Comparing Sentinel-5P TROPOMI NO2 column observations with the CAMS-regional air quality ensemble
Authors: Douros John , Eskes Henk , van Geffen Jos , Boersma K. Folkert , Compernolle Steven , Pinardi Gaia , Blechschmidt Anne-Marlene , Peuch Vincent-Henri , Colette Augustin , Veefkind Pepijn 
Published: June 2022

Abstract. The Sentinel-5P TROPOMI instrument, launched in October 2017, provides unique observations of atmospheric trace gases at a high resolution of about 5 km with near-daily global coverage, resolving individual sources like thermal power plants, industrial complexes, fires, medium-scale towns, roads and shipping routes. Even though Sentinel-5P (S5P) is a global mission, these datasets are especially well suited to test high-resolution regional-scale air quality (AQ) models and provide valuable input for emission inversion systems. In Europe, the Copernicus Atmosphere Monitoring Service (CAMS) has implemented an operational regional AQ forecasting capability based on an ensemble of 7 up to 11 European models, available at a resolution of 0.1° × 0.1°. In this paper, we present comparisons between TROPOMI observations of nitrogen dioxide (NO2) and the CAMS AQ forecasts and analyses of NO2. We discuss the different ways of making these comparisons, and present quantitative results in the form of maps for individual days, summer and winter months as well as a time series for European sub-regions and cities between May 2018 to March 2021. The CAMS regional products generally capture the ﬁne-scale daily and averaged features observed by TROPOMI in much detail. In summer, the quantitative comparison shows a close agreement between TROPOMI and the CAMS ensemble NO2 tropospheric columns, but in winter we ﬁnd a signiﬁcant discrepancy in the column amounts over much of Europe. The possible causes for these differences are discussed, focusing on the possible impact of retrieval and modelling errors. Apart from comparisons with the CAMS ensemble, we also present results for comparisons with the individual CAMS models for selected months. Furthermore, we demonstrate the importance of the free tropospheric contribution to the estimation of the tropospheric column, and thus include proﬁle information from the CAMS conﬁguration of the ECMWF’s global integrated model above 3 km altitude in the comparisons. We also show that replacing the global 1° × 1° a priori information in the retrieval by the regional 0.1° × 0.1° resolution proﬁles of CAMS leads to signiﬁcant changes in the TROPOMI retrieved tropospheric column, with typical increases at the emission hotspots up to 30 % and smaller increases or decreases elsewhere. As a spin-off, we present a new TROPOMI NO2 level-2 data product for Europe, based on the replacement of the original TM5-MP generated global a priori proﬁle by the regional CAMS ensemble proﬁle. This European NO2 product is compared with ground-based remote sensing measurements of 6 Pandora instruments of the Pandonia global network and 8 MAX-DOAS instruments. As compared to the standard S5P tropospheric NO2 column data, the overall bias of the new product is smaller owing to a reduction of the multiplicative bias, while compared to the CAMS tropospheric NO2 columns, dispersion and correlation parameters with respect to the standard data are superior.