm, meaning that the XCO(2 )from OCO-2 ACOSv11 retrievals tends to be 0.4 to 0.8 ppm lower across regions north of 60 degrees N than XCO2 from OCO-2 ACOS v10. Our analysis also suggests that the GLO-90 DEM has superior global continuity and accuracy compared to the other DEMs, motivating a post-processing update from OCO-2 v11 Lite files(which used NASADEM+) to OCO-2 v11.1 by substituting the GLO-90 DEM globally. We find that OCO-2 v11.1 im-proves accuracy and spatial continuity in the bias-correctedXCO2product relative to both v10 and v11 in high-latitude regions while resulting in marginal or no change in most regions within +/- 60 degrees latitude. In addition, OCO-2 v11.1 pro-vides increased data throughput after quality control filtering in most regions, partly due to the change in DEM butmostly due to other corrections to quality control parame-ters. Given large-scale differences north of 60 degrees N betweenthe OCODEM and NASADEM+, we find that replacing theOCODEM with NASADEM+ yields a similar to 100 TgC shift in in-ferred carbon uptake for the zones spanning 30 to 60 degrees N and60 to 90 degrees N, which is on the order of 5 % to 7 % of the esti-mated pan-Arctic land sink. Changes in inferred fluxes fromreplacing the OCODEM with the GLO-90 DEM are smaller,and given the evidence for improved accuracies from thisDEM, this suggests that large changes in inferred fluxes fromthe NASADEM+ are likely erroneous.