## 5 Conclusions and Outlook  
Complementing other ground-based remote sensing atmospheric composition monitoring networks, such as AERONET [66] or PGN, the NDACC MAX-DOAS UV-Visible network focuses on measurements of trace gases in the troposphere. Although instruments and QA/QC procedures comply with quality standards imposed by the NDACC, e.g., through participation in regular intercalibration campaigns, efforts to harmonise retrieval methods and data production were limited, and for this reason, NDACC MAX-DOAS measurements did not strictly comply with the requirements formulated within QA4 EO and imposed by the CEOS and space agencies to qualify as Fiducial Reference Measurements, i.e., reference measurements that meet the needs of the satellite community. For this reason, the NDACC UV-Vis working group, supported by ESA, initiated, in late 2016, the FRM4 DOAS project. Based on a consortium of expert scientists having demonstrated knowledge in MAX-DOAS instrument calibration and operation, data retrieval methods and large-scale automated data processing, the first central data processing system for MAX-DOAS type instruments delivering quality-controlled trace gas and aerosol data products with a time latency of 1 day was developed. This system incorporates advanced retrieval algorithms for lower tropospheric and stratospheric NO\({}_{2}\) vertical profiles, formaldehyde vertical profiles and total ozone columns, selected by community consensus following a round-robin protocol. Its design is highly modular, which enables the seamless integration of new tools or algorithm updates as well as its upscaling to enable the progressive integration of a large number of stations (up to about 100 instruments). During the first phase of the project, various activities took place including system design and implementation, performance assessment and various demonstration studies for satellite validation and model comparisons. A special emphasis was put on assessing the reliability of the generated data products through comparisons with results from alternative retrieval algorithms as well as independent measurements. Currently underway, the second R&D phase of the project focuses on improving existing algorithms and developing new ones, more specifically for cloud detection and classification, NO\({}_{2}\) measurements in urban areas and aerosol measurements. We also work on extending the system to generate a network-based stratospheric BrO data product, which would uniquely be available from FRM4 DOAS. In its current state, the FRM4 DOAS service has been funded up to a demonstrational status, implying a limited capacity of processing and operation (restricted to the NRT data stream) but full functionality in terms of level-2 data delivery to the NDACC rapid delivery (RD) database and support to level-1 data providers. Currently, FRM4 DOAS integrates 22 stations and delivers daily total ozone and tropospheric NO\({}_{2}\) profile datasets to the NDACC RD repository. As part of the service expansion planned from 2025 onwards, we intend to operationalise the retrieval of HCHO profiles and extend data processing at existing stations to provide historical datasets. At some sites, these data cover more than 20 years of observations. We also intend to progressively ingest data from a larger number of sites, thus improving the spatial coverage and representativeness of the network. In the longer term, further extensions to the service may be envisaged. These include the retrieval of aerosol extinction, cloud information, stratospheric BrO profiles and NO\({}_{2}\) columns in urban environments. These products are currently being developed as part of the FRM4 DOAS-2.0 R&D project. Further extensions could involve the retrieval of other gases of interest for satellite validation, such as glyoxal (CHOCHO), SO\({}_{2}\), OCIO or water vapour.