Impact of 3D Cloud Structures on the Atmospheric Trace Gas Products from UV-VIS Sounders – Part III: bias estimate using synthetic and observational data

Abstract. Three-dimensional (3D) cloud structures may impact atmospheric trace gas products from ultraviolet-visible (UV-VIS) sounders. We used synthetic and observational data to identify and quantify possible cloud-related bias in NO2 tropospheric vertical column densities (TVCD). The synthetic data were based on high-resolution large eddy simulations which were input to a 3D radiative transfer model. The simulated visible spectra for low-earth orbiting and geostationary geometries were analysed with standard retrieval methods and cloud correction schemes that are employed in operational NO2 satellite products. For the observational data the NO2 products from the TROPOspheric Monitoring Instrument (TROPOMI) were used while the Visible Infrared Imaging Radiometer Suite (VIIRS) provided high spatial resolution cloud and radiance data. Cloud shadow fraction, cloud top height, cloud optical depth, solar zenith and viewing angles, were identified as the metrics being the most important in identifying 3D cloud impacts on NO2 TVCD retrievals. For a solar zenith angle less than about 40° the synthetic data show that the NO2 TVCD bias is typically below 10 %. For larger solar zenith angles both synthetic and observational data often show NO2 TVCD bias on the order of tens of %. Specifically, for clearly identified cloud shadow bands in the observational data, the NO2 TVCD appears low-biased when the cloud shadow fraction > 0.0 compared to when the cloud shadow fraction is zero. For solar zenith angles between 50–60°, about 16 % of TROPOMI pixels with high quality value NO2 TVCD retrievals, were found to be impacted by cloud effects larger than 20 %.