New-generation OMI NO2 Standard Product: Algorithm description and initial results

We present our new and improved version (version 4.0) of the NASA standard nitrogen dioxide (NO2) product from the Ozone Monitoring Instrument (OMI) on the Aura satellite. This version incorporates the most important improvements proposed for regional OMI NO₂ products by expert users, and enhances NO2 data quality on a global scale through improvements in the Air Mass Factors (AMFs) in several ways. The algorithm is based on a conceptually new, geometry-dependent Lambertian surface equivalent reflectivity (GLER) operational product. GLER is calculated using the vector radiative transfer model VLIDORT, which uses as input high–resolution bidirectional reflectance distribution function (BRDF) information from NASA’s Aqua MODIS instrument over land and the wind-dependent Cox–Munk wave-facet slope distribution over water, the latter with a contribution from the water-leaving radiance. The GLER and our corresponding, consistently retrieved effective cloud fraction and O₂-O₂ optical centroid cloud pressures provide inputs to the new NO₂ AMF algorithm, which increases tropospheric NO₂ by up to 50% in highly polluted areas; the differences include both cloud and surface BRDF effects as well as biases between the MODIS and OMI-based surface reflectance data sets. We assess the new product using independent observations from ground-based and airborne instruments. The improved NO₂ data record could be beneficial for studies related to emissions and trends of nitrogen oxides (NO_x) and co-emitted gases.