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\section{Introduction}
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 \cite{Gurney_2002}. \citet{Yang_2007} and \citet{Stephens_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
\citep{Keppel-Aleks2011}. \cite{Keppel_Aleks_2011}. Column measurements can be performed from ground-based (usually stationary) or satellite platforms. Satellite GHG columns from instruments such as SCIAMACHY
\citep{Burrows1995} \cite{Burrows_1995} and GOSAT
\citep{Kuze2009} \cite{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
\citep{Rayner2001}. \cite{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
\citep{Oshchepkov2012,Oshchepkov2013}. \cite{Oshchepkov_2012}\cite{Oshchepkov_2013}.
The Total Carbon Column Observing Network (TCCON) is a network of high precision and accuracy ground-based Fourier Transform Infrared (FTIR) spectrometers, making 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\% \citep{Deutscher2010,Keppel-Aleks2007}. The accuracy of TCCON is achieved via calibration against independent aircraft profiles \citep{Deutscher2010,Messerschmidt2011,Wunch2010}. In addition to providing highly-precise and accurate ground-based column measurements, the TCCON serves as the primary validation for satellite retrievals and a transfer standard between satellite and surface in situ measurements.