Comparison of OCO-2 target observations to MUCCnet – is it possible to capture urban X$_{CO2}$ gradients from space?

In this paper, we compare Orbiting Carbon Observatory 2 (OCO-2) measurements of column-averaged dry-air mole fractions (DMF) of CO$_{2}$ (X$_{CO2}$) and its urban–rural differences against ground-based remote sensing data measured by the Munich Urban Carbon Column network (MUCCnet). Since April 2020, OCO-2 has regularly conducted target observations in Munich, Germany. Its target-mode data provide high-resolution X$_{CO2}$ within a 15 km × 20 km target field of view that is greatly suited for carbon emission studies from space in cities and agglomerated areas. OCO-2 detects urban X$_{CO2}$ with a root mean square different (RMSD) of less than 1 ppm when compared to the MUCCnet reference site. OCO-2 target X$_{CO2}$ is biased high against the ground-based measurements. The close proximity of MUCCnet's five fully automated remote sensing sites enables us to compare spaceborne and ground-based X$_{CO2}$ in three urban areas of Munich separately (center, north, and west) by dividing the target field into three smaller comparison domains. Due to this more constrained collocation, we observe improved agreement between spaceborne and ground-based X$_{CO2}$ in all three comparison domains. ... mehr

For the first time, X$_{CO2}$ gradients within one OCO-2 target field of view are evaluated against ground-based measurements. We compare X$_{CO2}$ gradients in the OCO-2 target observations to gradients captured by collocated MUCCnet sites. Generally, OCO-2 detects elevated X$_{CO2}$ in the same regions as the ground-based monitoring network. More than 90 % of the observed spaceborne gradients have the same orientation as the X$_{CO2}$ gradients measured by MUCCnet. During our study, urban–rural enhancements are found to be in the range of 0.1 to 1 ppm. The low urban–rural gradients of typically well below 1 ppm in Munich during our study allow us to test OCO-2's lower detection limits for intra-urban X$_{CO2}$ gradients. Urban X$_{CO2}$ gradients recorded by the OCO-2 instruments and MUCCnet are strongly correlated (R$^{2}$=0.68) with each other and have an RMSD of 0.32 ppm. A case study, which includes a comparison of one OCO-2 target overpass to WRF-GHG modeled X$_{CO2}$ , reveals a similar distribution of enhanced CO$_{2}$ column abundances in Munich. In this study, we address OCO-2's capability to detect small-scale spatial X$_{CO2}$ differences within one target observation. Our results suggest OCO-2's potential to assess anthropogenic emissions from space.