Abstract
Abstract. In recent years, space-borne observations of atmospheric carbon dioxide (CO2) have been increasingly used in global carbon-cycle studies. In order to obtain added value from space-borne measurements, they have to suffice stringent accuracy and precision requirements, with the latter being less crucial as it can be reduced by just enhanced sample size. Validation of CO2 column-averaged dry air mole fractions (XCO2) heavily relies on measurements of the Total Carbon Column Observing Network (TCCON). Owing to the sparseness of the network and the requirements imposed on space-based measurements, independent additional validation is highly valuable. Here, we use observations from the High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observations (HIPPO) flights from 01/2009 through 09/2011 to validate CO2 measurements from satellites (Greenhouse Gases Observing Satellite – GOSAT, Thermal Emission Sounder – TES, Atmospheric Infrared Sounder – AIRS) and atmospheric inversion models (CarbonTracker CT2013B, Monitoring Atmospheric Composition and Climate (MACC) v13r1). We find that the atmospheric models capture the XCO2 variability observed in HIPPO flights very well, with correlation coefficients (r2) of 0.93 and 0.95 for CT2013B and MACC, respectively. Some larger discrepancies can be observed in profile comparisons at higher latitudes, in particular at 300 hPa during the peaks of either carbon uptake or release. These deviations can be up to 4 ppm and hint at misrepresentation of vertical transport. Comparisons with the GOSAT satellite are of comparable quality, with an r2 of 0.85, a mean bias μ of −0.06 ppm, and a standard deviation σ of 0.45 ppm. TES exhibits an r2 of 0.75, μ of 0.34 ppm, and σ of 1.13 ppm. For AIRS, we find an r2 of 0.37, μ of 1.11 ppm, and σ of 1.46 ppm, with latitude-dependent biases. For these comparisons at least 6, 20, and 50 atmospheric soundings have been averaged for GOSAT, TES, and AIRS, respectively. Overall, we find that GOSAT soundings over the remote Pacific Ocean mostly meet the stringent accuracy requirements of about 0.5 ppm for space-based CO2 observations.
Highlights
Space-borne measurements of atmospheric carbon dioxide (CO2) can provide unique constraints on carbon exchanges between land, ocean, and atmosphere on a global scale
The HIPPO project, a sequence of five global aircraft measurement programs, sampled the atmosphere from the North Pole to the coastal waters of Antarctica, from the surface to 14 km a.g.l., spanning the seasons (Wofsy, 2011). This enables a comparison of individual sub-columns of air and of CO2 column-averaged dry air mole fractions, denoted XCO2, if the profile can be reliably extended above 14 km a.g.l
We look at XCO2, derived using absorption spectroscopy of reflected sunlight recorded by near-infrared spectrometers such as SCIAMACHY, Gases Observing Satellite (GOSAT), or Orbiting Carbon Observatory-2 (OCO-2)
Summary
Space-borne measurements of atmospheric carbon dioxide (CO2) can provide unique constraints on carbon exchanges between land, ocean, and atmosphere on a global scale. Accuracy requirements are very stringent (Miller et al, 2007), warranting large-scale biases of less than 0.5–1 ppm, being less than 0.3 % of the global background concentration This is one of the most challenging remote sensing measurements from space as we want to reproduce known average seasonal cycles and trends and small inter-annual deviations, resolved to subcontinental scales. The HIPPO project, a sequence of five global aircraft measurement programs, sampled the atmosphere from (almost) the North Pole to the coastal waters of Antarctica, from the surface to 14 km a.g.l., spanning the seasons (Wofsy, 2011) This enables a comparison of individual sub-columns of air and of CO2 column-averaged dry air mole fractions, denoted XCO2, if the profile can be reliably extended above 14 km a.g.l. As the troposphere dominates the variabil-. The data set covers a wide range of atmospheric CO2 profiles especially in the Northern Hemisphere where the strong biogenic cycle causes strong seasonality in CO2 fluxes
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