deletions | additions       diff --git a/section_Introduction_Knowledge_of_the__.tex b/section_Introduction_Knowledge_of_the__.tex  index 10607b7..8f12081 100644  --- a/section_Introduction_Knowledge_of_the__.tex  +++ b/section_Introduction_Knowledge_of_the__.tex 
...
\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 \citep{Gurney2002}. \citet{Stephens2007} \cite{Yang_2007}  and \citet{Yang2007} \cite{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}. Column measurements can be performed from ground-based (usually stationary) or satellite platforms. Satellite GHG columns from instruments such as SCIAMACHY \citep{Burrows1995} and GOSAT \citep{Kuze2009} 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}. 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}. 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.