Impact of synthetic spaceborne NO<sub>2</sub> observations from the Sentinel-4 and Sentinel-5p platforms on tropospheric NO<sub>2</sub> analyses

Abstract

<p><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 (NO<sub>2</sub>). Sentinel 4 is a geostationary (GEO) mission covering the European continent, providing observations with high temporal resolution (hourly). Sentinel 5P is a low-Earth Orbiting (LEO) mission providing daily observations but 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 NO<sub>2</sub> columns and surface ozone concentrations at 7 by 7 km resolution over Europe for two three-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 from the NR to evaluate their impact on modelled NO<sub>2</sub> 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 on analysed NO<sub>2</sub> columns and surface values, reflected in decreased biases, and improved correlations. The Sentinel 4 NO<sub>2</sub> observations with hourly temporal resolution benefit modelled NO<sub>2</sub> analyses throughout the entire day where the daily Sentinel 5P NO<sub>2</sub> observations have a slightly lower impact that lasts up to 3–6 hours 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 NO<sub>2</sub> products. We show that an accurate representation of the NO<sub>2</sub> 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 NO<sub>2</sub> analyses in view of the complementary benefits of hourly temporal resolution (GEO, Sentinel 4) and global coverage (LEO, Sentinel 5P).</p>