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

Abstract

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.

Potential of TROPOMI for understanding spatio-temporal variations in surface NO2 and their dependencies upon land use over the Iberian Peninsula

“…Consistent with this, the validation presented in Douros et al (2022) using European ground-based MAX-DOAS and Pandora instruments demonstrated that the negative bias was improved by 5-18% when using the CAMS a priori profiles. In the present study, we kept the original TROPOMI products, although we acknowledge that near future studies should probably explore such a use of alternative a priori profiles.…”

“…Following this observation-based study, the important next step is to assess the ability of state-of-the-art chemistry-transport models fed with most recent anthropogenic and natural emissions to reproduce the spatio-temporal variability of TrC-NO 2 over the Iberian Peninsula, including its dependency on urban and crops cover fraction. Douros et al (2022) recently highlighted some substantial discrepancies between the CAMS regional air quality ensemble and TROPOMI TrC-NO 2 , especially during wintertime, which could nonetheless points toward issues in both models and TROPOMI retrievals. Considering the aforementioned lack of surface monitoring stations in agricultural areas but potentially strong impact of soil NO emissions on the production of tropospheric O 3 , evaluating CTMs against TROPOMI in these specific areas is of particular interest, although this likely comes with some challenges due to the relatively weak and diffuse signal of this specific emission source compared to other anthropogenic sources.…”

Evaluating NOx emissions and their effect on O3 production in Texas using TROPOMI NO2 and HCHO

“…One can either apply the averaging kernel from the satellite instrument to the NO 2 column from the model simulation or use the NO 2 vertical profile from the model simulation and the averaging kernel to recalculate AMF and tropospheric NO 2 vertical column of the satellite measurement. Typically, studies use either one of the two methods; similar to Douros et al (2022), we use both.…”

Identifying Sources of NOx emissions from Aircraft through Source Apportionment and Regression Models

“…x 5.5 km 2 spatial resolution which is unable to capture the peaks of the NO2 pollution, especially the NO2 in narrow point source plume 42 . Previous work shows that the air mass factor is a small contributor to this low satellite bias in this area 30 , but this can vary by region 43 . The low satellite bias is consistent with comparisons completed by the TROPOMI Cal-Val team 44 .…”