Evaluation of OMI operational standard NO₂ column retrievals using in situ and surface-based NO₂ observations

We assess the standard operational nitrogen dioxide (NO₂) data product (OMNO₂, version 2.1) retrieved from the Ozone Monitoring Instrument (OMI) onboard NASA's Aura satellite using a combination of aircraft and surface in~situ measurements as well as ground-based column measurements at several locations and a bottom-up NOx emission inventory over the continental US. Despite considerable sampling differences, NO₂ vertical column densities from OMI are modestly correlated (r = 0.3-0.8) with in situ measurements of tropospheric NO₂ from aircraft, ground-based observations of NO2 columns from MAX-DOAS and Pandora instruments, in situ surface NO₂ measurements from photolytic converter instruments, and a bottom-up NOx emission inventory. Overall, OMI retrievals tend to be lower in urban regions and higher in remote areas, but generally agree with other measurements to within ± 20%. No consistent seasonal bias is evident. Contrasting results between different data sets reveal complexities behind NO₂ validation. Since validation data sets are scarce and are limited in space and time, validation of the global product is still limited in scope by spatial and temporal coverage and retrieval conditions. Monthly mean vertical NO₂ profile shapes from the Global Modeling Initiative (GMI) chemistry-transport model (CTM) used in the OMI retrievals are highly consistent with in situ aircraft measurements, but these measured profiles exhibit considerable day-to-day variation, affecting the retrieved daily NO₂ columns by up to 40%. This assessment of OMI tropospheric NO₂ columns, together with the comparison of OMI-retrieved and model-simulated NO₂ columns, could offer diagnostic evaluation of the model.