Chemical Transport Models (CTM) and air quality measurements provide complementary information on atmospheric composition. Their melding improves accuracy, important for air quality studies and to support policy makers. An assimilation scheme based on 3D-Var / OI has been developed and implemented for the CTM FARM, core of the MINNI modelling system, which is operational within the regional Copernicus Atmospheric Monitoring Service (CAMS). The scheme assimilates O3, NO2, CO, SO2, PM10 and PM2.5 concentration observations from ground stations and it can be adapted to other pollutants. Two assimilation experiments are validated with independent observations, and compared with a simulation and with CAMS Validated ReAnalysis (VRA) for the year 2018 over the European domain. Both experiments expectedly improve on the free simulation control run, and their scores are consistent with those of the VRA ensemble. In the second experiment the Spatial Consistency Test (SCT) is employed to precisely pinpoint observations affected by large errors and to prevent their assimilation. This significantly improves results that compare very well with the reanalysis ensemble median. © 2023 Elsevier Ltd

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