Assessing the Impact of Air Mass Factor on Satellite-Based Surface NO2 Concentration Estimates over India

Satellite retrievals of nitrogen dioxide (NO₂) vertical column density (VCD) are widely used to estimate human exposure to ambient NO₂ and related health impacts. The satellite retrieval of NO₂ VCD involves calculating the Air Mass Factor (AMF) to convert slant column densities into VCD. The AMF calculation requires a priori assumption of the vertical distribution of the species, which can be provided from a global or regional chemical transport model (CTM). The vertical profile of NO₂ simulated using a CTM can then be used to derive surface NO₂ concentrations. Previous studies have identified ​​AMF calculation as a significant source of uncertainty in NO₂ VCD retrievals, suggesting that AMF recalculation using high-resolution CTM simulations can improve both satellite-derived VCDs and surface NO₂ estimates. In this study, we assess the impact of AMF on satellite-based column and surface NO₂ estimation over a particular country, India. This region is significant, as satellite-based surface NO₂ concentration estimates over India are typically underestimated compared to regional in situ observations. Here, we use the TROPOspheric Monitoring Instrument (TROPOMI) retrievals and the GEOS-Chem global and nested regional CTM simulations to explore the impact of AMF on NO₂ VCD and surface data. We perform multiple model experiments using prior NO₂ vertical profiles derived from different spatial resolution CTM simulations (2° × 2.5°, 0.25° × 0.3125°, and 0.125° × 0.15625° latitude × longitude) and by varying the meteorological and emissions inputs in the model. The recalculated AMFs, using different NO2 vertical profiles from various model experiments, are then applied to generate VCD and surface NO₂ estimates and validated against available in situ measurements across India. Our preliminary results indicate that modified surface NO₂ concentration estimates generally show better agreement with in situ observations compared to those estimated using the standard TROPOMI VCD product. This study highlights the importance of regional-scale AMF recalculation in enhancing the accuracy of TROPOMI-derived NO₂ retrievals and providing a reliable representation of surface NO₂ over India.