Column-averaged dry-air greenhouse gas mole fractions measured at the TCCON site at Wollongong, Australia

Nicholas M. Deutscher, Voltaire A. Velazco, Ronald C. Macatangay, Clare Paton-Walsh, Nicholas B. Jones, Graham C. Kettlewell, Stephen R. Wilson, Debra Wunch, David W.T. Griffith.

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Knowledge of the distribution of global sources and sinks of the greenhouse gases (GHGs) CO\(_{\mathrm{2}}\) and CH\(_{\mathrm{4}}\) is usually based on surface in situ measurements of these gases (Gurney 2002). Yang et al. (2007) and Stephens et al. (2007) have shown that inversions using only these measurements are sensitive to model parameterization of vertical mixing. On the other hand, column measurements are relatively insensitive to vertical transport (Keppel-Aleks 2011). Column measurements can be performed from ground-based (usually stationary) or satellite platforms. Satellite GHG columns from instruments such as SCIAMACHY (Burrows 1995) and GOSAT (Kuze 2009) can achieve quasi-global coverage, and with sufficient precision and lack of spatial and temporal biases can provide further information about GHG fluxes (Rayner 2001). Achieving retrievals that are free of bias is a challenge, particularly because of the influence of scattering processes on reflected sunlight, as measured by these satellite instruments (Oshchepkov 2012, Oshchepkov 2013).

The Total Carbon Column Observing Network (TCCON, (Wunch 2011)) is a network of high precision and accuracy ground-based Fourier Transform Infrared (FTIR) spectrometers that make direct solar absorption measurements. Direct solar absorption measurements result in a significantly higher signal than those from reflected sunlight, and therefore scattering processes make an insignificant contribution to the measurement intensity, resulting in an inherently higher measurement precision. TCCON CO\(_{\mathrm{2}}\) column measurements have been shown to have a precision better than 0.1% (Keppel-Aleks 2007, Deutscher 2010). The accuracy of TCCON is achieved via calibration against independent aircraft profiles (Deutscher 2010, Messerschmidt 2011, Wunch 2010). In addition to providing highly-precise and accurate ground-based column measurements, the TCCON serves as the primary validation for satellite retrievals from the Greenhouse Gases Observing Satellite (GOSAT) and the Orbiting Carbon Observatory-2 (OCO-2), and acts as a transfer standard between satellite and surface in situ measurements.

TCCON not only provides validation for a number of satellite missions, but can also contribute to understanding of carbon cycle science, typically via use in conjunction with atmospheric chemical transport models. For example, TCCON CO\(_2\) data revealed an underestimate in the seasonal cycle of net ecosystem exchange (NEE) in an earlier version of the CASA biosphere model (Yang 2007). More recently, Wunch et al. (2013) showed a robust relationship between boreal surface temperature anomalies and the magnitude of summer CO\(_2\) drawdown in the northern hemisphere columns; the strength of that relationship is underestimated when using biospheric models or the CarbonTracker data assimilation system. In including TCCON X\(_{CO_2}\) data in an inversion, Chevallier et al. (2011) highlighted that Australia is a region where total column observations have greater power than surface in situ measurements to provide constraints on surface flux estimates.