# 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 (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).