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Nitrogen dioxide (NO 2 ) is an important contributor to air pollution and can adversely affect human health 1-9 . A decrease in NO 2 concentrations has been reported as a result of lockdown measures to reduce the spread of COVID-19 10-20 . Questions remain, however, regarding the relationship of satellite-derived atmospheric column NO 2 data with health-relevant ambient ground-level concentrations, and the representativeness of limited ground-based monitoring data for global assessment. Here we derive spatially resolved, global ground-level NO 2 concentrations from NO 2 column densities observed by the TROPOMI satellite instrument at sufficiently fine resolution (approximately one kilometre) to allow assessment of individual cities during COVID-19 lockdowns in 2020 compared to 2019. We apply these estimates to quantify NO 2 changes in more than 200 cities, including 65 cities without available ground monitoring, largely in lower-income regions. Mean country-level population-weighted NO 2 concentrations are 29% ± 3% lower in countries with strict lockdown conditions than in those without. Relative to long-term trends, NO 2 decreases during COVID-19 lockdowns exceed recent Ozone Monitoring Instrument (OMI)-derived year-to-year decreases from emission controls, comparable to 15 ± 4 years of reductions globally. Our case studies indicate that the sensitivity of NO 2 to lockdowns varies by country and emissions sector, demonstrating the critical need for spatially resolved observational information provided by these satellite-derived surface concentration estimates., (© 2022. The Author(s).)

Denitrification of the stratospheric vortex is a crucial process for the Antarctic Ozone Hole formation resulting in an analogous stratospheric "Nitrogen Hole". Here, 2018–2021 daily TROPOMI measurements are used for the first time for a detailed characterization of this Nitrogen Hole. Nitrogen dioxide total columns exhibit strong spatiotemporal and seasonal variations associated with both photochemistry as well as transport and mixing processes. Combined with total ozone column data two main regimes are identified: inner-vortex ozone and nitrogen dioxide depleted air and outer-vortex air enhanced in ozone and nitrogen dioxide. Within the vortex total ozone and total stratospheric nitrogen dioxide are strongly correlated which is much less evident outside of the vortex. Connecting both main regimes are what is defined here as "mixing lines", a third regime of coherent patterns in the total nitrogen dioxide column – total ozone column phase space. These mixing lines exist because of differences in three dimensional variations of nitrogen dioxide and ozone thereby providing information about vortex dynamics and cross-vortex edge mixing. On the other hand, interannual variability of nitrogen dioxide – total ozone characteristics are rather small except in 2019 when the vortex was unusually unstable. Overall, the results show that daily stratospheric nitrogen dioxide column satellite measurements provide an innovative means for characterizing and Polar stratospheric denitrification processes and vortex dynamics and potentially long term monitoring if the total nitrogen column data record is extended with past satellite observations. [ABSTRACT FROM AUTHOR]

The Sentinel-5P TROPOspheric Monitoring Instrument (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 powerplants, 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 several 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, and a time series for European subregions and cities between May 2018 and March 2021. The CAMS regional products generally capture the fine-scale daily and averaged features observed by TROPOMI in much detail. In summer, the comparison shows a close agreement between TROPOMI and the CAMS ensemble NO2 tropospheric columns with a relative difference of up to 15 % for most European cities. In winter, however, we find a significant discrepancy in the column amounts over much of Europe, with relative differences up to 50 %. The possible causes for these differences are discussed, focusing on the possible impact of retrieval and modeling 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 (European Centre for Medium-Range Weather Forecasts) 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 spinoff, we present a new TROPOMI NO2 level 2 (L2) 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 six Pandora instruments of the Pandonia Global Network and nine Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments. As compared to the standard S5P tropospheric NO2 column data, the overall bias of the new product for all except two stations is 5 % to 12 % smaller, owing to a reduction in the multiplicative bias. Compared to the CAMS tropospheric NO2 columns, dispersion and correlation parameters with respect to the standard data are, however, superior. [ABSTRACT FROM AUTHOR]

Sentinel-5 Precursor (S-5P), launched in October 2017, carrying the TROPOspheric Monitoring Instrument (TROPOMI) nadir-viewing spectrometer, is the first mission of the Copernicus Programme dedicated to the monitoring of air quality, climate, and ozone. In the presented study, the TROPOMI tropospheric nitrogen dioxide (NO2) level-2 (L2) product (OFFL v1.03.01; 3.5 km × 7 km at nadir observations) has been validated over strongly polluted urban regions by comparison with coincident high-resolution Airborne Prism EXperiment (APEX) remote sensing observations (∼ 75 m × 120 m). Satellite products can be optimally assessed based on (APEX) airborne remote sensing observations, as a large amount of satellite pixels can be fully mapped at high accuracy and in a relatively short time interval, reducing the impact of spatiotemporal mismatches. In the framework of the S-5P validation campaign over Belgium (S5PVAL-BE), the APEX imaging spectrometer has been deployed during four mapping flights (26–29 June 2019) over the two largest urban regions in Belgium, i.e. Brussels and Antwerp, in order to map the horizontal distribution of tropospheric NO2. For each flight, 10 to 20 TROPOMI pixels were fully covered by approximately 2700 to 4000 APEX measurements within each TROPOMI pixel. The TROPOMI and APEX NO2 vertical column density (VCD) retrieval schemes are similar in concept. Overall, for the ensemble of the four flights, the standard TROPOMI NO2 VCD product is well correlated (R = 0.92) but biased negatively by - 1.2 ± 1.2 × 10 15 moleccm-2 or - 14 ± 12 %, on average, with respect to coincident APEX NO2 retrievals. When replacing the coarse 1 ∘ × 1 ∘ the massively parallel (MP) version of the Tracer Model version 5 (TM5) a priori NO2 profiles by NO2 profile shapes from the Copernicus Atmospheric Monitoring Service (CAMS) regional chemistry transport model (CTM) ensemble at 0.1 ∘ × 0.1 ∘ , R is 0.94 and the slope increases from 0.82 to 0.93. The bias is reduced to - 0.1 ± 1.0 × 10 15 moleccm-2 or - 1.0 ± 12 %. The absolute difference is on average 1.3 × 10 15 moleccm-2 (16 %) and 0.7 × 10 15 moleccm-2 (9 %), when comparing APEX NO2 VCDs with TM5-MP-based and CAMS-based NO2 VCDs, respectively. Both sets of retrievals are well within the mission accuracy requirement of a maximum bias of 25 %–50 % for the TROPOMI tropospheric NO2 product for all individual compared pixels. Additionally, the APEX data set allows the study of TROPOMI subpixel variability and impact of signal smoothing due to its finite satellite pixel size, typically coarser than fine-scale gradients in the urban NO2 field. For a case study in the Antwerp region, the current TROPOMI data underestimate localized enhancements and overestimate background values by approximately 1–2 × 10 15 moleccm-2 (10 %–20 %). [ABSTRACT FROM AUTHOR]

Sentinel-5 Precursor (S-5P), launched in October 2017, carrying the TROPOspheric Monitoring Instrument (TROPOMI) nadir-viewing spectrometer, is the first mission of the Copernicus Programme dedicated to the monitoring of air quality, climate, and ozone. In the presented study, the TROPOMI tropospheric nitrogen dioxide (NO2) L2 product (OFFL v1.03.01; 3.5 km x 7 km at nadir observations) has been validated over strongly polluted urban regions by comparison with coincident high-resolution Airborne Prism EXperiment (APEX) remote sensing observations (~75 m x 120 m). Satellite products can be optimally assessed based on (APEX) airborne remote sensing observations as a large amount of satellite pixels can be fully mapped at high accuracy and in a relatively short time interval, reducing the impact of spatio-temporal mismatches. In the framework of the S5PVAL-BE campaign, the APEX imaging spectrometer has been deployed during four mapping flights (26-29 June 2019) over the two largest urban regions in Belgium, i.e. Brussels and Antwerp, in order to map the horizontal distribution of tropospheric NO2. For each flight, 10 to 20 TROPOMI pixels were fully covered by approximately 2800 to 4000 APEX measurements within each TROPOMI pixel. The TROPOMI and APEX NO2 vertical column density (VCD) retrieval schemes are similar in concept. Overall for the ensemble of the four flights, the standard TROPOMI NO2 VCD product is well correlated (R = 0.92) but biased negatively by -1.2 ± 1.2 x 1015 molec cm-2 or -14 % ± 12 %, on average, with respect to coincident APEX NO2 retrievals. When replacing the coarse 1° x 1° TM5-MP a priori NO2 profiles by NO2 profile shapes from the CAMS regional CTM ensemble at 0.1° x 0.1°, the slope increases by 11 % to 0.93, and the bias is reduced to -0.1 ± 1.0 x 1015 molec cm-2 or -1.0 % ± 12 %. When the absolute value of the difference is taken, the bias is 1.3 x 1015 molec cm-2 or 16 %, and 0.7 x 1015 molec cm-2 or 9 % on average, when comparing APEX NO2 VCDs with TM5-MP-based and CAMS-based NO2 VCDs, respectively. Both sets of retrievals are well within the accuracy requirement of a maximum bias of 25-50 % for the TROPOMI tropospheric NO2 product for all individual compared pixels. Additionally, the APEX data set allows the study of TROPOMI subpixel variability and impact of signal smoothing due to its finite satellite pixel size, typically coarser than fine-scale gradients in the urban NO2 field. The amount of underestimation of peak plume values and overestimation of urban background values in the TROPOMI data is in the order of 1-2 x 1015 molec cm-2 on average, or 10 %-20 %, in case of an urban scene. [ABSTRACT FROM AUTHOR]

We present an Observing System Simulation Experiment (OSSE) dedicated to the evaluation of the added value of the Sentinel-4 and Sentinel-5P missions for tropospheric nitrogen dioxide (NO2). Sentinel-4 is a geostationary (GEO) mission covering the European continent, providing observations with high temporal resolution (hourly). Sentinel-5P is a low Earth orbit (LEO) mission providing daily observations with a global coverage. The OSSE experiment has been carefully designed, with separate models for the simulation of observations and for the assimilation experiments and with conservative estimates of the total observation uncertainties. In the experiment we simulate Sentinel-4 and Sentinel-5P tropospheric NO2 columns and surface ozone concentrations at 7 by 7 km resolution over Europe for two 3-month summer and winter periods. The synthetic observations are based on a nature run (NR) from a chemistry transport model (MOCAGE) and error estimates using instrument characteristics. We assimilate the simulated observations into a chemistry transport model (LOTOS-EUROS) independent of the NR to evaluate their impact on modelled NO2 tropospheric columns and surface concentrations. The results are compared to an operational system where only ground-based ozone observations are ingested. Both instruments have an added value to analysed NO2 columns and surface values, reflected in decreased biases and improved correlations. The Sentinel-4 NO2 observations with hourly temporal resolution benefit modelled NO2 analyses throughout the entire day where the daily Sentinel-5P NO2 observations have a slightly lower impact that lasts up to 3–6 h after overpass. The evaluated benefits may be even higher in reality as the applied error estimates were shown to be higher than actual errors in the now operational Sentinel-5P NO2 products. We show that an accurate representation of the NO2 profile is crucial for the benefit of the column observations on surface values. The results support the need for having a combination of GEO and LEO missions for NO2 analyses in view of the complementary benefits of hourly temporal resolution (GEO, Sentinel-4) and global coverage (LEO, Sentinel-5P). [ABSTRACT FROM AUTHOR]

Land use regression (LUR) modelling has increasingly been applied to model fine scale spatial variation of outdoor air pollutants including nitrogen dioxide (NO 2 ). Satellite observations of tropospheric NO 2 improved LUR model in very large study areas, including Canada, United States and Australia. The aim of our study was to assess the value of satellite observations of NO 2 in modelling the spatial variation of annual average NO 2 concentrations in a small densely populated country. We used surface level annual average NO 2 concentration and geographic information system data from 144 monitoring sites spread over the Netherlands: 26 regional background, 78 urban background and 40 traffic sites for developing land use regression models. For the 144 monitoring sites we obtained the annual average tropospheric NO 2 concentration for 2007 from the Ozone Monitoring Instrument (OMI) satellite sensor. These OMI data reflect a spatial scale of about 10 × 10 km. We calculated the correlation between satellite and surface level NO 2 concentrations for all sites and for background sites only. We next evaluated whether adding satellite observations improved land use regression models. Annual average satellite observations of tropospheric NO 2 correlated well spatially with annual average urban plus regional background (R = 0.74, n = 104 sites) and especially regional background NO 2 concentrations (R = 0.88, n = 26). The correlation was moderate for all sites, including traffic locations (R = 0.51, n = 144). A LUR model including satellite NO 2 observations performed better (overall R 2 = 0.84) than LUR models including geographical coordinates or indicator variables (overall R 2 65–74%) in modeling concentrations at the 104 background sites across the Netherlands. Satellite NO 2 observations agreed well with measured surface concentrations at background locations and improved land use regression models, even in a small densely populated country. [ABSTRACT FROM AUTHOR]

The TROPOspheric Monitoring Instrument (TROPOMI) is a spectrometer measuring in the UV, visible, near-infrared and short-wave infrared, which allows the retrieval of trace gas species like O3, NO2, HCHO, SO2, CO, CH4 and aerosol aspects like the aerosol index. The Copernicus Sentinel-5P satellite, with TROPOMI as payload, was successfully launched on 13 October 2017. TROPOMI has a full global coverage each day, but with a much-improved resolution (3.5 x 7 km2) compared to the Ozone Monitoring Instrument (OMI) which is providing measurements since 2004. Because of the fine resolution, the TROPOMI observations are expected to be of great importance for estimating pollutant concentrations and emissions at the scale of smaller towns, individual power plants, wildfires and major infrastructures. Operational data products, including NO2, have become available July 2018 (doi: 10.5270/S5P-s4ljg54). The Nitrogen Dioxide (NO2) tropospheric columns are retrieved using the DOAS algorithm combined with an integrated modelling-retrieval-assimilation approach to derive the air-mass factors and to estimate the stratospheric column. This latter component is based on the TM5-MP chemistry-transport model operated at a resolution of 1x1 degree. Developments from the EU QA4ECV project (www.qa4ecv.eu) have been included in the retrieval software to ensure consistency with the datasets from this project, including OMI NO2. More information can be found at http://www.tropomi.eu/data-products/nitrogen-dioxide.In our contribution we will provide an overview of the TROPOMI NO2 retrieval with a focus on recent developments and plans. An assessment of the quality of the NO2 product will be provided, and the stability of the NO2 product since launch will be reported. Comparisons will be presented with surface remote sensing observations (e.g. MAX-DOAS), the OMI QA4ECV NO2 products, and with CAMS global and regional air quality models. At the time of the EGU, TROPOMI will have accumulated a full year of nearly continuous observations (April 2018 - March 2019). The benefits of the high-resolution TROPOMI NO2 observations for the monitoring of sources and air quality will be demonstrated. [ABSTRACT FROM AUTHOR]