## 2 Materials and Methods  
### Instruments and Network Requirements  
Although the overall measurement principle is the same for all UV-Vis DOAS systems operated worldwide, there is a wide diversity of such instruments, both in design and operation, since they are usually developed by individual research groups and institutes. Differences among instruments are primarily driven by the scientific applications for which they are developed (e.g., stratospheric research, air pollution monitoring, monitoring of volcanic emissions, power plant emission monitoring, etc. ), as well as by the cost of development and ease of deployment. Such different approaches to the design and operation can become limiting factors for the establishment of a network, and this needs to be addressed when integrating instruments into a centralised processing system. Hence, FRM4 DOAS provides recommendations and guidelines (best practices) for instrument operation and characterisation. In addition, as further described later, it focuses on the development of a near-real-time centralised data processing system for all participating instruments using homogenous retrieval settings. Currently, more than 20 ground-based DOAS instruments are centrally processed by FRM4 DOAS. Their locations and responsible institutes are listed in Table 1. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Station Name** & **County** & **Institute Acronym** & **Latitude (\({}^{\circ}\)N)** & **Longitude (\({}^{\circ}\)E)** & **Altitude (m a.s.l.)** \\ \hline Ny-Alesund & Norway & IUP Bremen 1 & 78.919 & 11.930 & 20 \\ Harestua & Norway & BIRA-IASB 2 & 60.216 & 10.753 & 600 \\ Bremen & Germany & IUP Bremen 1 & 53.103 & 8.849 & 46 \\ De Bilt & Netherlands & KNMI 3 & 52.101 & 5.177 & 22 \\ Cabauw & Netherlands & KNMI 3 & 51.967 & 4.927 & 3 \\ Uccle & Belgium & BIRA-IASB 2 & 50.799 & 4.360 & 125 \\ Vieslam & Belgium & BIRA-IASB 2 & 50.305 & 5.998 & 445 \\ Mainz & Germany & MPIC 4 & 49.990 & 8.229 & 160 \\ Heidelberg & Germany & IUP Heidelberg5 & 49.417 & 8.674 & 145 \\ Jungfraujoch & Switzerland & BIRA-IASB 2 & 46.547 & 7.985 & 3580 \\ San Pietro Capofiume & Italy & CNR-ISAC 6 & 44.650 & 11.619 & 12 \\ Toronto Downsview & Canada & ECCC 7 & 43.780 & \(-\)79.468 & 221 \\ Thessaloniki & Greece & AUTH-LAP 8 & 40.633 & 22.955 & 80 \\ Xianghe & China & BIRA-IASB 2 & 39.750 & 116.959 & 95 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Geographical location of ground-based UV–Visible stations that operationally submit data to the FRM4 DOAS processing system. The requirements for MAX-DOAS systems to be used for satellite Cal/Val are primarily driven by the needs of the satellite products, both in terms of relevance and the necessary accuracy and precision of the measured quantities. The wavelength coverage of the instruments' spectrographs as well as their spectral resolution are major factors in all DOAS applications since they impact the sensitivity of the measurements and the number of species that can be retrieved. Within FRM4 DOAS, both the UV and visible ranges of the spectrum need to be covered, typically from 300 to 550 nm, so that multiple absorbers can be measured. Furthermore, high signal-to-noise ratio (SNR) measurements are required (3000-4000 depending on the target species) to provide high-quality reference ground-based datasets for the validation of satellite observations. While twilight observations at a single viewing direction (zenith) are adequate for the retrieval of stratospheric trace gas concentrations [11; 12; 13], monitoring of tropospheric trace gases requires scanning across elevation angles [7; 8]. Hence, most instruments contributing to FRM4 DOAS are equipped with motorised systems, characterised by a pointing accuracy better than 0.2\({}^{\circ}\)[14] and allowing scanning from near the horizon up to the zenith. The field of view (FOV) of the instruments should also be restricted, usually by a telescope, to typically less than 1.5\({}^{\circ}\), ensuring that spectral radiances at different elevation angles do not overlap.