ROUGH DRAFT authorea.com/103049

# 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).

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% (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 and a transfer standard between satellite and surface in situ measurements.

# Site description

## Location

Wollongong, Australia is situated on a narrow coastal plain bordered by the Pacific Ocean to the east and a steep sandstone precipice to the west. Wollongong, population ~300,000, is located approximately 82 kilometres south of Sydney (population ~4.6 million). The measurement site is located at the University of Wollongong (34.406$$^{\circ}$$S, 150.879$$^{\circ}$$E, 30 m above sea level). The escarpment to the west rises approximately 400 m within 2 km of the measurement site, making Wollongong a challenging site for satellite validation.