A First Look at Target Mode Retrievals of CO2 from the Orbiting Carbon Observatory-2 (OCO-2)

Abstract:

The Orbiting Carbon Observatory-2 (OCO-2) is NASA’s first dedicated Earth remote sensing satellite to study atmospheric carbon dioxide from space, and was launched successfully on July 2, 2014. OCO-2 is designed to quantify the sources and sinks of CO₂by making highly precise measurements of its column abundance. OCO-2 has three science observation modes – nadir, glint and target. In the nadir mode, the satellite points the instrument to the local nadir, so that data can be collected along the ground track just below the spacecraft. In the glint mode, the spacecraft points the instrument toward the bright "glint" spot, where solar radiation is specularly reflected from the surface. In the target mode, the Observatory will lock its view onto a specific surface location, and will scan back and forth over that target while flying overhead. A target track pass can last for up to 9 minutes. Over that time period, the Observatory can acquire as many as 12,960 samples at local zenith angles that vary between 0° and 85°.

Here, we analyze target track measurements over several of the OCO-2 validation sites where ground-based solar-looking Fourier Transform Spectrometers are located. The target scan serves two purposes. The first is to collect a large number of measurements over surface calibration and validation targets to assess the precision and accuracy of the retrieved column averaged CO₂ dry air mole fraction (X_CO2). The second is to look for spatial variations in column averaged CO₂ dry air mole fraction (X_CO2) in the vicinity of the target that could compromise the value of the measurements for calibration or validation. To meet both requirements, we compare X_CO2 retrievals as a function of both observation angle and scan location, and investigate whether those variations are coming from instrument calibration, algorithmic deficiencies, aerosols/clouds or real CO₂variations. Further, simulated retrievals indicate that target observations can show large sensitivity to even small amounts of clouds and aerosols (especially if lofted). We investigate the sensitivity of OCO-2 target mode observations to clouds and aerosols and compare against the simulated results.

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