## 2 Materials and Methods
### Instruments and Network Requirements
MAX-DOAS systems that perform elevation scans at multiple azimuthal viewing directions (2D instruments) provide an improved spatial coverage compared to one-dimensional (1D) instruments that work at a single fixed azimuth. This allows for the investigation of possible horizontal inhomogeneities and for a better characterisation of the field around the measurement site [15; 16]. It should be noted that the requirements mentioned above are indicative and should not be treated as strict thresholds. The geographical layout of an optimal MAX-DOAS validation network is determined by the spatial variability and distribution of the parameters of interest. Species with long atmospheric lifetimes, such as carbon dioxide (CO\({}_{2}\)), need fewer stations than reactive gases, such as NO\({}_{2}\), which are characterised by high spatio-temporal variability. Thorough validation requires a spatial distribution of the ground-based measurements covering both hemispheres, all relevant latitudes from the tropics to polar regions, background regions and measurement sites where high concentrations are expected, as well as regions that are characterised by different conditions with respect to parameters that can potentially affect the quality of the measurements, such as surface albedo, topography, cloud coverage and aerosol loading. Figure 1 visualises the current geographical distribution of MAX-DOAS systems that are centrally processed by FRM4 DOAS. We anticipate that this network of instruments will grow with time when the FRM4 DOAS system moves into an operational phase. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Station Name** & **Country** & **Institute Acronym** & **Latitude (\({}^{\circ}\)N)** & **Longitude (\({}^{\circ}\)E)** & **Altitude (m a.s.l.)** \\ \hline Athens & Greece & IUP Bremen [1] & 38.049 & 23.860 & 532 \\ Izana & Spain & INTA [9] & 28.309 & \(-\)16.499 & 2393 \\ Kinshasa & RD Congo & BIRA-IASB [2] & \(-\)4.420 & 15.310 & 450 \\ La Reunion Maido & France & BIRA-IASB [2] & \(-\)21.059 & 55.380 & 2158 \\ Lauder & New Zealand & NIWA [10] & \(-\)45.037 & 169.684 & 374 \\ Neumayer & Antarctica & IUP Heidelberg [5] & \(-\)70.690 & \(-\)8.270 & 48 \\ Arrival Heights & Antarctica & IUP Heidelberg [5] & \(-\)77.830 & 166.649 & 192 \\ \hline \end{tabular}  
* \({}^{1}\) Institute for Environmental Physics, University of Bremen, Bremen, Germany; \({}^{2}\) Royal Belgian Institute for Space Aeronomy, Brussels, Belgium; \({}^{3}\) Royal Netherlands Meteorological Institute, Utrecht, Netherlands; \({}^{4}\) Max Planck Institute for Chemistry, Mainz, Germany; \({}^{5}\) Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germany; \({}^{6}\) Institute of Atmospheric Sciences and Climate, Bologna, Italy; \({}^{7}\) Environment and Climate Change Canada, Air Quality Research Division, Toronto, Canada; \({}^{8}\) Laboratory of Atmospheric Physics, Aristotle University of Thessaloniki, Thessaloniki, Greece; \({}^{9}\) Instituto Nacional de Tecnica Aeroespacial, Torrejón de Ardoz, Spain; \({}^{10}\) National Institute of Water and Atmosphere Research Centre, Lauder, New Zealand.  
\end{table} 
Table 1: _Cont._