Benefits and challenges of assimilating Sentinel-5P TROPOMI retrievals into the regional CAMS analysis with EURAD-IM

The regional CAMS (Copernicus Atmosphere Monitoring Service) analyses mainly profit from the assimilation of data from ground-based monitoring stations to obtain the best representation of the atmospheric state over Europe. Modern satellite missions such as Sentinel-5P and Sentinel-4, now provide high-resolution retrievals of atmospheric trace-gas columns. In order to assimilate such retrievals in EURAD-IM (European Air pollution Dispersion – Inverse Model), an observation operator is developed and tested. The challenge lies in the degrees of freedom how to distribute column values vertically across the different model levels. Furthermore, the assimilation of noise in the retrieval data poses risks to a meaningful representation of air pollutants in the model analysis. In this study, we assimilate nitrogen dioxide and sulphur dioxide TROPOMI retrievals in EURAD-IM, both with and without observations from the European ground-based monitoring network. The three-dimensional variational data assimilation technique is applied. Additionally, we test the potential of assessing emission with four-dimensional variational data assimilation. We compare and evaluate the simulation experiments using ground-based and airborne data. This demonstrates the added value of the satellite data assimilation. In this way, the EURAD-IM assimilation system is prepared for the upcoming hourly data of Sentinel-4.