Impact of synthetic space-borne NO2 observations from the Sentinel-4 and Sentinel-5P missions on tropospheric NO2 analyses

Timmermans, Renske ; Segers, Arjo ; Curier, Lyana ; Abida, Rachid ; Attié, Jean-Luc ; El Amraoui, Laaziz ; Eskes, Henk ; de Haan, Johan ; Kujanpää, Jukka ; Lahoz, William ; Oude Nijhuis, Albert ; Quesada-Ruiz, Samuel ; Ricaud, Philippe ; Veefkind, Pepijn ; Schaap, Martijn

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<p align=justify><strong>Abstract.</strong> 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 (<span class="inline-formula">NO<sub>2</sub></span>). 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 <span class="inline-formula">NO<sub>2</sub></span> 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 <span class="inline-formula">NO<sub>2</sub></span> 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 <span class="inline-formula">NO<sub>2</sub></span> columns and surface values, reflected in decreased biases and improved correlations. The Sentinel-4 <span class="inline-formula">NO<sub>2</sub></span> observations with hourly temporal resolution benefit modelled <span class="inline-formula">NO<sub>2</sub></span> analyses throughout the entire day where the daily Sentinel-5P <span class="inline-formula">NO<sub>2</sub></span> 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 <span class="inline-formula">NO<sub>2</sub></span> products. We show that an accurate representation of the <span class="inline-formula">NO<sub>2</sub></span> 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 <span class="inline-formula">NO<sub>2</sub></span> analyses in view of the complementary benefits of hourly temporal resolution (GEO, Sentinel-4) and global coverage (LEO, Sentinel-5P).</p>
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