29 Jun 2022
29 Jun 2022
Status: this preprint is open for discussion.

Comparing Sentinel-5P TROPOMI NO2 column observations with the CAMS-regional air quality ensemble

John Douros1, Henk Eskes1, Jos van Geffen1, K. Folkert Boersma1,2, Steven Compernolle3, Gaia Pinardi3, Anne-Marlene Blechschmidt4, Vincent-Henri Peuch5, Augustin Colette6, and Pepijn Veefkind1 John Douros et al.
  • 1Royal Netherlands Meteorological Institute, 3730 AE De Bilt, The Netherlands
  • 2Wageningen University, Meteorology and Air Quality group, 6708 PB Wageningen, The Netherlands
  • 3Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Ringlaan 3, 1180 Uccle, Belgium
  • 4Institute of Environmental Physics, University of Bremen, IUP-UB,Otto-Hahn-Allee 1, D-28359 Bremen, Germany
  • 5European Centre for Medium-range Weather Forecast (ECMWF), Sinfield Park, Reading, UK
  • 6National Institute for Industrial Environment and Risks (INERIS), Verneuil-en-Halatte, France

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.

John Douros et al.

Status: open (until 07 Sep 2022)

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John Douros et al.

John Douros et al.


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Short summary
This is an article that focuses on the challenges associated with comparing atmospheric composition models with satellite products such as tropospheric NO2 columns. The aim is to highlight the methodological difficulties and propose sound ways of doing such comparisons. Building on the comparisons, a new satellite product is proposed and made available, which takes advantage of higher resolution, regional atmospheric modelling to improve estimates of troposheric NO2 columns over Europe.