An Evaluation of the Tropospheric NO2 Air Mass Factors from TROPOMI Over Polluted Cities Using the NASA-GSFC NO2 Algorithm

Abstract:

The Tropospheric Monitoring Instrument (TROPOMI) was launched on October 13, 2017 on board the European Space Agency (ESA) and Copernicus Sentinel-5 Precursor (S5P) to measure atmospheric trace gas pollutants at a very high spatial resolution of 3.5 km x 7 km. TROPOMI provides an instrument capable of monitoring global air quality at sub-city scales. Most recently, TROPOMI data has been widely used to monitor changes in NO₂due to the effects of COVID 19 pandemic on retrieved tropospheric vertical column densities (VCD) around major population centers. The NASA OMI NO₂algorithm has been adapted to recalculate the air mass factors (AMF) from TROPOMI using improved auxiliary input. The algorithm then computes VCDs using TROPOMI slant column densities (SCD) obtained from the operational product provided by the Royal Netherlands Meteorological Institute (KNMI). The improved auxiliary inputs used in the calculation of the air mass factor (AMF) in the NASA algorithm, include: (1) a new daily and field-of-view (FOV) specific geometry-dependent Lambertian-Equivalent Reflectivity (GLER) product used in both NO₂and cloud retrievals; (2) daily Global Model Initiative (GMI) NO₂ profiles and ancillary pressure fields simulated at a resolution of 0.25 º x 0.25º,using the “replay” framework to constrain the dynamics for the MERRA-2 reanalysis; and (3) improved treatment over snow/ice surfaces. For this study, we will analyze the AMF calculation in comparisons with tropospheric AMFs and VCDs produced by the KNMI operational algorithm for some major cities and pollution hot spots. Since the NASA and KNMI algorithms both use the same SCDs, the differences in the VCDs can be primarily attributed to differences in both the AMF calculation and the applied troposphere-stratosphere separation scheme.