Impact of synthetic spaceborne NO2 observations from the Sentinel-4 and Sentinel-5p platforms on tropospheric NO2 analyses

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 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 NO2 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 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 on 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 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 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).