Goemetry-Dependent Lambertian-Equivalent Surface Reflectivity (GLER) Product for Tropomi NO2 Retrieval

The accuracy of trace gas retrievals from low earth orbiting and geostationary satellites requires accounting for the anisotropy and seasonal changes in the Earth’s surface reflectance. Currently most cloud and trace gas retrieval algorithms use angular independent minimal Lambertian-equivalent reflectivity (minLER) climatologies, which neglect the surface bidirectional reflectance distribution function (BRDF) effect, interannual variations, and may not sufficiently account for varying land cover and use changes. The concept known as Geometry-dependent LER (GLER) captures BRDF effects due to solar and satellite viewing angle dependence as well as daily, seasonal, and long-term changes in the surface reflectance. We show that using GLER in NO₂ retrievals in place of a climatological minLER improves the accuracy of air mass factor (AMF) calculations and increases tropospheric NO₂ retrievals in highly polluted areas by up to 50%. The use of GLER also enhances the retrieval accuracy of cloud fraction and cloud pressure in the O2-O2 cloud algorithm. GLER is calculated from simulations of Rayleigh-only top-of-atmosphere (TOA) radiances over non-Lambertian surfaces using the vector linearized discrete ordinate radiative transfer (VLIDORT) model. NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) BRDF product is used to calculate GLER over land while over water surfaces the wind-dependent Cox–Munk wave-facet slope distribution including water-leaving radiance is used to account for surface BRDF. GLER products have been developed for multiple satellite instruments including Ozone Monitoring Instrument (OMI) and Sentinel-5 Precursor (S5P) TROPOspheric Monitoring Instrument (TROPOMI) with several new improvements and updates. Some of the more recent improvements to GLER include the use of the Collection 6.1 MODIS BRDF product, GEOS FP-IT terrain height, improved snow and ice classification from the Interactive Multi-sensor Snow and Ice Mapping System (IMS), and improved version of VLIDORT. We demonstrate how the use of GLER is beneficial to TROPOMI’s high spatial resolution (up to 5.5 km x 3.5 km) measurements to monitor atmospheric trace gas pollutants down to the sub-city scale.