Quantification and Evaluation of OCO-2 measured XCO2 against COCCON

Carbon dioxide (CO₂) is the primary greenhouse gas emitted from anthropogenic activities. Although it is naturally present as a part of Earth’s carbon-cycle, human activities influence the ability of natural sinks to reduce CO₂ from the atmosphere, thus altering the carbon-cycle and necessitating the long-term monitoring of atmospheric CO₂. Precise, accurate and continuous measurements of CO₂ are important to this end.

The Orbiting Carbon Observatory-2 (OCO-2) was launched in 2014. It is NASA’s first Earth-orbiting satellite dedicated to making observations of CO₂ in the atmosphere and measuring its column-averaged dry-air mole fraction (X_CO2). The primary goal of the OCO-2 mission is to provide X_CO2 measurements with sufficient precision and accuracy alongside quantifying its seasonal and interannual variability. In this study, we use the new and improved OCO-2 B11 data set.

In the past, the space-based X_CO2 measurements from OCO-2 data have been validated against independent data sets such as the Total Carbon Column Observing Network (TCCON). In this study, we use independent measurements from the COllaborative Carbon Column Observing Network (COCCON) to identify potential biases and errors in the B11 data version and establish its robustness for use by the science community. COCCON uses portable Fourier-Transform InfraRed (FTIR) spectrometers (EM27/SUN) to measure greenhouse gases at several global sites.  

Comparison of OCO-2 measurements against COCCON sites indicate similar temporal trends in X_CO2 variability, with OCO-2 typically reporting higher values. Further, we evaluate the differences between the B11 OCO-2 and COCCON data sets. Finally, we analyze how OCO-2’s B11 version compares to selected COCCON and TCCON sites’ measurements in terms of capturing the seasonal cycle and growth rate of X_CO2.