## 5 Conclusions and Outlook
In addition to satellite validation, the generation of such data products could serve many different applications, such as the assessment of three-dimensional chemical transport model outputs, analysis of variability and trends in composition, detection of pollution events and so on. The FRM4 DOAS service recently went through a CEOS-FRM self-assessment process aiming at assessing its compliance against a set of criteria addressing instrumentation, operations, data sampling, metrology and verification. The purpose was to evaluate where the current system is mature and where evolution and effort are still necessary. Based on this exercise, FRM4 DOAS falls under class C according to the CEOS-FRM definition, which is a good score but also implies that improvements are needed to reach full compliance with FRM standards, i.e., class A. In the future, efforts will mainly concentrate on further operationalising the service and extending the number of stations and instruments being integrated into the processing system. The FRM4 DOAS centralised data processing system will also constitute an essential element of the ACTRIS Centre for Reactive Trace Gases Remote Sensing (CREGARS, [[https://www.actris.eu/index.php/topical-centre/cregars](https://www.actris.eu/index.php/topical-centre/cregars)]([https://www.actris.eu/index.php/topical-centre/cregars](https://www.actris.eu/index.php/topical-centre/cregars)), accessed on 27 November 2024), which includes a service to MAX-DOAS instruments operated at ACTRIS Reactive Trace Gases Remote Sensing National Facilities. Conceptualisation, M.V.R., F.H., M.M.F. and C.F. ; methodology, F.H., M.M.F., C.F., M.V.R., U.F., A.R., T.W. and S.B. ; software, M.M.F., C.F. and S.B. ; validation, A.R., M.M.F., T.B., A.M. and G.P. ; formal analysis, M.M.F., U.F., J.-L.T., T.B. and L.R. ; investigation, M.M.F., U.F., J.-L.T., F.H., A.P., A.R., K.K. and T.W. ; resources, S.B., U.F., M.N.C., O.P., M.Y., D.K., C.P.-R., S.Z., T.W., A.R., A.P. and A.M.; data curation, C.F., M.M.F. and F.H. ; writing--original draft preparation, M.V.R., M.M.F., U.F., A.M., G.P., A.R., T.W., D.K., C.P.-R., O.P. and M.N.C. ; writing--review and editing, M.V.R., A.B. and all; visualisation, M.M.F., M.V.R., D.K., U.F., G.P. and A.M.; supervision, F.H. ; project administration, F.H., M.M.F. and M.V.R. ; funding acquisition, M.V.R. All authors have read and agreed to the published version of the manuscript. This research and APC were funded by the European Space Agency's FRM Programme under grant agreement no. 4000118181/16/1-EF (FRM4 DOAS) and 4000135355/21/1-DT-lr (FRM4 DOAS 2.0). It was also supported by the ACTRIS European Research Infrastructure within its CREGARS Topical Centre as well as by the Belgian Federal Science Policy (Belspo) through the ACTRIS-BE project (contract no. FSIRI/00/AC1). The FRM4 DOAS level-2 products of total ozone and NO\({}_{2}\) columns and profiles are available from the NDACC rapid delivery data repository at [[https://www-air.larc.nasa.gov/missions/ndacc/data.html?RapidDelivery=rd-list](https://www-air.larc.nasa.gov/missions/ndacc/data.html?RapidDelivery=rd-list)]([https://www-air.larc.nasa.gov/missions/ndacc/data.html?RapidDelivery=rd-list](https://www-air.larc.nasa.gov/missions/ndacc/data.html?RapidDelivery=rd-list)), accessed on 27 November 2024. The authors wish to thank all the scientists and operators involved in the instrument development, operation and data analysis that made the FRM4 DOAS project possible. INTA's team acknowledges the support of the Spanish Ministry of Science through the different research projects that have allowed long-term observations from Izana. We also thank [PERSON] and [PERSON] from the European Space Agency for their continuous support of this project and their contributions to various meetings and discussions. Author [PERSON] was employed by the company BK Scientific. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.