Representativeness of TROPOMI tropospheric NO₂ columns near Asian airports constrained by IAGOS airborne measurements

Abstract. Tropospheric NO₂ data retrievals from the satellite-borne TROPOspheric Monitoring Instrument (TROPOMI) are sensitive to the vertical profile assumed in the air mass factor (AMF) calculation, especially in polluted lower-tropospheric environments. This study investigated the use of aircraft and ground-based profile information to evaluate this sensitivity near five major airports. We combined TROPOMI tropospheric NO₂ vertical column densities (VCDs), land-based Pandora observations, and airplane-based In-service Aircraft for a Global Observing System (IAGOS) descent profiles for selected airport–Pandora pairs in Japan, South Korea, and Thailand from March to July 2025. Before applying profile-based AMF recalculations, we examined whether the TROPOMI NO₂ field around each Pandora site was representative of that around the corresponding airport. The airport- and Pandora-region time series were generally coherent, although their mean NO₂ levels differed, indicating that temporal consistency and absolute spatial representativeness need to be considered separately.

The lower part of the a priori NO₂ profile obtained by the TM5-MP model was then replaced with IAGOS-derived profiles to recalculate TROPOMI AMFs and VCDs. The IAGOS-profile recalculation reduced the overall low bias relative to Pandora data and improved several validation metrics, but the response differed by site. Improvements were most evident where the IAGOS descent profiles, Pandora site, and satellite-observed NO₂ field sampled similar environments. In contrast, weaker or degraded performance occurred when the aircraft profiles represented an air mass different from that influencing the Pandora-acquired column. The Pandora-scaled-profile case produced the closest agreement with Pandora results, but the comparison is more suitable as a profile-sensitivity test rather than an independent validation. Residual diagnosis showed that the remaining differences after the Pandora-scaled-profile recalculation were not mainly controlled by horizontal NO₂ heterogeneity, pixel sampling density, mean quality assurance value, or the magnitude of the AMF correction. A slant column density (SCD)-equivalent closure analysis further suggested that profile replacement alone cannot fully reconcile TROPOMI and Pandora columns, because the recalculated VCD still depends on the TROPOMI effective tropospheric SCD and other non-profile retrieval inputs. These findings demonstrate the value of IAGOS and Pandora lower-tropospheric profile information for diagnosing TROPOMI NO₂ AMF uncertainty near airports, while underscoring the need to account for slant-column retrieval uncertainty, cloud and surface effects, and spatial representativeness.